Ship Structural FE Analysis

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Ship StructuralFE Analysis
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Global Model

• Full global FE model
• DNV rules for high speed craft requirement for
  Vessels gt 50m in length

Global model
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Design Loads - Multihull

Still Water Longitudinal hogging
moment Longitudinal sagging moment Transverse
split PCM Combined Longitudinal and PCM
Global model
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Design Loads - Monohull

Still Water Longitudinal hogging
moment Longitudinal sagging moment Torsion moment
Transverse racking
Global model
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Design Loads

• Based on Class Rules design values should be
  agreed between designer and class society prior
  to final analysis
• Alternative loads based on hydrodynamic analysis
  motions and loads calculations e.g. DNV program
  SWAN

Global model
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Design Loads

Hydrostatically balance vessel on wave Using
correct weight distribution (inc. dynamic
component) Determine sea forces
Global model
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Design Loads

• Necessary to show
• Required maximum BM
• Max shear at approx ¼ vessel length
• LCG approx in line with LCB
• If constraints used negligible reaction forces

Global model
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Modelling

• Model covers complete ship
• Geometrical hull shape
• Transverse bulkheads
• Decks
• Torsional box structures

Global model
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Modelling - Elements
Size, type and number of elements selected to
ensure effects of bending, shear and torsion of
the hull beam fully accounted for. If 4 noded
elements used typically max 3 elements per
longitudinal frame spacing and 3 per tier. Aspect
ratio of 13 is acceptable.
Global model
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Simplified Modelling

• Simplified modelling is acceptable must be
  clearly identified, e.g.
• curved plate modelled straight
• stiffeners lumped to nearest mesh line
• masses lumped as discrete points
• representation of cut-outs

Global model
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Boundary Conditions
Boundary conditions may reflect symmetry
Attention should be paid to stresses and
deflections resulting from the modelled BCs BCs
checked to ensure that they are in balance
without reaction forces and only rigid body
motions are prevented
Global model
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Boundary Conditions
Inertial relief may be best option for
restraining model. This constraint provides
stability by internally calculating and applying
an acceleration based on system mass to
counteract any unconstrained DOFs as
specified. (remember global accelerations will
need to be applied to the model to simulate
quasi-dynamic situation)
Global model
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Design Criteria

• Allowable global stresses
• Special attention to structural discontinuities
  esp. where coarse element mesh or simplifications
  in modelling
• Combination of global and local stresses
• Buckling capacity of various panels, stiffeners
  and girder systems as per class rules

Global model
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Local Model Analysis
Transverse web frame analysis, for typical frame
in the midship region, is DNV rules for high
speed craft requirement for vessels lt 50m in
length
For larger vessels several sections along length
of vessel should be considered
Local model
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Load Conditions

• Sea pressure, max load on decks (LC1)
• Symmetric bottom slamming (LC2)
• Asymmetric bottom slamming (LC3 LC4)
• Flat cross structure slamming (LC5)- multihull
  only
• Transverse racking (LC6) - monohull only
• Asymmetric deck load (LC7)

Local model
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Modelling

• Analyse structural strength of transverse web
  frame
• Length of one compartment in midship area
• From baseline to upper deck
• Extend from centre of one compartment to centre
  of next compartment

Local model
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Modelling - Elements

• Mesh fineness and element types must be
  sufficient to represent deformation pattern of
  actual structure with respect to
• effective flange (shear lag)
• bending deformation of beam structures
• 3-d response of curved regions

Local model
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Modelling Shear Lag
Compressive strain in upper edge and Tensile
strain in lower edge
Strain in flanges Strain in web at
flange/web join
Result -gt shear loading to edge of each flange
member
Local model
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Modelling - Elements

• Mesh fineness should represent true web frame
  structure
• Model plating, webs and flanges as separate
  elements
• 3 elements per height of web of frame
• Aspect ratio 13 is acceptable
• With curved flanges, element length stiffener
  spacing
• In areas with discontinuities, e.g. ends of
  flanges, brackets, use increased mesh fineness

Local model
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Boundary Conditions

• Symmetry conditions to be applied at each end of
  model
• If only half of breadth then symmetry conditions
  to be applied at centre line
• To obtain a balanced model use may be made of
  spring elements to restrain at boundaries

Local model
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Boundary Conditions
Local model
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Boundary Conditions

• Combine compartment model to beam simulating rest
  of vessel
• Joined using rigid elements
• Use inertial relief solution

Local model
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Design Criteria

• Allowable stresses dynamic loads and static
  loads
• Plate buckling of girder plate flange

Local model
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Additional Models Waterjet Ducts

• Reaction forces from waterjet nozzles transmitted
  into hull structure. Critical for strength and
  fatigue
• Load cases
• - Crash stop
• - max reversing load
• - max steering load
• - unit accelerated as cantilever in pitching

Local model
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ISSC Study FE Analysis of Transverse Frame

• Performed 9 analyses of the same tanker
  transverse frame
• Variety of programs, meshes, boundary condition
  methods etc.
• Deflection of bottom transverse ranged from 5.5
  mm to 44.0 mm
• Axial stress in flange ranged from 180 MPa to 227
  MPa

Local model