dc.description.abstract | Hywind Demo is the world s first full scale floating offshore wind turbine, developed and
operated by Statoil ASA. Throughout this thesis, a time domain model of Hywind Demo is
used to design an optimized mooring system which can reduce the overall cost of floating
wind turbines.
This thesis describes different mooring system design ideas, consisting of commonly used
configurations such as catenary and taut mooring, and a less common configuration which
uses buoyancy elements to increase the length of a fibre rope and increase flexibility without increasing the footprint.
Rules and regulations that govern the design of the mooring system for a floating offshore
wind turbine are described to their full extent with respect to ULS, ALS and FLS.
In accordance with the rules and regulations, two environmental conditions were established.
These correspond to rated wind velocity for the wind turbine and 50-year storm.
For these two conditions the significant wave height and peak period; current velocity; and
wind velocity were determined.
A time domain model of Hywind Demo with catenary mooring is established in SIMORIFLEX
through SIMA, this model is compared against a correlating frequency domain
model in MIMOSA. The results show that the SIMO-RIFLEX model and MIMOSA model
are in correspondence, and that the MIMOSA model yield more conservative results.
With the aim of maintaining the station keeping properties of the catenary system, several
fibre mooring designs were investigated, including plain taut mooring; fibre mooring with
clump weights; and fibre mooring with buoyancy elements. It was found that by combining
the elasticity of fibre ropes of 1000 m with the geometric flexibility added by a buoy with
30 t net buoyancy, the same restoring properties as the catenary system was achieved.
Environmental response analyses on the fibre mooring system was conducted, verifying
it s applicability and compliance with the governing rules and regulations.
The proposed fibre mooring system results in a cost reduction of 70% and a weight reduction
of 60% for the mooring system, while maintaining similar floater motion responses,
mooring utilization factors and footprint as the original, catenary mooring system.
A brief study on the use of fibre mooring in 130 m water depth was performed with regard
to Statoil ASA s proposed relocation of Hywind Demo. The study yielded positive results,
and the utilization of the fibre mooring system in shallow water is seen to be feasible,
though further investigation should be performed. | en |