02961E00_Eni Guidelines on Risk-Based Inspections

Eni S.p.A.Exploration & Production Division COMPANY STANDARD GUIDELINES FOR RISK BASED INSPECTIONS 02961.VAR.COR.SDS Rev. 0 March 2011 0 First issue TEMM TEMM TEMM 03/2011 REV. DESCRIPTION COMPILED VERIFIED APPROVED DATE ENGINEERING COMPANY STANDARD Documento riservato di proprietà di Eni S.p.A. Divisione Agip. Esso non sarà mostrato a Terzi né utilizzato per scopi diversi da quelli per i quali è stato inviato. This document is property of Eni S.p.A. Divisione Agip. It shall neither be shown to Third Parties not used for purposes other than those for which it has been sent. Eni S.p.A. 02961.VAR.COR.SDS Rev.0 March 2011 Exploration & Production Division Page 2 of 54 PREMISE Rev. 0 First issue March 2011 Eni S.p.A. 02961.VAR.COR.SDS Rev.0 March 2011 Exploration & Production Division Page 3 of 54 TABLE OF CONTENTS 1. GENERAL 1.1 Foreword 1.2 Scope 1.3 References 1.3.1 Codes and Standards 1.3.2 Eni Company Standards 1.3.3 Eni E&P Company Documents 1.4 Definitions of terms 1.5 Acronyms and abbreviations 2. INTRODUCTION 2.1 RBI targets and project phases 2.1.1 Base case 2.1.2 Asset integrity of existing facilities 2.2 Limitations and exclusions 2.3 RBI and Asset Integrity Management 2.4 Benefits of RBI 2.5 RBI overview 2.5.1 The RBI process 2.5.2 The RBI flow chart 2.5.3 The RBI team 2.5.4 Supports to RBI 3. RBI TARGETS AND EXTENT. DATABASE 3.1 RBI Targets 3.2 RBI Extent. Item identification 3.3 Database 3.3.1 Data collection and review 3.3.2 Database creation and management 4. DEGRADATION MECHANISMS AND RISK ANALYSIS 4.1 RBI and degradation mechanisms 4.2 Corrosion degradation mechanisms 4.2.1 Corrosion types 4.2.2 Corrosion rate and predictive models 4.2.3 Morphology of the degradation mechanisms 4.3 Other degradation mechanisms 4.3.1 Piping fatigue 4.3.2 Brittle fracture 4.4 Risk Analysis 4.4.1 Corrosion Risk Assessment 4.4.2 Other approaches to Risk Assessment 4.4.3 Corrosion risk matrixes 4.4.4 Confidence 4.4.5 Risk analysis results 5. INSPECTION PLANNING AND EXECUTION 5.1 Inspection Plan 5.2 Type of inspections and NDT methods 5.2.1 Intrusive and non-intrusive inspections 5.2.2 NDT inspection methods 5.2.3 Probability of Detection and inspection effectiveness 5.3 Criteria for selection of NDT inspection methods Eni S.p.A. 02961.VAR.COR.SDS Rev.0 March 2011 Exploration & Production Division Page 4 of 54 5.3.1 Vessels 5.3.2 Tanks 5.3.3 Heat exchangers 5.3.4 Pipework 5.3.5 Flowlines and Trunklines 5.4 Risk Classes and Inspection Level 5.5 Sampling criteria 5.5.1 Systematic sampling 5.5.2 Application to vessel inspection 5.5.3 Application to pipework inspection 5.5.4 Inspections in correspondence to defects 5.6 Inspections Program 5.6.1 First inspections planning 5.6.2 Inspections intervals 5.7 Requirements for inspection execution 6. RESULTS EVALUATION 6.1 Inspection results analysis 6.2 Statistical analysis 6.2.1 Extreme values analysis 6.2.2 Bayes’ theorem 6.3 Defects evaluation: acceptance, repair or replacement 6.4 Re-evaluation APPENDIX A. PRESSURE EQUIPMENT DIRECTIVE (PED) APPENDIX B. FORM FOR DATA COLLECTION APPENDIX C. CAUSES OF FAILURE IN OIL AND GAS PROCESS PLANTS APPENDIX D. NDT INSPECTION METHODS APPENDIX E. EXTREME VALUE ANALYSIS Eni S.p.A. 02961.VAR.COR.SDS Rev.0 March 2011 Exploration & Production Division Page 5 of 54 1. GENERAL 1.1 Foreword Risk Based Inspection (RBI) is presented as a method to optimize the inspection activities performed in Eni oil and gas production assets. The RBI approach is expected to improve the management of the assets integrity, reducing the overall costs for inspection and monitoring. The Document is strictly related to recently issued Company Standard dealing with Corrosion Integrity Management (Ref. /18/) and with Corrosion Risk Assessment (Ref. /19/). 1.2 Scope The present Company Standard illustrates the RBI approach for planning and executing inspections of in-service oil and gas (upstream) production assets. Specifically, the document is focussed on: − gathering and distribution flowlines and trunklines networks; − oil and gas treatment units and utilities, onshore and offshore. A similar approach is used for the inspection of pipelines, onshore and offshore, as described in the Company Standards N.11554.PLI.COR.PRG and 20415.SLI.OFF.SDS respectively (see Ref. /25/ and Ref. /29/). The Document is intended as a Guideline and contents shall be used for preparing Project Documents, in particular the Inspection Plans and Inspection Programs. In particular, the following issues are covered: − the RBI process; − corrosion mechanisms and morphologies; − criticality assessment; − inspection techniques; − inspection planning; − inspection results evaluation. Extensive reference is made to the International Normative (API and DNV), issued in last years on RBI and inspections. The Document does not cover: − intelligent pig inspections; − hydrostatic pressure testing. As far as cathodic protection is concerned, cathodic protection inspections is not covered in this Document. However, reference is made to applicable Company and International Standards where external degradation mechanisms are considered. 1.3 References 1.3.1 Codes and Standards Ref. /1/ API RP 580 Risk-Based Inspection. Ref. /2/ API RP 581 Risk-Based Inspection Technology. Ref. /3/ API RP 574 Inspection of Piping System Components. Ref. /4/ API RP 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. Ref. /5/ API RP 579 Fitness-for-Service. Ref. /6/ API 510 Pressure Vessel Inspection Code; Maintenance Inspection, rating, Repair, and Alteration. 4 Definitions of terms ALARP (As Low As A concept of minimization that postulates that attributes (such as risk) can only be reduced to a certain Reasonably Practical) minimum under current technology and with reasonable cost (Ref. /19/ 20557. Ref.3.OMS.COR. Ref. 1.0001 TMS. Ref. /10/ ASME B31G Manual for Determining the Remaining Strength of Corroded Pipelines.2 Eni Company Standards Ref. Asset All physical facilities required for operations (Ref.VAR. /15/ D. /23/ 20309. Ref. /7/ API 570 Piping Inspection Code. Maintenance Inspection. Rev. Rev. /30/ Eni E&P Doc N° 1. /13/ DNV-RP-G103 Non-Intrusive Inspection. /12/ DNV-RP-G101 Risk Based Inspection of Offshore Topsides Static Mechanical Equipment. Ref. N. /1/).COR.0 March 2011 Exploration & Production Division Page 6 of 54 Ref. /11/ ASTM G16 Practice for Applying Statistics to Analysis of Corrosion Data.DOC.COR.PRG Guidelines for drawing up the inspection and maintenance plan (IMP) for onshore pipelines. Repair.COR. Technology Management System Facilities Engineering Handbook.A02.L. rating. /24/ 20311. Ref. to the environment. Ref.VAR. Alteration.POS. Ref. 93/00 Attuazione della direttiva 97/23/CE in materia di attrezzature in pressione. /9/ API 653 Tank Inspection.COR.SDS Corrosion Integrity Management. Ref.Eni S. Repair.0.PRG Cathodic protection of buried structures in plant facilities. 29/06/2005.STD Cathodic protection measurements and surveys for on-land buried pipelines. Ref.M.SLI.VAR. /25/ 11554.TMS.SDS Cathodic protection underwater inspection.SDS Facility Functional Units. 02961. /16/ D. /17/ UNI/TS 11325-1 Messa in Servizio e Utilizzazione delle Attrezzature e degli Insiemi a Pressione. /29/ 20415.0001 Opportunity and Production Operation System Handbook. Ref.COR.COR.FUN Technical Specification for on-shore pipeline external survey. /18/ 20602.OFF. including effects on failure Health and Safety. Visual testing. 329/04 Regolamento Recante Norme per la Messa in Servizio ed Utilizzazione delle Attrezzature a Pressione e degli Insiemi di cui all'Articolo 19 del Decreto Legislativo 25 febbraio 2000. /26/ 11555.SDS Guideline for sealine and riser inspection and maintenance program.GEN. /20/ 06215. Ref.COR. Ref. Ref. MA. /22/ 20555. /14/ EN 13018 Non-destructive testing.SDS Corrosion risk assessment methodology.PLI.VAR.3 Eni E&P Company Documents Ref. of employees as well as of the public.PRG Internal Corrosion Monitoring Specification. Corrosion Parameters and Classification of the Fluid. and Reconstruction Ref. Consequence of The consequence of failure through the unintentional release of hazardous fluids.A02. 1. Rev. to the operability of the .VAR. Ref. /30/).COR. /8/ API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks. Internal Corrosion. Ref.MA. /32/ SVI.PLI. Ref.COR. Ref. Ref. 1. dated September 2009.PRG Design Criteria.3.SDS Rev. and Alternation.VAR. 93. /27/ 11557. Ref. Ref.COR.VAR. /21/ 02555.SDS Guidelines for planning cathodic protection surveys of on-land buried pipelines. /28/ 11559. /31/ SVI.VAR.08 General Requirements for HSE Asset Integrity Management.3. General principles.A.p.PLI. Ref. 29/10/2004. 00. and pitting (Ref. Damage (type) The observed effect on a component of the action of a degradation mechanism. /6/). or criticality. neither of which necessarily imply leakage (Ref. for instance by spot UT readings. or exceedance of a set risk limit. /6/). The point at which a component ceases to fulfil its function and the limits placed on it.e. Extreme value Statistical method applicable for evaluating inspection results. Corrosion allowance The thickness of material which can safely be allowed to corrode having regard for the operating environment and applied stresses Corrosion risk An assessment of the susceptibility of the structure under investigation to all in-service degradation assessment (CRA) mechanisms that may affect it. Inspection A description of the ability of the inspection method to detect the damage type inspected (Ref.SDS Rev. Design life That period during which an item or component is intended to remain fit for service under the specified design and operating process conditions. Criticality A function of the risk associated with the inspected equipment. or to a part of it. Inspection An activity carried out periodically and used to assess the progression of damage in a component. The damage type gives rise to the failure mechanism of a component. An example would be the monitoring of . effectiveness Inspection level The Inspection Level is an attribute given to an item in the inspection planning phase which reflects the risk class.Eni S. loss of containment). for example.VAR.COR. conventional magnetic particle (MPI) or dye penetrant (DPI) inspection of welds. /6/). analysing moreover the consequences of such anomalies and the actions to be taken to limit them to the smallest possible area (Ref. It shall be determined in the longitudinal and in circumferential direction. or may be announced and detected by any number of methods at the instance of occurrence (announced failure) (Ref.. Inspection program A summary of inspection activities mainly used as an overview of inspection activity for several years into the future. Failure may be unannounced and undetectable until the next inspection (unannounced failure). attributed to the item itself. The failure condition must be clearly defined in its relationship to the component. for instance as spacing of spot measurements or size and number of sample areas to be inspected. /6/). /6/). assessed Profile through inspection measurements. HAZOP (Hazard and Qualitative methodology that identifies possible deviations from the correct functioning of the process Operability Analysis) and of the plant services. and normally includes the estimation of the conditions that give rise to failure(Ref. where applicable. incorporating likelihood of degradation occurring and associated consequences (Ref. /32/). Damage rate The development of damage over time (Ref.A. /6/). Failure Termination of the ability of a system. ultrasonic. or component to perform its required function of containment of fluid (i. Critical Thickness Profile of the penetration if a defect through the wall thickness of a vessel shell or pipe. Internal Visual This is considered as an intrusive close visual examination of all internally accessible plate material Inspection (IVI) and. Monitoring An activity carried out over time whereby the amount of damage is not directly measured but is inferred by measurement of factors that affect that damage. Damage model A mathematical and/or heuristic representation of the results of degradation. The CRA is not restricted to simply those degradation mechanisms related to corrosion (Ref. radiographic (Ref. /6/). This may express the accumulation of damage over time as functions of physical or chemical parameters. in particular UT residual thickness analysis measurements (see also APPENDIX E in this Document). See also Remnant life. /13/).p. Inspection can be by means of technical instruments (NDT) or as a visual examination (Ref. 02961. structure. /1/). Examples of damage include cracking.0 March 2011 Exploration & Production Division Page 7 of 54 asset and to the company reputation. Failure can be expressed. It is used to define the extent of the inspection. in terms of cracking. /13/). Defect A defect is an incompliance with project specifications. uniform wall thinning. Inspection methods The means by which inspection can be carried out such as visual. Flaw The physical manifestation of a degradation mechanism. pitting or wall loss etc. in terms of non-compliance with design codes. The terms “non-invasive” and “non-intrusive” are often used interchangeably (Ref. /13/). Inspection of components using equipment to reveal the defects. allowing for the elapsed service life.COR. Environment GEV (-) Generalized Extreme Value distribution LDEFECT (-) Length of a defect LS (-) Recommended profile spacing LRUT (-) Long Range Ultrasonic MFD (-) Material Flow Diagram MFL (-) Magnetic Flux Leakage MPI (-) Magnetic Particle Inspection NDT (-) Non Destructive Testing OPDS (-) Opportunity and Project Development System OPOS (-) Opportunity and Production Operation System p (mm) Through wall corrosion penetration P&ID (-) Process Instrumentation Diagram PDEFECT (-) Penetration of a defect PEC (-) Pulsed Eddy Current .0 March 2011 Exploration & Production Division Page 8 of 54 CO2 content in a process stream in relation to CO2 corrosion.5 Acronyms and abbreviations Symbol or Unit Definition abbreviation ACFM (-) Eddy Current ACFM AE (-) Acoustic Emission CHIME (-) Creeping Head Inspection CR (mm/y) Corrosion rate CorrRA (-) Corrosion risk assessment CSCC (-) Chloride Stress Corrosion Cracking CTP (-) Critical Thickness Profile CUI (-) Corrosion Under Insulation CVI (-) Close Visual Inspection D (-) Diameter (vessel or piping) DL (years) Design life or remnant life DMS Development Management System DPI (-) Liquid Penetrant Inspection EMATs (-) Electromagnetic Acoustic Transmission EVT (-) Extreme Value Theory Fc (-) Corrosion factor FOC (-) Overall consequence factor HAZOP (-) Hazard and Operability Study HSE (-) Health. such as magnetic particles or ultrasonic methods. 