Resistance to Accidental Ship Collisions

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Resistance to Accidental Ship Collisions
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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

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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..

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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

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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

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SHIP COLLISION
How much energy has been dissipated? What is the
extent of damage to the platform?
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Grane- impact events to be simulated on Row 2
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Grane - potential impact locations - Row A
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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

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SHIP COLLISIONDesign principles

• Strength design
• Installation resists collision without
  deformation- ship deforms and dissipates major
  part of energy
• Ductility design
• Installation deforms and dissipates major part
  of energy- ship remains virtually undamaged
• Shared energy design
• Both ship and installation deform and contribute
  substantially to energy dissipation

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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.

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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.

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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.

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Stern corner -column collisionDistribution of
energy dissipation- ductile vs. strength design
Weak column left (Alt. 1) Strong column right
(Alt.2)
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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

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External collision mechanics
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External collision mechanics
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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
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SHIP COLLISIONForce-deformation curves for
supply vessel (TNA 202, DnV 1981)
Note Bow impact against large diameter columns
only
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SHIP COLLISIONContact force distribution for
strength design of large diameter columns
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SHIP COLLISIONSupply vessel - stern corner
force/distribution

• Total force

• Local force subset of total force distributed
  over smaller area

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SHIP COLLISIONStrength 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
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Energy dissipation modes in jackets
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Local denting tests with tubes
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Yield line model for local denting
Measured deformation
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Resistance curves for tubes subjected to denting
Approximate expression including effect of axial
force
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Resistance curves for tubes subjected to denting
Include local denting
If collapse load in bending, R0/Rc lt 6 neglect
local denting
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Relative bending moment capacity of tubular beam
with local dent (contribution from flat region
is conservatively neglected)
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SHIP COLLISIONPlastic resistance curve for
bracings collision at midspan
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SHIP COLLISIONElastic-plastic resistance curve
for bracings collision at midspanFactor c
includes the effect of elastic flexibility at
ends
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Strength of connections (NORSOK N-004 A.3.8)
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Strength of adjacent structure
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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

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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)

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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)

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Critical deformation for tensile fracture in
yield hinges
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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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Global integrity during impact