fpso full ship analysis - an overview

Fan (Joe) Zhang, Sesam BD Manager, DNV Software

October 15, 2012

FPSO Full Ship Analysis

Integrated Strength and Hydrodynamic Analysis using Sesam

© Det Norske Veritas AS. All rights reserved.


October 15, 2012

Topics

Strength assessment of FPSOs and related software from DNV

Global modelling

Hydrodynamic analysis

Ultimate strength analysis

Submodelling

Fatigue analysis

2



October 15, 2012

FPSO Package for design and analysis

3

Proven solutions in use by major

companies around the world

Topside

GeniE

Main scantlings

Nauticus Hull

Risers

DeepC

Turret

Local analysis

GeniE

Hydrodynamics

• Seakeeping

• Wave loads

HydroD

Fatigue

Simplified, Spectral

Nauticus Hull

Stofat

Mooring

Mimosa

3D Hull modelling

GeniE

Risk Analysis

Safeti



October 15, 2012

SESAM strength assessment analysis system and interfaces

4

Workflow manager

Modelling, structural analysis and code check

Stability andwave load analysis

Mooring and riser analysis

Local analysis

GeniE HydroD DeepC

Global analysis 1

Global analysis 2

Wave load 1

Wave load 2

Wave load nGlobal analysis n

Analysis 1

Analysis 2

Analysis n

Stability 1

Stability 2

Stability n

Model

Loads

Model

Results



October 15, 2012

Sesam – a fully integrated analysis system

5

FE analysis

4. Global stress and deflection

& fatigue screening

1. Stability and wave load analysis

Wave scatter diagram

2. Pressure loads and accelerations

Lo

ad

tran

sfe

r

3. Structural model loads (internal + external pressure)

Local FE analysis

5. Local stress and deflection

& fatigue



October 15, 2012

Sesam Workflow Manager

Key features

- Model and file management

Benefits

- Automatic re-run of analysis hierarchy to re-produce

analysis after model updates

- Facilitate alternate engineers to re-run analysis

- Documentation/description of models and analysis

can be linked into the explorer

- Supports best engineering practice and workflow

6



October 15, 2012

GeniE

Key features

- Modeller for all hydrodynamic and structural applications

within the Sesam system

- User interface for FE analysis, post-processing and code

checks for both hull, topside and jacket

Benefits

- One common model for strength and hydrodynamics

- Efficient modelling and code checks within one user

environment

- Easy to implement updates and changes to geometry

and properties

- Different level of detailing of FE model derived from one

global model by adjusting mesh densities

- Mesh automatically adapts to changes in the model

7



October 15, 2012

HydroD

Key features

- Hydrostatics and stability calculations

- Linear and non linear hydrodynamics

Benefits

- Handling of multiple loading conditions and models through one user interface and

database

- Sharing models with structural analysis

- Direct transfer of static and dynamic loads to structural model

8



October 15, 2012

Analysis Overview

Task Purpose Input Output

Global modelling Make global model for

hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for

fatigue and ultimate

strength

Global FE model

Wave data

Load files for

structural

analysis

ULS analysis Calculate hull girder

strength

Global FE model

Snap shot load files

from HydroD

Ultimate strength

results

Spectral fatigue

analysis

Fatigue screening on

nominal stress

Local fatigue analysis

Global FE model

Frequency domain

load files from HydroD

Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

9



October 15, 2012

Analysis Overview



hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural

analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model

Frequency domain


Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

10



October 15, 2012

Creating the Global Model

The global model is used to calculate

loads and strength and must represent

the actual properties of the ship

For direct strength calculations

essential properties are

- Buoyancy and weight distribution

- Compartment loads

- Structural stiffness and strength

Modelling of hull form

Creating compartment and loads

Mass tuning

11

Challenges Model requirements



October 15, 2012

Global Modelling with GeniE

12



October 15, 2012

Benefits of GeniE for Global Modelling

One common model for hydrodynamic and

structural analysis

Geometry modelling

- Advanced surface modelling functions

- Re-use data from CAD

- Parametric modelling using JavaScript

- Use of units

Compartment and loads

- Compartments are created automatically

- GeniE calculates tank volumes and COG

- Loads are generated from compartment fillings

and automatically applied to tank boundaries

Mass tuning

- Scaling mass density to target mass

13



October 15, 2012

Analysis Overview



hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural

analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model

Frequency domain


Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

14



October 15, 2012

Hydrodynamic Analysis

Hull shape as real ship

Correct draft and trim

Weight and buoyancy distribution

according to loading manual

Mass and buoyancy in balance

Obtain correct weight and mass

distribution

Balance of loading conditions

15

Challenges Model requirements



October 15, 2012

HydroD

17



October 15, 2012

Benefits of HydroD

One common program and model for - Stability calculations

- Linear hydrodynamic analysis

- Non-linear hydrodynamic analysis

- With or without forward speed

Supports composite panel & Morrison models

Support both standalone and integrated analysis - Models can made in HydoD or based on structural models

