Onbottom Stability of JacketsONBOTTOM STABILITY OF JACKETS 5/24/2014 1 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets OUTLINE Onbottom Stability Piling Sequence Mudmat Concepts Stability Requirements Design 5/24/2014 2 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets ONBOTTOM STABILITY What is Onbottom Stability ? When the jacket is floated and upended from horizontal floating position, it shall stand vertically on the seabed. The stability of the same shall be maintained until its is fixed on to the seabed by piles. This temporary phase is called “Unpiled Stability” or “Onbottom Stability”. The jacket with pile segment and hammer should be able to stand without, sliding, settling and overturning due to external forces. 5/24/2014 3 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets PILING SEQUENCE Planning a piling sequence can reduce the offshore construction and installation time to considerable extent. Since the piles or pile segment and hammer is temporarily supported on to the jacket, the weight of the same needs to be taken in to consideration during the onbottom stability. Further, during this period, external environmental forces from wave, current and wind also needs to be considered. 5/24/2014 4 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 one would consider the piling sequence 2 (refer to table) 5/24/2014 5 Dr. S. A2. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Lift hammer place on top of pile at corner A1 and drive to target penetration.Onbottom Stability of Jackets Preferred Piling Sequence A1 For example. extent. 4A 3A Place pile at corner A2 and release the crane hook. 3A 2B Lift hammer place on top of pile at corner B2 and drive to target penetration Place pile at corner B1 and release the crane hook. 2A 2A Place pile at corner B2 and release the crane hook. the handling time approximately 3 to 6 hours. following sequence can be adopted. B1 and B2) needs to installed on to a jacket. 1A 1B Lift hammer place on top of pile at corner A1 and drive to target penetration Place pile at corner B2 and release the crane hook. multiple rigging and derigging activities. This is due to manual handling of rigging for the pile and hammer. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Preferred Piling Sequence ID Piling sequence 1 Piling sequence 2 ID 1A Place pile at corner A1 and release the crane hook. Lift hammer place on top of pile at corner A2 and drive to target penetration 3B 4B Lift hammer place on top of pile at corner B1 and drive to target penetration Lift hammer place on top of pile at corner B1 and drive to target penetration 4B 5/24/2014 6 Dr. To avoid. B1 A2 B2 Each time the crane lifts the pile including rigging and de-rigging. S. Place pile at corner A2 and release the crane hook. 1B 3B Lift hammer place on top of pile at corner A2 and drive to target penetration Lift hammer place on top of pile at corner B2 and drive to target penetration 2B 4A Place pile at corner B1 and release the crane hook. if four corner piles (A1. Place pile at corner A1 and release the crane hook. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets 5/24/2014 8 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 4A .Onbottom Stability of Jackets PILING SEQUENCE 1A 5/24/2014 2A 7 3A Dr. S. S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets 5/24/2014 10 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S.Onbottom Stability of Jackets 5/24/2014 9 Dr. Onbottom Stability of Jackets LOAD COMBINATIONS FOR BEARING CHECK For bearing pressure check and mudmat design maximum possible gravity loads shall be considered. Case Set 900 Minimum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into A1. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Case Set 700 Minimum Jacket weight Installation Wave and current (8 directions) Case Set 800 Minimum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg. S. S. The dead loads of the jacket shall be considered without any contingency. Case Set 100 Maximum Jacket weight Installation Wave and current (8 directions) Case Set 200 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg Case Set 300 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg Main pile section P1 inserted into A1 Leg Case Set 400 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg Main pile section P1 inserted into A1 Leg Main pile section P1 inserted into B1 Leg 5/24/2014 11 Case Set 500 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg Main pile section P1 inserted into A1 Leg Main pile section P1 inserted into B1 Leg Main pile section P1 inserted into A2 Leg Case Set 600 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 Leg Main pile section P1 inserted into A1 Leg Main pile section P1 inserted into B1 Leg Main pile section P1 inserted into A2 Leg Pile section P2 stabbed and welded on B2 leg Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets LOAD COMBINATIONS FOR STABILITY CHECK For sliding and overturning stability checks minimum gravity loads applicable shall be considered. The dead loads of the jacket shall be considered including contingency applied at that stage. B1 and A2 Leg Pile section P1+P2 stabbed and welded on B2 leg 5/24/2014 12 Dr. 5Be from the bottom of the Mudmat Be – Effective Mudmat width Le – Effective Mudmat length D – Depth of Embedment of the Mudmat below seabed Undrained shear strength at depth 0. However. 