Computations of Wave-Current Interaction and their Effects on Floating Offshore Wind Turbines

This study investigates the phenomenon of wave-current interaction and its impact on Floating Offshore Wind Turbine (FOWT) platforms by use of the computational fluid dynamics approach. The governing equations for a laminar flow model are solved by use of the finite volume approach in an open-source computational fluid dynamics package, namely OpenFOAM. Modifications are made to an existing wave-making toolbox, waves2Foam, to generate combined nonlinear waves and currents in the two-dimensional computational domain. Various wave conditions are considered by systematically changing the wave height and the wavelength. Several current profiles are studied as polynomial functions of water depth following the profiles and magnitudes of the available ocean current data. First, the interaction of nonlinear waves with currents in deep and finite water depths is studied. Overall, it is observed that opposing current has a remarkable impact on the wave field, and the particle velocity and wave height are affected the most from the presence of the current. Following currents are found to generally increase the parameter under observation and opposing currents reduce them. This is followed by a study of the effect of various forms of currents on regular nonlinear waves in shallow water. It is found that presence of the current can alter the wave field significantly, and the current profile and direction play a significant role in the change of the wave field. A following current in shallow water increases the peak of surface elevation, horizontal particle velocity and pressure, along with an increase in wavelength and wave height, while an opposing current reduces these. Differences observed between deep and shallow water wave-current interactions leads to the study where two nonlinear waves, one in deep water and one in shallow water, interact with the same set of currents. It is seen that in shallow water conditions wave height increases with following current and decreases with opposing current, while reverse is observed in case of deep water conditions. Following currents increase the wavelength and opposing currents decrease it in both shallow and deep water conditions, however, currents have a stronger influence on wave length in deep water conditions. Effect of wave-current field on a barge platform housing a two-dimensional 5-MWNREL turbine is investigated computationally. It is found that following currents have a stronger influence on the structure than opposing currents. Loads on and responses of the structure increases with larger current speeds while effect of current profile is found to be negligible. Presence of tower has a strong influence on the behaviour of the structure in the cases investigated in this study, causing a reduction in surge and pitch responses. Lastly, an assessment of wave-structure interaction and wave-current-structure interaction in a three-dimensional computational domain is carried out. Comparative assessment of the surface elevation in the domain with and without current indicates that the presence of the structure likely generates reflections which modify the incident wave at certain locations in the 3D domain more significantly than other locations. Study of forces on the structure shows that horizontal force has a larger magnitude than vertical force and the presence of currents has an increasing effect on the horizontal force.