Loading conditions - Multiple loading conditions by changing compartment

contents

Balancing the model - Auto balance of loading conditions by draft and trim or

compartment fillings

Built in roll damping module - Stochastic linearization

- Quadratic damping

Strong postprocessing and graphical results presentation

Load transfer to FE analysis - Snap shot or frequency domain

- With splash zone correction for fatigue

18



October 15, 2012

Analysis Overview



hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural

analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model

Frequency domain


Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

19



October 15, 2012

Ultimate Strength Analysis

Global structural analysis with load transfer

from hydrodynamic analysis

Snap shot load transfer of non linear loads

for selected design conditions

Yield and buckling check with PULS

Benefits of global analysis with direct load

transfer

- Eliminate effect of boundary conditions

- Loads applied as a simultaneous set of sea and

tank pressures according to the calculated

design wave No need for conservative and/or

uncertain assumptions

- Integrated buckling check

20



October 15, 2012

Design Wave Determination – Example

1. Calculate long term response (100 years return period for FPSO)

100 years wave bending moment: 2.184E9 Nm

2. Find peak value, phase and corresponding peak period in transfer function

Peak value: 2.33E8 Nm

Phase angle: ϕresp= 128 deg (relative to incoming wave)

Period: 12 s

3. The design wave is then

Amplitude 100 year response/peak value

2.184E9/2.33E8*2=18.75 m

Period 12 s

Phase: ϕwave = 360 - ϕresp = 360 – 128 = 232 deg.

Resulting values:

232 deg hogging

52 deg sagging

- Which is sagging and hogging must be evaluated separately

21



October 15, 2012

Verify the applied loads

Reaction forces Sestra.lis

- Reacting forces “close to zero” compared to the global excitation forces (<~1%)

- E.g. stillwater load case

Max Fz < mass*g/100

22



October 15, 2012

Verification of applied loads

Visual check using Xtract

- Pressure mapping - external and internal pressures

- Deflections

- Nominal stress level

23



October 15, 2012

Verification of applied loads

Global sectional loads Cutres

- Cutres calculates and integrates the force distribution of cross sections and is ideal to

evaluate the hull girder shear forces and bending moments

- Large deviations Improper load balance

- Small deviations will occur since HydroD only consider vertical loads (mass) from internal tank

pressures

- NB! Check position of neutral axis in HydroD sections and Cutres results

24

0 50 100 150 200 250 300 350

Distance from AP

Vert

ica

l sh

ea

r fo

rce

WASIM

CUTRES

Vertical shear force distribution

0 50 100 150 200 250 300 350

Distance from AP

Vert

ical

be

nd

ing

mo

men

t

WASIM

CUTRES

Vertical bending moment distribution



October 15, 2012

Cutres – main features

Relevant for long and narrow structures that may be viewed as beams, e.g. ships

Two basic features:

- Presentation of stress resultant diagrams for user defined sections through the FE model

Integration of stress resultants over sections and presentation of force and moment

graphs along ship axis

26



October 15, 2012

Sections created in Cutres

27



October 15, 2012

Display section diagram

28



October 15, 2012

Still water bending moment

29



October 15, 2012

Vertical shear force and bending moment

30

Postresp: 2.33E8 * 9.375 = 2.18E9

Cutres: 2.207E9



October 15, 2012

Cross sections positions adjust

31



October 15, 2012

PULS – Advanced Buckling & Panel Ultimate Limit State

32

PULS is a code for buckling

and ULS assessments

of stiffened and unstiffened

panels



October 15, 2012

Benefits of PULS

Characteristics

- Higher accuracy than traditional rule formulations

and classic buckling theory

- Quick and easy-to-use design tool for calculation of

ULS capacity

- Valuable information about failure mode and

buckling pattern

- Effective to evaluate

Benefits

- Design optimization with increased control of safety

margins

33

0

50

100

150

200

250

0 20 40 60 80 100 120 140

s2 (MPa)

t12 (

MP

a)

Abaqus

PULS

DNV Rules

GL Rules

Py

Px



October 15, 2012

PULS - Element library

34

Un-stiffened plate element

Stiffened plate element (S3)

Corrugated plate element (K3)

Stiffened plate element (T1)



October 15, 2012

PULS Code Check in GeniE

Buckling capacity panels are

automatically generated from the plate

and beam concepts

Colour code presentation of Utilization

Factors (UF)

