Numerical and experimental investigation of novel foundation systems for offshore wind turbines

Faizi, Koohyar (2020). Numerical and experimental investigation of novel foundation systems for offshore wind turbines. University of Birmingham. Ph.D.

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Abstract

The next generation of offshore wind turbines (OWTs) greatly depend on the development of reliable foundations which will enable the utilisation of generators with larger capacity at greater water depths.

Traditionally, pile foundations have been used to support superstructures in the offshore wind industry. However, recently, suction caissons are being increasingly considered as alternative foundations for supporting offshore renewable structures. The arrangement options for these suction caisson foundations could be a monopod, tripod or quadropod. In general, caisson foundations for offshore wind turbines are subjected to combined loadings of lateral, vertical and overturning moment. The most unfavourable loading condition results in a large overturning moment for monopods, whereas the structural design approach for a tripod must take into account the fact that the most unfavourable conditions involve the possibility of tensile loads in the caissons induced by the overall overturning moment. To guarantee the normal operation of offshore wind turbines (OWTs), the foundations of OWTs are required to resist significant lateral loads and overturning moments generated by wind and currents.

This research presents an innovative type of suction caisson, a "winged suction caisson", as a monopod foundation for offshore wind turbines, which has the ability to provide a larger overturning capacity compared with standard suction caisson designs. In order to assess the behaviour of the winged caissons, a series of laboratory works was conducted under 1-g and centrifuge conditions. The experimental campaign was complemented by detailed numerical studies employing finite element analyses (FEA). The short-term cyclic performance of a winged caisson foundation, installed in sand, was also investigated using a series of small-scale laboratory tests under 1-g condition. Different models with various wing sizes and different soil densities were tested in the laboratory under an overturning loading and the results were compared with a conventional suction caisson. The moment-rotation performance of the foundation under both monotonic and cyclic loading were examined to assess the potential benefits of adding wings to suction caisson foundations. The results showed that there is a significant increase (up to 75%) in overturning capacity provided by the novel foundation, demonstrating its great potential over standard suction caissons for their use in offshore wind turbine foundations.

It is known that mono-pod caissons, have a limited maximum capacity which prohibits their use in very large foundations particularly when lateral loading governs the design. Multi-pod suction bucket foundations are rapidly expanding as a foundation system for OWTs, therefore, this research has proposed a novel capacity improvement system for a tripod arrangement of suction caissons.

Tripod suction bucket foundations have the potential to increase the bearing capacity and overturning resistance of the foundation for offshore wind turbines. However, existing tripod suction bucket foundations, as utilised for offshore wind turbines, are required to resist significant lateral loads and overturning moments generated by wind and currents with the most optimized foundation dimensions. This research presents an innovative type of tripod bucket foundation, a ‘hybrid tripod bucket foundation’, for foundations of offshore wind turbines, which has the ability to provide a larger overturning capacity compared with conventional tripod buckets. The proposed foundation consists of a conventional tripod bucket foundation combined with three large circular mats attached to each bucket. Several numerical models of varying geometries were validated with very good agreement against the conducted laboratory tests. The results of experimental and numerical studies performed on the proposed hybrid tripod bucket foundations installed in loose sand, and subjected to overturning moments, are discussed. The experiments were conducted on small-scale models under 1-g conditions in sand. Different circular mat diameter sizes with various bucket spacing under an overturning loading were considered and the results were compared with a conventional tripod bucket foundation. The results showed that there is a significant increase in overturning capacity provided by the novel foundation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):