Numerical Study on Wave Drift Loading on Slender Marine Structures
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- Institutt for marin teknikk 
This master thesis investigates the wave drift forces acting on slender structures through numerical simulations. Basic theory is presented along with an overview of how the problem is treated in the industry and a description of the most commonly used methods. The numerical study of how the wave drift forces act on a 2D geometry is performed using the computational fluid dynamics (CFD) program OpenFOAM. The effect of viscosity is also investigated. This is an in-depth study focusing on the methodology behind the problem and presents the first step of the comprehensive study of wave drift forces on semi-submersible platforms. The study is a continuation of the preliminary project performed from January to March 2017. The recent mooring line failures in the North Sea occurring at lower loads than the design loads indicate the need for an investigation of the existing load prediction methods. A full understanding of the problem is not yet obtained, even though many studies have been carried out during the recent years. Several studies suggest that the wave drift forces are higher than initially anticipated, especially the viscous contribution. Wave drift forces are of higher order and can be difficult to calculate accurately. Therfore, simplified potential flow theory is often applied together with common viscous theory. However, these methods have shown to under-predict the drift forces. The viscous model is often based on the drag term of Morison s equation. The drag coefficient is case dependent and difficult to predict accurately without the use of model tests. Predictions of wave drift forces using model tests, CFD analysis and investigation of empirical formulas are still an active research field with the purpose of understanding the underlying physics properly. The analysis using OpenFOAM is performed for a 2D column of quadratic cross section mimicking one of the columns of a semi-submersible platform. The column is fixed in space, meaning that the diffraction problem is investigated. The column is subjected to both regular and bi-chromatic waves with wave height 4.5, 6 and 7 m. The wave conditions have been chosen based on parameters presented in relevant literature. The ratio wave length to body size is such that the wave classifies as long wave. The waves are generated in two ways; by a numerical wavemaker and by using the built-in feature of the toolbox waves2Foam, both based on first order wave theory. A brief convergence study is performed in order to investigate the correct time step to be used. The choices made during the modelling and simulations are discussed along with the presented results. Selected inviscid cases have also been simulated in order to investigate the effect of viscosity. The presented results show that the expected dominating frequencies are present in the spectra of the forces and wave elevation. For the bi-chromatic wave cases, the mean drift force in surge is proportional to more than the wave amplitude squared. The slow drift forces are also clearly increasing for higher wave amplitudes. Furthermore, the results show that the viscous contribution to the wave drift loads is small and that non-viscous flow separation is present at the sharp corners at the bottom of the column. The grid resolution might have been too low in order to fully capture the viscous effects, and a finer mesh could have been applied in order to investigate this further. Future investigation of this problem would benefit from building up a 3D case to study the effects and forces in 3D, the element size should be reduced further and a larger area below the bottom of the column should contain a refined grid.