Integrated hydrodynamic and structural analysis webinar presentation tcm4 601490

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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Introduction to the tool used in the case study:

Sesam for Floaters

DNV GL © 2014

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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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DNV GL © 2014

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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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Internal dynamicsQuasi-static approachFull handling of internal fluid dynamics

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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


25 compartments with 4 for liquid cargo tanks

balancingAutomatic balancing

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LNG carrier study


– Surge, heave and pitch not so affected

– Sway, yaw and roll affected both for full and half tank

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Case study: Analysis of an FPSO

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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°


• 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°


• 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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Loads around the waterline

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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

Example

Pressure reduction zone

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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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Fatigue analysis workflow and Submod properties

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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

Integrated hydrodynamic and structural analysis webinar presentation tcm4 601490

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Transcript of Integrated hydrodynamic and structural analysis webinar presentation tcm4 601490