5/24/2014 13 Dr.5Be below the Mudmat bottom shall be evaluated using the linear interpolation of the shear strength of layers. Usually made of steel plate and reinforced by steel beams. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Mudmat Mudmats are temporary floor support for the jacket immediately after the jacket has been upended from floating horizontal position prior to supported by piles. Need to designed with adequate surface area and sufficient strength strength to avoid excessive settlement of the jacket. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S.2 1 0.Onbottom Stability of Jackets Ultimate Bearing Capacity D B qu 5Su 1 0. alternate materials like Timber and FRP has been used to reduce weight and cost 5/24/2014 14 Dr.2 e Be Le Where Su – Undrained shear strength at 0. The design requirement for Cathodic Protection will also be reduced 5/24/2014 15 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . They will reduce the weight considerably. S.Onbottom Stability of Jackets Advantages of FRP and Timber Mudmat FRP and Timber mudmats are used when lift weight is a concern. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Large Timber Mudmat 5/24/2014 16 Dr. Onbottom Stability of Jackets FRP Mudmat 5/24/2014 17 Dr. S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets MUDMAT CONCEPTS 5/24/2014 18 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Triangular Mudmat 5/24/2014 20 Dr.Onbottom Stability of Jackets Jacket with Rectangular Mudmat 5/24/2014 19 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Circular Mudmat 5/24/2014 22 Dr.Onbottom Stability of Jackets Rectangular Mudmat 5/24/2014 21 Dr. S.Onbottom Stability of Jackets Triangular Mudmat 5/24/2014 23 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Mudmat Panels Mudmat panels can be any one of the following. Flate Plate (Steel) Corrugated Plate (Steel) Timber Plank Profiled Panel (FRP) These panels will be appropriately supported by steel structural members attached to the jacket structure 5/24/2014 24 Dr. S. S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Timber Plank 5/24/2014 26 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 .Onbottom Stability of Jackets Flat Steel plate 5/24/2014 25 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets FRP PANEL 5/24/2014 28 Dr. S. S.Onbottom Stability of Jackets Corrugated Steel plate 5/24/2014 27 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . it shall satisfy following requirements Stability against bearing Stability against sliding Stability against overturning Structural members shall have adequate strength 5/24/2014 29 Dr. S.Onbottom Stability of Jackets Design Requirements When the jacket is resting on seabed. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Design Loads Dead loads Bouyancy Loads Wave and Current Loads Wind Loads Loads from Pile stabbing sequence 5/24/2014 30 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Stability Against Bearing As explained earlier. This has two parts. it shall satisfy following requirements (API RP 2A) Stability against bearing Stability against sliding Stability against overturning Sometimes it is also called “Unpiled Stability” since this is prior to the piling of the jacket after which the jacket is firmly fixed to the seabed by piles 5/24/2014 31 Dr.Onbottom Stability of Jackets Design Requirements When the jacket is resting on seabed. Geotechnical Requirement Structural Requirement 5/24/2014 32 Dr. stability against bearing is to have adequate bearing area to avoid excessive settlement of jacket / failure of mudmat. S. S. 0 for loads arising from dead weight of the jacket only and 1. Q F . then the effect shall be included as moment component. buoyancy tanks and AM is the total mudmat area If the Jacket is not symmetrical and has self weight acting at an eccentricity of ex. 5/24/2014 34 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Applied Mudmat Pressure (Dead Load) The applied mudmat pressure can be calculated for dead loads alone very easily. and not at the geometric centre of mudmat.S P u a Where Qu is the ultimate bearing capacity of soil and Pa is the applied pressure The minimum Factor of Safety shall be 2. 5/24/2014 33 Dr. W eW H P A I 2 S x S a M yy Where WS is the total submerged weight of the jacket including ballast water on any compartments of legs.Onbottom Stability of Jackets Factor of Safety against Bearing The Factor of Safety against bearing shall be calculated as below.O.5 for dead weight + environmental loads. S. S. 5/24/2014 36 Dr. F . S. The minimum FOS of 1.Onbottom Stability of Jackets Applied Mudmat Pressure (Dead Load + Environment Load) The applied mudmat pressure can be calculated for dead loads alone very easily. