A Methodology for Engineering Criticality Assessment (ECA) for Offshore Pipelines

A Methodology for Engineering Criticality Assessment (ECA) for Offshore Pipelines Ben Lee PhD, J P Kenny, Inc.Paul Jukes PhD, J P Kenny, Inc. Ayman Eltaher PhD PE, J P Kenny, Inc. James Wang MSc, J P Kenny, Inc. J P Kenny, Inc. 15115 Park Row, Houston, TX 77084 Abstract This paper presents a methodology for an Engineering Criticality Assessment (ECA), and results from a study, for the flaws in the welded joints of gas export offshore pipelines. The ECA analyses were performed based on the assumption of various concrete stiffnesses, which considers full concrete stiffness and some degree of concrete stiffness degradation during the service. A series of field surveys were conducted to assess the existing defect sizes, joint KP, field joint number, weld number, pipe/seabed profile, TDP location, span length and water depth, etc. Based on the survey data, finite element model was developed using ABAQUS to calculate the natural frequencies, stresses and mode shapes. The in-house MathCad spreadsheet was developed based on DNV-RP-F105 to generate stress ranges. The results obtained from ABAQUS and MathCad program were used as an input data to ECA analysis. The new technology in this paper is the use of advanced finite element analysis tools to determine the inputs for the ECA. In order to determine the maximum allowable flaw sizes at critical weld locations under operating and extreme environmental load conditions, TWI software Crackwise 4 was used in the assessment. Crackwise 4 is a Windows based integrated software package, which automates the fracture and fatigue assessment procedures based on BS 7910:2005. For fracture and fatigue analysis of pipelines, a level 2 analysis was conducted using the stress concentration factor and reference stress solution. The ECA assessment assumed a welding residual stress condition allowing relaxation and different stress concentration factors due to weld misalignment. The acceptance criteria were developed under extreme environmental load condition as a guideline. The final flaw sizes induced from the fatigue analyses were compared with the established acceptance criteria to assure the structural integrity of the pipelines. The survey assessed weld defect sizes were also compared with the acceptance criteria to make an engineering decision for fitness-for-service. Keywords: DNV (Det Norske Veritas); ULS (Ultimate Limit State); FLS (Fatigue Limit State); WIF (Wave Induced Fatigue); FM (Force Model); VIV (Vortex Induced Vibration); RM (Response Model); Mode Shape; Natural Frequency; Unit Stress; CSF (Concrete Stiffness Factor); ECA (Engineering Critical Assessment); FAD (Failure Assessment Diagram); CTOD (Crack Tip Opening Displacement); NDE (Nondestructive Evaluation); AUT (Automated Ultrasonic Testing); TWI (The Welding Institute) 2. Introduction Offshore pipelines which have been damaged locally due to the free spanning may not satisfy its intended design life.1 Field Data Assessment Field survey data were assessed and these raw data were processed in order to eliminate noise and invalid data before use in the FEA modeling. Engineering Criticality Assessment (ECA) software. screening determination and . The pipeline integrity assessment is very important. residual tension and interaction between pipe and soil etc is critically important in order to estimate accurate strength and fatigue life. In order to include the welding defects or grown fatigue cracks in the assessment of structural integrity of free spanning pipelines. was adopted as well as advanced finite element tool such as ABAQUS. Any free spanning pipelines may be able to withstand the extreme storm condition as long as the accumulated fatigue damage is below unity (1. stiffness of pipeline. Welding induced defects or propagated cracks may be large enough to fail the free spanning pipelines under design extreme condition which cannot be considered in the conventional S-N curve method. Since the crack propagation life is affected by the pipeline static strength which decreases with the crack propagation.1. S-N curve method cannot reflect the pipeline static strength degradation with service. soil properties. Correct interpretation of surveyed data such as wave/current. In-house program was also developed to better align with industry standard practices such as DNV-RP-F105 and DNV-OS-F101. 