Bushra Rasheed, Aaqib Gulzar Khan, Hoppe Marine GmbH, Hamburg
Offshore wind-turbines have emerged as a promising solution for harvesting energy from oceans. Wind-turbines are usually supported by either a monopile or jacket structures in shallow waters. In deep waters, floating semi-submersible platforms are the most common type for anchoring offshore wind turbines. All floating semi-submersible platforms are posed to a threat of excessive motions in roll (rotation about platform’s longitudinal axis), pitch (rotation about transversal axis), and heave (vertical motion). Out of these three critical motions, roll and pitch motion can lead to increased fatigue loads on turbine tower and blades, hence compromising their structural integrity. Hoppe is investigating a solution to dampen excessive roll and pitch motions of floating platforms, in the form of U-shaped and box-shaped roll damping tanks, carefully designed for roll/pitch periods. Roll damping tanks operate by generating controlled sloshing motion within their reservoirs. The sloshing motion is precisely tuned to resonate at frequencies that enable the mass of the fluid to counterbalance roll/pitch motion of the platform. To investigate these tuned frequencies, multiple experiments need to be conducted across a range of frequencies. This leads to a laborious analysis for evaluating damping moments through experiments. Utilizing Computational Fluid Dynamics (CFD) can serve as a viable alternative to efficiently obtain damping moments across a broad frequency range. As a first step, roll damping coefficients are calculated from RANSE-CFD simulation in OpenFoam and moment/phase curves are obtained. A parametric model is built so that the dimensions of the tank can be optimized according to a given platform geometry and load distribution. In the final stage, sea-keeping analysis is performed using NEMOH, an open-source tool pioneered by Ecole Centrale de Nantes and the results are validated against pre-existing data. The principal axes of tank are identified in sea-keeping analysis, which helps analyze the tank’s response to external moments induced by waves. The motion of the platform with and without the use of roll damping tanks is investigated and the effectiveness of the damping system in reducing roll motions of platforms is evaluated. This study serves as a guideline for evaluating the effectiveness of roll damping tanks in offshore platforms and verify its design using open-source CFD and sea-keeping software.