Dynamic response analysis of floating offshore wind turbines with failed moorings

Abstract

Offshore wind energy is turning to a mainstream energy source due to the advantages of low emission, high efficiency and abundant reserves. The deployment of offshore wind turbines has accelerated during the past decade in the globe. Currently, most of the commercialized offshore wind farms are installed at the shallow water area using the fixed bottom support structure. However, wind energy in the ocean at water depths deeper than 40 m is estimated to account for 81% of total offshore wind electricity generation potential. To harvest the resource over the deep water, floating offshore wind turbine (FOWT) technologies are considered as a more cost-effective alternative compared to the fixed-bottom wind turbines such as monopiles or jacket type designs. At present, the FOWT technologies have been brought from the stage of concept proof toward pre-commercial phase. To improve the reliability and confidence of FOWT technologies and achieve a more widespread utilisation of floating wind farm, it is urgent to enhance the understanding of loads from the complicated ocean environment and the resulting dynamic responses of the coupled system.

This study aims to provide an insight to dynamic characteristics of FOWT system by means of coupled numerical analysis and physical model experiments with special focus on the effect of mooring failure. Two case studies were carried out focusing on two different floating platform configurations. In Case Study I, the dynamic responses of a Hywind spar FOWT were investigated by employing the coupled aero-hydro-servoelastic- mooring simulation tool, FAST. The source code of the program was modified to achieve the disconnection of mooring line at specific position and point of time. A set of simulations were conducted based on the modified numerical model. The results indicate that the breakage of one mooring line can lead to a long drift of the platform and cause a significant transient deflection at the moment of the accident. A mitigation strategy was proposed to reduce the large transient pitch response, which was then proved to be effective in various environmental conditions.

In Case Study II, a novel multi-column tension leg platform (TLP) was developed aiming at a water depth of 60 m to support the same 5-MW wind turbine as used in Case Study I. The dynamic characteristics of the FOWT were evaluated by means of numerical simulation and experiments. The modified FAST program was utilised to establish the numerical model of the TLP FOWT, and the prototype was scaled based on the Froude law and tested in the wave basin of the State Key Laboratory of Coastal and Offshore Engineering at Dalian University of Technology. Simulations and laboratory tests were conducted for various environmental conditions with intact and broken tendons. Good agreements in results between the numerical simulations and laboratory tests for the dynamic characteristics of the two models were achieved. The results show that the performance of the newly designed multi-column TLP satisfy the design criteria, thus providing a possible alternative for the deployment of FOWT in the intermediate water depth other than the most commonly used semi-submersible configurations. Compared to the result of spar presented in Case Study I, the effect of tendon failure on the horizontal motion (surge) of the TLP in Case Study II was found to be insignificant. The most remarkable impact was observed in the increase of tension force in the tendon adjacent to the broken one. The safety factors of the corresponding tendons were therefore checked and found to satisfy the design standard in this case.

To sum up, this research aimed to investigate the dynamic responses of two types of FOWT using the numerical and experimental approaches with special focus on the effects of mooring/tendon failure. The results presented in this thesis have contributed to newer knowledge in the understanding of the coupled analysis of spar and TLP FOWT and the impact of mooring/tendon failure. The research methodologies proposed, and the results obtained through this research can potentially help address the technical challenges of FOWTs.

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