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . current and wind and h is the height from seabed at which the environmental loads are applied and Iyy is the moment of inertia of the mudmat system about YY axis.S W s F e Where Fe is the total environmental loads applied and μ is the friction coefficient between the soil and mudmat system.5 shall be required. 5/24/2014 35 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Factor of Safety against Sliding The Factor of Safety against sliding shall be calculated as below. S.O. P a W eW H Fh H A I 2 I 2 S x M S e yy yy Where Fe is the total environmental loads from wave. Wx F .O. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets SLIDING STABILITY IN UNDRAINED CONDITION API RP 2A requires the mudmat sliding stability to be considered using undrained methodology. CA F . The minimum FOS of 1.O.5 shall be required. S.S F u m e Where Fe is the total environmental loads applied and Cu is the undrained shear strength and Am is the total area of the mudmat.5 shall be required. 5/24/2014 37 Dr. 5/24/2014 38 Dr.Onbottom Stability of Jackets Factor of Safety against Overturning The Factor of Safety against Overturning shall be calculated as below (for each edge). Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 .S Fh s e Where x is the distance between the vertical load (jacket submerged weight) and the geometric centre of mudmat system at mudline. S. The minimum FOS of 1. S. 5/24/2014 39 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . The FOS against this combined loading for drained and undrained conditions are shown in figure below. S.Onbottom Stability of Jackets COMBINED HORIZONTAL AND VERTICAL STABILITY The revised APRI RP 2GEO requires consideration of combined effect of horizontal and vertical loading on the stability of jackets. Hence the only immediate settlement using elastic theory will suffice. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Jacket Settlement Most of Settlement will take place immediately after the jacket has been placed on seabed. 5/24/2014 40 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 .Onbottom Stability of Jackets W 5/24/2014 41 Fe Dr. The mudline framing will be subjected to constant upward force on the members The conductor guide if any will be submerged in to mud thus driving conductors will become difficult Boulder if present at shallow depth may damage structural braces The jacket cut-off level will get affected 5/24/2014 42 Dr. This will lead following issues. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Jacket Settlement Settlement of jacket is an important criteria in designing the mudmat system as excessive settlement woill lead submergence of bottom framing in to the soil. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S. qB (1 ) I E 2 s Where q is the uniform applied pressure. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Settlement of Circular Footing Vertical settlement of circular footing is given by 1 Q 4GR Where = Vertical displacement Q = Vertical load G = elastic shear modulus of the soil υ = poisson’s ratio of the soil R = radius of the base 5/24/2014 44 Dr. ν is the poissons ratio and Is is the influence coefficient and shall be calculated depending on the shape of the mudmat. B is the width of the mudmat.Onbottom Stability of Jackets Jacket Settlement Elastic settlement of jacket on to the seabed can be calculated as below. E is the Modulus of the soil. 5/24/2014 43 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . S. S.Onbottom Stability of Jackets Rectangular Mudmat system Am 4bh Pa Ws M ( y ) M ( x) Am I xx I yy 4bh 3 I xx 4bh( H / 2 h / 2) 2 12 4b 3h I yy 4bh( B / 2 b / 2) 2 12 Where x and y are co-ordinates of points at which the mudmat pressure is required 5/24/2014 45 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Circular Mudmat system Am 4 Pa 4 D2 Ws M ( y ) M ( x) Am I xx I yy 4 4 4D 2 H 2 I xx D 2 64 4 4 4 4 D 2 B I yy D 2 64 4 5/24/2014 46 2 Dr. Onbottom Stability of Jackets Triangular Mudmat system bh Am 4 2 Pa Ws M ( y ) M ( x) Am I xx I yy 4bh3 I xx 2bh H 1 b 2 3 36 4bh3 I yy 2bh B 1 b 2 3 36 5/24/2014 2 2 47 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets Triangular Mudmat system bh Am 4 2 Pa Ws M ( y ) M ( x) Am I xx I yy 3bh3 I yy 2bh B b 2 2 48 2 2 2 bh 2 H 3bh 3 I xx bh H 2 h 1 h 3 3 2 3 36 2 5/24/2014 48 Dr. S. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 . Onbottom Stability of Jackets Determine the factor of safety against bearing. S. Estimate the immediate settlement of jacket. W X COG Y COG = 12000 kN = -6 m = 1. S. sliding and over turning for the jacket shown in the sketch subjected to environmental forces as shown in the table.0 m Direction Force Centre of force F1 2000 kN 52 m F2 2800 kN 45 m F3 2500 kN 48 m 5/24/2014 49 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 Onbottom Stability of Jackets 5/24/2014 50 Dr. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36 .