Advanced Methods for ULS and FLS

Torbjørn Lindemark, Nauticus Product Manager

Advanced Methods for Ultimate and Fatigue Strength of Floaters

DNV Software

© Det Norske Veritas AS. All rights reserved.


2

Agenda Strength assessment of FPSOs and related software from DNV

Introduction to direct load and strength calculations

Deterministic vs. spectral analysis

Fatigue loading and critical details for FPSOs

Case study and software demo on direct strength calculations of a ship shaped FPSO



3

FPSO - What is required?

FPSO - Complex design process - Ships and Offshore Rule requirements - Regulatory requirements - Seakeeping, Hydrodynamic analysis - Long operation life without docking - Topside & Topside/Hull interaction - Turret area - Risers & Moorings - Deep water

Tools for assessment of - Conversion of tanker to FPSO - FPSO newbuilding

Tools for maintenance of FPSO’s in operation

We deliver a package that ties it all together and provide a complete, integrated toolkit, tailor made for FPSOs



4

Challenge of FPSO New Build and Conversion Conversions

- Increase certainty that the chosen vessel is suitable for conversion,

- Determine how much steel should be replaced during conversion/maintenance,

- Identify where to focus surveys.

New Builds

- Selection corrosion protection strategy to determine a rational material thickness

- Identify comprehensive analysis requirements for design

- Develop Inspection Plans - Choice of turret design



5

FPSO Package for design and analysis

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 Sesam/Stofat

Mooring Mimosa

3D Hull modelling

GeniE

Risk Analysis Safeti



6

FE analysis

4. Global stress and deflection & fatigue screening

Direct Calculations in an Integrated Analysis System

1. Stability and wave load analysis

Wave scatter 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



7

Wave Load Analysis Input

- Models - Panel &/or Morrison model - Mass model - Compartments - Structural model for load transfer

- Loading conditions - Compartment fillings, draught and trim

- Wave and environmental data - Scatter diagram - Wave spectrum - Directionality and spreading - Current - Water depth

Output - Load transfer functions (Response Amplitude

Operators – RAOs) - Motions in 6 dof (+ derived velocities and

accelerations) - External wave pressures - Internal tank pressures - Morrison forces - Sectional loads

- Load statistics - Derived by combining the load RAOs with wave data - Design values for ULS/ALS - Long term load distribution for simplified fatigue

calculations - Load files for transfer to structural model

- Design waves for deterministic ULS and/or FLS analysis

- Load RAOs for stochastic ULS and FLS analysis - Both containing accelerations, external and internal

pressures



8

Finite Element Analysis

Input - Global and local FE models - Design wave load transfer files (or long term

loads by manual input)

Output - Stress response for a given design wave/load

Input - Global and local FE models - RAO based load transfer files - Wave and environmental data

- Scatter diagram - Wave spectrum - Directionality and spreading

Output - Stress transfer functions (Response Amplitude

Operators – RAOs) - Stress statistics

- Derived by combining the stress RAOs with wave data

- Short and long term distribution - Design values for specified probability level/return

period

Deterministic Analysis Spectral Analysis



9

Fatigue Analysis by Cumulative Damage

Input - Long term stress distribution

- Described by Weibull distribution or stress histogram - The Weibull distribution is described by

- Stress at a given probability level - Weibull parameter - Zero crossing frequency

- S-N curves

Output - Calculated fatigue life or damage

Input - Stress transfer functions (Response Amplitude

Operators – RAOs) - Wave and environmental data

- Scatter diagram - Wave spectrum - Directionality and spreading

- S-N curves

Output - Calculated fatigue life or damage

- Fatigue calculations performed based on short term statistics by summing up part damage for each cell in the scatter diagram the uncertainties involved in Weibull fitting are avoided

Deterministic Analysis Spectral Analysis



10

Simplified vs. direct fatigue calculations

Wave Load Analysis:

Stress analysis:

Environment

Long term Weibull distribution by rule formulas

Direct calculated loads - 3D potential theory

Fatigue damage analysis:

Wave scatter diagram and energy spectrum

Accelerations, pressure and moments on 10^-4 or 10^-8 probability level by rule formulas

Load transfer to FE model. Stress transfer function implicit in FE model

Rule formulations for stresses and correlation of different loads

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 Analysis



11

Fatigue loads and stress components Global wave bending moments Hull girder stress Stress in topside supports due to global hull

deflections Stress in turret and moonpool areas due to hull

deflections

Wave pressure Shell plate local bending stress Local stiffener bending stress Secondary stiffener bending due to deflection

of main girder system Local peak stresses in knuckles due to

deflection of main girder system

Vessel motions (accelerations) Liquid pressure in tanks Stress in topside support from inertia forces Mooring and riser fastenings



12

Moonpool areas

Long. stress in deck (noshear lag effect)

CL

Nominal stresslevel

Actual stressdistribution

Long. stress in deckuniform deck thickness

Long. stress in deckwhen plates near side

are increased

Increased platethickness



13

In-service Experience on Fatigue Critical Details Stiffener end connections

Root source of cracking Global hull girder bending Local dynamic pressures Relative deflections caused by bending of

girder system Stress concentration at stiffener toe and

heel

Longitudinal

Stiffener Web-plating



14

In-service Experience on Fatigue Critical Details

Cracks under development

Repair example

Knuckles in inner structure (hopper knuckle)

