fosls anatomy

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Posted Chapters of Bjørn Haugens 1994 Thesis
Title Buckling and Stability Problems for Thin
Shell Structures Using High Performance Finite
Elements
AFEM Ch 31 - Thesis Ch 4 Triangular ANDES Shell
Element AFEM Ch 32 - Thesis Ch 5 Quad ANDES
Shell Element AFEM Ch 33 - Thesis Ch 6-8
Numerical Examples and
References
Complete Thesis (in PDF) available on request
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A New Sandwich Design Concept for Ships
ADMOS 2003, Göteborg, Sweden

• Pål G. Bergan
• Det Norske Veritas, Høvik, Norway
• and
• NTNU, Trondheim, Norway

26
Topics of the Lecture

• Some examples of challenges in ship modelling and
  simulation
• Some general problems
• Container ship
• Liquid natural gas ship
• A new concept for building ships using steel and
  light-weight concrete design
• Some conclusions

27
Characteristics of Ship Structures

• Many pieces of steel welded together, e.g. more
  than 100 000 in a large ship
• Many types of structural elements
• Outer skins, internal skins
• Bulkheads
• Integrated ballast tanks
• Girders, frames, stringers
• Stiffeners, brackets, lug-plates, cut-outs
• Cutouts, surface grinding and polishing
• Numerous stress concentrations
• Corrosion serious problem

28
Particular Considerations for Modeling and
Analysis

• Enormous scale effects from overall ship beam
  (e.g. more than 400 meters long) to stress
  concentrations around weld or crack
• Good modeling of ship beam requires inclusion of
  a significant number of secondary and tertiary
  structural elements
• Fatigue and fracture analysis requires and
  detailed and accurate analysis of stress
  concentrations and cracks
• Dynamic response analysis integrated with
  hydrodynamic simulation
• Ultimate strength analysis by way of buckling
  and/or nonlinear simulation

29
Typical Analysis Steps for Ship Analysis
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Container ship
Global structural model 3-and 4-node
elements Containers with low E-modulus Modelled
in PATRAN/NASTRAN, transferred to SESAM
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Hydrodynamic Model
32
Hydrodynamic Load Analysis
Dynamic pressures for head sea and max hogging
condition
33
Ultimate load state (ULS) checks
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Hot spot stress analysis at hatch corner
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Liquid Natural Gas (LNG) Ship
Global finite element model
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Stepwise Construction of Global Model
37
Wave Motion and Pressures
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Hot Spots
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Structural Problems Bulk Carrier
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Steel Light-weight Concrete Sandwich
From complex steel structure to clean sandwich
structure

The main idea is to replace stiffened steel
panels by steel-concrete sandwich elements for
main load carrying structural components
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Cellular Sandwich
The light-weight concrete is filled into the
space between the surface steel sheets to
completely occupy the internal space and bond to
the steel along all sides

• The steel sheets provide the major part of the
  structural strength
• The concrete provides some strength and stiffness
  in compression, but not in tension (conservative
  assumption)
• The concrete provides a stiff spacing between the
  surface sheets and supports against surface skin
  buckling
• The need for secondary stiffeners is eliminated
• The concrete has sufficient strength to transfer
  relevant transverse shear forces in plates
• The number of details prone to coating failure
  with subsequent corrosion and fatigue is greatly
  reduced
• A concrete with a density below approximately 900
  kg/m3 is preferred to keep down the total weight

Steel plate
Light weight aggregate concrete
Steel plate
Thin walled steel spar box
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Using Experience from Other Applications

• Steel-concrete sandwich elements have been used
  successfully for bridge structures, which are
  also exposed to large dynamic loads and demanding
  environmental conditions
• Composite sandwich structural elements are used
  in air plane wing structures, wind turbine wings,
  trains, naval ships, and other severely loaded
  structures as a particularly efficient design
  solution
• Shipbuilding should learn from successful
  experiences in other industries

43
Panmax Bulk Carrier
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Some Characteristics of the Concept

• Longitudinal girder stiffened double bottom
  structure
• Solid sandwich structure in deck
• Continuous hatch coaming beam structure
• Partly hollow sandwich elements in ship sides,
  transverse bulkheads, and double bottom
• Traditional fore and aft ship design in the
  present study
• Ballast water carried primarily in cargo holds
• HT 36 steel throughout cargo area
• Minimum steel skin plate thickness 10 millimetre
• Concrete properties (example)
• density 900 kg/m3
• compressive cube strength 14 MPa
• tensile splitting strength 2.5 MPa
• failure strain in compression 2-2.5
  similar to yield strain for steel
• E-modulus 6000 MPa
• More than 50 of concrete strength achieved
  after a few days

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Cross-section of ship beam

• Global and local load cases from DNV Steel Ship
  Rules
• Initial scantlings selected
• Linear FEM analysis to determine sectional forces
  with stiffness contribution of concrete in
  both compression and tension
• Scantling optimisation of sections assuming no
  tensile concrete strength safety factor 1.4 for
  concrete compressive strength
• DNV Steel Ship Rule longitudinal strength
  requirements satisfied without including
  contribution from concrete
• Confirmation that all local buckling modes are
  eliminated
• Depth of sandwich minimum 70 millimetre to avoid
  global buckling of deck slab outside the hatch
  coaming

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LNG carrier
Primary barrier 9 Ni Steel or Invar steel
Insulation layer e.g. geomaterial
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LNG carrier
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Tanker for oil or chemicals
Sandwich deck
Easy to clean ballast cells
Ice strengthened side structure
Stainless steel primary barrier
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Safety and Structural Attributes

