Dynamic response of 15 MW floating wind turbine with non-redundant and redundant mooring systems under extreme and accidental conditions

This work focuses on examining the dynamic behaviour of large floating offshore wind turbine (FOWT) exposed to extreme loading and accidental loading. The FOWT studied in this work is the 15 MW reference turbine recently released by the International Energy Agency. The 15 MW turbine is supported on the UMaine Volturn US-S semi-submersible platform which is stationed using catenary mooring lines. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (3-line) and redundant (6-line) systems are studied and compared. The coupled multi-body dynamic system is solved using an open-source code, OpenFAST. When simulating the mooring line failure, both the operating and extreme loading conditions are considered. Failure of one mooring is considered at a time. The response of the coupled system due to breakage of the mooring indicate high displacements in surge and sway directions in comparison to the intact system especially for the nonredundant mooring system. Furthermore, failure in mooring leads to change in the platform yaw angle, which in turn results in rotor misalignment with respect to the incoming wind. Also increased tension in the other intact mooring lines is observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large wind turbines mounted on semi-submersible platforms. In addition, insights on using non-redundant and redundant mooring configuration for such large structures with respect to extreme and accidental loading is also discussed.