1. Resistance to Accidental Ship Collisions

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

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…..

4. NORSOK STANDARD DESIGN AGAINST ACCIDENTAL LOADS
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

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”

6. Recent trends: Location sometimes close to heavy traffic lanes

7. Present trend for supply vessels: bulbous bows & increased size

8. The outcome of a collision may be this….

9. ..or this….

10. 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

11. SHIP COLLISION Design principles- analysis approach
Strength 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.

12. SHIP COLLISION Design principles - analysis approach
Ductility 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.

13. SHIP COLLISION Design principles - analysis approach
Shared 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.

14. Grane - potential impact locations -

15. Collision Mechanics External collision mechanics
Convenient to separate into
External collision mechanics
Conservation of momentum
Conservation of energy
Kinetic energy to be dissipated as strain energy
Internal collision mechanics
Distribution of strain energy in installation and ship
Damage to installation

16. External collision mechanics

17. External collision mechanics

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

19. SHIP COLLISION - according to NORSOK Force-deformation curves for supply vessel (TNA 202, DnV 1981)
Force – deformation curves from 1981 – derived by simplified methods
Now: NLFEA is available!
Analysis of bulbous bow required
Note: Bow impact against large diameter columns only

20. Supply vessel bow ~ 7500 tons displacement
Dimension: Length:
L.O.A m
Lrule m
Breadth mld 18.80m
Depth mld 7.60m
Draught scantling m

21. Finite element models

22. Material modeling
Bow: Mild steel – nominal fy = 235 MPa, apply fy = 275 MPa
Column: Design strength fy = 420 MPa
Strain hardening included – relatively more for bow

23. Impact location 1 Max strain 12%
Bow is crushed – relatively small deformations in column
Max. column strain – 12% - at bulb location
Strain level close to rupture
Column strain at superstructure location is 7%

24. Force deformation curve for bow
Bulb
Bow superstructure
The crushing force in the bulb is larger than the superstructure for the crushing range analyzed
The crushing force increases steadily for the superstructure
The bulb attains fast a maximum force followed by a slight reduction

25. Pressure-area relation for design
Pressure-area relation analogy with ice design is found from collision analysis
Provide recommendation for design against impact
P=7.06A-0.7
Total collision force distributed over this area
Plots of collision force intensity
Pressure-area relation for design

26. Ship collision with FPSO
Only the side of one tank is modeled
Three scenarios established w.r.t. draughts
Scenario 1
Scenario 2
Scenario 3

27. 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

28. Bow collision with braces Can the brace be designed to crush the bow?
Strong bow- tube and bow deforms
Medium strength bow - tube undamaged

29. Ship collision with oblique brace

30. Ship collision with brace

31. Ship collision with brace Energy dissipation in bow versus brace resistance
Brace must satisfy the following requirement
Joints and adjacent structure must be strong enough to support the reactions from the brace.

32. Energy dissipation modes in jackets
Elastic
Plastic

33. Local denting tests with tubes

34. Yield line model for local denting
Measured deformation

35. Resistance curves for tubes subjected to denting
Approximate expression including effect of axial force

36. Resistance curves for tubes subjected to denting
Include local denting
If collapse load in bending, R0/Rc < 6 neglect local denting

37. Relative bending moment capacity of tubular beam with local dent (contribution from flat region is conservatively neglected)

38. SHIP COLLISION Plastic resistance curve for bracings collision at midspan

39. 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 w
F - R
Rigid-plastic

40. Example: supply vessel impact on brace

41. Example: supply vessel impact on brace
Kinetic energy absorbed by brace prior to rupture: 6 ~ 7 MJ

42. Strength of connections (NORSOK N-004 A.3.8)

43. Strength of adjacent structure

44. Ductility limits Ref: NORSOK A.3.10.1
The 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.
If 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.
If 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

45. Local buckling of tubes undergoing large rotations
D/t -ratio

46. Ductility limits Ref: NORSOK A.3.10.1
To ensure that members with small axial restraint maintain moment capacity during significant plastic rotation it is recommended that cross-sections be proportioned to Class 1 requirements, defined in Eurocode 3 or NS3472.
Initiation of local buckling does, however, not necessarily imply that the capacity with respect to energy dissipation is exhausted, particularly for Class 1 and Class 2 cross-sections. The degradation of the cross-sectional resistance in the post-buckling range may be taken into account provided that such information is available
For members undergoing membrane stretching a lower bound to the post-buckling load-carrying capacity may be obtained by using the load-deformation curve for pure membrane action.

47. Tensile Fracture
Plastic deformation or critical strain at fracture depends upon
material toughness
presence of defects
strain rate
presence of strain concentrations
Critical strain of section with defects
- assessment by fracture mechanics methods.
Plastic straining preferably outside the weld
- overmatching weld material

48. Assumption: Bilinear stress-strain relationship
Stress-strain distribution - bilinear material
e k
Axial variation of maximum strain for a cantilever beam with circular cross-section
Assumption: Bilinear stress-strain relationship

49. Local buckling does not need to be considered if the follwowing conditions is met Assumption: Membrane tension larger than compression in rotation (NORSOK N-004)

50. Critical deformation for local buckling (NORSOK N-004)

51. 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)

52. 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)

53. Critical deformation for tensile fracture in yield hinges

54. Tensile fracture in yield hinges Determination of H

55. Tensile fracture in yield hinges
Proposed values for ecr and H for different steel grades
Steel grade ecr H
S %
S %
S %

56. Tensile fracture in yield hinges comparison with NLFEM