2. however. however. Crackwise. A VIV screening analysis was then conducted to determine the maximum allowable span length for in-line and cross-flow directions under both current and wave conditions. analysis procedures may be complicated because of the interpretation of surveyed data. A span survey data was assessed initially to prepare all the necessary input data in order to perform static and dynamic ULS (Ultimate Limit State) checks based on DNV-OS-F101. This paper presents a methodology from the screening analysis to ECA analysis to assess the integrity of offshore pipelines with the use of advanced finite element tools.0). The fatigue life of offshore pipelines consists of both crack initiation and crack propagation life. Free spanning analysis procedures are specified in DNV-RP-F105 for strength and fatigue evaluation due to in-line and cross-flow Vortex Induced Vibration (VIV) and direct wave loading. The detailed fatigue analysis is based on the S-N curve method which is a quick and reliable method. Assessment of Free Spanning Pipelines Free spanning pipeline analyses was performed based on DNV-RP-F105. Finally a fatigue analysis was performed for the spans that exceed the maximum allowable span length. and application of the advanced tools. the static strength assessment is an integral part of the analysis. single span and interacting span. The maximum allowable span lengths are normally determined in the screening analysis by VIV onsets at both inline and crossflow directions defined in DNV-RP-F105 and by assuming ULS unity of 1 based on the procedures described in DNV-OS-F101. Boundary condition: “single span on seabed” is used for a single span. For the interacting span case with “pinned-pinned”. The key survey data are described as follows: • • • • • • • • • Free span length Span location Height (gap) between the pipeline and seabed Pipe profiles Touchdown point Wave / current data with associated number of occurrences Wave / current directions Soil data Seabed profiles Perpendicular interactions between wave/current and pipeline are assumed for conservative results if directional information is not available. concrete coating thickness and free span length would be different along the pipeline sections. The purpose of screening analysis is to define a criterion that is applied to free spans in order to capture any free spans which exceed screening criteria. 2. VIV load and direct wave load were considered in the calculation. The following governing criteria were used to calculate the maximum allowable span lengths: • • • • Fatigue due to in-line VIV Fatigue due to cross-flow VIV Static ULS check Dynamic ULS check The screening analysis was performed using the in-house spreadsheet which considers the following features: • • Static load. The assessment is performed yearly basis to accommodate the variation of field data.fatigue life calculation. The effective span length is defined as the equivalent length considering soil stiffness and . The pipeline route is divided into several sections to accommodate different concrete coating thickness. In each section.2 Screening Analysis The environmental data. Based on DNV-RP-F105. the effective span length is used only for the “single span” case. The most onerous spans can be prioritized for detailed analysis. and the “pinned-pinned” is used for interactive spans. the apparent span length is used. water depth. an average water depth was selected and maximum environmental data was applied for conservative analysis. water depth and environmental data. 3. 