Root source of cracking: Deflection on main girder system High stress concentration



15

In-service Experience on Fatigue Critical Details Shell plating

Root source of cracking Local pressure



16

In-service Experience on Fatigue Critical Details Main deck openings and attachments

Root source of cracking Global hull girder stress Stress due to hull girder deflection and stiff topside

lattice construction Stress from topside inertia forces Local stress concentrations



17

Summary Fatigue Critical Details Main deck openings, attachments and topside support

Moonpool area

Knuckles and discontinuities in the main girder system

Stiffener end connections

Side shell plating



18

A few useful ratios Ratio Stress factor

(equivalent stress reduction)

Fatigue Damage factor

Base / Weld - SN curve

(10^12.89) / (10^12.65)

0.83 1.74

World wide / North Atlantic ocean

0.8 / 1.0 0.8 2.0

Non-corrosive / corrosive environment

(10^12.65) / (10^12.38)

0.81 2.0

Mean / Design SN curve

(10^12.09) / (10^11.63)

0.7 3.0



19

Part 2 – Case Study and Demos

Direct strength ULS and FLS calculations of a ship shaped FPSO



20

Why direct load and strength calculations Rule loads are not always the truth Modern

calculation tools give more accurate loads - Ultimate strength loads - Fatigue loads - Phasing and simultaneity of different load effects

Design and strength optimizations based on analysis closer to actual operating conditions

Improved decision basis for - In-service structural integrity management - Life extension evaluation

0

500000

1000000

1500000

2000000

0 0.2 0.4 0.6 0.8 1

[kN

m]

VBM (linear)

0

50000

100000

150000

0 0.2 0.4 0.6 0.8 1

[kN

]

VSF (linear)

Pressure

Rule −−−

Direct −−−

Time

Stre

ss

Vertical BendingMomentSea Pressure

Double Hull Bending

Total Stress



21

Direct calculated loads vs. rule loads Fatigue loads:

0.00

0.20

0.40

0.60

0.80

1.00

1.20

VerticalBending

HorizontalBending

Pressure WL Vert. Acc.

DirectDNV RuleCSR



22

Spectral vs Simplified Fatigue Analysis Comparison of fatigue damage by DNV rules and Common Scantling Rules relative

to spectral fatigue calculations:

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Bottom atB/4

Side atT/2

Side at T TrunkDeck

Comp. Stoch.DNV RuleCSR



25

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


Long term stress



26

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

Challenges Model requirements



27

Demo – Global Modelling with GeniE



28

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



29




Global FE model















Long term stress



30

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

Challenges Model requirements



31

Demo – HydroD



32

Benefits of HydroD One common model for

- Stability calculations - Linear hydrodynamic analysis - Non-linear hydrodynamic analysis - With or without forward speed

Supports composite panel & Morrison models

Model shared with structural analysis

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



33




Global FE model















Long term stress



34

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



35

Cutres - Verification of Applied Loads

0 50 100 150 200 250 300 350

Distance from AP

Ver

tical

she

ar fo

rce

WASIMCUTRES

Vertical shear force distribution

0 50 100 150 200 250 300 350

Distance from AP

Vert

ical

ben

ding

mom

ent

WASIMCUTRES

Vertical bending moment distribution

Cutres calculates and integrates the force distribution of cross sections and is ideal to evaluate the hull girder structural response



36

PULS – Advanced Buckling & Panel Ultimate Limit State

PULS is a code for buckling and ULS assessments

of stiffened and unstiffened panels



37

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

0

50

100

150

200

250

0 20 40 60 80 100 120 140

σ2 (MPa)

τ12 (

MPa

)

AbaqusPULSDNV RulesGL Rules

Py

Px



38

PULS - Element library Un-stiffened plate element

Stiffened plate element (S3)

Corrugated plate element (K3)

Stiffened plate element (T1)



39

Demo – PULS Code Check in GeniE



40




Global FE model















Long term stress



41

Stochastic Fatigue Analysis

Fatigue Life

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

S-N Fatigue Curves


Stress Transfer Functions Fatigue Damage Calculation

- Input: Stress transfer functions (FE results file), wave data - Output: Calculated fatigue life



42

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)

Head Sea



43

Demo - Stofat

Calculated fatigue damage by nominal stress and user defined SCF for an LNG carrier



44

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



45

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

Concept model with mesh densities

Local fine mesh model

Calculated fatigue life



46

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

Local model with new bracket

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

Fatigue results



47




Global FE model















Long term stress



48

Stochastic ULS Analysis

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


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



49

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

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



50

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 for global & local strength analysis and for

hydrodynamic analysis - Same model basis for hydrostatics and frequency and time domain

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



51

Safeguarding life, property and the environment

www.dnv.com

Date post:	18-Jan-2015
Category:	Technology
Upload:	joao-henrique-volpini-mattos
View:	2,149 times
Download:	7 times

Advanced Methods for ULS and FLS

Technology