For current offshore wind farms, monopiles are by far the most popular support structure type. However, for deeper water and/or larger turbines, the fatigue loading is becoming critical and the monopile dimensions are exceeding the current economical feasibility. Therefore the paper focuses on an integrated optimization process for a 5MW offshore wind turbine design on a monopile. The chosen site with 25m water depth is considered to be challenging for such a large and heavy turbine type. The approach presented in this paper is to integrate an optimization for load mitigation in the design process of offshore support structures. A reference design of the support structure is made following a conventional design approach and using data from measurements at a site in the Dutch sector of the North Sea. The focus is on the reduction of the dominant hydrodynamic loads on the support structure. The implemented load mitigation concept leads to significant reductions in loading, allowing considerable material savings and therefore a more cost-effective structural design. Undesired side effects, such as increased wear of turbine components are unlikely as other system loadings and characteristics remain within an acceptable range. Even if some of the rotor-nacelle-assembly loads are slightly increased by the applied controller, the increases are low and probably still within the margins of the type-class fatigue loads. Furthermore a significant increase in energy output could be obtained by applying an extended cut-out range.
The UpWind project looks towards wind power of tomorrow and towards the design of very large turbines in wind farms of several hundred MW. The work presented here is part of WP4, which deals about offshore foundations and support structures. The primary objective of WP4 is to develop innovative, cost-efficient wind turbine support structures to enable the large-scale implementation of offshore wind farms.