2. A finite element analysis is necessary in order to apply more accurate loads and boundary conditions and calculate more accurate natural frequency and combined unit stresses under various vibration modes. boundary conditions are applied as specified in DNV-RP-F105. linear stiffness is used to consider the vertical . ABAQUS was used in the analysis. The steady current velocity and wave-induced flow velocity are added together with relevant probability of occurrence for each velocity. The pipe surface is generated from the pipe elements and act as a slave surface and seabed surface is defined as a rigid body with the stiffness between the pipe and soil and act as a mater surface. Alternative ways to distinguish between the single and interactive span is to use the free span classification defined in DNV-RP-F105.3 FEA Modeling The code base calculation of natural frequency and unit stresses may be enough during the screening analysis because code base results are usually conservative. the pipe surface and seabed surface. The determination of single and interacting spans is based on the initial survey data and engineering judgment. therefore. it is only applied during the screening analysis. Initially element size of 1 meter was used and had been fine meshed via iteration based on span length and higher order modes. A contact model includes two contact surfaces. Both environmental cases are checked and governing ULS values are selected as a result. The apparent span length is the original span length identified based on the seabed profile and pipeline information.2 Loads and Boundary Conditions ULS checks are considering environmental loading of 100 year current/1 year wave or 1 year current/100 year wave.• boundary conditions. In the contact model. flow conditions due to current and wave action at the pipe level govern the response of free spanning pipeline. a contact model is used in the static analysis and the spring model is used in the dynamic analysis.5 meters) at one end of the pipe. The maximum gap is used in the screening analysis for conservative results. It may not represent actual boundary conditions. In some cases. A single span boundary condition is assumed when the pipe is fully embedded into the soil at the span shoulders. 2. In order to mimic the interactions between the pipe and soil in FE analysis. The interacting span barely touches the soil (less than 0. Non-linear finite element software. a screening analysis was performed based on the assumption of interacting span boundary conditions in order to generate conservative allowable span lengths. 2. For fatigue analysis.1 Pipe Element The 2-node pipe element was used in the analysis. In a screening analysis.3. EIconc : Young’s Modulus and Inertia of Concrete Coating EIsteel : Young’s Modulus and Inertia of Steel Pipe 2. The equation has been calibrated using the separate FEA model and applied to ULS checks and fatigue calculation.1.direction boundary conditions and constant friction factors are used to accommodate the pipe and soil interaction. non-linear spring elements are applied to the nodes between pipe and seabed in axial. . Both soil stiffness and friction factors can be obtained from DNV-RP-F105.4 ULS Check The purpose of ULS check is to ensure the pipeline is within the corresponding pipeline specification limit by using more accurate results such as natural frequency and unit stress obtained from the FEA.  EI  SCF = 1 +  conc  (1)  EI   steel  where. 2. lateral and vertical directions.3. The concrete and pipe are bonded together right after installation and representing intact condition which has full concrete bending stiffness.75 An example of the ULS check results along the pipeline length is shown in Figure 2. The discontinuity of concrete coating at the field joint was considered by applying stress concentration factor (SCF) using the equation defined below. The loads used in the calculation are the following: • • • Static load – weight VIV loads from in-line and cross flow vibrations Direct wave load – direct drag and inertia effects 0. In the spring model. The ULS calculation is performed based on the DNV-OSF101 using the extreme environmental loading conditions. Large bending strains are generated at span area which induce cracks in the concrete and reduce bending stiffness of concrete. Some degree of concrete stiffness degradation during the service is considered in the analysis.3 Concrete Modeling The concrete was modeled as an outer pipe relative to the steel pipe using pipe-in-pipe (PIP) model. If the difference of these span heights at the mid-point is within a certain limit (∆1 and ∆2 in Fig. 20 periods and 8 directions with relevant probabilities. both the in-line and cross-flow are considered in the current and wave induced VIV. The pipeline profile measured by survey data can be matched with FE predicted pipeline static profile by changing the residual tension.5.20 0. m Figure 2. 