Transient Response of a TLP-type Floating Offshore Wind Turbine under Tendon Failure Conditions

A tension-leg platform (TLP)-type floating offshore wind turbine (FOWT) has deteriorated stability and experiences transient responses under tendon failure. This should be considered as part of accidental limit state checks at the design stage. The WindStar TLP FOWT is considered in this investigation of the transient effects of tendon failure on its system responses. Tendon failure is first modeled using the fully coupled simulation tool FAST. Subsequent numerical simulations consider operational and parked conditions, and the response statistics of key design parameters are analyzed. The numerical results show several responses of the FOWT under operational conditions are higher than those under 50-year extreme condition. The design check shows that the remaining tendons have adequate redundancy to withstand the maximum tensions encountered during the transient responses following the failure of one tendon. These findings are relevant for the accidental limit state design of the considered TLP FOWT. INTRODUCTION Currently, the design and manufacture of floating offshore wind turbines (FOWTs) adopt practices and technology employed in the offshore oil and gas industry (Kaldellis et al., 2016). There are three major types of support platforms for FOWTs namely, spar, semisubmersible, and tension-leg platform (TLP) support platforms. A series of numerical and experimental studies of TLP-type FOWTs has demonstrated their excellent hydrodynamic performance (Vijay et al., 2018; Zhao et al., 2018; Hsu et al., 2019). The mooring system keeps support-platform response within a safe operational zone to ensure it maintains a proper position and orientation. A TLP is stabilized by vertical mooring cables called tendons, which are essential to the safety of the whole system. Maritime history and industrial practice show that the mooring system may fail in a wide range of regions and environmental conditions (mild to severe sea states) (Vijayaraghavan, 2018). During its lifecycle, a mooring system may fail as a result of fatigue, accumulated corrosion, accidental collision, or extreme storms (Ma et al., 2013; Gordon et al., 2014; Prislin and Maroju, 2017). In 2005, hundreds of oil-drilling and production platforms in the Gulf of Mexico were hit by hurricanes Katrina and Rita, with some TLPs' tendons failing (Cruz and Krausmann, 2008). After mooring failure, platform position and orientation change substantially, possibly causing the capsize or sink of the fault platform. Additional risks include loose floating structures colliding with other nearby structures.