Support for multi-core parallel buckling

calculations

Numeric and colour code presentation

of result

“Worst case” - colour code presentation

of the maximum UF from all load cases

Buckling panels can be exported to

PULS Advanced Viewer and PULS

Excel for further postprocessing

35



October 15, 2012

Combination of still water and wave loads results

Design wave load transfer

from Wadam results in

separate result cases for

static and hydrodynamic

loads

- RC1 still water load

- RC2 wave loads

Create result combination in

GeniE

- Alternative: Combine results in

Prepost

36



October 15, 2012

Code check

Code check according to DNV-RP-C201 Pt 2 (PULS) by “CSR Tank”

Note: Default parameters are according to CSR Tank and must be modified

according to the RP

37



October 15, 2012

Modified CSR Tank PULS code check

FPSO ULS 3 hold or global model

- Design wave approach with direct load transfer from HydroD

or

Maximum hogging and Maximum sagging condition (according to DNV-RP-C102, App D).

- Check only longitudinal structure

- Method 1 (Ultimate Capacity), according to DNV-RP-C201 Pt 2 (PULS)

- Stiffened panel type

- Allowable usage factor = 0.8 (loadcase design condition must be harbour or seagoing)

- Meshing rules: Gross scantlings, (don’t use co-centric stiffeners).

38



October 15, 2012

Code check result

39



October 15, 2012

Analysis Overview



hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural

analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model

Frequency domain


Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

40



October 15, 2012

Simplified vs. spectral fatigue

41

Wave loads

Stress calculations:

Environment Long term rule Weibull distribution

Direct calculated loads - 3D potential

theory

Fatigue damage

calculation:

Actual wave scatter diagram and

energy spectrum

Rule formulations for accelerations,

pressure and moments on 10-4

probability level

Load transfer to FE model. Complete

stress transfer function.

Hotspot stress models for SCF

Rule formulations for stresses.

Rule correlations.

Based on expected largest stress

among 10^4 cycles of a rule long term

Weibull distribution

Based on summation of part damage

from each Rayleigh distributed sea

state in scatter diagram.

Simplified Spectral fatigue



October 15, 2012

Stochastic Fatigue Analysis

Wave Load Analysis - Input: Global model, wave headings and frequencies

- Output: Load transfer functions (RAOs)

Stress Response Analysis - Input: FE models and load file from wave load analysis

- Output: FE results file with load cases describing complex

(real and imaginary) stress transfer functions (RAOs)

Fatigue Damage Calculation - Input: Stress transfer functions (FE results file), wave data

- Output: Calculated fatigue life

42

Fatigue Life

Direct Load

Transfer

S-N Fatigue

Curves

Wave scatter

diagram

Stress Transfer Functions



October 15, 2012

Typical workflow

43

Hydrodynamic

analysis

Load transfer

RAO’s

•External pressure

•Rel. wave elevation

•Accelerations

•Full load / intermediate/ ballast

• ->800 complex lc

Global FE-model

Hydrodynamic

model

Local model

boundary

conditions

Global +

local FE-

model

RAO’s

•External pressure

•Internal pressure

•Accelerations

•Adjusted pressure for

intermittent wetted areas

Global structural

analysis Global

stress/deflection

RAO’s

•Global stress/deflections

•Entire global model

Deflection transfer

to local model

Global deflections as

boundary conditions on

local model



October 15, 2012

Typical workflow

44

Local structural

analysis

Stress

extrapolation

Stress distribution for

each load case

RAO’s

•Local stress/deflections

Local

stress/deflections

Input

•Hot spot location

Result

•RAO

•Principal hot spot stress

Principal hotspot

stress

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



October 15, 2012

Global Frequency Domain Analysis

Loads from HydroD

Static load case

- For verification of load balance and static

shear and bending compared to loading

manual

- Enables automatic calculation of mean

stress effect in fatigue calculartions

- Enables possibility for to calculate long

term extreme loads including static stress

Dynamic load cases

- Number of complex dynamic load cases =

number of wave headings x number of

wave periods (e.g. 12 x 25 = 300)

45



October 15, 2012

Pressure reduction zone

46

CN 30.7

Zwl = ¾*5.626E04/(1025*9.81)