2. span gap and environmental data.00 0. static load. For fatigue life assessment. residual lay tension and concrete stiffness are input parameters for FE analysis.1 Unit check results along the pipeline length 2.90 0.60 0. 2.10 0.1 Natural Frequency Determination The natural frequency and mode shapes are calculated from the FE analysis to perform accurate fatigue life calculation. the calculation is based on Morison’s equation for direct in-line loading. The pipeline mid-point is selected and predicted span height and surveyed span height are compared. fatigue analysis is conducted for each span corresponding to the applicable changes in concrete coating thickness. the averaged differences between the predicted span height and the surveyed span height along the span length are also used as the match criteria. The soil stiffness.Unit Check Results 1. Alternatively. tension may be determined by trial and error.00 0 50 100 150 200 250 300 350 400 Pipeline Length. The wave information contains 8 significant wave heights. seabed topography. VIV loads and direct wave loads are considered in the calculation.40 0. both “Response Model” and “Force Model” are used in the calculation.70 0. The span gap is calculated as average value over the central third of the span suggested by DNV-RP-F105.2). it is assumed that pipeline profile matching is achieved and residual tension is selected.5 Fatigue Calculation Procedures Similar to the span screening analysis. If residual tension is not available for existing pipeline. The current information contains 8 directions with relevant probabilities.50 0. water depth. In the “Response Model”. For the “Force Model”.30 0. . In the fatigue analysis.80 Unit Check Results 0. As a result. Depending on the pipe size and the total weight. . the following features are included: • • • • Current and wave modules accounting for the current and wave effects in VIV Force model calculations for direct wave load response Multimode response for VIV calculations Fully compatible with DNV-RP-F105 Several primary parameters that influence the fatigue life in general are the natural frequencies. mode shapes. and survey accuracy. 2. concrete conditions (intact or damaged). water depth and weld type etc. mostly due to the variation in concrete thickness.2 S-N Fatigue Software The environmental data and FE results are used as an input data to DNV FatFree software version 10 and fatigue life was calculated for the span.tolerance Concrete condition – Young’s modulus: nominal +/.5. The sensitivity study accounts for the uncertainty due to soil properties. In the FatFree software. the dynamic soil stiffness can vary with one location to another. thereby varying the inputs.tolerance The tolerance for each item varies from case to case. span lengths. a range of tension and frequency values are calculated and is used in the subsequent fatigue analysis. 2. To ensure a conservative estimate of the residual lay tension.Fig.2: Pipe Profile Comparison The soil stiffness is assessed based on the DNV codes. sensitivity studies are performed to determine a range of residual tension that produces a matching pipeline profile. current / wave data. The range of the parameters is initially taken as the following: • • • The soil static stiffness: nominal +/.tolerance in a certain percentage Survey accuracy: (height) +/. Force Model (FM) 7. The final flaw sizes induced from the fatigue analyses were compared with the established acceptance criteria to assure the structural integrity of the pipelines. Response Model (RM) 6. Structure calculations (pipe submerged weight. The stress ranges from in-line / cross-flow VIV and direct wave load are required for ECA. DNV-RP-F105 parameters. and oneyear near-bottom extreme current. The acceptance criteria were developed under extreme environmental load condition as a guideline. Stress range output (stress ranges from RM and FM) This calculation procedure is a DNV-RP-F105 code based calculation using FE results. 