1. Safety. It may be performed on-stream or off-stream.VAR. Risk is defined as the product of probability and consequences when probability and consequence are expressed numerically (Ref.SDS Rev. Risk The combination of the probability of an event and its consequences.p. Risk Based A decision making technique for inspection planning based on risk – comprising the probability of Inspection (RBI) failure and consequence of failure. Non-Intrusive This refers to any inspection performed from the outside of the vessel without having to break Inspection (NII) containment and/or not requiring vessel entry. NDT Non-destructive testing.A. Remnant life That period during which it is judged that an item or component will remain safe to operate.Eni S. 02961. In some situations risk is the deviation from the expected. /1/). Eni S.COR. 02961.0 March 2011 Exploration & Production Division Page 9 of 54 PED (-) Pressure Equipment Directive PFD (-) Process Flow Diagram POD (-) Probability of detection RT (-) Radiography RTR (-) Real Time Radiography RVI (-) Remote Visual Inspection SLFEC (-) Saturated Low Frequency Eddy Current SSC (-) Sulphide Stress Corrosion Cracking t (mm) Nominal wall thickness (referred to a pipe or a vessel) tCA (mm) Corrosion allowance tCD (mm) Declared design corrosion allowance.SDS Rev. Is the grid spacing for spot NDT readings kPC and kPL (-) Circumference and longitudinal spacing for spot NDT readings on pipework .A.VAR.p. tMIN (mm) Minimum required thickness TOFD (-) Time of Flight Diffraction TT (-) Thermography UI (-) Ultrasonic Imaging UT (-) Conventional Ultrasonic Testing k (-) Skip. focused on a specific target. the present Guideline has been prepared for specific areas of applicability and under a number of limitations.COR. RBI can be adopted with different targets and contexts which are reviewed here below. like asset re- qualification or acquisition or as preliminary activity for future interventions on the asset. 2. where RBI is requested as part of the Asset Integrity process. In Italy. 02961.A. already operating. − hydrostatic pressure tests. INTRODUCTION 2. RBI is planned as method to execute periodical inspections (Inspection Programs). First inspection campaign is typically executed within the first year of the operating life. 2.p. for instance. Besides.2.VAR. Integrated to the RBI process. based on design data and documentation (see Figure 2. RBI is performed as unplanned activity.1. In these contexts. Inspection frequency and requirements are then updated based on available inspection results and the RBI process is periodically re-executed in accordance with the Inspection Programs. the European Directive for Pressure Equipment (PED) applies. − intelligent pig inspections. starting from the development phase. The following types of inspections are not covered by the present Guideline: − cathodic protection inspections. . as reported in Table 2.2 Limitations and exclusions RBI methodology is intended to be applied to all Static Pressure Equipment. Actually. − asset re-qualification.0 March 2011 Exploration & Production Division Page 10 of 54 2.1). The present Guideline does not intend to comply with National Regulations for pressurized systems. The RBI results are then integrated in the database and used to update the criticality levels of the evaluated items.2 Asset integrity of existing facilities This is the case of existing facilities.1 RBI targets and project phases RBI applies to in-service facilities and it is a process mainly pertinent to operation phase of a Project. Corrosion Risk Assessment is planned and executed from the development phase. with the aim to provide base- line data as well as to confirm the facility is free from construction defects.1 Base case The case for adopting the RBI approach is for new assets where.1.Eni S. 2. Examples of this case are: − evaluation after asset acquisition. an overview of the PED methodology is given in APPENDIX A.SDS Rev. etc. − Utilities Units. pipework and heat Exclusions: exchangers − structural items including supports. including: Exclusions: − weight loss corrosion. /18/) has been established to adequately handle the corrosion issues in the engineering phase (OPDS .Eni S. as illustrated in this Document applies but shall be adapted to underwater situations. − instrumentation. Figure 2. phase.3 RBI and Asset Integrity Management RBI is defined as (see Par. The Corrosion Integrity Management System (Ref. Failure mode Degradation mechanism. Therefore the risk class shall be inherited from the line which they belong. injection. − internal components (covered only in case of intrusive inspections).0 March 2011 Exploration & Production Division Page 11 of 54 Table 2. flowlines. − Oil and gas process Units. Pipe Line End Manifold (PLEM). or TASKS. 1. flanged connections Remark: − at the moment no valid RBI methodologies are available to evaluate the risk classes. require underwater interventions and are covered by dedicated Company Standard. general and localized. /29/. like subsea wellheads. − Pipelines inspections are covered by other − Distribution networks (gas and water Company Standard (see Ref. − erosion corrosion and wear abrasion.Opportunity and Production Operation System) through a number of well-defined and organised activities.1illustrates the positioning of the tasks of the Corrosion Integrity Management System with respect to the project phases. − fatigue.A. 02961. /25/). risers. RBI procedure.1 – Applicability and limitations of this Guideline.p. water disposal).VAR.Opportunity and Project Development System) and during production (OPOS . RBI and Monitoring and Inspections are . − defectiveness occurred in construction NOTE 2 − corrosion fatigue. 2.SDS Rev.4) ‘a decision making technique for inspection planning based on risk – comprising the probability of failure and consequence of failure’. − stress corrosion cracking.COR. as for instance Ref. − Storage tanks. tanks. − non-static equipment (pumps. Corrosion Risk Assessment. and the frequency of inspection shall be the highest between the one suggested by RBI and the one required by the Maintenance. NOTE 1 Location Onshore and Offshore (topside) Not applicable for subsea facilities Equipment Pressure vessels. RBI is one of the Tasks of Asset Integrity Management and it contributes to prevention of major accidents and to maintain the safe operability of oil and gas production facilities. − accidental events. gaskets. − low temperature brittle fracture − follow-up of previously detected defects. − Oil and gas gathering networks. − creep. NOTE 2 Defects originated in the construction phase is covered by dedicated documents. − high temperature corrosion (T>500°C). compressors. /24/ and Ref. /25/ and Ref. NOTE 1 Inspection of subsea facilities. Seals. Parameters Area of applicability Exclusions and remarks Facilities Typical facilities covered by RBI are: − Wellhead and wells in general. In this Document RBI is presented as a process of planning the inspection requirements through the assessment of risk.). skirts and saddles. Construction defects cannot be correlated to operating conditions of the item and cannot be predicted and risk assessed. Preparation to Operation Production & Improvement Decommissioning Corrosion control philosophies Materials and corrosion control design Corrosion monitoring and inspection design Laboratory and field testing Data management Corrosion management CORROSION RISK ASSESSMENT RISK BASED INSPECTIONS MONITORING AND INSPECTIONS Asset Integrity review Figure 2. through the identification of most critical positions and parameters to be monitored. The RBI approach can lead to a reduction of the inspection costs. 2. − monitoring optimization. PROJECT PHASES Development Operation TASKS Commissioning.p. The reasons for selecting a risk based approach to inspection planning are: − to focus inspection efforts on items where the safety. Evaluation Concept Selection Concept Definition Execution Start-up. − to identify the appropriate inspection or monitoring methods according to the identified degradation mechanisms. which are: − definition of RBI extent and targets.4 Benefits of RBI RBI is intended to provide beneficial effects in the following area: − increased operability through increase in asset availability and reduction of shutdowns.1–Corrosion Integrity Management tasks and project phases (simplified from Ref.VAR. .1 The RBI process RBI is a process consisting of independent and correlated steps.SDS Rev. /18/). − reduced risks of failure and associated consequences. set by the operator. through optimization of number of items and positions to be monitored and frequency of inspection. − inspection optimization. Tests Handover to First Period Running Product. the tasks begin in the development phase and continue during the whole operation phase. whilst similarly reducing the effort applied to low risk systems.5.0 March 2011 Exploration & Production Division Page 12 of 54 the tasks involved in the RBI process. at any time. including safety and environmental impact. − to ensure that the overall installation risk does not exceed the risk acceptance limits. or tasks. as shown in same figure.A. 2. Eni S. 02961.COR. However. − creation of database.5 RBI overview 2. cost reduction is neither the target nor the motivation of RBI. economic or environmental risks are identified as being high. Each step is analysed in next Section of the Document.SDS Rev.Eni S. − update of database and re-evaluation.3.COR.A. − inspection planning. Figure 2.0 March 2011 Exploration & Production Division Page 13 of 54 − risk and criticality assessment. − inspection results evaluation. 2.2 The RBI flow chart The detailed flowchart of the RBI process is illustrated in Figure 2. − inspection execution.2 – Sub-tasks of the RBI process. 02961. .5.VAR. DEFINITION OF RBI EXTENT AND TARGETS CREATION OF DATABASE RISK AND CRITICALITY ASSESSMENT UPDATE OF DATABASE INSPECTION PLANNING AND RE-EVALUATION INSPECTION EXECUTION INSPECTION RESULTS EVALUATION Figure 2.2 shows the main sub-tasks of the RBI process and sequence of their execution.p. Database Identify and List the Codify the Items Collect Data and ITEMS Create the Database Risk Analysis Identify the Perform Corrosion Perform Consequence Risk Matrixes Criticality Assessment corrosion mechanisms Analysis Analysis Inspection planning Identify Applicable Establish Sampling For Each Item Define Fix Inspection Inspection Methods Criteria Inspection Level and Requirements Coverage Inspection execution Perform Inspections Review and Check Inspection Results Inspection results evaluation Evaluate Inspection Perform Statistical Evaluate Defects Perform Re- Results Analysis evaluation and Re- Assessment Fitness-for-service Fitness for Service Accept. etc. . 2.SDS Rev. HSE Manager.0 March 2011 Exploration & Production Division Page 14 of 54 RBI extent and targets Define the Asset and Establish the targets battery limits for RBI of RBI Item Identification.).A. Corrosion Manager. 02961. De-rate or Replace Figure 2.3 – RBI process flowchart.5. Data collection. Maintenance Manager.3 The RBI team The RBI is a complex process which involves several parties. Typically the team for RBI execution would include the following Functions and responsibilities. − the discipline Specialists. Repair.VAR.p. − the Company in charge for operations through its representatives (Operation Manager.Eni S. including: − the Owner of the asset.COR. Inspection Manager He contributes to all aspects related to inspection execution. 02961.4 Supports to RBI A number of supports are available and shall be used along the execution of the RBI process.Eni S. − Company software tools. − International standards. • fluid treatments with corrosion inhibitors.p. including: • construction materials. • item redundancy.A. he is responsible that remedial recommendations are implemented. including Contractors. Corrosion Manager He contributes to all aspects related to corrosion assessment of the item under evaluation.COR. also depending on the Parties involved. 2. • management of the inspection data results and loading in the database. • impact of production losses.5. Operation Manager He contributes to all aspects related to operability. these include: − Company standards and Procedures. • corrosion monitoring. . • corrosion rate prediction. • identification of expected degradation mechanisms. including: • compliance of inspection works with inspection plans. Based on RBI results. The above description of Function and responsibilities shall be obviously adapted to each specific context where the RBI is executed and in case integrated with additional contributions. • to ensure proper training of inspection personnel. including: • shut downs. − Data management systems.0 March 2011 Exploration & Production Division Page 15 of 54 HSE Manager He contributes to the identification of the consequences in case of failure of the item covered by the RBI.VAR.SDS Rev. like Separation. grouping of items is admitted in principle. however.Eni S. For risk analysis. − P&IDs. operation.COR.SDS Rev. Item identification The asset object of the RBI process shall be clearly defined and battery limits established. sizes – thickness in particular. 3.3 Database 3.) − requirements from the legislation of the country where the asset is located.2 RBI Extent.VAR. RBI target shall be identified and agreed amongst the Parties involved considering all aspects affecting the RBI process and use of the results. Typical source documents are the following: − design premises and design codes. Based on the asset under study. it has to be verified their equivalence from all viewpoints. − type of inspections: if routine Periodical) or extraordinary. These typically include: − internal. decommissioning. − layout drawings . Stabilization. DATABASE 3. − external – atmospheric − external – soil. Application of the RBI procedure is mandatory for pressure vessels and pressure pipework.0 March 2011 Exploration & Production Division Page 16 of 54 3. RBI TARGETS AND EXTENT. − materials selection reports. Utilities. RBI typically covers pressure vessels. a list shall be created of the item to be included in the RBI process. Gas Compression and Treatment. the exposure side(s) to be investigated shall be established. Water Treatment. . − cost related aspects.1 Data collection and review In order to allow the most appropriate corrosion risk analysis. The most significant case is of process Units. or TAG. it is recommended to include ALL the items of the Units.A.3. Once identified the asset and the battery limits. − PFDs. exposure conditions (present and past). but are not limited to: − phase of the asset within the project life (commissioning. tanks. pipework and heat exchangers. 02961. re-qualification. new asset operated by the Company. operating parameters. 3. etc. leaving to corrosion risk analysis to exclude the less risky items. These include. project data shall be collected and validated. − under thermal insulation.1 RBI Targets Definition of the target of an RBI represents the first step of the RBI process. − heat and material balances. − material flow diagram. − piping class specifications. As first choice.p. − corrosion prevention philosophy studies. Each selected item shall be univocally identified by a code. − design and remnant life. including: material. − design and operating parameters (actual and past). − fluid treatments with chemicals: types. The creation and update of the database for the asset under evaluation is a key point of the RBI process (see Ref. − dimensional data.0 March 2011 Exploration & Production Division Page 17 of 54 − vessel data-sheets. − cathodic protection specification. − process treatments. − fluid chemical analysis and physical parameters (actual and past). past and forecast. In APPENDIX B an example is reported of FORM for data collection. − cathodic protection data (actual and past). /18/). inspection and failures data. injection mode. . shall be reviewed and validated and used to create the RBI database to be used in next steps of the RBI process.SDS Rev.2 Database creation and management The documents listed in previous paragraph. − coating and painting. − monitoring. 