Effects of Heaving Plate on the Dynamics of a Novel Floater Concept for FOWT

Floating Offshore Wind Turbine (FOWT) offers a doable solution to harness renewable energy in deep-water regions. The dynamic behavior of FOWTs is significantly influenced by hydrodynamic forces, with added mass playing a critical role in determining rigid motions, natural frequencies and dynamic stability. Here, we examine a novel floater design for a 15 MW floating offshore wind turbine, to be integrated with multipurpose energy harvesting and utilization at sea, which is part of the Floating Energy Archipelago, a breakthrough concept for the Marine Renewable Energy, conceptualized in 2015 at CNR by one of the authors and systematically studied within the Ricerca di Sistema Research Project since 2019. For this novel floater concept, designed by Fincantieri to address the needs for a caisson-type structure, typical of the shipbuilding industry, heaving plates on the pontoons become relevant to improve the dynamic response of the floater. Without them, the natural period of heave and pitch motions is aligned closely with the wave period of the Mediterranean Sea, posing a potential risk to the structure due to increased sea loads, fatigue, and vibrations. This study examines the influence of the added mass, related to the flat plate use, on the natural frequencies of vertical motions of the platform.. Numerical analyses with various flat plate geometries (variable width and thickness) are conducted to examine the changes in added mass and their effects on the natural vibration period for both heave and pitch motions. Five configurations were compared using an in-house numerical code based on potential flow theory and a boundary element method (BEM) to solve the relevant hydrodynamic equations. The code discretizes the structure into panels and solves the boundary value problem for the Laplace equation to address linear wave diffraction and radiation problems. The investigation highlights the beneficial role of heaving plates in increasing the natural period of the vertical motions. Damping effect, induced by the vortex shedding at the edges of the flat plates, not considered within the present analysis, is expected to matter and will be discussed during the conference. Further investigations are necessary to characterize the structural stress induced at the root of the plates.