3.1. DNV FatFree software generates span fatigue life only and related stress ranges are not available from FatFree.1 Analysis Procedure Two spans at KP “A” and KP “B” are analyzed to output the stress ranges for ECA analysis. inertia moment etc. A flow chart for stress range calculation procedures are shown in Figure 3. and in-line direct wave loading are considered in the analysis. in-house program was developed to generate stress ranges and number of vibration cycles per year based on the Response Model and Force Model. . ECA for Offshore Pipelines The objective of the Engineering Critical Assessment (ECA) is to determine the maximum allowable flaw sizes for surface and embedded flaws in the weld metal of pipelines under operating and extreme loadings. The calculation procedures for stress ranges are described as follows: 1. The survey assessed weld defect sizes were also compared with the acceptance criteria to make an engineering decision for fitnessfor-service. cross-flow VIV. In-line VIV. ECA is necessary to make an engineering judgment whether it is acceptable or not for the continuing service. The schematic diagrams of overall calculation procedures from the screening analysis to ECA analysis are described in Figure A1 of Appendix A. The results are based on the metocean data with wave directionality. Therefore.) 3. current velocity at pipe location) 5. 3. Environmental calculations (wave induced velocity. environmental parameters.If any crack is initiated during the service and propagated to a certain size due to the cyclic stresses. metocean parameters) 2. Soil calculation 4. Input data (pipeline parameters. 3.2.1 Flow Chart for Stress Range Calculation Stress histograms are obtained at two (2) critical locations. 3. The analysis was performed with an assumption of full concrete stiffness and 30% concrete stiffness degradation. The directional probability of occurrence data for waves is used in the analysis. Generalized Failure Assessment Diagram (FAD) option was selected to characterize the mechanical behavior of weld metal as actual stress and strain curve data for the weld metal was not available. The current and wave induced flow components are assumed co-linear as DNV-RP-F105 indicated.3 ECA Software .2 Analysis Type The ECA was performed in accordance with BS 7910:2005 using Level 2A analysis.7 Stress Range Cycles per Year (Response Model) Stress Range Cycles per Year (Force Model) Stress Ranges and Cycles Stop Figure 3.Start ABAQUS/FEA: Natural Frequency Mode Shape Excel Spreadsheet: Equivalent Stress Factor DNV-RP-F105 (2006): Section 5. KP “A” and KP “B” in the pipeline. Crack growth assessment shall consider cyclic stress histograms determined from wave induced fatigue (WIF) analysis and vortex induced vibration (VIV) analysis. Secondary stresses are defined as those stresses which are self-balancing and do not contribute to plastic collapse. or machining stresses.4 Input Data Required by Crackwise 4 The following input parameters are necessary in order to perform an ECA analysis in Crackwise 4: • • • • • Geometry / Flaw Selection – used for setting geometry. Table 3.1 Summary Table for Input Data SCF due to weld misalignment 1. such as welding residual stresses. Young’s modulus and Poisson’s ratio. which automates the fracture and fatigue assessment procedures in the BS 7910:2005.3 Extreme Condition In-line VIV considered Extreme Condition Cross-flow VIV = 0 Axial Force Obtained from FEA Bending Moment due to Submerged Weight Obtained from FEA Bending Moment due to In-line VIV Obtained from MathCad Bending Moment due to Direct Wave Load Obtained from FEA CTOD 0. • • • • • Table 3. including weld cap width.0. 3. and which contribute to plastic collapse.5 mm Corrosion Allowance 0 mm (No corrosion) Pipe Material Grade API 5L X65 Steel’s SMYS 448 N/mm2 . edge and bar / bolt flaws.1 summarizes the input data. Primary Loading – used for input of primary membrane and bending stress components. Cyclic Stresses – input of cyclic stresses for fatigue and fatigue-fracture analysis. including through-thickness. surface.45 mm Pipeline Outside Diameter 660. Secondary Loading – used for defining secondary stresses. Primary stresses are generally those which do not self-balance across the section. thermal stresses. Partial Safety Factor.4 mm Pipeline Wall Thickness 16. Crackwise is a Windows based integrated software package.The ECA was performed using TWI software Crackwise 4. Fatigue Crack Growth Constants – used to define fatigue crack growth properties. full penetration weld or fillet weld and flaw type. corner. long surface. Misalignment – used for the calculation of additional misalignment induced bending stresses. embedded. tensile strength. Material Tensile Properties – yield strength. Flaw Dimensions – used for setting flaw dimensions. Toughness – input fracture toughness of material. 