Motions and Loads on a Moored Floating Ocean Structure in Large Waves and Strong Currents

Abstract All structures in offshore areas are susceptible to a range of environmental loadings and motions due to the presence of very large waves and strong currents that can have substantial impacts on their operational activities. Green renewable energy sources are critical to the fight against global warming and the reduction of greenhouse gas emissions. A consistent wind supply is always an important element for carbon-free energy production utilizing a wind turbine, both onshore and offshore. Offshore wind is generally more reliable and effective than onshore wind because of the lack of onshore impediments. Therefore, an efficient way to harness clean energy is through offshore wind farms. A moored carrier structure on which a wind turbine is generally installed may encounter high waves and strong currents and its interaction with local hydrodynamic elements may create hazardous operating conditions. This study investigates the motions of a moored floating structure, as well as the tensions in the mooring lines caused by large waves, strong currents, and their interactions. A custom-developed tool with 3D mass, momentum, and energy equations was used for numerical simulations of the flow field in the presence or the absence of current. The results obtained from the above 3D model for various cases were used in the commercial software OrcaFlex™ which uses lumped mass elements to simplify calculations, to evaluate structural motions and loadings. To investigate various situations, the incidence wave and current conditions were systematically altered in the simulations, while the shape of the structure, mooring lines, airgap, and still water depth remained constant. Various combinations of waves, currents, and incident angles are utilized for a better understanding of the motions and tensions. In the simulations, a moored floating structure having 4 legs and 4 cross bars is attached with 4 catenary types mooring lines by one end at the legs’ underside while other ends are fastened at the sea bottom.