3. − operating data. injection points.Eni S. past and actual: − failure track records and failure analysis reports. − painting. dosages (actual and past). − flow rate data. − materials and grades. 02961.VAR.COR.A.p. − repair interventions history and reports. − inspection history and reports. together with all useful sources. the database shall contain the following categories of data (for each item): − codes and extent (from/to for pipework only). − criticality level (from risk matrixes). − cathodic protection inspection reports.3. − bacterial analysis (for waters). As a minimum. coating and insulation specifications. − risk matrixes by homogeneous items and Process Units. corrosion mechanisms – weight loss.2 Corrosion degradation mechanisms 4. is covered by API 571 (Ref. 1 A comprehensive description of degradation mechanisms met in industry.Eni S. However. /4/). nickel alloys. /6/). − stress corrosion (environmental) cracking. but also corrosion resistant alloys (CRA) can be met (stainless steels. A further category of degradation mechanism is classified as ‘high temperature corrosion’.1 Main categories are: − internal and external corrosion mechanisms. In oil and gas production. for instance by fracture. Several degradation mechanisms are met in oil and gas production facilities.COR. Carbon and low alloy steel (CS) have a dominant position in the facilities under consideration. pitting. − mechanical and metallurgical mechanisms. 02961. cracking – represent the main time dependent corrosion mechanism.SDS Rev. Non-time dependent degradation mechanisms occur as sudden rupture. and morphology (see Table 4. 4. at least in part. can be easily extended and adapted to all expected degradation mechanisms.A. which occurs only at temperature above 500 °C. applies to time-dependent degradation mechanisms where risk prediction can be performed. in particular in refinery.p.1). RBI. which can be grouped in accordance to different criteria.1 Corrosion types Corrosion mechanisms result by a combination of environment and metallic material. if internal or external. Amongst the degradation mechanisms.2. in its most appropriate and complete interpretation. as for instance oxidation.0 March 2011 Exploration & Production Division Page 18 of 54 4.1 RBI and degradation mechanisms The degradation mechanisms are defined as the means by which a component degrades thus reducing its ability to carry out its function (Ref. . distinction can be made between time dependant and non- time dependent mechanisms. − sand erosion. titanium alloy). DEGRADATION MECHANISMS AND RISK ANALYSIS 4. Corrosion mechanisms are classified depending on exposure side. and inspection criteria based on inspection intervals cannot be applied. Examples of failure data in offshore facilities are reported in APPENDIX C.VAR. the RBI approach. copper alloy. It is the case of cracking mechanisms in general and of some localised types of pitting and crevice corrosion. for some of these mechanisms models are available to predict corrosion rate. CRA Electrical interference (DC and AC) CS.Eni S. The propagation modes (through wall penetration. the failure event.VAR.COR. CRA Localized pitting and crevice corrosion CRA Sand erosion CS. CRA Galvanic CS. − incubation-propagation mechanisms: corrosion occurs after an incubation time. 02961. For corrosion mechanism which proceeds in a time dependent manner. time) for typical corrosion forms met in oil and gas production facilities are illustrated in Figure 4. the corrosion attack proceeds regularly.2. For incubation- propagation mechanisms. the failure event is at end of incubation time.SDS Rev.2 Corrosion rate and predictive models Corrosion in general is a time dependent phenomenon. at rate which depends on value assumed by the operating and environment parameters. if the full wall thickness or corrosion allowance. depend on which wall thickness is taken as reference. CRA Internal Elemental sulphur corrosion CS. CRA Sea water corrosion CS.1. However. which proceed at a fast or instantaneous rate. depending on the specific mechanism. CRA Soil corrosion CS. after which the damage occurs at high rate or instantaneously.1 Exposure side Corrosion mechanism Affected materials Uniform or localized loss of thickness CO2 corrosion CS Microbial Induced Corrosion (MIC) CS H2S corrosion CS Oxygen corrosion CS Erosion corrosion CS. p. which results from the combination of given metal and environment. and time-to-failure. CRA Hydrogen Induced Cracking CS Amine cracking CS Chloride Stress Corrosion Cracking (CSCC) CRA External Atmospheric CS. and prediction is often issued in binary terms (pass – no pass) based on compatibility verification. CRA Amine corrosion CS.A. case (a).0 March 2011 Exploration & Production Division Page 19 of 54 Table 4. typically as weight loss. vs. .p. as cracking phenomena. In particular the following types can be identified: − progressive time dependent mechanisms: for a given corrosion system. CRA Environmental cracking Sulphide Stress Cracking (SSC) CS. CRA Corrosion under insulation (CUI) CS. variable to zero up to infinite. CRA Carbonate-bicarbonate stress corrosion cracking CS 4. different types of dependence of corrosion with time exist. Progression with time of these mechanisms is difficult to predict. penetrates through the wall thickness. 5. Identification of the expected corrosion morphology is integral part of the risk analysis as inspection techniques shall be selected based on the type of defect to be looked for.VAR. Failure occurs as leak before break. Morphology is expressed as ranges of probability for the size parameters of the corrosion defect: LDEFECT is the length of the defect (or the equivalent diameter of the corroded area) and PDEFECT is the penetration of the defect through the wall thickness. − uniform weight loss corrosion: it occurs on a large area and it affects the pressure/load bearing capacity of the equipment wall.0 March 2011 Exploration & Production Division Page 20 of 54 P P P localized propagation propagation uniform incubation incubation time time time CO2corrosion (Carbon Steel) Pitting Corrosion (Stainless Steels) Cracking Erosion Microbial Corrosion Oxygen corrosion (Carbon Steel) (a) (b) (c) Figure 4.1 – Through wall penetration modes for different corrosion mechanisms. 4.COR. . Conservatively. failure can occur as wall perforation or as consumption of the corrosion allowance (different definitions of the corrosion allowances are possible: see Ref. the knowledge on morphology of corrosion defects has to be considered in the phase of NDT selection (see Par. Prediction of corrosion morphology is intrinsically uncertain. − cracking: one or more crack. single or ramified. Both localised and uniform weight loss corrosion produce a wall thickness reduction. − pitting corrosion: it is typical of stainless steels and corrosion resistant alloys. however. Expected morphology for main weight loss (internal) corrosion forms are reported in Table 4. general rules are available.3). giving leak or structural failure respectively. the corrosion damage assumes different morphologies.Eni S.SDS Rev. corrosion occurs at a very small area and it develops through the wall thickness. /19/). failure shall be referred to the consumption of the design corrosion allowance.p.2. The following ones are the most typical: − localised weight loss corrosion: localized attacks have a minor impact on pressure/load bearing capacity. Within the RBI process.2. Depending on the mechanism.A.3 Morphology of the degradation mechanisms Knowledge and prediction of the morphology of defects produced by a given degradation mechanism supports the selection of most convenient inspection method. 02961. 5 0.9 Erosion LDEFECT/PDEFECT Uniform (elbows.2 0 ~10 0. Par. thermal-.2 1. The main ones are: − fatigue (for piping): mechanical-.e.3 Other degradation mechanisms Although corrosion degradation mechanisms have been identified as the main mechanisms affecting the durability of the assets considered in this document. − brittle fracture. purely mechanical or aggravated by corrosion (corrosion-fatigue).0 4. 4.5 0 ~10 0. can occur in high thickness items.SDS Rev.8 0. /2/) – Part 2.1 Piping fatigue Mechanical fatigue failures can typically occur in installed piping systems connected to reciprocating pumps and compressors which cause vibrations and cyclic stresses.9 Pitting Corrosion (Stainless LDEFECT/PDEFECT Localised Steels) «1 1.3. − audible or visible piping vibrations.8 0 ≥100 0 0 Oxygen corrosion (Carbon LDEFECT/PDEFECT Localised Uniform Steel) ~1 0. fatigue cracks. like pumps or compressors. valves) Localised ~1 0 0 ~10 0. i. or to high pressure drop valves. corrosion-.1 ≥100 0 0. Fatigue is a time dependent degradation mechanism.0 March 2011 Exploration & Production Division Page 21 of 54 Table 4.9 0 Microbial Corrosion LDEFECT/PDEFECT Carbon Steel Stainless Steels ~1 0.COR. The two mechanisms are reviewed in the paragraphs here below. 02961.A. − connection to reciprocating equipment. The norm API RP 581 (Ref. In these cases.0 ~10 0. the component shall be evaluated from view point of its susceptibility to mechanical fatigue damages.VAR. caused by low temperature or low toughness. 25. in particular if unexpected low temperatures are experienced.1 0 ≥100 0. tees.Eni S. other degradation mechanisms exist which shall be adequately considered within the risk analysis and the inspection tasks. Main factors for identification of piping mechanical fatigue failures are: − evidences of previous fatigue failures. Brittle fracture. .2 – Predicted corrosion morphology for main corrosion forms.p. in particular during transitory operations. Corrosion form Size parameters Probability CO2 corrosion (Carbon Steel) Uniform (mesa at bottom-of- LDEFECT/PDEFECT Localised line) ~1 0. although not time dependent. provides guidelines to assess the probability of piping fatigue failures.1 ≥100 0 0. Brittle fracture occurs as a sudden failure usually initiated at a crack or defect. other methods exist for risk analysis which are based on same approach but executed at different levels. by combining) the probability of failure occurrence and the entities of the consequence in case of failure.SDS (Ref. − confidence level. with limits which depend on the alloy and its microstructure.VAR. welding procedures. − applied loads and wall thickness. the Eni Company Standard 20557. impact test resistance.A.VAR. /19/). in particular during temporary phases. during both normal operations and upsets. 21. the requirements for the execution of Corrosion Risk Assessment (CorrRA) are covered in a dedicated Eni Document: 20557. 4.Eni S.0 March 2011 Exploration & Production Division Page 22 of 54 4. Distinction. . /9/). Par.COR. Low temperature conditions can be experienced due to environmental conditions.2 Brittle fracture Toughness of metals and alloys decreases with temperature.VAR.SDS represents the base and recommended reference for risk assessment.COR. as for instance upsets.3. risk assessment consists in separately determining. In particular. 02961. depending on the approach followed to assess probability of failure and the entity of the consequences. as for instance in artic regions. − consequence analysis.4. /2/) – Part 2. heat treatments.p. Accordingly. Procedures for the assessment of brittle fracture occurrence are available in several international codes and standard. 4.1 Corrosion Risk Assessment The risk associated with (corrosion) failure is defined by multiplying (or. − minimum operating and design temperatures. Main factors affecting brittle facture and to be considered are: − metallic material: type.4. the probability of failure and the entity of the consequences. can be made between qualitative and quantitative risk assessment as limit cases.COR.SDS covers the following topics: − corrosion analysis in oil and gas production facilities. The following are mentioned here: − API RP 581 (Ref. − corrosion risk matrixes and results from corrosion and consequence analysis. for a given item. The following results are expected by the risk analysis: − expected degradation mechanisms and corrosion forms. − risk matrixes and criticality levels. 4. − API RP 653 (Ref.SDS Rev. post-weld heat treatment.VAR. − criticality level (for individual item). different definitions are given for risk assessment.4 Risk Analysis Risk analysis is the task within the RBI process whose outputs allow to orient and optimize the inspection activities.COR. in fact. which shall be adopted for the execution of RBI as integral part of this Guideline.2 Other approaches to Risk Assessment Although the above mentioned Eni Company Standard 20557. in general. − morphologies of expected damages and defects. Within the body of the Eni Corporate Standard. microstructure. or because operating conditions. where: − qualitative risk assessment is intended as performed using a judgment-based approach to the assessment. the corrosion rate or the corrosion morphology are recognized. CRA shall cover for each item: − the degradation types (all). qualitative and quantitative. Assignment of confidence is a subjective judgement. modification and repair history. Confidence can be expressed through three categories: high. − the metallurgy. eventually assigning intervals for probability of failure and consequence of failure. based on the following guidelines: High confidence.3 Corrosion risk matrixes Depending on the corrosion risk assessment performed different types of corrosion risk matrixes can be produced. In DNV-RP-G103 (Ref. their locations and rates.SDS Rev.Eni S. /19/) shall be based on at least 5 levels of risk. with no previous inspection results available. Same as above.3). medium and low. In all cases. − the process fluid composition and operating conditions. and assigning risk values to risk ranks. The approach illustrated in the Eni 20557 VAR. − corrosion preventive measures. 4. More than one of above requirements cannot be fulfilled.VAR. see Par. a thorough CRA (quantitative or semi- quantitative) has been executed giving confidence that all relevant mechanisms and their likely locations have been predicted.4 Confidence Confidence is a qualitative attribute that expresses the ability of the corrosion analysis to predict corrosion types and probability of occurrence. The recommended risk matrix (see also Ref. /13/) for instance definitions are given for different types (four) of corrosion risk assessment.2. or risk classes.0 March 2011 Exploration & Production Division Page 23 of 54 − quantitative risk assessment should use numerical value calculated with units of measurement.4. Very high Probability (Æ increasing) Corrosion Severity or High Medium Low Very low (Å increasing)Consequence of Failure RISK CLASSES Figure 4. − the results of at least one previous inspection performed on the item.2 – Corrosion risk matrixes and risk classes. as shown in Figure 4. Medium confidence.A. . 