NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 1 Resistance to Accidental Ship Collisions

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 2 Outline  General principles  Impact scenarios  Impact energy distribution  External impact mechanics  Collision forces  Energy dissipation in local denting  Energy dissipation in tubular members  Strength of connections  Global integrity

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 3 DESIGN AGAINST ACCIDENTAL LOADS Verification methods –Simplified (“back of the envelope methods) Elastic-plastic/rigid plastic methods (collision/explosion/dropped objects) Component analysis (Fire) –General calculation/Nonlinear FE methods USFOS, ABAQUS, DYNA3D…..

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 4 General –“The inherent uncertainty of the frequency and magnitude of the accidental loads as well as the approximate nature of the methods for their determination as well as the analysis of accidental load effects shall be recognised. It is therefore essential to apply sound engineering judgement and pragmatic evaluations in the design.” SS NORSOK STANDARD DESIGN AGAINST ACCIDENTAL LOADS

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 5 NORSOK STANDARD DESIGN AGAINST ACCIDENTAL LOADS “If non-linear, dynamic finite element analysis is applied all effects described in the following shall either be implicitly covered by the modelling adopted or subjected to special considerations, whenever relevant”

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 6 How much energy has been dissipated? What is the extent of damage to the platform? SHIP COLLISION

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 7 Grane- impact events to be simulated on Row 2

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 8 Grane - potential impact locations - Row A

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 9 Principles for ALS structural design illustrated for FPSO/ship collision  Strength design - FPSO crushes bow of vessel (ref. ULS design)  Ductility design - Bow of vessel penetrates FPSO side/stern  Shared energy design - Both vessels deform Fairly moderate modification of relative strength may change the design from ductile to strength or vice verse

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 10 SHIP COLLISION Design principles lStrength design Installation resists collision without deformation- ship deforms and dissipates major part of energy lDuctility design Installation deforms and dissipates major part of energy- ship remains virtually undamaged lShared energy design Both ship and installation deform and contribute substantially to energy dissipation

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 11 SHIP COLLISION Design principles- analysis approach lStrength design: The installation shape governs the deformation field of the ship. This deformation field is used to calculate total and local concentrations of contact force due to crushing of ship.The installation is then designed to resist total and local forces. Note analogy with ULS design.

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 12 SHIP COLLISION Design principles - analysis approach lDuctility design: The vessel shape governs the deformation field of the installation. This deformation field is used to calculate force evolution and energy dissipation of the deforming installation. The installation is not designed to resist forces, but is designed to dissipate the required energy without collapse and to comply with residual strength criteria.

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 13 SHIP COLLISION Design principles - analysis approach lShared energy design: – The contact area the contact force are mutually dependent on the deformations of the installation and the ship. –An integrated, incremental approach is required where the the relative strength of ship and installation has to be checked at each step as a basis for determination of incremental deformations. –The analysis is complex compared to strength or ductility design and calls for integrated, nonlinear FE analysis. –Use of contact forces obtained form a strength/ductility design approach may be very erroneous.

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 14 Stern corner -column collision Distribution of energy dissipation- ductile vs. strength design Weak column left (Alt. 1) Strong column right (Alt.2) ColumnStern corner

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 15 Collision Mechanics Convenient to separate into äExternal collision mechanics –Conservation of momentum –Conservation of energy ïKinetic energy to be dissipated as strain energy äInternal collision mechanics –Distribution of strain energy in installation and ship ïDamage to installation

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 16 External collision mechanics

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 17 External collision mechanics

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 18 Ship collision- dissipation of strain energy The strain energy dissipated by the ship and installation equals the total area under the load-deformation curves, under condition of equal load. An iterative procedure is generally required

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 19 SHIP COLLISION Force-deformation curves for supply vessel (TNA 202, DnV 1981) Note: Bow impact against large diameter columns only

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 20 SHIP COLLISION Contact force distribution for strength design of large diameter columns Total collision force distributed over this area Area with high force intensity Deformed stern corner

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 21 SHIP COLLISION Supply vessel - stern corner force/distribution Total force Local force subset of total force distributed over smaller area

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 22 SHIP COLLISION Strength design of large diameter columns- supply vessel stern impact For strength design the column shall resist maximum local concentrations of the collision force imposed by the deforming supply vessel. The forces are assumed uniformly distributed over a rectangular area

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 23 Energy dissipation modes in jackets Elastic Plastic

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 24 Local denting tests with tubes

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 25 Yield line model for local denting Measured deformation

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 26 Resistance curves for tubes subjected to denting Approximate expression including effect of axial force

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 27 Resistance curves for tubes subjected to denting If collapse load in bending, R 0 /R c < 6 neglect local denting Include local denting

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 28 Relative bending moment capacity of tubular beam with local dent (contribution from flat region is conservatively neglected)

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 29 SHIP COLLISION Plastic resistance curve for bracings collision at midspan

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 30 SHIP COLLISION Elastic-plastic resistance curve for bracings collision at midspan Factor c includes the effect of elastic flexibility at ends Bending & membrane Membrane only k k w F - R Rigid-plastic

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 31 Strength of connections (NORSOK N-004 A.3.8)

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 32 Strength of adjacent structure

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 33 Ductility limits Ref: NORSOK A lThe maximum energy that the impacted member can dissipate will – ultimately - be limited by local buckling on the compressive side or fracture on the tensile side of cross-sections undergoing finite rotation. lIf the member is restrained against inward axial displacement, any local buckling must take place before the tensile strain due to membrane elongation overrides the effect of rotation induced compressive strain. lIf local buckling does not take place, fracture is assumed to occur when the tensile strain due to the combined effect of rotation and membrane elongation exceeds a critical value

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 34 Tensile Fracture The degree of plastic deformation at fracture exhibits a significant scatter and depend upon the following factors:  material toughness  presence of defects  strain rate  presence of strain concentrations Welds normally contain defects. The design should hence ensure that plastic straining takes place outside welds (overmatching weld material)

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 35 Tensile Fracture The critical strain in parent material depends upon:  stress gradients  dimensions of the cross section  presence of strain concentrations  material yield to tensile strength ratio  material ductility Critical strain (NLFEM or plastic analysis)

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 36 Critical deformation for tensile fracture in yield hinges

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 37 Tensile fracture in yield hinges Proposed values for e cr and H for different steel grades Steel grade  cr H S % S % S %

NUS July 12-14, 2005 Analysis and Design for Robustness of Offshore Structures NUS – Keppel Short Course 38 Global integrity during impact