A parametric optimization approach for the initial design of FOWT’s substructure and moorings in Brazilian deep-water fields

As Floating Offshore Wind Turbines (FOWT) continue to demonstrate their technical feasibility in different projects, the conceivable scenarios for the application of this technology rapidly expand. Outside the mainstream perspective of floating farms connected to onshore grids, several projects currently study the technical and economic feasibility of adopting FOWTs to supply part of the electrical power required by the offshore production of oil and gas. Having this goal as a main target, the present work addresses initial results obtained in a comprehensive research project that aims to prospect potential uses for FOWT technology in the context of offshore oil and gas production in the Brazilian oil fields. In this context, a parametrical optimization procedure has been adopted to provide suitable candidates for the floating substructures and their mooring systems. The optimization framework was designed for automatic generation of geometrical concepts already considering the equilibrium involving hull weight, ballast and the tensions applied by the mooring lines. The latter are modeled in hybrid configuration, composed of sections of steel chains and polyester rope, considering both catenary and taut-leg configurations. The main goal is to minimize CAPEX costs but, knowing that the acceleration levels in the rotor hub are inevitably linked to indirect costs that are hardly predictable at this stage, a multi-objective approach is preferred, taking the minimization of the nacelle acceleration as one of the drives. Moreover, for a better resolution of the site-specific design, the optimization is done not only for a few pre-selected metocean conditions, but considering a long-term series of the simultaneous action of seas, swell waves, currents and winds. In this paper, two illustrative concepts with different floater geometries are presented, both of them optimized for a water depth of 600m. A discussion on the mooring configurations is made and the rationale involved in the optimization procedure that leads to the final floater dimensions is investigated by connecting the floater motion responses to the wave characteristics in the chosen field.