02961.COR. from negligible to very high. 4. − track record of past history of the item shall be known and documented using reliable inspection techniques (high Probability of Detection – POD. The term semi-quantitative is also used to define combinations of the two approaches.SDS is typically semi- quantitative.4.2. but with no previous inspection results available. the difficulty in calculating the probability of corrosion failure.COR.p. 5. As part of the integrity management activity. Low confidence. As whole result of the corrosion risk analysis.0 March 2011 Exploration & Production Division Page 24 of 54 The confidence level shall be assigned to the performed corrosion analysis and the criteria adopted for assignment declared. 02961.4. − localization of the attacks. i. the following results are provided: − expected corrosion degradation mechanisms. and then to assign a corrosion risk class. as part of the RBI process.A.SDS Rev.p.VAR. to the item. inspection planning.COR. 4. . for each evaluated item. − expected corrosion morphologies.Eni S. − ranking by risk classes for homogeneous items (for instance vessels and pipework for homogeneous process Units). examples are: bottom-of-line for CO2 corrosion. Above results are results to the next step of RBI process. or criticality level. to position the item in the risk matrix through the relevant coordinates of failure likelihood and consequence. elbows for erosion corrosion.5 Risk analysis results The corrosion risk analysis allows.e. − risk class (or criticality) and confidence. i.1 Intrusive and non-intrusive inspections Inspections can be classified as intrusive and non-intrusive. it inherits results of the risk analysis and it takes into account the RBI targets and the specific context. − process. − general requirements for inspection execution. − expected degradation mechanisms.0 March 2011 Exploration & Production Division Page 25 of 54 5. − priorities for the inspection. − where necessary. etc. − inspection drawings (recommended for main vessels). by separate documents. however. and shut down costs. in order to avoid impact on operations.p. oil and gas production are designed for continuous operation. − access is avoided into area which can be hazardous.VAR. . Corrosion Risk Assessment and Inspection Planning. The acceptability and benefits of non-intrusive inspection for a particular item will depend on a number of factors including: − geometry and presence of non-accessible parts (for instance.A.1 Inspection Plan Inspection planning is the key task of RBI process. − historic inspection data. RBI is mostly based on the non-intrusive approach. this can be acceptable in case of scheduled plant shut down. The Inspection Plan is the project document which defines the requirements for the inspection execution.2 Type of inspections and NDT methods 5. shutdown duration may be reduced. The advantages of performing non-intrusive inspection include: − shutting down the plant or system is avoided. the following issues shall be covered by the Inspection Plan: − definition of inspection extent (items to be inspected) and targets. 5. it is preferable to cover the two tasks. − inspection costs. can be prohibitive because of high costs associated with loss of production. − reporting requirements. − impact on operations are minimized. INSPECTION PLANNING AND EXECUTION 5. − extent and coverage of the inspections.2. in case of vessels the shell surface in correspondence to supports). Actually. − time schedule. 02961. − selection for each type of item (vessels. − locations and sizes of damages of concern.e.) of the inspection methods. − confidence in inspection capability. pipework.COR. The Inspection Plan can include the Corrosion Risk Assessment as part of it. − ranking of items to be inspected based on risk class.SDS Rev. aimed to establish the requirements for inspection execution within a cost effective frame.Eni S. scheduled or not. however. Intrusive inspection implies the internal access to the item to be inspected. − construction materials. − Surface and Wall thickness up to 300 mm. Testing (shear wave and reduction. Eni S.1 – NDT inspection methods (adpted fromRef. For same methods. local and general. APPENDIX D in this Document provides a brief description of the NDT methods used in oil and gas production considered in this document with indication on their maturity and Probability of Detection (POD).5 mm allowed. girth weld- Electromagnetic Acoustic EMATs − As UT. . local and general. − Blistering.1/0. local and circumferential area It works better on straight pipes. ‘Guided Waves’ (Lamb reduction. crawler.COR. Anything visible 20 m/day Detects surface cracks Inspection Clean surface needed Magnetic Particle MPI − Surface cracks. Quite Wall thickness up to 600 mm general variable depending on items to be inspected and mechanization level of the tool (robot. Waves) general.1/0. 02961. etc. Liquid Penetrant DPI − Surface cracks.5 mm (at spot) 1000 readings/day (spot) Lower sensitivity than conventional UT Transmission Acc.2.1 a number of features are collected useful for their selection.) Time of Flight Diffraction TOFD − Wall thickness Depends on 40 m/day (weld) - reduction.5 mm (at spot) 1000 readings/day (spot) Access for spot readings limited by item compression wave) general. embedded cracks (shear wave).2 NDT inspection methods Inspection methods applicable in oil and gas production industry include a wide number of techniques. Anything visible 20 m/day Detects surface cracks Inspection Clean surface needed Applicable to magnetic alloys only Magnetic Flux Leakage MFL − Wall thickness 30% min. Real Time Radiography RTR − Wall thickness 2% of wall thickness Faster than Film Radiation safety restrictions reduction. reduction.1/2. . Scan speed 1 m/min - reduction.p. Defects detected Productivity Remarks and limitations methods Accuracy Close Visual Inspection CVI − Anything visible .A.5 mm Eddy Current ACFM ACFM − Cracks (also under Crack depth > 1mm. Table 5. Radiography Acoustic Emission AE − Growing cracks Detects growing Moderate 60°C max.: ±0. more if 4÷10 mm wall thickness reduction. normally referred as Non-Destructive Testing (NDT). 0. or LRUT − Wall thickness 5÷10% of pipe wall 1 km/day as typical Piping from 2” up to 48”. local and geometry general Acc.: ±3 mm Pulsed Eddy Current PEC − Wall thickness 5% of wall thickness 1000 readings/day (spot) Detects general corrosion and erosion if reduction.0 mm Phased-Array PA − Wall thickness - reduction. Repeatability 2% of wall thickness Saturated Low Frequency SLFEC − Wall thickness 15% of wall thickness 10÷50 m2/day Fast large area scanning Eddy Current reduction. NDT Inspection Sensitivity and Abb. in Table 5.5 mm configuration.: ±0.5 mm Greater than manually Color wall thickness map produced. cracks . local and general Creeping Head Inspection CHIME − Wall thickness .0 March 2011 Exploration & Production Division Page 26 of 54 5.VAR. measurements Acc. of wall 10÷50 m2/day.1/0. thickness mechanized scan Thermography TT − Wall thickness - reduction.SDS Rev. Suitable for crack detection and coating) length > 10 mm. - Conventional Ultrasonic UT − Wall thickness 1÷5 mm (weld) 20 m/day (weld) Adherent coating up to 1. Film Radiography (Ir 192. Probe ring at least 1 m from nearest − Cracks.: ±0. /13/). RT − Wall thickness 2% of wall thickness low Radiation safety restrictions Co 60) reduction. local and 0.: ±0. Long Range UT.5 mm performed UT. local and Acc. Accuracy 5% of wall damage area exceeds 500 mm2 thickness. Ultrasonic Imaging UI − Wall thickness 0. In same table productivity data are also reported to be considered for planning of inspection duration and budgetary costs assessment. − Embedded flaws. Acc. 0 March 2011 Exploration & Production Division Page 27 of 54 5. non-intrusive inspection shall be performed in order to avoid plant shut-down. stiffeners. etc. The second number denotes the confidence level for detecting the flaw. conventional or advanced. Effectiveness is expressed qualitatively (high. whose application shall be evaluated case by case based on specific project requirements. 5. The first number in the series denotes the probability that the flaw will be detected.A. column. scrubbers. − inspection constraints: inspection tool and personnel availability. . which depending on the method are up to 300 mm.3 Probability of Detection and inspection effectiveness Probability of Detection (POD) is a measure of the capability of an inspection method to detect a given type of defect in the area covered by the inspection method.2. Vessel internals can be inspected only in case of shut down of the vessel (intrusive inspection). lifting lugs. Ultrasonic methods. 02961. Inspection shall be focussed on shell and nozzles. The most common and critical type of damage met in vessel in oil and gas treatment plant is wall thinning.SDS Rev. and in particular the expected degradation mechanisms. Actually. distinguishing between internal side and external. pipeline. pipework. − applicable NDT methods and features. uniform or localised.) normally are within the UT applicability limits.3 Criteria for selection of NDT inspection methods NDT inspection methods shall be selected considering the results of the corrosion analysis. 90/90. above ground tank. 5.1 Vessels Pressure vessels represent the main item in oil and gas treatment plants. 90/95.3. often sophisticated. dehydrators. − item to be inspected: vessel. organic or metallic 2 The POD is normally expressed as a ratio of a probability of detecting a flaw with a confidence level (e. desalters. selection of most convenient method(s) shall be performed considering the following factors: − type of expected defects and degradation mechanisms. or cracking. medium and low). if present. as wall thinning. also reported as loss of wall thickness.VAR. other. is suitable to detect specific corrosion defects.p.2 The effectiveness of an inspection depends on the POD of the adopted NDT methods and on the degree of coverage. Access limitations can be caused by saddle plates.g. their probability to occur and their morphology. which is given as a percentage. costs and duration of the inspection. Non-intrusive inspection can be performed with probes contacting the external side of the vessel shell. are the preferred applicable NDT methods. Wall thickness of vessels (separators. Hereinafter base criteria are given for main type of item. applicable methods are intended as proven techniques.Eni S. thermal insulation. Basically. mainly caused by internal corrosion. including: separators. Each method in fact. specific targets of the inspection activities. 90/75 or 90/50) depending on the requirements of the application.COR. columns. uniform or localised. UT methods can be applied also in presence of paints. etc. can limit accessibility to surfaces to be inspected. distinguishing between recommended (base case) inspection methods and applicable methods. 1 – Set-through and se-on design for nozzles. Table 5. Recommended and applicable methods for vessel inspection. In both cases cathodic protection is normally foreseen.SDS Rev. removal shall be considered. 5. Set-through Set-on nozzle nozzle design design Vessel shell Vessel shell Figure 5. VESSELS NDT inspection methods Expected damage Remarks Recommended Applicable Internal wall thinning (general or UT and UI RT − Internal CVI recommended during localised) RVI shut downs.3. Nozzles can be set- on nozzles or set-through nozzles (see Figure 5.p. 02961. is required.). etc. with corrosion being possible from the internal side. Nozzles welds can be preferential point for defects.VAR. shall be considered. potable. − Local removal of thermal insulation. the type of nozzles shall be considered in inspection execution. The tank bottom represents the most critical part from durability viewpoint. .0 March 2011 Exploration & Production Division Page 28 of 54 coating (galvanized or flame spray). caused by separated water. if MFL present. demineralized. Thermography (TT) is applicable Cracking ACFM TOFD - DPI and MPI AE UT (shear waves) Wall thinning or cracking at nozzles UT RT − UT shear waves type shall be used welds DPI and MPI for cracks detection.Eni S.COR. like wall thinning or cracking.2 Tanks In oil and gas treatment plants above ground tanks exist for storage of liquid hydrocarbon and water (firewater. In presence of thermal insulation or high built passive fire protection coating. or from the lower side in contact with soil or foundations. design of the nozzle.1). Recommended and applicable methods for vessel inspection are given in Table 5. if set-on or -through.2. External wall thinning (general or CVI TT − In presence of thermal insulation CVI localised) require insulation removal.A.2. alternatively. sea water. Crude oil storage tanks represent the most common type. From the external side. UT IRIS . 02961. in particular of the lower part of the shell (first and second shell course from bottom) where internal corrosion can occur caused by separated water. Recommended and applicable methods for heat exchangers. RVI ACFM .Eni S. 5.3.COR.3. the same guidelines above reported for vessels apply (see Par.VAR. CVI applicable to upper side and shell localised) (*) UT and UI (*) internal during shut downs. with production of maps of the investigated area. and this implies the shutdown of the tank for intrusive inspection. Recommended and applicable methods for above ground storage tanks. Close visual inspection can be integrated with UT inspection.0 March 2011 Exploration & Production Division Page 29 of 54 Inspection of the tank bottom is possible only from the internal side. of lower side of the tank bottom and of the internal surfaces.1). HEAT EXCHANGER (TUBE BUNDLES) NDT inspection methods Components Remarks Recommended Applicable Shell. 5. tube bundle. Type of defects and damage of tubes include: erosion at pipe inlets. non-intrusive inspections can be performed on shell and roof. reference can be made to applicable Company and International Standards: API 651(Ref. both methods are applicable using mechanised devices which allow scanning the entire surface. The recommended methods for tank bottom inspection are Magnetic Flux Leakage and UT.p. general or localized wall thickness reduction along the tube (internal or external). Eni 20309.3. ABOVE GROUND STORAGE TANKS NDT inspection methods Expected damage Remarks Recommended Applicable (*) Tank bottom thinning (general or MFL .A. Inspection of tube-sheet and tube bundle can be only performed during shut downs (intrusive inspection). /23/). CVI Tank shell and roof thinning UT and UI EMAT CVI applicable to external side. tube-sheets.VAR. Heads. For the inspection and control of cathodic protection systems.3 Heat exchangers Inspection of shell and tube heat exchanger has to be referred to the specific components to be inspected: shell. vibration induced fatigue cracks at supports. Applicable techniques for tube-sheet and tubes inspections are given in Table 5. cracks at tube-sheet connection. heads and nozzles. Recommended methods for tubes are UT and ACFM. heads and nozzles. Table 5.SDS Rev.COR. or a portion of it.4. if present. Nozzles As per vessel (see Table 5. (general or localised) CVI (*) Inspection to be performed during shut downs only and by access into the tank.Internal Rotating Inspection System (ultrasonic) uses a rotating probe for 360° inspection of tubes.4.2) Tube-sheet CVI DPI and MPI − Inspection feasible only during Tubes UT-IRIS equipment shut downs. For non-intrusive inspection of inspection of shell.PRG (Ref. Table 5. /8/). VAR. . by flanges along the piping route and by the moderate POD. Recommended and applicable methods for inspection of above-ground pipework are given in Table 5. 