Dynamic response analysis of floating offshore wind turbines with failed moorings
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Date
19/12/2022Embargo end date
19/12/2023Author
Ren, Yajun
Metadata
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. Restricted Access