Slamming Response of Large High Speed Catamarans

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Slamming Response of Large High Speed Catamarans Powered By Docstoc
					Ship Structural
 FE Analysis
       Global Model
      • Full global FE model
      • DNV rules for high speed craft requirement for
      Vessel’s > 50m in length

Global model
Design Loads - Multihull

                 Still Water
                 Longitudinal hogging moment
                 Longitudinal sagging moment
                 Transverse split
                 Combined Longitudinal and PCM

                                        Global model
       Design Loads - Monohull

                       Still Water
                       Longitudinal hogging moment
                       Longitudinal sagging moment
                       Torsion moment

                                     Transverse racking
Global model
       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
 Design Loads

Hydrostatically balance vessel on wave
Using correct weight distribution (inc. dynamic
Determine sea forces
                                                  Global model
       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
      Model covers complete ship
      • Geometrical hull shape
      • Transverse bulkheads
      • Decks
      • Torsional box structures

Global model
       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 1:3 is acceptable.

Global model
       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
       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
       Boundary Conditions
      Inertial relief may be best option for restraining
      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

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

Local model
        Load Conditions
      1. Sea pressure, max load on decks (LC1)
      2. Symmetric bottom slamming (LC2)
      3. Asymmetric bottom slamming (LC3 & LC4)
      4. Flat cross structure slamming (LC5)- multihull only
      5. Transverse racking (LC6) - monohull only
      6. Asymmetric deck load (LC7)

Local model
       Analyse structural strength of transverse web
      •       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
          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
          Modelling – Shear Lag
        Compressive strain in upper edge and Tensile
          strain in lower edge

Strain in flanges = Strain
in web at flange/web

Result -> shear loading
    to edge of each
    flange member

Local model
          Modelling - Elements
       Mesh fineness should represent true web frame
      •    Model plating, webs and flanges as separate
      •    3 elements per height of web of frame
      •    Aspect ratio 1:3 is acceptable
      •    With curved flanges, element length = stiffener
      •    In areas with discontinuities, e.g. ends of flanges,
           brackets, use increased mesh fineness

Local model
        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

Local model
        Boundary Conditions
      2.6(n  1)
         8 As
   l  dist between tr bulkheads
   n  number of loads
   A s  actual shear area

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

Local model
        Design Criteria
      • Allowable stresses – dynamic loads and
        static loads
      • Plate buckling of girder plate flange

Local model
        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
        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

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