02961.COR. Piping conveying hydrocarbon are the most representative in oil and gas process units. In horizontal piping. PIPEWORK NDT inspection methods Expected damage Remarks Recommended Applicable Internal wall thinning (general or UT and UI RT − Local removal of thermal insulation. but localization shall be evaluated in the corrosion risk analysis phase. with mechanisms which depend on fluids and operating conditions. Table 5.4 Pipework Pipework in oil and gas process plant can be internally exposed to a wide range of fluids. alternatively.3. also if partly buried.0 March 2011 Exploration & Production Division Page 30 of 54 5.p. fuel gas. Piping Regions inspected Transducer Flange Figure 5. corrosion likelihood is greater at bottom-of-pipe. access limitation exists and long range UT (guided waves) represents the most effective inspection method. allows to quickly inspect significant lengths of piping.SDS Rev. cathodic protection conditions can be verified in accordance with Eni 20309.COR. The principle of the method is shown in Figure 5.VAR. /23/).2.Eni S. is required. Thermography (TT) is applicable Cracking ACFM TOFD − DPI AE MPI UT In case of buried pipework. flare system.A. crude stabilization. uniform or localized. Limitations are represented by presence of thick coating or thermal insulation. if localised) LRUT RVI present. Recommended and applicable methods for pipework. like separation.2 – Schematic of application of Long Range UT to piping inspection. Pipework inspection are preferably based on UT methods. represents the main damage. Wall thinning by weight loss corrosion. conventional or advanced.5.5. gas compression and dehydration.PRG (Ref. When underground piping are cathodically protected against external corrosion. Long Range UT (LRUT) or Guided Waves. With respect to manually operated UT conventional probe. MFL External wall thinning (general or CVI TT − In presence of thermal insulation CVI localised) requires insulation removal. or criticality. that is the costs of the non-intrusive inspection works.3.5).5. 5. and it is calculated as k = N / n. However. Results of the corrosion analysis. type of coating.5 Flowlines and Trunklines For inspection of flowlines and trunklines same methods apply recommended for pipework. presence of bends.Eni S.0 March 2011 Exploration & Production Division Page 31 of 54 5. however. The aim of using a statistical approach to sampling is to optimize the number of measurements. Guide wave performed from above ground positions allow inspection of buried parts of the pipe.A. focussing the inspection works on most critical items.5) are associated to the Inspection Level. but. can be further integrated with an approach for sampling of the positions of the items to be inspected based on statistical techniques. k is the sampling interval. with length which depends on pipe sizes.1 Systematic sampling Systematic sampling relies on selecting elements at regular intervals. inspection is limited to above-ground portions. 02961. Selection of the portions. or samples. or samples.4 Risk Classes and Inspection Level The results of the corrosion risk analysis (see Par. 5. in which every kth element of the ideal frame is selected. Otherwise. or skip. as follows: Risk Class (Criticality) Inspection Level Severe A High B Medium C Low D Negligible or Safe E The Inspection Level reflects the risk class. 5. with respect to the quality of the information gained from inspection results.COR. attributed to the item and is used to express the requirements for inspection execution and the frequency of inspection once the inspection methods have been chosen. often inspections cannot be carried out on the entire (surface of the) item.5 Sampling criteria Risk analysis provides a robust basis for optimization of the inspection costs. to further optimize costs. the greater will be the Inspection Level. in fact. Using this procedure . Cathodic protection inspection shall be performed in accordance with same Company Standard available for pipelines (see Ref.4.SDS Rev. as for instance in correspondence to manifold and wellhead area or to valves and chambers if present along the pipe route. 4. /26/ and Ref. The Inspection Level is used for instance to fix the spacing of positions for spot NDT readings (see Par. The most common form of systematic sampling is an equal-probability method. Actually.VAR.p. the higher is the item criticality. local excavations shall be planned. designed by capital letters. flowlines and trunklines are normally buried and severe limitations exist for access them. /27/). 5. they shall be limited to previously selected portions. where n is the sample size and N is the population size (for instance surface area expressed in m2). to be inspected shall combine knowledge on the expected degradation mechanisms with statistics (sampling theory). Inspected length extends from a few meters up to 50 m maximum. These readings are mainly intended to confirm the design thickness datum and to be used as reference actual thickness.2 Application to vessel inspection Figure 5.SDS Rev. Different homogeneous zones of the vessel are then identified based on a local corrosion analysis. Dividing the population into distinct and independent strata (the different zones) allows to draw inferences about specific subgroups (zones) that may be lost in a more generalized random sample.3. 02961. 2 cross sections (A-A’ and B-B’ in figure) are randomly selected to verify the different degree of internal corrosion (see Figure 5. because units are uniformly distributed over the population.3 – Schematic view of a three-phase separator with indication of most critical area. top of vessel wet gas wetted gas phase weir water-oilemulsion water water-oilemulsion bottom of vessel water wetted bottom of vessel emulsion wetted Figure 5. in longitudinal direction. corresponding to k = 30 degree.p. In case shown in figure.5. 5. for production separator (see Figure 5. Moreover. Systematic sampling can be applied only if the given population is homogeneous. 12 readings each section. . The spacing between two adjacent readings (the skip k) is fixed for instance of 0. − top: corrosion likelihood = moderate.COR.VAR. the thickness readings will be taken on one row. This makes systematic sampling functionally similar to simple random sampling. for a total of N. less severe conditions are foreseen at bottom. N. after weir. − bottom – after weir: corrosion likelihood = high.0 March 2011 Exploration & Production Division Page 32 of 54 each element in the population has a known and equal probability of selection. In case of selection of conventional UT methods for measurements of punctual residual thickness.30 m. different sampling scheme can be applied for each homogeneous zone. this corresponds to a linear sampling approach. in contact with emulsified water in oil and at top of the vessel in contact with wet gas. A the top of the vessel. for instance every 30 degree. different sampling approaches can be applied to different strata since each stratum is treated as an independent population.4): − bottom – before weir: corrosion likelihood = very high. shows a schematic view of a separator vessel: corrosion analysis allows identifying portions with different internal corrosion likelihood.4): spot measures are taken in a systematic way.Eni S. in contact with separated gas. where less severe corrosion is expected.A. the area of the vessel most severe from corrosion viewpoint is the portion of the bottom in contact with separated water (before weir). a stratified sampling method can lead to more efficient statistical estimates. This sampling involves a random start and then proceeds with the selection of every kth element from then onwards. p. Within each sub-area some convenient areas.0 March 2011 Exploration & Production Division Page 33 of 54 At the bottom of the vessel. Each grid point represents a position for thickness reading. of the whole sub-zone. with grid points at the intersections of the line segments. . a grid consists of two sets of parallel line segments. are randomly selected. intersecting at some angle.Eni S.A. Each examination area of 1 m2 is then split into sub-areas (say 0. which is eventually divided in homogenous sub-zone.SDS Rev. in case of 20 %.VAR.10 m × 0. The numbers of areas to select are proportional to the sub-zone extension till to reach the target coverage.10 m squares) via an ideal grid of intersecting line segments covering the selected area. for instance of 1 m2 each. in contact with separated formation water. These sampled areas are assumed to be representative of the condition of the whole sub-area of the vessel being considered. A coverage area for instance of 20 % minimum of the whole area is recommended3. A B inspection inspection inspection sub-area 1 sub-area 2 sub-area 3 A’ B’ SECTION A-A’ SECTION B-B’ Figure 5. with readings taken on an ideal grid of given spacing. This is a systematic sample where the spacing (Euclidian distance) between measures is k (see Figure 5. 3 The indicated percentage of 20 % shall be intended qualitatively as minimum percentage to obtain a sample of readings representative from statistical viewpoint of the inspected surface area. In general.4 – Identification of zones with different Risks and Inspection Levels in a production separator (example). where internal corrosion is expected to be more severe. 02961.COR. bi-dimensional sampling is adopted.5 here below). Table 5. Grid spacing for spot Item type Inspection Level NDT readings A 0. Number of readings on pipe circumference will depend on nominal pipe diameter.5 m For vessels. typically at bottom-of-line (for horizontal piping).1 m B 0. for spot reading inspection. 6.5. 02961.Eni S. When. . it is recommended to issue inspection drawings.6): − on ideal pipe circumferences. 5.7.3 Application to pipework inspection NDT spot readings are taken on piping in two modes: on pipe ideal circumferences and longitudinally.1). the spacing for the two modes are kPC and kPL.A. The spacing parameters kPC and kPL are correlated to the calculated Inspection Level as indicated in Table 5.0 March 2011 Exploration & Production Division Page 34 of 54 k a Figure 5. readings on pipe circumference are repeated with spacing kPC.6 – Inspection Level and UT grid spacing for vessels.SDS Rev.p.4 m E 0.2. based on corrosion analysis. Longitudinal readings at bottom of line are taken with spacing kPL. using UT this corresponds in reporting the lower wall thickness value within the considered area. with skip = k. NDT spot inspections of pipework are performed taking spot readings (see Figure 5. The skip for each sub-area is then in inverse proportion with respect to risk (see Table 5.2 m Vessels C 0.6).5) and in reporting only the most severe one. The grid spacing varies with the resulting Inspection Level which depends on Risk Class and local corrosion likelihood. localised wall thickness reduction is expected.5 – Ideal grid. with detailed indications of the zones to be covered by the inspections and of the Inspection Level to be used.3 m D 0. − longitudinally. a complementary approach consists in taking several readings within a given inspection surface (for instance the square a in Figure 5.VAR.COR. These inspection results can be reviewed using extreme value statistical analysis (see Par. 3 m D 8m 0. as above. Item type Inspection Level spacing for spot NDT readings circumferential.0 March 2011 Exploration & Production Division Page 35 of 54 Table 5. kPC longitudinal.A. equal to 20%).VAR.1 m B 2m 0. The coverage of each piping item is performed randomly selecting a piping inspection length as percentage of the total pipework length (for instance. kPL A 1m 0. .7 – Inspection Level and UT spacing for piping.5 m DN < 12” DN ≥ 12” Detail A Detail B spacing.p. 02961. Spacing parameters.Eni S.2 m Pipework C 5m 0. KPL See Details A or B Detail C Figure 5.SDS Rev.6 – Spot inspection of pipework. kPC A A A’ A’ See Details A or B Spacing.4 m E 10 m 0.COR. readings of the wall thickness in correspondence to the defect shall be taken at regular intervals along the inspection plane longitudinal and circumferential (see Figure 5. .2.p. In accordance with the RBI approach. tNOM nominal wall thickness (mm).7). an accurate mapping shall be performed of the defect sizes. can be modified based on the actual sizes of the flaw.0 March 2011 Exploration & Production Division Page 36 of 54 5. D inside vessel or pipe diameter (mm).4 Inspections in correspondence to defects In case of significant internal corrosion attacks.COR.Eni S. LS. longitudinal and circumferential. shall be determined by spot thickness measurements.3) leading to new and updated requirements for future inspections. /4/): LS = MIN [0. tMIN minimum required thickness (mm).36·(D·tMIN)½. /4/). C1 C2 C3 C4 C5 C6 C7 M5 M4 M3 M2 M1 Line M: path of minimum Line C: path of minimum thickness readings in thickness readings in longitudinal direction circumferential direction Figure 5.6 Inspections Program The Inspection Program is defined (DNV G101) as ‘a summary of inspection activities mainly used as an overview of inspection activity for several years into the future’.A. not visually accessible.4 In case of localized corrosion attacks or pitting or grooves (mesa corrosion) the critical thickness profiles (CTP).5. planning of the inspection along the operating life of an asset is a dynamic process. The spacing of the readings. 2·tNOM] where: LS recommended profile spacing (mm).VAR. as for instance UT.SDS Rev. 02961. where the results of each inspection are used within an iterative loop (see Figure 2.7 – Method for determining the Critical Thickness Profiles in correspondence to a defect (adapted from Ref. 4 This is the case for instance of defects found exceeding the acceptance limit (see Par. The spacing distance for spot readings along each inspection plane can be determined by the following formula (Ref. 4. detected by spot inspections methods. To determine the CTP.4). 5. presence of contaminants. a more conservative approach is convenient. flow rates.VAR. Similar situation can be met of existing assets where for any reasons previous inspection have not been performed or whose results are not available. . repairing costs.Eni S.4). 