5 of BS 7910.5. Mpa 71. Mpa 60.3 Max bending stress.5 Primary membrane and bending stresses for KP “B” (70% Concrete Stiffness) Membrane stress.0 Max bending stress. MPa 89.6 Static Stresses Static primary membrane and bending stresses at KP “A” and KP “B” were tabulated in Table 3.4 Primary membrane and bending stresses for KP “A” (70% Concrete Stiffness) Membrane stress.30% will be considered) 7850 kg/m3 1025 kg/m3 -850 mV (Ag/AgCl) Level 2A All defects in the analysis were assumed to occur at fusion lines between the weld and HAZ. 3. Mpa 70. stress concentration at the weld toe were taken into account based on 2D or 3D solution outlined in Annex M.4 mm 60 mm 3. MPa 104.Steel’s SMTS Steel’s Modulus of Elasticity External Corrosion Coat (FBE) Thickness Concrete Coating Thickness Concrete Elastic Modulus Steel Pipe Density Seawater Density Cathodic Protection Analysis Type 3. External surface flaw and embedded flaw were assumed to be oriented circumferentially in the girth weld.9 Max bending stress.5 Max bending stress. The embedded flaws were re-characterized as surface flaws before assessed in accordance to the Annex E of BS7910.2 Table 3. Mpa 86.13E+04 MPa (+/. For external surface flaws. Table 3.2 Primary membrane and bending stresses for KP “A” (100% Concrete Stiffness) Membrane stress.2 and Table 3.3 Primary membrane and bending stresses for KP “B” (100% Concrete Stiffness) Membrane stress. MPa 84.8 Table 3.5 .3 Table 3. MPa 82.5 Defect Locations 531 N/mm2 207000 N/mm2 0. The location of the embedded cracks for all cases was assumed to be 2 mm from the external surface. 0E+05 0.0E+05 2.0E+05 6.5 21.3 Annual Cyclic Stress Ranges for WIF and VIV Acting on KP “A” (70% Concrete Stiffness) Location KP "B" (100% ) 1.0E+00 1.0E+05 4.6 Stress (MPa) Figure 3.2E+06 # of cycle/year 1.2 42.2 11.4 Annual Cyclic Stress Ranges for WIF and VIV Acting on KP “B” (100% Concrete Stiffness) . Crack growth assessment shall consider cyclic stress histograms determined from wave induced fatigue (WIF) analysis and vortex induced vibration (VIV) analysis.4 and Figure 3.3.6E+06 1.0E+05 0.0E+05 6.6 Stress (M Pa) Figure 3.7 26.1 37.2 Stre ss (MPa) Figure 3.5 21.7 26.0E+06 8.3 for 100 % concrete stiffness case. The relevant stress ranges from WIF and VIV are shown in Figure 3.0E+05 4.0E+06 8.8E+06 1.4E+06 1.4 16.4E+06 1.0E+06 8.0E+05 0. Location KP "A" (100% ) 1.0 6.1 37.0E+05 4.4 16.2 11.5 21.2 11.0E+05 2.4E+06 1.0E+05 6.6E+06 1.2 and Figure 3.2E+06 # of cycle/year 1.7 Cyclic Stresses Stress histograms are obtained at two (2) critical locations. KP “A” and KP “B” in the pipeline.5 shows relevant stress ranges for 70 % concrete stiffness case.9 32.1 37.4 47.6E+06 1.4 16.2 42.7 26.0E+00 1.0 6.4 47.2 Annual Cyclic Stress Ranges for WIF and VIV Acting on KP “A” (100% Concrete Stiffness) Location KP "A" (70% ) 1.2E+06 1.0E+05 2.9 32.9 32.0E+00 # of cycle/year 1. Figure 3.0 6. 5 Annual Cyclic Stress Ranges for WIF and VIV Acting on KP “B” (70% Concrete Stiffness) 3. The fracture analysis was performed under two (2) different concrete stiffness assumptions which are full concrete stiffness and 30% concrete stiffness degradation. The maximum allowable flaw size that will grow to the critical size over the design life of the structure with a safety factor of five (5) which is 100 years shall be determined.4 16.0E+05 2.4E+06 1.7 26.6E+06 1. The two (2) most critical welded locations were selected for offshore pipeline at KP “A” and KP “B”.10 x 10-17 Stage 2 290 2. 3.9 32.4 47.Location KP "B" (70% ) 1. 