Investigation of global and local structural response of Semi-submersible FOWT using hydro-structure interaction in the frequency domain

The impact of wave loads on the structural integrity of floating foundation for wind turbine is crucial. However, the structural design standards of hull for floating offshore wind turbine (FOWT) are typically derived from the design specifications of oil and gas platforms, which leads to uneconomical designs and high steel consumption. The economic design of the floater will provide a new approach to the cost reduction of FOWT. Therefore, it is of great importance to better understand the structural response characteristics such as the relationship between internal loads and wave parameters under different wave loading conditions. To achieve this goal, one of the most difficult problem is the interaction between hydrodynamic and structural analysis because the philosophies of these methodologies are completely different. In this study, a typical 5 MW Semi-submersible FOWT is selected, the finite element model is established for the floater. Given the primary emphasis on wave-induced structural response in the frequency domain, the impact of wind and current loads is not considered. Therefore, the tower and rotor nacelle assembly of the wind turbine are simplified as an equivalent concentrated mass point. An implicitly balanced model is proposed, the hydrodynamic pressure based on the 3D diffraction and radiation theory is recalculated at structural points, and different pressure components are separately transferred from the hydrodynamic to the structural model. Global motion response are validated by comparing the results of numerical simulation and a 1:50 Froude scaling model test. Wave-induced global structural response amplitude operator (RAO) and local stress RAO are calculated, the long-term extreme stress analysis based on 2,592 sea-states from a scatter diagram is performed. The mechanism and characteristics of structural response and waves are investigated. Results indicate that the internal loads are significant when the corresponding wavelength satisfies some relations with the geometry dimensions of the Semi-submersible floater, which is credited to the phase difference of hydrodynamic pressure. Stress hot spots appear at the intersection between the floater and tower, column and bracing, and hull around the still water level due to various causes e.g. hydrodynamic pressure, and internal loads. These findings can guide the engineering design and optimization of the Semi-submersible floater.