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## PowerPoint Slideshow about 'Resistance to Accidental Ship Collisions' - kerry

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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

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…..

NORSOK STANDARDDESIGN 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

NORSOK STANDARDDESIGN 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”

Principles for ALS structural designillustrated 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

SHIP COLLISIONDesign 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.

SHIP COLLISIONDesign 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.

SHIP COLLISIONDesign 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.

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

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

SHIP COLLISION - according to NORSOKForce-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

Supply vessel bow ~ 7500 tons displacement

Dimension: Length:

L.O.A. 90.70m

Lrule 85.44m

Breadth mld 18.80m

Depth mld 7.60m

Draught scantling 6.20m

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

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%

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

Pressure-area relation for design

- Pressure-area relation analogy with ice design is found from collision analysis
- Provide recommendation for design against impact

Total collision force distributed over this area

Plots of collision force intensity

Pressure-area relation for design

Scenario 3

Scenario 1

Ship collision with FPSO- Only the side of one tank is modeled
- Three scenarios established w.r.t. draughts

distributed over this

area

Area with high force

intensity

Deformed stern corner

SHIP COLLISIONContact force distribution for strength design of large diameter columnsBow collision with bracesCan the brace be designed to crush the bow?

Strong bow- tube and bow deforms

Medium strength bow - tube undamaged

Ship collision with braceEnergy 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.

Yield line model for local denting

Measured deformation

Resistance curves for tubes subjected to denting

Approximate expression including effect of axial force

Resistance curves for tubes subjected to denting

Include local denting

If collapse load in bending, R0/Rc < 6 neglect local denting

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

SHIP COLLISIONPlastic resistance curve for bracings collision at midspan

Bending & membrane

Membrane only

F - R

k

k

w

SHIP COLLISIONElastic-plastic resistance curve for bracings collision at midspanFactor c includes the effect of elastic flexibility at ends

Example: supply vessel impact on brace

Kinetic energy absorbed by brace prior to rupture: 6 ~ 7 MJ

Ductility limitsRef: 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

Ductility limitsRef: 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.

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

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

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

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)

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)

Tensile fracture in yield hinges

- Proposed values for ecr and H for different steel grades

Steel grade ecr H

S 235 20 % 0.0022

S 355 15 % 0.0034

S 460 10 % 0.0034

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