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

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.

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 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.

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 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)

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

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

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

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

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

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 Wave run-up and air gap Extreme wave loads Tether ringing Overlap study involving both Atkins and DNV systems to allow benchmarking

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

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

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