2-Span case was selected as a controlling load case for fracture analysis of KP “A” and 1-Span case was selected as a controlling load case for KP “B”.3 to assess the maximum allowable flaw sizes in order to assure the structural integrity of the pipelines.2E+06 # of cycle/year 1.6 Paris Law Parameters A ∆Ko (N/mm3/2) m Stage 1 63 5.0E+05 4.6.1 37.2 11.6 Stress (MPa) Figure 3.0E+05 0.2 42.1 2.0 6.67 2.45 mm was assumed in the analysis. Table 3.8 Fracture and Fatigue Analysis Engineering Critical Assessments (ECAs) have been performed using static stresses and cyclic stresses for the welded joints of offshore pipelines.5 are used in the analysis for all assessments of flaws in welded joints as recommended in BS 7910.1 Fracture Analysis under Extreme Condition Fracture analysis was performed under extreme environmental condition at KP “A” and KP “B” with SCF = 1. TWI software Crackwise 4 was used to determine the maximum allowable flaw sizes. The material toughness parameter CTOD = 0. The .3 is used in ECA analysis.5 21. As welded condition and marine environment condition with cathodic protection at -850 mV (Ag/AgCl) were assumed for pipeline weld with Paris law parameters as described in Table 3.0E+06 8. The stress concentration factor SCF = 1.0E+00 1.8.02 x 10-11 The upper bound (mean + 2SD) values for R ≥ 0.0E+05 6. 0 12. The maximum allowable flaw sizes are shown in Figure 3. Figure 3.0 4.9 shows the maximum allowable flaw sizes at KP “B” with 30% concrete stiffness degradation and SCF = 1.0 Flaw Depth (mm) 10.9.3) Maximum Allowable Flaw Size at KP "A" 14.0 8.0 4. Figure 3. For 70% concrete stiffness case.0 0.0 4. Maximum Allowable Flaw Size at KP "A" 14.8 Fracture Analysis for Surface Flaw under Extreme Condition at KP “A” .6 shows the maximum allowable flaw sizes at KP “A” with 100% concrete stiffness and SCF = 1.0 2.0 2.0 6.8 shows the maximum allowable flaw sizes at KP “A” with 30% concrete stiffness degradation and SCF = 1.0 8.7 Fracture Analysis for Surface Flaw under Extreme Condition at KP “B” (100% Concrete Stiffness and SCF = 1.0 0 20 40 60 80 100 120 140 160 Surface Flaw Length (mm) Fracture only (100 Year Event) Figure 3.0 Flaw Depth (mm) 10.0 12.3 under extreme condition. Since the surface flaw is a governing case compared to the embedded case.0 8.6 through 3.3) Maximum Allowable Flaw Size at KP "B" 14.3 under extreme condition.3 under extreme condition.0 0 20 40 60 80 100 120 140 Surface Flaw Length (mm) Fracture only (100 Year Event) Figure 3.primary membrane stresses used were 286 MPa and 278 MPa respectively for KP “A” and KP “B” for 100% concrete stiffness. the fracture analysis was performed for surface flaw only.0 12.0 0.6 Fracture Analysis for Surface Flaw under Extreme Condition at KP “A” (100% Concrete Stiffness and SCF = 1. Figure 3.7 shows the maximum allowable flaw sizes at KP “B” with 100% concrete stiffness and SCF = 1.0 0 20 40 60 80 100 120 140 Surface Flaw Length (mm) Fracture only (100 Year Event) Figure 3. 260 MPa and 254 MPa were used as primary membrane stresses for KP “A” and KP “B”.0 2.0 Flaw Depth (mm) 10.0 6. Figure 3.0 6.3 under extreme condition.0 0. 0 0.0 6.0 3.0 12.0 0. Conclusions This paper focuses on the methodology for an Engineering Criticality Assessment (ECA) for the flaws in the welded joints of gas export offshore pipelines.9 Fracture Analysis for Surface Flaw under Extreme Condition at KP “B” (70% Concrete Stiffness and SCF = 1.0 Flaw Depth (mm) 5.0 1.0 3.3) 3.7 and 3.0 2. ECA tools and industry standard codes.0 Flaw Depth (mm) 5.0 5 10 15 20 25 30 35 40 Surface Flaw Length (mm) 70% concrete stiffness 100% concrete stiffness Figure 3.0 4.(70% Concrete Stiffness and SCF = 1.0 8. Maximum Allowable Flaw Size at KP "A" 7.0 0 20 40 60 80 100 120 140 160 Surface Flaw Length (mm) Fracture only (100 Year Event) Figure 3.10 Maximum allowable surface flaw sizes at KP “A” Maximum Allowable Flaw Size at KP "B" 7.0 4.11.3 was used for all cases.2 Fatigue Analysis Fatigue analysis was performed using the information in section 3.8.0 0. .0 4.8.3) Maximum Allowable Flaw Size at KP "B" 14. The following conclusions are obtained from this study.0 6.0 2.0 5 10 15 20 25 30 35 40 Surface Flaw Length (mm) 70% concrete stiffness 100% concrete stiffness Figure 3. The maximum allowable flaw sizes for KP “A” and KP “B” with 100% concrete stiffness were shown in Figure 3. 