EXPRO-CFD: Development and Validation of CFD based Co-simulation of Spar/CALM Buoy Fluid. Structure Interaction. Peter Woodburn1, Paul Gallagher,1 ...
Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25 –30, 2003 Copyright © 2003 by The International Society of Offshore and Polar Engineers ISBN 1 –880653 -60 –5 (Set); ISSN 1098 –6189 (Set)
EXPRO-CFD: Development and Validation of CFD based Co-simulation of Spar/CALM Buoy Fluid Structure Interaction Peter Woodburn1 , Paul Gallagher,1 Pierre Ferrant2 , Jean-Paul Borleteau3 1 Atkins Process, Epsom, UK. Ecole Central de Nantes, France 3 Sirenha, Nantes, France
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KEY WORDS:
Fluid-structure interaction, co-processing, CFD, Spar, viscous effects.
ABSTRACT This paper provides a description of the development and validation of a practical system for simulating the fluid loading and response of floating production systems using computational fluid dynamics coupled to conventional diffraction and hydromechanics models. It is one of a number of papers submitted to ISOPE 2003 by the various participants of the EXPRO-CFD project [1]45][6]. This $3.5m project, part funded by the European Commission, covers the development of fluid structure interaction co-simulation software systems, experimental work in particle image velocimetry (PIV) on a vertical cylinder and a FPSO (Floating, Production, Storage, Offloading) vessel in waves, validation of the CFD, case studies and the development of practical working guidelines.
INTRODUCTION
This particular paper describes a fluid-structure co-processing system based upon the CFX4 CFD code and the AQWA suite of hydromechanics software, the validation of this system using experimental data from the EXPRO-CFD project, and review of working methodologies for practical application for predicting the motions of Spar/CALM buoys.
It could be argued that current general purpose CFD software should meet all of ht ese requirements. However, much of the literature devoted to potential flows, particularly with non-linear free surface shows that there are many subtle effects that need to be understood and dealt with carefully. The simulation of the free surface in general RANSE (Reynolds-averaged Navier Stokes Equation) solvers, particularly using VOF methods, might not be capable of reproducing all of these effects unless specific particular numerical schemes are used.
This paper describes some of the work carried out to date within the EXPRO-CFD project to develop simple and robust Comp utational Fluid Dynamics (CFD) based models for fluid-structure interaction as related to floating offshore systems. The aim of the development work is to provide offshore engineers with a simulation tool for the motion response of floating production systems in waves, subject to all sources of fluid forces, without resorting to empirical correlations for drag, inertia or lifting effects and vortex induced vibration.
We describe the co-simulation system in some detail, including the manner in which the CFD and vessel response models are coupled. In addition, the development of algorithms to improve free surface definition, boundary conditions, and adaptive meshing procedures are described. The performance of these coupled dynamic systems for simple motion extinction and regular wave excitation tests are given.
Similarly, the coupling of a CFD code to the simulation of body motion dynamics requires and understanding of how certain “classical” effects, such as fluid added inertia, wave damping etc, appear within a CFD solution. This understanding is fundamental to the development of a correct form of dynamical equations of motion and their associated time integration scheme.
We also report on the validation of the CFD model through the use of experimental data derived from the test programme carried out at Ecole Centrale de Nantes. The tests consisted of measurements in regular and bi-chromatic waves on a vertical surface piercing circular cylinder. Wave loads, surface pressures, near-field wave elevations and fluid velocities were measured. CFD results for surface pressures, wave elevation and local fluid velocities are compared with the experimental data for key cases of interest.
The overall technical benefits of such a simulation system are considerable, reducing empiricisms and modeling uncertainty to those that can be better understood and managed (indeed they are a key subject in the general CFD literature), and are less application dependent. The potential disadvantages include however factors such as the need for increased technical know-how (in CFD), greater computational times and cost.
The paper provides conclusions on the most appropriate application areas for these systems and current levels of confidence.
In this paper we hope to demonstrate that such systems are capable of reproducing solutions to classical fluid structure interaction problems in offshore engineering. This will then serve as the basis for the ongoing
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work of the EXPRO-CFD project, to further develop these systems as practical tools for the design of offshore floating production platforms.
The system still allows for the modeling of risers and mooring systems using 1D Finite Element approaches, but at present assumes that their hydrodynamic influence is small on the scale of the structure as a whole. Thus a one-way data exchange of fluid velocities, acceleration and pressure gradients is all that is required for given nodal locations. Similarly, items of structure which are considered to be too small to include in the CFD model, but which might contribute to hydrodynamic forces, can be added to the overall model as additional “sub-grid scale” elements. They may contribute to (for example) drag through empirical equations, and their effect on the flow modeled as momentum sinks and turbulence sources.
METHODOLOGY The modeling system and data exchange . The key elements that comprise the coupled computational model are: • • •
A linear or non-linear potential flow based diffraction model An unsteady RANSE model based on 3D finite volume methods Dynamical models for the motions of floating bodies in waves, with attached mooring and riser systems.
All communication takes place via the control box. Codes communicate with the control box rather than with each other to maintain the modular basis of the system.
The intention was to employ generally available software tools since they would bring the benefits of practicality, robustness, existing validation data and known working methodologies.
The main advantages of this approach were seen as: •
A number of possible approaches exist for coupling or co-processing between the various software tools. They range from simple data exchange through live backing files combined with a logical sequence of program run – wait states, to the coupling of software at the object module level and the sharing of common memory space. The latter requires extensive sharing of information between software developers and vendors and was not felt to be appropriate or feasible for this project.
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Free-surface modeling One objective of the project was to investigate the advantages and disadvantages associated with the use of Volume of Fluid (VOF) methods to represent the free surface.
An alternative system design was developed based upon a more flexible computational model in which the individual software tools remain separate, run separately, and communicate through backing files and a system control unit accessed through a graphical user interface (GUI). This was possible largely due to the objective of the project to look only at rigid body motions for the floating systems, and hence the requirement for interaction at the 3D field data level is replaced by simpler integrated loads and overall body motions.
VOF techniques are common where more than one fluid, or a combination of a fluid and a gas, is to be modeled. The volume fraction F (of water in this case) in each grid cell is stored as a scalar variable. If its value is 1, the cell is full, if 0 it is empty, and if 0
