Comparison of Potential Flow and CFD for a Column With Heave Plate

Abstract

Floating offshore wind turbines can only withstand a limited amount of (heave) motions before the equipment fails. In order to reduce the heave motion, the DeepCwind floater for offshore floating wind turbines makes use of heave plates. This semi-submersible floater consists of three cylindrical columns with a heave plate attached to the bottom of each column. Potential flow models are often used in order to assess the response. However, potential flow theory does not take into account the viscosity and the vorticity of the fluid. Therefore, this thesis examines the effect of a heave plate on a cylindrical column's response in heave direction and subjected to wave loads with both a potential flow model and a fully nonlinear numerical wave tank. Specifically, the difference between a potential flow model and a fully nonlinear numerical wave tank is examined.

The simulations with the fully nonlinear numerical wave tank have been carried out using the open source computational fluid dynamics (CFD) software package OpenFOAM (version 1606+). An unresolved direct numerical simulation (DNS) approach is used throughout this work. Best practices for the dimensions of the wave tank, the mesh and settings of the solver where obtained from Bruinsma (2016) and Rivera-Arreba (2017). The OpenFOAM waves2Foam toolbox (developed by Jacobsen et al. (2012)) has been used to generate waves in the wave tank. The two phase solver interDyMFoam for moving bodies was coupled to the waveFoam solver from the waves2Foam toolbox in order to simulate a moving body under wave loads. The simulations in OpenFoam were carried out on a 1:50 scale. The potential flow model WAMIT has been used in order to obtain the response amplitude operator (RAO), added mass, damping and wave excitation forces from potential flow theory.

A single cylindrical column has been tested in the numerical wave tank both with and without heave plate. Firstly, a heave decay test has been carried out. As a result, the linear damping ratio and the linear and quadratic damping coefficients have been determined. Secondly, the structure was exposed to incoming waves. The response of the structure has been assessed under three different wave periods, which were selected in order to align with Rivera-Arebba (2017). The response of the structure was measured, filtered on the frequency of the incoming wave and compared with the RAO from the potential flow model. Also, the wave excitation forces of the potential flow model have been compared with wave loads from the numerical wave tank, based on simulations where heave motion of the structure was constrained.

It was found that both the wave excitation forces and the RAO of the potential flow model are in agreement with the CFD model results. The viscous effects included in the CFD model affect the response of the structure only very lightly. The largest differences between the potential flow and CFD model were found around the heave cancellation wave frequency. At the heave natural period of the structure, the heave plate increases the linear damping coefficient with ca. 50%. The damping at this period was dominated by viscous effects. In general, the potential flow model produces an accurate RAO, due to the fact that the system is lightly damped and the damping therefore plays a minor role in the structure's response.

The outcome of this work contributes to the understanding of the effects of heave plates in general and can be used to assess the added value of computational expensive CFD software in the design process of floating wind systems.