dnv integrated hydrodynamic and structural analysis webinar presentation_tcm4-601490
TRANSCRIPT
DNV GL © 2014 SAFER, SMARTER, GREENERDNV GL © 2014
29 April 2014Torgeir Vada
SOFTWARE
Integrated hydrodynamic and structural analysis
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DNV GL Tech Talk webinar
DNV GL © 2014
About the presenter
Name: Torgeir Vada
Position: Product Manager for floating structures
Background:
– PhD in Applied mathematics/Hydrodynamics from University of Oslo, 1985
– Worked in DNV since 1985, with Sesam since 1997
– Worked as developer and in various line management roles
– Member of technology leadership committee for hydrodynamics in DNV GL
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Agenda
Introduction to the tool used in the case study: Sesam for Floaters
Internal dynamics
o Quasi-static approach
o Full handling of internal fluid dynamics
Case study: Analysis of an FPSO
o Loads around the waterline
o Checking load transfer quality
o Submodelling and fatigue
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FE analysis
4. Global stress and deflection & fatigue screening
Sesam – a fully integrated analysis system
1. Stability and wave load analysis
Wavescatter diagram
2. Pressure loads and accelerations
Loa
d tr
ansf
er
3. Structural model loads
(internal + external pressure)
Local FE analysis
5. Local stress and deflection & fatigue
DNV GL © 2014
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The Sesam floating structure package
Linear structural analysis of unlimited size
Hydrostatic analysis including stability code checking
Hydrodynamic analysis
Buckling code check of plates and beams
Global to sub-model boundary conditions
Fatigue analysis of plates and beams
Coupled analysis, mooring and riser design
Marine operations
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The Sesam floating structure package – main tools
Sesam GeniE for modelling and structural analysis
Sesam HydroD for hydrostatics and hydrodynamics
Sesam Manager for managing the analysis workflow
Sesam DeepC for umbilicals, mooring and riser analysis
Sesam Marine for marine operations
Sesam CAESES for parametric modelling and optimization
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What can you do with Sesam HydroD?
Model environment and prepare input data for hydrostatic and hydrodynamic analysis
Perform hydrostatics and stability computations (including free surface)
Calculate still water shear and bending moment distribution
Perform hydrodynamic computations on fixed and floating rigid bodies, with and without forward speed
Calculate wave load statistics and determine design loads
Transfer hydrostatic and hydrodynamic loads to structural analysisHydroD D1.3-04 Date: 31 May 2005 15:01:34
0 50 100 150
-2-1
01
23
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GZ-Curve
Heel Angle [deg]
GZ
[m]
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Why Sesam HydroD?
Advanced modeling features– Anchor and TLP elements simulation
– Multi-body analysis – hydrodynamic, stiffness and damping coupling are included
Second order motions and forces– Mean drift force
– Quadratic transfer function (QTFs) for motions and forces
Non-linear time domain analysis– Hydrostatic and Froude-Krylov pressures to
instantaneous free surface
– Exact handling of gravity and inertia according to vessel motions
– Morison drag force considered in time domain
– 5th order Stokes wave for shallow water
– Quadratic damping coefficients
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Quasi-static method
The internal fluid is regarded as rigid body, no internal waves or relative motion wrt. hull structure
Internal free surface is accounted for with additional restoring matrix.
Tank fluid Mass added to the total hull mass, to be balanced with buoyancy force.
Filling fraction is defined in pre-processing.
Reference points for each tanks shall be pre-calculated.
– “Acceleration point” (CoG of the tank fluid)
– “Zero level point” (geometry center of the internal free surface, or roof center for a full tank)
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Full dynamic method, introduction
The internal radiation is solved for each tank.
_ _
_ _ _
The acceleration point is not needed anymore for calculating local pressure.
More accurate.
Sloshing mode to be captured (Linear).
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Only known as a global load
Computed from a distributed load
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Comparison with Molin’s experiment
Two rectangular tanks next to each other with the same geometry.
The fluid level are set as 19cm for both tank in case1
The fluid level are set as 19cm for one tank and 39cm for the other in case2
Roll motion to be investigated.
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Experiment layout
Panel model in HydroD(filling height 19cm & 39cm)
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Comparison with Molin’s experiment, continue
The 1st peak corresponds to the eigen period of the hull in water
The 2nd & 3rd peaks relate to the sloshing modes of the tanks
Smaller filling fraction, smaller sloshing frequency
Sloshing modes captured very well. Linear effects only.