3.6. A methodology contains whole procedures from the screening analysis of free span pipelines to ECA analysis with the use of advanced finite element analysis tools.0 2.0 6.0 1.11 Maximum allowable surface flaw sizes at KP “B” 4. The stress concentration factor SCF = 1.0 Flaw Depth (mm) 10.10 and 3. 11. In order to overcome the limitation of FatFree. Acknowledgement The authors would like to appreciate G. “Environmental Conditions and Environmental Loads” DNV-RP-F105:2006. pipeline profile. The outside surface crack cases are the controlling case compared to the embedded flaw size.7 Analysis User’s Manual API RP 579. The accurate estimation of residual tension. natural frequency and mode shape using the advanced numerical FE tools is important as part of the methodology since FE results are employed directly to the industry standard codes. The maximum allowable flaw sizes for outside surface crack are smaller than those of embedded cracks. The current version of FatFree can only generate S-N fatigue life. in-house MathCad program was developed to generate stress ranges for ECA analysis and concluded that it is a valuable tool as part of the methodology since the program aligns with the industry codes. “Fatigue Design of Offshore Steel Structures” DNV-RP-C205:2007. pipeline bending stiffness. Appropriate mitigation plan is necessary if weld defect size is close to or larger than maximum allowable. The assessment of maximum allowable flaw sizes provides guidelines of AUT acceptance criteria during fabrication and in-service inspection. “Submarine Pipeline Systems” DNV-RP-C203:2005. The results of single span and interacting span cannot be compared directly as shown in Figure 3. seabed profile and gap as shown in the results of Figure 3. bending stiffness and seabed stiffness plays an important role in determination of fatigue life.10 and Figure 3. Weld defect can be repaired immediately or stress ranges and number of cycles can be mitigated via fairing/strake or sand bags/gravel dump to reduce fatigue damage. Duan and SK Park for their assistance. dominant factors of fatigue life for each case are not easy to define unless actual analyses are performed. References ABAQUS Version 6. angle. “DNV FatFree User Manual”.10 and Figure 3.11 because its physical phenomena is complex. 1999 BS 7910:2005. 2003 DNV-OS-F101:2007. Fitness-for-Service. Since the fatigue life is affected by many factors such as contact shoulder length. tension.• • • • • • • • Residual tension of the pipeline. “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” DNV FatFree Software Version 10. The fracture mechanics fatigue life of a component is generally shorter than S-N curve fatigue life. The established acceptance criteria under extreme environmental loading condition are good reference guidelines to check surveyed weld defect sizes for fitness-for-service. “Free Spanning Pipeline” . P. S. G. Jukes.. Wang.0 User Manual” Wang. J... “Crackwise 4. Duan. “Efficient Assessment of Subsea Pipelines and Flowlines for Complex Spans” .TWI.. environments •Operating pressure & temperature Span Screen Analysis using spreadsheet based on DNV-RP-F105 & DNV-OS-F101 •Evaluate max allowable span length •Prioritize the most onerous span (No FEA used.weight •VIV loads (IL & CF) •Direct wave loads (Drag & Inertia) •Static & dynamic ULS check (Both FEA & code are used.) Fatigue Life Only In-House Program •Used all equations in DNV-RP-F105 •Same input as FatFree Stress Ranges ECA Analysis using Crackwise •Geometry/flaw selection •Flaw dimensions •Misalignment •Primary/secondary loading •Material properties •Toughness •Fatigue crack growth constants Fitness-For-Service .) Static & Dynamic ULS Check using spreadsheet based on DNV-OS-F101 •Environmental loads •Combined loads (internal & external overpressure) •Static load . Only code is used for screening.) ABAQUS Results •Mode shape •Natural frequency •Unit stress •Effective axial tension •Bending moment FatFree Software based on DNV-RP-F105 •Environmental information (wave/current) •Mode shape •Natural frequency (Both FEA & code are used.APPENDIX A Figure A1 Schematic Diagrams of Overall Calculation Procedures Span Survey Data •Free span length •Span location •Year to year seabed gap variation •Operational data •Soil data FE Modeling •Contact boundary conditions •Apply different concrete coating. water depth.