Optimising the Design of a Steel Substructure for Offshore Wind Turbines in Deeper Waters

Abstract

In the need for more green energy a prominent role is reserved for wind energy. Offshore wind energy in deeper waters capitalises on more efficient wind properties and increased public acceptance compared to onshore wind energy and wind farms close to shore. In the coming years the offshore wind market is expected to evolve rapidly, especially in the deeper water range of thirty to sixty meter. In a business case preceding to this study as first reference a jacket type substructure was designed for a 6 MW turbine in a water depth of sixty meter. The goal of this thesis is to reduce the cost of this reference design in total use of material and assembly. Also the transportation and installation of the substructure are taken in consideration. First a step back is taken to reconsider the structural concept of the reference design. Several substructure concepts, like tripods and straight-leg jackets, have passed the review and firstly qualitative weighed against primary criteria in a Multi Criteria Analysis and subsequently by FEM based in-place analysis. The outcome of the total substructure weight and natural frequency with respect to frequency of wave loading and turbine excitement determined the decision to further investigate a three-leg and four-leg battered jacket. Thereto a fatigue analysis was performed. The calculation method used at the original reference design to determine the total fatigue damage due to turbine and wave loading was proven to be too optimistic and therefore modified. In relation to the reference design several optimisations have been proposed, including applying a horizontal brace just above mudline level, applying double sided butt welds and adopting K-bracing instead of X-bracing. Here the four enclosed pictures can be placed. (number 1 upmost left, number 4 upmost right) Finally, four designs have been worked out; the reference design (without optimisations), an optimised four-leg jacket, four-leg jacket with k-braces and a three-leg jacket. The total assembling cost of each design is calculated by considering the handling time and the welding volume with corresponding welding time of each weld. Together with the material use the total fabrication cost is assessed. The jacket shall be transported offshore by a standard North Sea barge. The dimensions of this barge potentially enable the transportation of three four-leg jackets and four three-leg jackets. Depending on the wind farm location this may lead to reduction of one tug and transport barge case of the three-leg jacket. Further consequence of the three-leg jacket is that a foundation pile less needs to be driven. Thereto is the installation time of the three-leg jacket reduced, resulting in less installation cost. By combining fabrication, transport and installation cost it is possible to compute an overview for substructures cost in a complete wind farm. Final conclusion is that the fabrication cost are decisive compared to the installation and transport cost. The four-leg jacket with K-braces turns out to be the most inexpensive design, respectively followed by the thee-leg jacket, the optimised four-leg jacket and the reference design. It is expected that the four-leg jacket with K-braces brings total cost reduction of approximately nine percent compared to the reference design.