• Reduced number of fatigue and corrosion prone
  details
• Buckling failure modes virtually eliminated
• Increased hull torsion stiffness
• Increased energy absorption in case of collision
  or grounding
• Increased strength to withstand explosions and
  accidental loads
• Increased stiffness of aft ship to avoid
  vibrations and propeller shaft bearing damages

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Safety and Operational Attributes

• Increased resistance against damage from cargo
  handling equipment
• Better damping of dynamic stresses and response
  from hydrodynamic loads
• Enhanced damping of noise and vibrations from
  machinery and propulsion system
• Simplified hull structure maintenance
• Significantly reduced coating area
• Increased service life
• Highly fire resistant and insulating hull

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Sandwich Application Potential

• Sandwich design can be adapted to many different
  ship types
• Sandwich design can be introduced for parts of a
  ship
• The sandwich concept can be used for
  reinforcement of existing ships
• The sandwich concept can be used for repair and
  strengthening of degradation and damage

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Initial Cost and Life Cycle Cost

• Building
• Price competitive design where 40 of the steel
  weight is exchanged with cheaper concrete
  material
• Much fewer fabrication details and less welding
• Potential for automization and modular
  construction
• Significantly reduced coating area and cost
• Operation
• Hull maintenance cost expected to be reduced
• Other operational advantages because of layout?
• Scrap value uncertain

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Conclusions

• There are still major challenges in practical
  modeling and simulation of ship structures
• The complexity and mere size of these structures
  offer particular difficulties
• Practical analyses require coupling of several
  analysis tools
• A new idea for building ships using a steel
  -concrete sandwich concept has been presented
• This concept seems to offer a wide range of
  advantages, but further development of the
  technology is required

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Combining High Performance Thin Shell and Surface
Crack Finite Elements For Simulation of Combined
Failure Modes

• Bjørn Skallerud
• Kjell Holthe
• Bjørn Haugen
• The Norwegian Universitey of Science
  Technology
• Dept. of Structural Engineering, Trondheim,
  Norway
• FEDEM Technology, USA

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Application free spanning oil/gas pipelines
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Two Bending-Induced Pipeline Failure Modes
Mode 1 Ovalization plastic buckling on the
compressive side
Mode 2 Wall crack on the tensile side
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Solid FE Modeling of Pipe Wall
Advantages accurate, no additional
modeling needed. Disadvantages time consuming
as regards preprocessing and simulation
3D Solid Model (ANSYS)
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Thin Shell Model of Pipe Wall
Bjørn Haugens corotational quad thin shell
element used (preferred to triangle since mesh
generation is easy for a pipe - all elements are
rectangles) Plastic buckling failure mode
small-strain elastoplasticity (stress resultant
or thickness-integrated) Tensile cracking
fracture mechanics by link elements
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Design Rules are Very Conservative for Tension
Solution use two-parameter fracture mechanics
(constraint correction) and direct numerical
simulation
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Formulation works well for large disp/rot, e.g.
inelastic collapse of pinched cylinder
From Haugens thesis, note that triangles are
used here
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A comment on elastic-plastic analysis, stress
resultants versus integration through thickness

• Run Ninc up to max load, elastic analysis
  CPUelastRun Ninc up to max load, elasti-plastic
  analysis CPUelast-plastgt CPUplast
  CPUelast-plast - CPUelast

Number of integration points over thickness 1
2 3 5 7 10
12
1.0 1.23 1.40 1.55 1.75 2.25
2.22 1.0 1.28 1.38 1.49 1.62 1.85
2.00 1.0 1.02 1.13 1.29 1.33
1.51 1.75 1.0 1.22 1.31 1.52 1.80
2.03 2.21
Plate bending Scordelis-Lo Plate
buckling(Q) Plate buckling (R)
1.0 1.19 1.30 1.46 1.63 1.90
2.05
Plasticity model Integration over thickness
(using 5 integr. points) approximately 50 more
time consuming than Stress resultant plasticity
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Fracture By Line Spring Finite Element
Reduces 3D crack problem to 2D, has a sound
fracture mechanics basis from slip line analysis
of the crack ligament
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Line spring relationships
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Line spring fe discretization, 8 DOF, elongation
and rotation (opening of the crack)
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Summary of Formulation

• Quadrilateral ANDES FE, co-rotated kinematics,
  consistent tangent
• Stress resultants, linear hardening for the shell
  element, consistent tangent
• Rect line spring FE, co-rotated kinematics, power
  law hardening, alternative stress updates tried
  (expl, impl euler), yield surface with corners,
  calculates fracture mechanics quantities such as
  J-integral, CTOD, T-stress(constraint)
• Increm-iterative solution of global eqs using
  Newton-Raphson and a simplified arc-length method

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Some Test Cases
ANSYS 3D bricks
Corotational quad shell link elements
CPU for 3D, half of full model 60000 sec CPU
for shell/link full model 100 sec
67
Visualisation of J-integral in Crack
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CTOD versus Strain
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Load-Displacement Response in Bending, D/t80
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Failure Modes Plastic Buckling vs Fracture
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J-Integral versus Load
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Conclusions

• A very feasible tool for assessment of critical
  compressive strains and fracture mechanics
  quantities (by means of two-parameter fract mech)
• Mesh generation requires only 6 input parameters
  (providing automatic meshing of shell and crack)
• Needs special treatment for short cracks (a/t lt
  0.15, which is the most interesting sizes for
  practical applications and assessments)
• Further work nonlinear hardening for the shell
  material, ductile tearing of the crack (both a
  semi-elliptical crack growing through thickness,
  and further along the circumference as a through
  crack)