4. can be attributed to the results risk analysis only if previous inspection data are available (see Par. Inspection intervals depend on several factors. the recommended inspection intervals are given hereinafter: Time for inspection Risk Class (Criticality) Inspection Level (months) Severe A ≤6 High B ≤12 Medium C ≤24 Low. like: water cut. When previous inspection data were not available.p. − uncertainties in future operating parameters. most of which should be incorporated in the risk analysis and in the resulting Risk Class.6. For inspection management purposes.6. an inspection shall be programmed within first 3 years (36 months) since start-up. A base case is defined.4. these inspection results do not address for defects caused by degradation mechanisms active during operations.8 in this paragraph to be followed for long term inspection planning. however. Examples are: − the confidence attributed to the risk analysis. however. Inspection intervals can be also affected by the aspects of the involved degradation mechanisms which not always can be conveniently addressed by the risk analysis. etc. with tentative intervals to be further confirmed based on the results of each inspection.A. 02961. − uncertainties in predicting the rate of deterioration or the effectiveness of the preventive measures. production losses. a high confidence. However. 5.1 First inspections planning Frequency of inspection for each item shall be defined based on risk analysis results and in particular on the risk class attributed to the item. however. the latter combining several aspects like environmental issues. Independently from the Risk Class. the inspections performed after erection should be available. Negligible or Safe D and E ≤36 Accordingly for items identified as belonging to the risk class severe or high. safety issues. the confidence on risk analysis results is low being the analysis based on theoretical models only. planning of the next inspection shall be the result of the RBI process. inspection shall be performed urgently and in any case no later than one year. a long term inspections program shall be issued. is the combination of the likelihood of occurrence of the degradation mechanisms and consequences in case of failure. − uncertainties on construction material properties: it is the case for instance of old plants with poor project documentation available.2 Inspections intervals Once previous inspection results are available.0 March 2011 Exploration & Production Division Page 37 of 54 First inspection however cannot benefit of previous inspection results. 5. The Risk Class. In case of new assets. Standing the above outlined factors.SDS Rev. guidelines are given in Table 5. In case of first inspection.COR. in fact. with inspection interval ranging from 24 months up to 60 months depending on the risk class the item belongs. the following issues shall be dealt with: − applications to appropriate Authorities and permits. proximity to environmental − Facilities handling H2S or lethal sensitive or protected area.Eni S. Risk Class Reduced Relaxed Influencing factors (Å) Base case Influencing factors (Æ) (Criticality) intervals intervals − Facilities located in high − Facilities located in desert or low Severe 12 population density area. 30 − Facilities with high redundancy. − Quality.COR. 24 population density area. . − High confidence risk analysis. severity of the degradation mechanisms are expected to be moderate to low. High 24 − Offshore facilities. − safety rules to be adopted in the operating area. which are usually performed on more frequent basis (see Ref. Low 42 48 60 − Low confidence risk analysis. − Works reporting and deliverables: Safety. and inspection.p. inspection intervals shall be conveniently planned based on the maintenance schedule. based on the features of the case under evaluation. Daily reports. − HSE. 36 − Facilities located in close − On-land facilities. can lead to reduced or relaxed inspection intervals. rates. Medium 30 fluids. In case of inspections works performed through Contractors. 42 − Unmanned facilities.VAR.8 – Inspection intervals and influencing factors.A. two scenarios are defined for inspection intervals. Inspection Procedures. Preliminary and Final reports. The case of above ground storage tanks shall be evaluated separately. − Facilities with low redundancy. 36 − Low corrosivity conditions with 48 − Facilities handling explosive high predictability of propagation fluids. − personnel qualification. − Facilities handling H2S or lethal − Low cost impact of shut downs. 02961. can be performed only with the tank not in service. defined reduced and relaxed. Table 5. The base case intervals can be modified by considering a number of influencing factors which. in particular of the tank bottom. 60 84 The given intervals DO NOT apply for cathodic protection inspections and controls. − Instrumentation. − Low POD of applied NDT Negligible or Safe 48 methods. in fact.7 Requirements for inspection execution The Inspection Plan shall cover the requirements for the execution of the inspections. Accordingly. fluids.SDS Rev. − High cost impact of shut downs. /26/).0 March 2011 Exploration & Production Division Page 38 of 54 With respect the base case. The table applies in particular for inspection of pressure vessels and pipework performed using non- intrusive methods and without interrupting the operations. 5. In particular. for instance due to impossibility to perform inspections in some zones (presence of obstacles. in case under study inspection results. Complete and univocal indications shall be reported of: item. . any other useful information.2. When corrosion defects are expected and detected originated by localised corrosion mechanisms. the extreme value technique consists in analysing the tail of the normal distribution.0 March 2011 Exploration & Production Division Page 39 of 54 6. Example on how to process data is given in DNV-RP-G103. depending on type of NDT methods. In case of deviations. RESULTS EVALUATION 6. Plot the function Y = -ln[-ln(F1(x))] where F1(x) is the double exponential. (Ref.A. 6. 4.Eni S. 6.1 Inspection results analysis Results of RBI inspection shall be adequately validated and compliance with inspection plans verified. comparing it with the cumulative data distribution. calculate γ (slope) and α (intercept at Y=0). time. Type of elaboration and parameters adopted for representing set of data. instrumentation employed. 3. 6. Inspection results shall be available in convenient format. From the plot of Y vs. Depth of corrosion. 02961. Main steps of the analysis are listed here below.SDS Rev. Method for gathering inspection data suitable for extreme value analysis was given in Par.2. for each item. Take extreme value reading.p. /13/).2 Statistical analysis Statistics can be effectively used. etc. shall be preferably reported in spread sheets.) this shall be recorded and motivated. from frequency data calculate values for probability density function and the cumulative distribution function. 7. shall be evaluated case by case. Spot readings. as for instance average or standard deviation. 2. 1. inspection positions. inaccessibility. i.COR.e. 5. Use the Gumbel function to calculate the probability of having given defect size (depth). /11/). or Gumbel. predicted corrosion forms and severity of the corrosion damages shall be confirmed through inspection results. Verify if the extreme value data set fits a Gumbel distribution function. NDT operator.5. for instance using UT. a powerful statistical approach is the extreme values analysis (see also APPENDIX E).1 Extreme values analysis Extreme values analysis allows investigating the extreme values of a set of data.VAR. Practice for applying statistics to corrosion data is covered by ASTM G16 (Ref. depth of corrosion vs. Using a spreadsheet. in particular in case of large set of inspection result data. typically UT inspection results. Create the frequency distribution histogram. Frequency. Appendix B. Plot the Gumbel function using calculated γ and α. distribution: 5. Inspection results shall be analysed and compared with results of the corrosion analysis. assuming that the whole inspection data fit a normal distribution. of an event A given that observed an event B. the conditional and marginal probabilities of events A and B. at least in principle. can be calculated as: That is confidence that no critical defects exists has increased from 80% to 97.COR. Bayes' theorem expresses the conditional probability.80 A’ critical defects exist. Then. If defects were found exceeding the corrosion allowance. It is also called the posterior probability because it is derived from or depends upon the specified value of B. compromised as the design mechanical thickness will be interested.5%. which shall be available and indicated in the Inspection Plan.Eni S. in terms of the prior probability of B.p. − p(A|B) is the conditional probability of A. p(B|A)=1. the operability of the item shall be considered.. /13/). It is prior in the sense that it does not take into account any information about B. In case previous inspection results were available. or an equivalent predefined limit. . The key idea is that the probability of an event A given an event B (e. in fact. the confidence that no critical defects exist after inspection execution.e. different (i. before inspection: p(A’)=0. 6. − p(B|A) is the conditional probability of B given A. or posterior probability. the probability of corrosion given that I measured corrosion in a past survey) depends not only on the relationship between events A and B but also on the (marginal) probability of occurrence of each event.0 B the probability of not detecting a defect. corrosion allowance.g. provided that the probability of B does not equal zero. p(A|B).20 B|A no critical defects detected | no critical defects exist. and the conditional probability of B given A. represents the design limit which the defects caused by degradation mechanisms cannot exceed. a new value for confidence can be calculated using the Bayes’ theorem. Let: A no critical defects exist. 02961.VAR. repair or replacement Defect evaluation shall be primarily based on the corrosion allowance value of the inspected item.2. given that a correlated event B has been observed (defects measured in a past survey).A. given B. An interesting application to inspections is to calculate confidence on presence of defects. If the item is then inspected using a NDT procedure with 90% probability of detecting a critical defect.2 Bayes’ theorem Bayes' theorem is a useful tool to update the probability of an event A (critical defect). this statement could come from an expert analysis or from a probabilistic corrosion analysis. before inspection: p(A)=0. It is also called the likelihood. Assume that there is 80% confidence that no critical defect is present. and no critical defects are reported. at least locally.Ref. is: where − p(A) is the prior probability or marginal probability of A.0 March 2011 Exploration & Production Division Page 40 of 54 6. and acts as a normalizing constant. p(B)=[p(B|A)xp(A)]+[p(B|A’xp(A’)]. less restrictive) acceptance limits can be used.SDS Rev.3 Defects evaluation: acceptance. based on inspection results (see DNV-RP-G103. Bayes' theorem commonly applied in science and engineering. Appendix B . Based on Bayes’ theorem. by defect penetration. − p(B) is the prior or marginal probability of B. 0 March 2011 Exploration & Production Division Page 41 of 54 In case of defects found exceeding the acceptance limits fixed for the inspection. of the inspected items. − defect is acceptable at design conditions. Defect analysis shall be carried out using applicable codes. as for instance API-RP-579 (Ref.4). . as for instance: − attribution of risk class. The analysis can provide the following results. − defect is acceptable after de-rating of the item.Eni S. 5.p. the database of the asset under evaluation shall be updated with inspection results.COR.4 Re-evaluation Inspection results.5.VAR. together with the actions taken based on inspection results. or criticality. /4/) which provides guidance for conducting Fitness-for-Service assessment for in-service pressurized items containing a flaw or a damage. Accordingly. − defect is NOT acceptable and shall be repaired. re-assessment based on inspection results will have an impact in several area. represent feed information for future corrosion risk assessment and RBI process. − planning of next inspection. − inspection intervals. − defect is NOT acceptable and the item shall be abandoned and replaced. In general. 02961. dedicated investigations shall be carried out aimed to verify the fitness-for-service of the item containing a defect with known sizes (maximum depth and Critical Thickness Profiles: see Par.A.SDS Rev. 6. L.6): the user shall send to ISPESL and to the competent ASL. The outcome of the requalification defines also the frequency of future controls for requalification. 329/04): “Regulation carrying rules for the pressure equipment and assemblies putting into service and use in respect of the section N. 329/04 requirements.Eni S.12) and operating controls (Section N. Integrity controls (Section N. In Italy the directive has been adopted through the government decree issued under parliamentary delegation N. safety devices. the user shall verify the appropriate operation of safety devices and that the real working conditions of pressure equipment and assemblies comply with the commissioning attestation. PRESSURE EQUIPMENT DIRECTIVE (PED) The PED directive (Pressure Equipment Directive) is directive applied in European Countries for the design. 19 of the D. gas and liquid piping. N.13). Accordingly. the controls and verifications on pressure equipment and assemblies in Italy are regulated by the ministerial decree D. − a classification of the equipment and fluid in accordance with the D. maintenance manual instructions. Following the implementation of the section N.L.0 March 2011 Exploration & Production Division Page 42 of 54 APPENDIX A. The decree also establishes the following requirements for the operating life: − mandatory controls at start-up: appropriate installation of pressure equipment and assemblies shall be verified.14). 93/00 (D. 5 Disposizioni per la messa in servizio e l’utilizzazione delle attrezzature a pressione e degli insiemi.8): periodical requalification shall be carried out of pressure equipment and assemblies. 329/04 provides.SDS Rev. For the operating controls (Section N. − Periodical requalification (Section N. an attestation and a Commissioning Report together with other operating data and handled fluids. Specifically. 329/04 also points out the requirements for liquid vessels and piping in service before the 29th May 2002 and not certified in accordance with the D. operating conditions.12) include external visual inspections and internal inspection visual. − an assessment of integrity and efficiency of the equipment. .VAR. outcomes from previous controls.). the construction and the conformity assessment of pressure equipment with maximum pressure greater than 0. among duties to be abided to the putting into service and use of the pressure equipment and assemblies.A. D. Hydraulic test and the internal visual inspection are not mandatory for piping. including: gas and liquid vessels. or through thickness measurements or other adequate inspection methods.M. (D. 02961. − Periodical controls (Section N. 19 of the D.M. D. 329/04.p. th − Pressure equipment and assemblies in-service before the 29 May 2002 and homologated by ISPESL in accordance with the previous legislation. steam or superheated water generators. etc. 93/005. in force since May 2000.5 bar. 16). th − Liquid vessel and piping in use before the 29 May 2002 and never homologated. identification codes.8): it includes integrity controls (Section N. for non-certified vessels and piping handling liquids in service before 29th May 2002 it is mandatory to provide first periodical requalification. 