Support structures are one of the main cost drivers offshore, and since other offshore rules apply with regards to design and site conditions, different methodologies have to be developed to mitigate the loads on the support structure and therefore to reduce the associated component costs. Detailed onshore studies and field tests have already shown that advanced control algorithms can be an effective way to reduce fatigue and extreme loading on a wind turbine. Still, the overall cost‐effectiveness for large offshore wind farms has not been studied in detail yet. In the scope of this paper, studies of new controller concepts are integrated within the design process of offshore support structures in order to stretch the applicability of monopiles to larger water depths or simply to reduce the structural weight and associated costs. A reference design of the support structure is made for a site in the Dutch North Sea. The focus is on reducing the dominant hydrodynamic excitation 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 design. The presented approach allows material reductions of more than 9% for the studied support structure. 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. Copyright © 2012 John Wiley & Sons, Ltd.

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. Especially in cases of misaligned wind and waves, the side-to-side fatigue loading at the support structure can become a design driver. Since the industry aims to use standardized rotor-nacelle-assembly (RNA) designs, the goal of this paper is to show the effectiveness of using the RNA as adjustable controller device for site-specific offshore support structure configurations by using active control devices.

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Abstract. An in-depth study has been completed to study the effects of slender, flexible blades in combination with high rotor speed operation on load mitigation, targeted at cost reductions of the structural components of large wind turbines, consequently lowering the levelized cost of energy. An overview of existing theory of sensitivity of turbine fatigue loading to the blade chord and rotor speed was created, and this was supplemented by a proposed theory for aboverated operation including the pitch controller. A baseline jacket-supported offshore turbine (7MW) was defined, of which the blade was then redesigned to be more slender and flexible, at the same time increasing rotor speed. The blade redesign and optimisation process was guided by cost of energy assessments using a reduced loadset. Thereafter, a full loadset conform IEC61400-3 was calculated for both turbines. The expected support structure load reductions were affirmed, and it was shown that reductions of up to 18.5 % are possible for critical load components. Cost modelling indicated that turbine and support structure CapEx could be reduced by 6%. Despite an energy production reduction of 0.44% related to the thicker airfoils used, the blade redesign led to a reduction in Cost of Energy. IntroductionA key challenge to improve the economics of offshore wind energy -and thereby sustained public and political support for large programs -is lowering the cost of the structural components of large wind turbines and their support structures. The hypothesis is that increasing design rotor speeds in combination with significant reductions to blade chord lengths and blade stiffness can mitigate loading on these components, hence reducing their cost and wind farm cost of energy. The authors have completed an in-depth assessment to these effects, the results of which are presented here.The study was conducted throughout 2013 in the context of the DNV GL Garrad Hassan FORCE project (FOR Reduced Cost of Energy), a research project aimed at fully integrated design of large offshore wind turbines. This FORCE project consisted of two phases: phase I, where the use of advanced control strategies was demonstrated, and phase II, which focused on the blade redesign and is discussed in this paper. This research aims to compliment other studies in the field of integrated turbine design [3] [4].The research discussed in this paper uses two approaches. The first approach, described in section 2, is a theoretical approach; an investigation to the scaling relations that describe the dependence of turbine fatigue loading to changes in design rotor speed and blade chord lengths. The second approach, described in section 3, is a full design study comparing a high speed, slender rotor turbine to a baseline turbine, using aero-hydro-elastic time history based load calculations and assessment of cost effects using cost models.