Session: Hydrodynamic and Coupled Analyses II

Paper Number: 98001

98001 - Numerical Analysis of the Semi-Submersible Using Absorbing Boundary Conditions 

Floating Offshore Wind Turbines (FOWTs) are a promising technology to harness wind energy in deepwater offshore regions. One of the critical design aspects is the accurate prediction of hydrodynamic loading on the substructures, leading to the precise estimation of the Fatigue Limit State(FLS) and Ultimate Limit State(ULS) ensuring a safe design of the floater. While engineering tools can sufficiently predict these load states for the preliminary design stage, design optimisation requires accurate values. Moreover, engineering tools are limited to linear or weakly non-linear analysis, thereby failing to capture non-linear hydrodynamic effects predominant in extreme sea states. Additional studies have revealed that accuracy can be improved by using computational fluid dynamics(CFD). However, Navier-Stokes based solvers are computationally expensive. A method to reduce the computational cost is outlined in this work.

The fluid flow is represented by a two-phase incompressible Navier-Stokes solver, and the air-water interface is tracked by the Volume of Fluid (VOF) method. For an accurate representation of the wave system near the structure, spurious wave reflections from the boundaries of the computational domain need to be prevented. A common approach is to impose relaxation zones or numerical damping zones(in the order of multiple wavelengths) to dissipate wave energy and prevent reflections effectively. The strength of the relaxation-zone technique is that the amount of reflection can be easily reduced by applying more extended zones; however, this comes at the cost of an increase in computational times due to (i) an increase in the total number of computational cells and (ii) the evaluation of the target solution in a larger relaxation zone. While the former argument is mentioned frequently in the literature, the latter is rarely mentioned. It nevertheless is a severe drawback of the use of relaxation zones in engineering applications involving complex, irregular incident wave spectra. The target solution for irregular waves is typically the linear superposition of many wave components, so the larger the relaxation zone, the more computational time it takes to evaluate the linear superposition of wave harmonics. Keeping the computational domain size as small as possible would significantly improve the numerical efficiency for extreme wave cases. Hence, the absorbing boundary condition (ABC) developed by the waves2Foam toolbox is tested for this work.
 

In the current research, the simulations of the “DeepCwind” semi-submersible subjected to regular waves are performed in a CFD toolbox, OpenFOAM extended with waves2Foam package for wave generation and wave absorption. The computational grid is created using the snappyHexMesh utility in OpenFOAM. The effect of ABC applied to the numerical simulation of the semi-submersible is compared with the simulations performed with relaxation zones. Further, the performance of the ABS is investigated by comparing the free surface at a specific position in the simulations with different domains (one with a long domain and the other with a significantly small domain size). The computational effectiveness of ABC against the relaxation zone method is established. Furthermore, optimal domain size with absorbing boundary conditions is deduced in this research. The future work encompasses studying the effect of ABC in irregular sea states.

Presenting Author: Likhitha Ramesh Reddy Delft University of Technology