93(Section N.M. operability is allowed only in case of favourable outcome of each control.L. The liquid vessels and piping can be operated only with the favourable outcome of the first periodical requalification. 93/00” The fields of application of DM 329/04 are: − pressure equipment and assemblies.COR. for accessible items.M. the user shall submit to ISPESL a report with the following information: − a shortly description of the vessel or piping (plant description. 93/10. 93/00). sizes. the commissioning attestation (Section N.M. − Controls and inspections after repairs or modifications (Section N.13).L. fluids. and convenient non- destructive tests can be performed. Controls frequencies are based on D.L. 25/02/2000 N. the UNI/TS 11325 norm states that the integrity and efficiency assessment can be carried out through standardized procedures based on hazard analysis for the risk assessment. . as for example the an RBI process or other equivalent procedures.M.VAR. However different time intervals and alternative inspections can be established provided that an equivalent level of protection was guaranteed. Furthermore. The technical study of integrity includes a study of all known and predictable damage mechanisms and issue and implementation of an inspection plan. The time intervals between periodical requalification are given in D. The technical study of efficiency goes along the technical study of integrity again and it defines the consequences of the all known and predictable damage mechanisms along the time using practical and empirical comparison or numeric modelling. obviously. provides the instructions and requirements for periodical requalification of pressure equipment and assemblies. Accordingly.COR. − technical verification of integrity. the need of repair interventions or de-rating shall be assessed.p.A. issued on March 2009. In this respect. thus recognizing the importance of adopting standardized methods in the preparation of the Inspection Plan as the RBI methodology assures. it include the following tasks : − technical study of efficiency.0 March 2011 Exploration & Production Division Page 43 of 54 The Italian Standard UNI/TS 11325. − technical verification of efficiency. During the technical verification of integrity the availability of project data shall be verified and the inspection results shall be analysed and the stability of the equipment verified. shorter intervals shall be considered. 02961. The following phases are defined for evaluating the integrity and efficiency conditions: − integrity assessment including the following tasks: − technical study of integrity. − efficiency assessment. the controls of pressure equipment and assemblies can be developed within an RBI process.Eni S. During the technical verification of efficiency (see Figure) the capability of the equipment to respect and maintain the minimum requirements shall be assessed for the time between two periodical integrity requalification. 329/004 (Tables A and B). chemical or thermal process that can bring to the structural degradation of the equipment under study. Damage mechanism is defined as any mechanical. In case. the verification of the requirements as well as the issue of an exhaustive reporting of the performed controls remain mandatory.SDS Rev. to be performed only in case of favourable outcome of the integrity assessment. the extent of the inspections. SDS Rev. 02961. .p.VAR.A. The Inspection data and the inspection results shall be provided by the Inspection Company in Microsoft Excel or compatible format. FORM FOR DATA COLLECTION Hereinafter an example of Form is reported with indication of the main data to be collected along the execution of the RBI procedure and to be loaded in the RBI database.Eni S.COR.0 March 2011 Exploration & Production Division Page 44 of 54 APPENDIX B. The form complies the structure of Inspection Manager. 02961. Procedure . [°C] 3 Vessel Volume [m ] Line diameter [in] From To RBI Database Fluid treatments with chemicals Type Injection points Injection modes Dosages Cathodic Protection Impressed current/ sacrificial anode system Insulating joints location and type Monitoring Test Point location Refer.SDS Rev. Inspection Drawing Ref. electrode Measurements Corrosion Monitoring Monitoring Point location Monitoring Probe type Corrosion rate results Bacteria presence [yes/no] Other water analyses Risk Analysis Results Failure\degradation Mechanisms Corrosion morphologies Localization of the attacks RBI Output Data Damage likelihood Consequences Factor Risk level Confidence Factor Inspection Plan Risk Analysis results Failure\degradation Mechanisms Notes RBI output Data Risk level Confidence Factor Inspection Plan Data Inspection Methods Methods Description Inspection Priority Insp. Insp.VAR.p.Eni S. Unit Fluid Code Fluid Description Fluid Phase P&ID Name General and Operational Data Baseline Thickness Corrosion Allowance Wall Thickness measurement points Specification Rating Materials and Grades Insulation Code Painting code Outer coating Inner coating Insulation Code Insulation thickness Heath tracing type Tracing temperature Operating pressure [bar] Design pressure [bar] Design Temperature [°C] Operating Temp.COR.A.0 March 2011 Exploration & Production Division Page 45 of 54 Asset Database Name and Plant TAG Area FFU Facility Funct. Frequency Last Inspection date Next Inspection Ref. 0 March 2011 Exploration & Production Division Page 46 of 54 APPENDIX C. pipework or fitting Flange/Joint Leak Instrument taping compressors and Heat Exchangers Pipework Failure Valve Loss of Fired Heaters Containment Vessels and or Failure Pumps. corrosion of metallic materials. Loughbrough University. Table C. 174 failures and 22% of total.COR.SDS Rev. corrosion and related phenomena (erosion and pinhole) represents the first cause of incidents. the distribution for causes of the failure and type of equipment. Bruceand D. G. John. Except for failures from leaking gaskets. which represents a well-defined and easily detectable type of failure. Offshore Technology Report. 1999/064. 1997.1 – Ranking of causes of incidents vs. Reported in: J. fatigue or in-service stress 21 4 2 16 2 0 0 0 45 8 Seal failure 0 7 0 1 29 4 0 0 41 9 Other miscellaneous failure 1 20 0 10 1 2 1 0 35 10 Mechanical failure 0 3 1 1 27 2 0 0 34 11 Poor design or construction or manufacture 0 2 8 12 1 0 1 0 24 Total 186 157 153 144 91 27 28 14 800 % 23% 20% 19% 18% 11% 3% 4% 2% 100% 6 From: R. 2001. Table C. 02961. “Evaluation of Hydrocarbon Leaks due to Corrosion/erosion in Offshore Process Plant”. bolting. K. plug or gland 1 22 37 20 4 2 2 0 88 5 Incorrect or deficient procedure or specification 9 3 23 13 2 3 0 0 53 6 Poor or deficient maintenance procedure 1 6 13 19 5 0 1 1 46 7 Vibration. for 800 inventoried mechanical failures. erosion or pinhole leak 123 16 3 10 1 3 7 8 171 3 In service failure – no specific cause 30 7 7 26 9 1 4 5 89 4 Loose connection. Corrosion risk assessment and safety management for offshore processing facilities.VAR. Capcis.p. in particular of carbon and low alloy steels which still are the main construction material. Tanks Total fans 1 2 3 4 5 6 7 8 1 Leaking gasket at gland or O-ring 0 67 59 16 10 10 12 0 174 2 Corrosion. Patel. A Safety Practical Project. . Diploma in Occupational Health and Safety Management. Dawson.1 6 shows.A. CAUSES OF FAILURE IN OIL AND GAS PROCESS PLANTS Amongst the degradation mechanisms which affect the asset integrity. has been recognized as main one. type of equipment.Eni S. Exploration & Production Division APPENDIX D. as for instance DPI or MPI. - or heat exchangers.A. each pulsed individually. Maurice Silk. the depth of a crack tip can be calculated automatically by simple trigonometry. for instance by pulsing the reduction. This document is property of Eni S. invented in the UK in the 1970s for the nuclear industry by Dr. which can be also connected to video recorder.p. the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. It shall neither be shown to Third Parties not used for purposes other than those for which it has been sent. and the data from multiple beams are put together to make a visual image showing a slice through the object.Eni S. Divisione Agip. The beam is swept electronically like a search-light through the object being examined. rigid or flexible. Ultrasonic wall thickness mapping allows to produce wall thickness maps where different reduction. up to 1 m. POD high Testing (shear wave and 15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to embedded cracks compression wave) determine the wall thickness of inspected item. This method is even more reliable than Radiographic testing of a weld. there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Blistering- Time of Flight Diffraction TOFD Time of Flight Diffraction (TOFD) method of Ultrasonic inspection is a very sensitive and accurate Surface and Consolidated. POD high method for non-destructive testing of welds for defects. elements one by one in sequence along a row. typical tools optic fibre boroscopes. result in a beam at a set angle. All visible defects Remote Visual Inspection RVI CVI can be also performed using specific supports tools to extend the access inside piping or vessels and damages.) and performed in combination with other inspection methods. One of the probes emits an ultrasonic pulse that is picked up by the probe on the other side.p. Ultrasonic Imaging UI Ultrasonic probes are combined with hardware and software suitable to provide imaging of vessel and Wall thickness Consolidated. By varying the timing. The use of TOFD enabled crack sizes to be measured more accurately. so that expensive components could be kept in operation as long as possible with minimal risk of failure. lights. In undamaged pipe. Creeping Head Inspection CHIME Creeping Head Inspection method consists in the transmission of ultrasound between two probes Wall thickness POD medium placed a distance apart. - It can be supported by tools (measuring instrument. In a TOFD system.A. in parallel walled material. When a crack is present. a pattern of constructive interference is set up that Cracks. Esso non sarà mostrato a Terzi né utilizzato per scopi diversi da quelli per i quali è stato inviato. Wall thickness reduction: blistering (compression wave). (shear wave). covering the 100% of wall volume ENGINEERING COMPANY STANDARD Documento riservato di proprietà di Eni S. Divisione Agip. a pair of probes sit on opposite sides of a weld. TOFD is a computerized system that was embedded cracks. very short ultrasonic pulse-waves with center frequencies ranging from 0.1. thickness values are associated to different colors. NDT INSPECTION METHODS Inspection Technique Abbreviation Description in short Defects detected Method maturity and POD Close Visual Inspection CVI It is intrusive when referred to internal side of an item. . Phased-Array PA Phased Array technique is an advanced method of ultrasonic testing. etc.A. Conventional Ultrasonic UT In ultrasonic testing (UT). Using the measured time of flight of the pulse.Surface and Consolidated. non-intrusive if applied to the external surfaces.p. POD high pipe walls. The PA probe consists of small Wall thickness ultrasonic elements. pipe or vessel. This field penetrates through the weather sheeting and magnetizes the pipe wall. movement of the probes to cover large surface areas.p. small detectors or sensors are also built into the probe. Eddy Current ACFM ACFM The Alternating Current Field Measurement (ACFM) technique is an electromagnetic non-contacting Cracks (also under Consolidated. causing a sudden drop in the magnetic field.SDS Rev. the CHIME method is a medium range Cracks. Long Range UT (Lamb LRUT Long-range ultrasonic methods use so-called guided ultrasonic waves. Typically frequencies around 50kHz are used compared with around 5MHz for conventional thickness testing. technique which provides instantaneous coverage of the full volume between the probes. The detection of additional mode converted signals from defects aids discrimination between pipe features and metal loss. A magnetic field is present above the surface associated with this uniform current and this will be disturbed if a surface- breaking crack is present. Cracks. The probe is scanned longitudinally along the weld with the front of the probe parallel and adjacent to the weld toe. This gives an indication of length. Blistering. cause reflections which are detected by the transducer. Pulsed Eddy Current PEC Pulsed Eddy Current is an electromagnetic method to determine wall thickness of electrical Wall thickness POD medium conductors. Hence metal loss defects from corrosion/erosion inside the pipe or corrosion on the outside of the pipe can be detected. similar to the Lamb waves Wall thickness POD medium Wave) which may be generated in plates and in common pipe thicknesses are necessarily of much lower reduction. These transducers do not need embedded cracks contact nor coupling fluid. scaled or heavily painted surfaces. As a . POD low Transmission receive ultrasound instead of the traditional piezoelectric probes. and they can be used on rough. The PEC instrument probe is placed against the metal weather sheeting of an insulation reduction. It was realised that if these disturbances could be measured they should have some relationship to the defects that had caused them.0 March 2011 Exploration & Production Division Page 48 of 54 Inspection Technique Abbreviation Description in short Defects detected Method maturity and POD between the probes.Eni S. Any changes in the thickness of the pipe. either on the inside or the outside.A. With a crack present. the current would flow around the ends and the faces of the crack. It is possible to make quantitative measurements of the magnetic field disturbances and relate them to the size of the defects which produced them. the Bx along the length of the defect which responds to changes in surface current density and gives an indication of depth when the reduction is the greatest and Bz which gives a negative and positive response at either end of the defect caused by current generated poles. A magnetic field is created by an electrical current in the transmitting coil of the probe. The electrical current in the transmission coil is then switched off. 02961. frequency than that used for normal ultrasonic tests in order to generate the appropriate wave modes. In contrast to conventional ultrasonic inspection techniques which require reduction.VAR. POD high technique which has been developed to detect and size surface breaking defects in a range of coating) different materials and through coatings of varying thickness. as on hot surfaces (up to 460 in constant contact). These waves have the property that they can travel many metres with minimal attenuation and therefore offer the potential of testing large areas from a single point using a pulse-echo transducer bracelet wrapped around the pipe. which measure the magnetic field disturbances. Wall thickness reduction. as well (shear wave). When a uniform current is introduced into the area under test if the area is defect free the current is undisturbed. Electromagnetic Acoustic EMATs The technique is based on using electromagnetic acoustic transducers (EMATs) to generate and Surface and New.COR. Special techniques are used to induce these electric currents and the components used are built into the ACFM probes. Two components of the magnetic field are measured. The basis of the technique is that an alternating current flows in a thin skin near to the surface of any conductor. spraying. but links to tank floor . The particles may be dry or in a wet suspension. most commonly pipelines and storage tanks. Consolidated. