Semi-submersible floating platforms are designed to support wind-energy generation in deep water. Movable water ballast can be deployed to minimise mean pitch due to turbine thrust, but there is also potential to use a finer-tuned redistribution of water to reduce dynamic pitch, improving human workability and reducing fluctuating structural stresses. In this paper a linearised mathematical formulation for coupled platform and water-ballast dynamics with wave and wind forcing is derived, with water redistributed between outer columns by pitch-dependent pumping. Results for the VolturnUS-S 15 MW platform without wind reveal a strong dependence on forcing frequency. In particular it is found that: a passive (unpumped) system could only provide benefit if forcing frequencies lie between platform and water-column natural frequencies, whereas typical wave frequencies are higher, and the fixed turbine moment lower, than either; a “no-effect” frequency exists where rate of change of angular momentum exactly balances the turning moment from displaced water ballast; a suitably optimised pumping system can provide a modest reduction in pitch for longer period waves. When combined with turbine load in a full platform model, including wave excitation and radiation and mooring forces, the pumping system is effective at bringing mean pitch to zero and providing beneficial dynamic pitch performance for longer-period wave spectra. Peak pump power is modest, whilst average power is near zero, the system extracting energy from the flow for part of the time. Hence, a combined pumping/energy-recovery system is recommended.
Apsley et al. (Mon,) studied this question.