Effects of Second-order Wave Force on Fatigue Damage Assessment of a TLP-type Floating Wind Turbine

Resonant and transient responses of a tension leg platform (TLP)-type floating offshore wind turbine can be excited by high-frequency wave forces. This paper investigates the damaging effect of second-order wave forces on structural fatigue in the WindStar TLP system. Dynamic responses in the time domain are analyzed using the program FAST. To explore the mechanisms of different combinations of wind and wave loads inducing damage, five scenarios are considered: wind only, first-order wave only, second-order wave only, joint wind and first-order wave, and joint wind and second-order wave. The results show the dominant effect of second-order waves in tower structural fatigue damage. Additionally, wind and wave coupling becomes stronger under second-order wave loads. 1. INTRODUCTION A trend in wind farm development is the shifting of turbines from land to offshore deep water, where conditions are better for wind-energy generation. In addition to being plentiful, wind on the ocean is more stable with lower turbulence than that on land. The development of offshore wind turbines therefore represents a strong contribution to clean and renewable electricity generation as well as eliminating the negative visual and acoustic impacts of turbines. With increasing water depth, the economic feasibility of traditional fixed turbines, supported by such as jacket-based and gravity-based structures, dramatically decreases. Therefore, several concepts of floating offshore wind turbine (FOWT) have been proposed. Three representative types, categorized by the method of attaining static stability, are spar-type, semi-submersible type, and tension-leg-platform (TLP) type (Robertson and Jonkman 2011). The TLP type uses a vertically tensioned mooring system. The natural periods of heave, pitch, and roll are restricted to between 3–5 s, while those of surge, sway, and yaw are >20 s, all different to waveenergy frequencies. This ensures that a TLP-type FOWT has small wavefrequency responses, providing a stable foundation for the wind turbine (Suzuki et al. 2011). However, in realistic sea conditions, the resonant and transient responses (springing and ringing) of TLP-type FOWTs can be excited by high-frequency wave forces, especially during storms (Zou 1998). Severe sea states have more wave energy at higher and lower frequency ranges than calmer seas due to nonlinear wave effects (Xu et al. 2019). These high-frequency responses can undoubtedly induce greater structural fatigue damage and thus reduce the safety of a TLP-type FOWT.