Abstract
A comprehensive study has been carried out to understand the system motion and mooring tension response under various environmental conditions for the 15MW offshore reference wind turbine. The publicly available data of IEA 15MW offshore reference wind turbine and the UMaine VolturnUS-S reference semisubmersible platform and its associated chain catenary mooring system are used for this study. The simulations are run in the time domain using OrcaFlex for a coupled aero-servo-hydro model. To simulate the stochastic turbulent wind as observed in the real world, wind fields are generated by TurbSim and imported into OrcaFlex. The simulations account for various environmental conditions which include individual and combined wind, waves, and current. Response statistics are summarized to study the impact of individual environmental component on the system responses. Furthermore, the responses are also inspected in the frequency domain. It can be shown that the responses under a general combined wind, waves, current condition can be decomposed into the responses in three frequency regions (low frequency, wave frequency, and high frequency). Each region is dominated by either a specific (or up to two) environmental component(s) or the resonant responses of the superstructure (tower and turbine). A further investigation also reveals that, in operational conditions, the high-frequency motions from the superstructure could contribute significantly to the tension response of the mooring line. Given the large number of cycles and amplitudes of this response, severe fatigue damage could be accumulated in a short time and leads to premature mooring failure. The underlying physics of this high-frequency tension response is examined, and a possible solution is proposed to mitigate the issue.
