EXPRO-CFD

1. EXPRO-CFD An Overview of European Research in CFD-Based Fluid Loading and Fluid Structure Interaction

2. 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

3. 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.

4. 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.

5. 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

6. Key Technical Aspects The main technical features of the project fall into three areas: • The coupling, or integration, of existing software tools and methodologies, • The validation and tuning of the models within the coupled system through experiments and some detailed studies, • Concept design case studies to provide benchmarks, demonstrate practicalities and establish application guidelines.

7. 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.

8. The Work-plan 6 Work-packages • Integration and Testing of CFD/diffraction and platform dynamics programs. • Key technical studies (Cylinder LES and FPSO modeling) • Validation experiments • Tuning and validation of the hydrodynamic systems • Design evaluation case studies • Benchmark tests.

9. 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.

10. Work Packages (1) Stage 1 – First 18 months • Integration and testing of CFD/diffraction and platform dynamics programs (Atkins, DNV, CIMNE). • Key technical studies (ECN, Imperial College, University College) • Validation experiments (SIREHNA, ECN, MARIN)

11. Systems Integration

12. 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.

13. 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

14. Experimental Work

15. 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

16. 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

17. Example Simulation ECN Cylinder Example case Vertical cylinder in regular waves: Period = 1.26 seconds Amplitude = 0.127m

18. Early Validation Results

19. 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

20. 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

21. 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

22. 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

23. 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

24. AKER KVAERNER TLP KEY ISSUES Wave run-up and air gap Extreme wave loads Tether ringing Overlap study involving both Atkins and DNV systems to allow benchmarking Case Studies (1)

25. Case Studies (2) • LMG MARIN FPSO KEY ISSUES • Wave Drift Damping • Fishtailing • Green water (if time allows) • The DNV system only to be applied to this case

26. Case Studies (3) • SBM – Export Buoy KEY ISSUES • Skirt damping • Extreme loads • Mooring/floater interaction • The Atkins system only to be applied in this case

27. 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