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Case 1 19cm in both tanks Case 2 19cm & 39cm
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LNG carrier study
Dynamic pressure in compartments
Compartments included in Panel model to calculate internal hydrodynamics
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LNG carrier study
Dynamic pressure in compartments
25 compartments with 4 for liquid cargo tanks
balancingAutomatic balancing
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LNG carrier study
Dynamic pressure in compartments
– Surge, heave and pitch not so affected
– Sway, yaw and roll affected both for full and half tank
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FPSO ULS and FLS analysis modelled in Sesam Manager
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Wave load computation
+ Structural analysis
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The load transfer workflow
This is the core workflow in both ULS and FLS analysis
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Compute hydrodynamic loads
Transfer loads to FEM model
Load transfer verification
Structural analysis
HydroD
Sestra
Cutres (global model only)
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Loads transferred from HydroD to Sestra
Hydrodynamic pressure on the outer hull
Hydrodynamic pressure from internal fluid
Inertia and gravity loads
Line loads on beams (Morison’s equation)
Nodal loads
– Anchor and TLP elements
– Pressure area elements => axial loads on beams
Ma = F => sum of all transferred loads should (ideally) be zero
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The FPSO used in this study
Length 165.7 mBeam 43 mFull load condition: All cargo compartments full
All ballast compartments empty
Mass 111,180 tonneCOG (78.6m, 0, 12.35m)Radii of gyration (19.6m, 95m, 95m)Draft 15.5 m
Half load condition: All compartments half filled
Mass 77,047 tonneCOG (78.6m, 0, 7.5m)Radii of gyration (19.7m, 95m, 95m)Draft 10.8 m
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FEM model
Compartment model
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Half filled compartments – rigid body motions
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Blue: Dynamic Red: Quasi-static
Surge Sway Heave
RollPitch Yaw
Wave heading: 135°
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Full compartments – rigid body motions
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Blue: Dynamic Red: Quasi-static
Surge Sway Heave
Roll Pitch Yaw
Wave heading: 135°
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Half-filled compartments – pressure distribution on foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• Significantly lower pressures in dynamic solution
• Zero pressure at waterline in dynamic solution
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Full compartments – pressure distribution on foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• Up to 10% lower pressures in dynamic solution
• In general quite similar
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Half-filled – MVonMises stress distribution on foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• 20% lower maximum stresses in dynamic solver
• Lower stress level in most of the bulkhead in dynamic solver
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Half-filled – stresses on all bulkheads
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Wave period = 10s
Wave heading: 135°
• 30% lower maximum stresses in dynamic solver
• Lower stress level in most of the bulkheads in dynamic solver
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Full load – MVonMises stress distribution on foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• 10% lower maximum stresses in dynamic solver
• Difference within a few per cent on most of the bulkhead
• Much smaller difference on stresses than on pressures
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Full load – stresses on all bulkheads
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Wave period = 10s
Wave heading: 135°
• In general very small differences
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Pressure scaling for waterline elements
Retain correct pressure, but get incorrect force
– Constant pressure centroidOr Scale pressure at waterline elements to get correct force
– Area adjusted
padjust = A1/A2 x poriginal
– where A1 is the wet area of the element and A2 is the total area of the element
– This is applied whether or not the centroid is below the free surface
A2A1
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Pressure reduction zone
Modify pressure in the area +/- A around the free surface level to account for the presence of water above the mean waterline and absence of water below. Default: A=0.0 (i.e. no change)
DNV class note 30.7:
AzAzr wl
p 2
Reduction factor when |z-zwl| < A
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User defined pressure reduction region
User defined wall-sided part
Apply a user defined pressure reduction region on a selected part of the vessel
– The method is only recommended on the part of the vessel which is wall-sided and should thus be controlled by the user
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Reaction forces – global load balance
These are quite small in all four analyses indicating good global balance in the load transfer
– Example below is for half filled condition with dynamic solver
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Half-filled – sectional load comparison – load distribution consistency – internal dynamics
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Fx Fy Fz
Mx My Mz
Blue: HydroD - load integration Red: Cutres – stress integration
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Recommendations for load transfer to FEM model
Avoid loads with unknown distribution
– E.g. additional damping or restoring matrices
Use FEM model as mass model for the hydrodynamic analysis
– To obtain identical mass matrices
Avoid putting the fixed nodes close to “interesting” parts of the structure
– There will always be some imbalance which will create artifical reaction forces at these nodes
Convergence of local loads may require a finer mesh than convergence of global responses
– Use fine mesh in areas with large curvature
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Sub-modelling procedure
Do first the global analysis in Sesam Sestra
Then create the sub-model in e.g. Sesam GeniE
– With prescribed displacement boundary conditions where geometry is cut
Submod:
– Reads the sub-model
– Reads the global analysis results file
– Compares the two models and fetches displacements from global analysis
– Imposes these as prescribed displacements on the sub-model boundaries with prescribed b.c.
Perform the structural analysis for the sub-model, this is a standard Sesam Sestra analysis
It is important to perform load transfer from Sesam HydroD to the local model since the loads must be the same as on the global model.
Slide 53
analyseanalyse
analyseanalyse
SubmodSubmod
global model
sub-model
prescribed b.c.
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Sesam HydroD to compute local wave loads
Rerunning Sesam HydroD for the sub-model is easy:
– Panel and mass models are typically the same as for the global model
– Wave periods and directions must be the same as for the global model
– The basic hydrodynamic results from the global analysis can be reused so the local analysis is much faster
– Structural model in Sesam HydroD:Simply replace global model with sub-model
– Pressure loads for panels outside the sub-model are discarded
– Also needed if there are no wet surfaces since the inertia and gravity loads will still apply
Slide 54
Global model
Sub-model
Panel model
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Typical workflow – Local analysis
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Local structuralanalysis
Stress extrapolation
Stress distribution foreach load case
RAO’s•Local stress/deflections
Local stress/deflections
Input•Hot spot location
Result•RAO•Principal hot spot stress
Principal hotspot stress
Principal stress
0.E+00
1.E+07
2.E+07
3.E+07
4.E+07
5.E+07
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0Wave period [ s]
0
45
90
135
180
Local stress transfer functions
Fatigue calculations
Input•Wave scatter diagram•Wave spectrum•SN-curve•Stress RAO
•=> Fatigue damage
Stress
Hot spot
Geometric stress
Geometric stress athot spot (Hot spot stress)
Notch stress
Nominal stress
Scatter diagram
SN data