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC). After adequate penetration time has been allowed. The eddy currents diffuse inwards and decrease in strength. The piece can be magnetized by direct or indirect magnetization. where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping. LPI is used to detect casting.VAR. This article currently focuses mainly on the pipeline application of MFL.p. also called liquid penetrant inspection (LPI) or penetrant testing (PT). The penetrant may be applied to all non-ferrous materials and ferrous materials. plastics. attack (metal loss) even at the subsurface can be detected from the surface side. nickel. The thicker the wall. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. DPI is based upon capillary action. but for inspection of ferrous components magnetic-particle inspection is also preferred for its subsurface detection capability.0 March 2011 Exploration & Production Division Page 49 of 54 Inspection Technique Abbreviation Description in short Defects detected Method maturity and POD result of electromagnetic induction.Eni S. Indirect magnetization occurs when no electric current is passed through the test object. the longer it takes for the eddy currents to decay to zero. a developer is applied. or missing metal. the magnetic field "leaks" from the steel. Saturated Low Frequency SLOFEC The SLOFEC inspection technique uses the eddy current principle in combination with amagnetic Wall thickness POD medium Eddy Current field. forging and welding surface defects such as cracks. The process puts a magnetic field into the part. and fatigue cracks on in-service components. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector.COR.SDS Rev. In an MFL tool. The particles will build up at the area of leakage and form what is known as an indication. The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. or ceramics). eddy currents will be generated in the pipe wall. Ferrous iron particles are applied to the part. If an area of flux leakage is present the particles will be attracted to this area.A. At areas where there is corrosion (localized). or brushing. By superimposed DC-magnetisation the depth of penetration is increased so that corrosion reduction. Inspection is performed under ultraviolet or white light. Magnetic Particle Inspection MPI Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface Surface cracks. The indication can then be evaluated to determine what it is. a magnetic detector is placed between the poles of the magnet to detect the leakage field. 02961. POD high and subsurface discontinuities in ferroelectric materials such as iron. and what action should be taken if any. POD high is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non- porous materials (metals. Liquid Penetrant Inspection DPI Dye penetrant inspection (DPI). the excess penetrant is removed.fluorescent or non-fluorescent (visible). surface porosities. Magnetic Flux Leakage MFL Magnetic flux leakage (MFL) is a magnetic method of non-destructive testing that is used to detect Wall thickness POD high/medium corrosion and pitting in steel structures. depending upon the type of dye used . Surface cracks Consolidated. The technology is an inspection method for detection of topside and underside corrosion in thin and thick walled plates and pipes. cobalt. and some of their alloys. what may have caused it. The basic reduction principle is that a powerful magnet is used to magnetize the steel. and leaks in new products. but a magnetic field is applied from an outside source. The decrease of eddy currents is monitored by the PEC probe and is used to determine the wall thickness. Analysts interpret the chart recording of the leakage field to identify damaged areas and hopefully to estimate the depth of metal loss. Radiography is based on the Wall thickness Consolidated. is a NDT method where the image is produced Wall thickness POD medium electronically.p. which propagate to the surface and are recorded by sensors placed on the surface of the inspected item. .SDS Rev. The variations in heat emission can be local reduction. Acoustic Emission AE Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden Propagating POD medium/low redistribution of stress in a material. or temperature). load. 02961. In most instances.A. embedded (as it typically occurs on welds). When a structure is subjected to an external stimulus (change in cracks pressure. so that very little lag time occurs between the item being exposed to reduction.COR.VAR. rather than on film. cause a variation of local thermal conductivity. Thermography TT In passive thermography heat distribution on a vessel surface is measured. if Wall thickness - present. Real Time Radiography RTR Real-time radiography (RTR). wall thinning defects. localized sources trigger the release of energy. POD high 60) different absorption of the radiations in presence of variable thickness values. radiation and the resulting image. or real-time radioscopy. Film Radiography (Ir 192. defects. also if reduction. in the form of stress waves.Eni S. Co RT Radiographic inspection uses X-rays.0 March 2011 Exploration & Production Division Page 50 of 54 Inspection Technique Abbreviation Description in short Defects detected Method maturity and POD examination are provided at the end. Iridium 192 and Cobalt 60 are the most common radiation sources for NDT inspections. The technique can be conveniently Thermal insulation used on thermally insulated items. vessels or piping. insulation. i. measured with a sensible video camera and hot spots identified. the electronic image that is viewed results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs.e. Embedded flaws. to detect defects in the wall or in the thermal breakdowns. gamma-rays or neutron radiation. and minor attention is paid to the distribution of the extreme corrosion defects. where and are sequences of constants with . parameter of scale positive and parameter of shape defined in ( ).A. Divisione Agip. If converges in distribution to a non-degenerate variable . It shall neither be shown to Third Parties not used for purposes other than those for which it has been sent. a way to study is to estimate F from the whole available data and then to substitute this estimation in the previous formula to estimate . namely the most penetrating ones. EXTREME VALUE ANALYSIS Introduction Analysis of inspection data of items where localised wall thinning mechanisms are expected. This type of problem also arises in other engineering areas such as ocean engineering (wave height). is primarily focussed on detecting the most important defects. providing information on this part of a distribution of data.Eni S. etc. Esso non sarà mostrato a Terzi né utilizzato per scopi diversi da quelli per i quali è stato inviato. Then is the maximum of the observed corrosion process over n space units. Exploration & Production Division APPENDIX E. One alternative approach is to estimate directly from the extreme data. The traditional statistical methods tend to ignore extreme events and focus on risk measures on the whole empirical distribution. One simple way of characterizing the behaviour of extremes is by considering the behaviour of the maximum order statistics .p. A linear normalization of is used. rainfall). For example. . Divisione Agip. as inspection measurements are. variables represent values of the corrosion process measured on a regular time-scale or space-scale. it is often assumed that corrosion defects are normally distributed. meteorology (temperatures.A. Different values of the parameter of shape define three classes of distributions named the extreme value distributions.p. hydraulics engineering (floods).A. which are the most critical ones. Extreme Value Theory (EVT) is a specialist branch of statistics that applies to the tails of a distribution. In particular: − The type I (Gumbel distribution) is obtained by letting − The type II (Fréchet) distribution is obtained when ENGINEERING COMPANY STANDARD Documento riservato di proprietà di Eni S. Model formulation Suppose is a sequence of independent random variables having a common distribution function . The problem of this approach is that small deviances in the estimation of F lead to large discrepancies for .p. The distribution function of verifies:       Thus. Following this way it is necessary to study the behaviour of as tends to infinity. In practice. This document is property of Eni S. structural engineering (earthquakes). then automatically has a distribution belonging to the generalized extreme value distribution (GEV):       Defined an such that and with parameter of location defined in ( ). fatigue strength (workloads). When data are taken in spatial interval represents simply the level of corrosion that is expected to be exceeded with as associated probability of . then represent the annual maxima. Appendix B.   Practical implementation Example on how to process data is given in DNV-RP-G103 (Ref. Finally the GEV distribution is fitted to this series of block maxima .Eni S. like normal. we can calculate the quintile function. being sufficiently large. Weibull  The three limit types have different forms of tail behaviour.COR.0 March 2011 Exploration & Production Division Page 52 of 54 − The type III (Weibull) distribution is obtained when The uncertainty in the estimation of the parameter measures the lack of certainty in the choice of one of the three models. is the level that is exceeded by the annual maximum in any particular year with probability . Scope of the analysis is to model the extreme values of a series of independent and identically distributed observations . lognormal. Frechet. exponential and gamma (see Figure D. The Gumbel type is the domain of attraction for many common distributions.p.1). once every years. Equivalently. Main steps of the analysis are listed here below. for example every year.SDS Rev. That is. In such case is called the return level associated with the return period .1 . .Probability density of GEV distributions: Gumbel. Then the maxima of each block is considered. Once the GEV distribution has been fitted. is the level that is expected to be exceeded. in average. The first step consists in blocking the data into sequences of observations. 02961.VAR. Figure E. /13/). . The density of Gumbel distribution decays exponentially and the density of Fréchet distribution decays polynomially. These three types of distributions are the only possible limits for the distributions of the normalized maxima regardless of the distribution F for the population. for the maximum distribution observing that . If the maxima are taken in a time interval space.A. They depend on the shape parameter values (Smith. which is rarely the case in real applications of extreme value modelling. − when the maximum likelihood estimators are unlikely to be obtainable. The maximum likelihood estimates of the parameters are obtained maximizing the log-likelihood functions trough numerical optimization algorithms. − when the maximum likelihood estimators can be obtained in general but they do not have the standard asymptotic properties.COR. In practise points should lie close to the first diagonal if data fit well the model. 1985): − when the maximum likelihood estimators have the usual asymptotic properties. When the length of the blocks is small. Long blocks on the other hand generate only few extreme data leading to large estimation variance. . 02961.0 March 2011 Exploration & Production Division Page 53 of 54 Inference The choice of the length of blocks implies a trade-off between bias and variance.A. The method most commonly used to estimate the parameters is the likelihood method.p. assessment can be done with reference to the observed data. Confidence intervals are obtained from this approximate normality of the estimator.Eni S.SDS Rev. Graphical model checking Though it is impossible to check the validity of an extrapolation based on the GEV model. The most used graphical method of validation are the probability and the quintile plot.VAR. The classical theory of maximum likelihood estimation establishes that the distribution of the estimated parameter is approximately normal with mean and variance covariance matrix equal to the inverse of the observed information matrix evaluated at the maximum likelihood estimate. The probability plot (Figure D. then the approximation of the distributions by the limit is not so good and this lead to bias in estimation.2) is a comparison of the empirical (derived from data) and fitted distribution functions (theoretical). One difficulty of this approach is that the regularity conditions for its application are not always satisfied by the GEV distributions. The case with corresponds to distributions with a very short bounded upper tail. SDS Rev. Mallor. E. R Development Core Team (2008).org. The quintile plot is much more sensitive to slight departures from model accuracy in the upper tail then the probability plot. The return level plot is the graph of empirical estimates of the return level function. Application to calculate extreme wind speeds. An introduction to statistical modelling of extreme values. 1999.VAR.. A quintile plot that deviates greatly from a straight line suggests that the model assumptions may be invalid for the data plotted. F. 2. 2009. 02961. Omey.COR. that is plotted against . the plot is convex in the case of and the plot is concave for and has not finite bound. R: A language and environment for statistical computing. Extreme Value Theory and Application. Stuart Coles University of Lancaster.Eni S.  The quintile plot compares the model quintiles against the data (empirical) quintiles..0 March 2011 Exploration & Production Division Page 54 of 54 Figure E.A. HU Brussel Research Paper 2009/36.Example of graphical diagnostic. R Foundation for Statistical Computing.R-project. The importance of return periods in engineering is due to the fact that the return period is used as a design criterion. E. Austria. References 1. 3.p. This plot also includes 95% confidence intervals. Coles. The plot is linear in the case of . Vienna. Nualart. .2 . This graph is useful as validation tool as well as a way of presenting the fitted model. Stuart. URL http://www.