|dc.description.abstract||This thesis considers the possibilities of cost reduction of the mooring system for a
floating semi-submersible wind turbine by inclusion of fibre rope instead of, or together
with, chain mooring lines. Polyester fibre rope is cheaper, has lower weight and
better fatigue life performance compared to chain, but has significantly lower abrasion
The mooring system considered is made for the semi-submersible substructure OOStar
Wind Floater, designed by Dr. Techn. Olav Olsen, with a 6MW wind turbine and
for operation at 100 meters water depth. The work comprehended initial modelling of
the substructure and tower as rigid bodies, implementation of the finite element wind
turbinemodel, given by Statoil ASA, and establishment of an initial catenary mooring
system, made of chain. Next, different configurations of taut leg mooring systems with
polyester fibre rope were considered and compared with the initial system. The performance
of the chain system and the fibre rope system were analysed through constant
wind tests, free decay tests, ultimate strength analysis and fatigue life analysis.
Finally, basic cost estimations of both systems were completed.
The chain mooring system consisted of three 730 m long lines of 147 mm studless
chain links. The pretension of the system was 50 tonnes.
The first system analysed with fibre rope was a pure fibre rope mooring system, but
the simulations showed slack in the mooring lines, as well as extensive pitch motions.
Hence, the system was not approved. One clump weight were included on each mooring
line to avoid slack loads. To increase the abrasion resistance, chain segments were
included in the ends of the fibre ropes. Different configurations of systems with chain,
polyester fibre rope and clump weights were analysed. The final fibre rope system
consisted of polyester fibre rope mooring lines with end segments of chain (80 m in
total) and clump weights of 140 tonnes. The total mooring line length was 600 m and
the pretension was 384 tonnes. Polyester fibre rope with minimum break load of 1600 tonnes was used, while the minimum break load for the chain links which were included
was 2160 tonnes.
The simulations of the fibre rope system showed a satisfactory behaviour and an improved
performance with respect to pitch motions relative to the chain system. The
analyses of the 50-year storm condition proved to be most critical for both systems
and the largest responses were found during this condition. In particular, the extreme
condition analyses showed a prominent coupling between surge and pitch in both
systems, but the two systems responded differently to this coupling. This was an interesting
and unexpected finding that should be investigated further. The two systems
had relatively similar behaviour in the other performance aspects that were analysed.
The cost estimations of the components were simplified, but assumed to be correct
relative to the other components. The fibre rope mooring system was dominated by
the certification cost when only considering one turbine. But for a wind park with
several wind turbines, this one-time cost has less relative importance. For a wind park
of 50 wind turbines, the chain system had a total cost of 136 mEUR, while the fibre
rope mooring system cost for 50 wind turbines was 48.7 mEUR. This implies a cost
reduction of 87mEUR, i.e. 64%.
The thesis work has shown that the mooring system with fibre rope, clump weights
and chain segments has a satisfactory behaviour and significantly lower cost. Therefore,
further design and analyses of this type of system is recommended.