A multi-body approach for distributing first-order hydrodynamic and hydrostatic loads on flexible structures, with application to coupled simulations of large floating wind turbines

Optimized future floating wind turbines (FWTs) are expected to be both larger and relatively lighter than conventional offshore platforms, thus more flexible. The common practice of modeling the platform as a rigid body in coupled dynamic simulations of FWTs can then be questioned. Specifically, natural frequencies of the elastic modes of large flexible platforms can be close to the frequency range of excitation loads. Additionally, platform flexibility can have a significant effect on the natural modes involving significant tower deformation. Considering the platform’s flexibility in coupled simulations of large FWTs requires distributing the hydrodynamic and hydrostatic pressure loads on the flexible model of the platform, instead of the traditional approach of lumping the loads at a single point. This work presents a rational method to evaluate the first-order added mass, radiation damping, and excitation coefficients for a multi-body representation of the platform and develops an energy-conserving distributed formulation for the hydrostatic loads. Assuming small flexible deformations, a decoupled radiation damping matrix is used to model radiation loads for better computational efficiency, while a fully coupled infinite-frequency added mass matrix is used to ensure a stable model. The decoupled radiation coefficients can either be obtained from a single-body or a multi-body diffraction/radiation analysis. A case study of the INO OptiFLEX 22MW semisubmersible FWT is used to illustrate and verify the implementation of the proposed approach. Compared to a baseline model with rigid floater, the results show that introducing platform flexibility significantly affects the high-frequency dynamics of the tower and can also potentially affect mooring line tensions. Moreover, platform flexibility was shown to influence roll and pitch dynamics. These findings highlight the need to model platform flexibility in coupled simulations when analyzing and designing future large FWTs, which can be achieved through the proposed methodology.