EXPRO-CFD

An Overview of European Research in CFD-Based Fluid Loading and

Fluid Structure Interaction

Contents

Background. The Main Objectives The Partnership Main Technical Features

The Workplan

The Deliverables

Progress to date Systems development

Experimental programme

Early validation results

Current Work and Case Studies

Further Work

Background

EXPRO-CFD was instigated through consultation with

oil companies and contractors who need:

improvements in the prediction of wave loads on

floating systems

more account to be taken of non-linearities in design

better validation

better models for dealing with deep water behavior

improved integration with other design tools

application guidelines specific to CFD for offshore

engineering applications.

The Main Objectives

To develop new methodologies in offshore

hydrodynamic analysis based on: Coupling or co-processing systems,

Using readily available, commercial CFD codes,

Existing hydrodynamic diffraction tools,

Existing vessel response and riser/mooring system models.

To ensure that we have the right working methodologies

for these systems.

To validate, by carrying out detailed flow visualization

experiments.

To critically review via case studies.

The Partnership

Atkins Process* - Coordinators

Det Norske Veritas* MARIN* Statoil*

BP Single Buoy Moorings*

Aker Kvaerner LMG Marin

Imperial College* Sirehna* CIMNE

Ecole Centrale de Nantes University College

London

SIREHNA

Key Technical Aspects

The main technical features of the project fall into three

areas:

1. The coupling, or integration, of existing software tools and

methodologies,

2. The validation and tuning of the models within the coupled

system through experiments and some detailed studies,

3. Concept design case studies to provide benchmarks,

demonstrate practicalities and establish application

guidelines.

Typical Offshore Problems of Concern

Slow drift damping and viscous drag effects

Extreme or steep wave loads - viscous effects in

trough to crest region,

Vortex shedding at all scales - riser interactions

Wave impact, run-up and air-gap.

Tether ringing and springing.

Local, non-linear free surface problems.

The Work-plan

6 Work-packages

1 Integration and Testing of CFD/diffraction and platform dynamics programs.

2 Key technical studies (Cylinder LES and FPSO modeling)

3 Validation experiments4 Tuning and validation of the hydrodynamic systems5 Design evaluation case studies 6 Benchmark tests.

The Deliverables

Systems for coupling unsteady RANSE, diffraction

and systems response models, built from readily

available tools.

New experimental data for flow and loading on

floating offshore systems.

Guidelines for the application of these systems aimed

at specific design and safety related problems.

A software environment for set-up and control.

Demonstration case studies.

Work Packages (1)

Stage 1 – First 18 months

1. Integration and testing of CFD/diffraction and platform dynamics programs (Atkins, DNV, CIMNE).

2. Key technical studies (ECN, Imperial College, University College)

3. Validation experiments (SIREHNA, ECN, MARIN)

Systems Integration

The Scope of the Developed System

Provide engineers with a single point of entry to the modelling system (i.e. using CAD)

Should provide a common grid generation capability and common interfaces to commercial CFD tools

Allow traditional hydrodynamic tools to be used independently Rigid body motions only for floater, but with mooring, riser,

tether models included. Should allow set up and control of the simulation parameters

through a single interface. Common post-processing, but again – interfaces to other

systems.

Example coupled system(Atkins solution)

At present based on AQWA/CFX4 with GiD front end “Control box” determines which module runs when Data to files in a common directory Use of files is the most flexible way of reading / writing data Indirect interaction between individual modules Individual modules can easily be replaced

GiD launcher

Control box

Potential flow solver

RANS solver Motions solver Moorings Risers

Experimental Work

Measurements - FPSO

Measurements by MARIN Freely floating 1:80 scale FPSO model 5 wave periods/2 wave heights wave directions at 90,135 degrees Measurements of global loads and

moments, wave profiles, PIV velocity measurements

Incoming waves

PIV FoV

y

x

FPSO at 90°

FPSO at 135°

Camera housing

Measurements – vertical cylinder

Fixed vertical cylinder Tests by ECN and Sirenha Regular waves Monochromatic: 9 combinations of L

and H Bichromatic: 2 combinations of 2 wave

periods PIV measurements of velocities on

radial slices Pressure measurements at vertical and

horizontal sets of pressure tappings Overall forces and moments

Example Simulation ECN Cylinder

Example case

Vertical cylinder in regular waves:

Period = 1.26 seconds

Amplitude = 0.127m

Early Validation Results

Vertical cylinder tests

Fixed vertical cylinder as per WP3 experiments

‘Beach’ downstream Extent of domain limited at

present – testing to determine optimum extents will be undertaken.

Grid movement to follow potential flow free surface to aid propagation of wave through the domain

Vertical cylinder – preliminary results

Comparisons with experimental data

Noticeable effects of VOF, and grid dependency

Wave kinematics at inlet, and propagation – test of consistency

pressures - experiments

pressures – coupled system

ECN data - pressures - l2a20ld10

-1000

0

1000

2000

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4000

5000

6000

10 12 14 16 18 20

Time (s)

Pre

ss

ure

(P

a)

p1

p3

p4

p5

p6

p7

p8

p9

p10

p11

p12

p14

p15

p16

p17

CFD data - pressures - l2a20ld10

-1000

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1000

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0 2 4 6 8 10

Time (s)

Pre

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ure

(P

a)

p1

p3

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p8

p9

p10

p11

p12

p13

p14

p15

Coupled model

Floating cylinder depth 1m, mass 205kg

1D motion with simple AQWA-NAUT model

Extinction tests in surge and heave

Surge added mass from CFD 95% (AQWA-LINE 92%)

Heave added mass from CFD 17% (AQWA-LINE 16%)

increased damping over radiation/diffraction only AQWA-LINE – comparable to additional viscous effects

1DoF surge extinction

-0.12

-0.10

-0.08

-0.06

-0.04

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0.00

0.02

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0 1 2 3 4

Time (s)

Dis

pla

ce

me

nt

(m)

radiationonly

CoupledSystem

Surge in waves

Two cases: k = 4000, 1000N/m Comparison with AQWA-NAUT

with no added viscous effects – radiation / diffraction only.

After initial transients both calculations reach a motion of constant amplitude

Coupled system with effects of viscosity included shows expected reduced amplitudes

Displacement - surge only - k = 4000n/m

-0.15

-0.10

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Time (s)

Dis

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(m)

Coupledsystem

diffraction /radiation

Displacements - surge only - k = 1000n/m

-0.15

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0.00

0.05

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Time (s)

Dis

pla

cem

en

t (m

)Coupledsystem

diffraction /radiation

Current Work

Completing the process of validation and tuning

Work Package 6 – Design evaluation case studies Focus has shifted from development to prototype

application. Tools to be applied to case studies within a design

environment.

Independent physical testing of 2 cases at MARIN and ECN.

Guidelines for application to be developed. Completion in January 2004

Case Studies (1)

AKER KVAERNER TLP

KEY ISSUES

1. Wave run-up and air gap

2. Extreme wave loads

3. Tether ringing

Overlap study involving both Atkins and DNV systems to allow benchmarking

49.0

9.5

31.0

1000

951

Z

XCoG

10

Case Studies (2)

LMG MARIN FPSO

KEY ISSUES1. Wave Drift Damping

2. Fishtailing

3. Green water (if time allows)

The DNV system only to be applied to this case

Case Studies (3)

SBM – Export Buoy

KEY ISSUES

1. Skirt damping

2. Extreme loads

3. Mooring/floater interaction

The Atkins system only to be applied in this case

Further Information

Publications Forthcoming OTC – Overview including more technical

details ISOPE – Session of 6 papers dedicated to the project Additional publications by ECN, Imperial College and UCL EXPRO-CFD web site.

Further Developments EXPRO-CFD Brochure – available on request EXPRO-CFD Participation Programme – aimed at

exploiting deliverables EXPRO-CFD II