Abstract
In a severe sea state, nonlinear wave loads can excite resonant responses of floating wind turbines either at high (structural) or low (rigid body motions) natural frequencies. In the present work, a computational fluid dynamics (CFD) model and an engineering model based on potential-flow theory with Morison-type drag are developed to investigate nonlinear wave loads on a stationary, rigid semi-submersible wind turbine under regular and irregular waves. The numerical results are validated against experimental measurements. A trimmed floater is modelled to examine the change in nonlinear wave loads due to the mean pitch angle which occurs during operation of a floating wind turbine. Furthermore, wave loads on each column are investigated numerically. Compared to the experimental measurements, the CFD model gives better estimations than the engineering model for the first, second and third order wave diffraction loads. The engineering model based on the first- and second-order potential-flow theory has large discrepancies in the phase of high order wave diffraction loads and underpredicts the amplitude of low-frequency wave loads. In the CFD simulations for the studied wave period (12.1 s), the second and third harmonic surge forces on the starboard columns are significantly larger than those on the upstream column, while first harmonic results are consistent with potential flow. The trim angle (5°) results in an increasing surge force and pitch moment but a decreasing heave force.
Highlights
There has been a huge increase in the use of wind turbines for generating electricity
Nonlinear wave diffraction loads on a semisubmersible floating wind turbine (FWT) are studied in detail, and all the results are shown at full scale
Nonlinear diffraction wave loads on a semi-submersible FWT are studied using two numerical approaches based on computational fluid dynamics (CFD) and potential flow theory as well as experimental measurements
Summary
There has been a huge increase in the use of wind turbines for generating electricity. In deep water, the costs of bottom-fixed foundations rise sharply, so a wide variety of floating wind turbine (FWT) concepts have been proposed, such as spar, semisubmersible and tension leg platforms (TLP). FWTs may be exposed to harsh environments and steep waves which induce highly nonlinear wave loads on the floater of FWTs. The high-frequency loads can cause springing and ringing, while the low-frequency loads can lead to the resonance in surge, sway and yaw of a moored platform. Mercier et al [1] showed the importance of high order wave loads on TLPs through experiments. Coulling et al [2] used experiments and numerical tools to stress the importance of second-order difference-frequency wave forces in capturing the global response of a semi-submersible FWT. As the responses of FWTs are largely affected by nonlinear wave loads, validated modelling tools should be developed to predict these loads more accurately while keeping the computational efficiency at a reasonable level
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