= 4.196

Postresp Long Term Prediction



October 15, 2012

Load Transfer to Global Model

48



October 15, 2012

Fatigue Calculation Program - Stofat

Performs stochastic (spectral) fatigue calculation

with loads from a hydrodynamic analysis using a

frequency domain approach

Deterministic fatigue under development

Structures modelled by 3D shell and solid

elements

Assess whether structure is likely to suffer failure

due to the action of repeated loading

Assessment made by SN-curve based fatigue

approach

Accumulates partial damages weighed over sea

states and wave directions

49

POSTPROCESSING

RE

SU

LT

S I

NT

ER

FA

CE

FIL

E

ST

RU

CT

UR

AL

RE

SU

LT

S I

NT

ER

FA

CE

FIL

E

Stofat

Shell/plate

fatigue

Stofat

database



October 15, 2012

Global Screening Analysis

Fatigue calculations based on nominal

stress from global analysis and stress

concentration factors

Typical use

- Identify fatigue sensitive areas

- Determine critical stress concentration

factors for deck attachment and topside

supports

- Determine location of local models and fine

mesh areas

- Decide extent of reinforcements based on

SCF from local analysis

51



October 15, 2012

Fatigue Screening Analyses

Fatigue Damage in Lower Hopper Knuckles

- Global screening scaled by results from local analysis

Lower Hopper Knuckle

0.000

0.250

0.500

0.750

1.000

1.250

100425 120425 140425 160425 180425 200425 220425

Distance from AP [mm]

Fati

gu

e D

am

ag

e [

-]

Screening Result

TBHD Pos.

Local Model Result



October 15, 2012

Global Screening

53



October 15, 2012

Local Fatigue Analysis

Local fine mesh model created

from global GeniE model by

changing the mesh density in

the location of the crack

Hot spot stress RAOs at the

location of the crack

established by spectral FE

calculation

Submodelling techniques is

used to transfer the results

from the global FE analysis to

the boarders of the local model

Fatigue damage/life calculated

using Stofat

54

Concept model with mesh densities

Local fine mesh model

Calculated fatigue life



October 15, 2012

Submodelling

55



October 15, 2012

Fatigue Strengthening and Screening of Extent

Soft bracket added in the local model

of the stringer at crack location

Re-run sub-model analysis and

fatigue calculation to check effect of

strengthening proposal

Necessary extent of repair evaluated

by fatigue screening of global

Stress concentration factor used in

global screening calculated by the

ratio of long term stress from local

and global analysis

57

Local model with new bracket

Results from fatigue screening of global model to evaluate extent of repair

Fatigue results



October 15, 2012

Analysis Overview



hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural

analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model

Frequency domain


Calculated

fatigue lives

Spectral ULS

analysis

Calculate long term

stress based on

spectral method

Global FE model

Frequency domain


Long term stress

58



October 15, 2012

Stochastic ULS Analysis

59

Long term stress

Wave Load Analysis

- Input: Global model, wave headings and frequencies

- Output: Load transfer functions (RAOs)

Stress Response Analysis

- Input: FE models and load file from wave load analysis

- Output: FE results file with load cases describing complex (real and

imaginary) stress transfer functions (RAOs)

Direct Load

Transfer

Wave scatter

diagram

Stress Transfer Functions Long Term ULS Load Calculation

- Input: Stress transfer functions (FE results file), wave data

- Output: Calculated long term stress

Challenge: Determine ULS design wave for areas subjected to a combination of different load

effects (e.g. turret area)

Typical way: Selection of one or several design waves Uncertainties

New solution with Stofat: Spectral stress analysis to determine long term stress distribution directly



October 15, 2012

Stofat – Features and Benefits

Features

- Stochastic fatigue calculations based on wave statistics

- Supports all common wave models

- Predefined and user defined S-N curves

- Option for implicit mean stress correction (by static load case)

- Statistical stress response calculations

- Calculation of long term stress and extreme response including static loads

- Graphical presentation of fatigue results and long term stress directly on FE model

Benefits

- Unique functionality for spectral fatigue and stochastic long term stress and extreme response calculations

- Flexible – support all your needs

- Transparent – all calculation steps can be documented

60

Calculated fatigue damage by nominal stress

and user defined SCF for an LNG carrier

Calculated long term stress amplitude (left) and fatigue

damage (right) for the hopper knuckle in an oil tanker



October 15, 2012

Local fatigue check result

61



October 15, 2012

Benefits of Sesam for Advanced Analysis

Complete system – Proven Solution

- Cover your needs for strength assessment of ship and offshore

structures

- 40 years of DNV experience and research put into software tools

Concept modelling

- Minimize modelling effort by re-use of models for various

analysis

- Same concept model can be used for global & local strength analysis,

stability, linear and non-linear hydrodynamic hydrodynamic analysis

Same system for offshore and maritime structures

- Minimizes the learning period and maximizes the utilisation of

your staff

Process, file and analysis management by Sesam Explorer

62



October 15, 2012

63

Safeguarding life, property

and the environment

www.dnv.com

fpso full ship analysis - an overview

Documents