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WindFloat: A oating foundation for offshore wind turbinesDominique Roddier,1,a Christian Cermelli,2 Alexia Aubault,2 and Alla Weinstein11 2

Principle Power, Inc., Seattle, Washington, USA Marine Innovation and Technology, 2610 Marin Ave., Berkeley, California 94708, USA

Received 8 January 2010; accepted 2 May 2010; published online 15 June 2010

This manuscript summarizes the feasibility study conducted for the WindFloat technology. The WindFloat is a three-legged oating foundation for multimegawatt offshore wind turbines. It is designed to accommodate a wind turbine, 5 MW or larger, on one of the columns of the hull with minimal modications to the nacelle and rotor. Potential redesign of the tower and of the turbine control software can be expected. Technologies for oating foundations for offshore wind turbines are evolving. It is agreed by most experts that the offshore wind industry will see a signicant increase in activity in the near future. Fixed offshore turbines are limited in water depth to 30 50 m. Market transition to deeper waters is inevitable, provided that suitable technologies can be developed. Despite the increase in complexity, a oating foundation offers the following distinct advantages: Flexibility in site location; access to superior wind resources further offshore; ability to locate in coastal regions with limited shallow continental shelf; ability to locate further offshore to eliminate visual impacts; an integrated hull, without a need to redesign the transition piece between the tower and the submerged structure for every project; simplied offshore installation procedures. Anchors are signicantly cheaper to install than xed foundations and large diameter towers. This paper focuses rst on the design basis for wind turbine oating foundations and explores the requirements that must be addressed by design teams in this new eld. It shows that the design of the hull for a large wind turbine must draw on the synergies with oil and gas offshore platform technology, while accounting for the different design requirements and functionality of the wind turbine. This paper describes next the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind turbine aerodynamic forces. Three main approaches are presented: The numerical hydrodynamic model of the platform and its mooring system; wave tank testing of a scale model of the platform with simplied aerodynamic simulation of the wind turbine; FAST, an aeroservoelastic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. Finally, this paper focuses on the structural engineering that was performed as part of the feasibility study conducted for qualication of the technology. Specically, the preliminary scantling is described and the strength and fatigue analysis methodologies are explained, focusing on the following aspects: The coupling between the wind turbine and the hull and the interface between the hydrodynamic loading and the structural response. 2010 American Institute of Physics. doi:10.1063/1.3435339I. INTRODUCTION

Currently, there are a number of offshore wind turbine oating foundation concepts in various stages of development. They fall into three main categories: Spars, tension leg platforms TLPs,a

Author to whom correspondence should be addressed. Electronic mail: Tel.: 510200-0530 ext 101. 2, 033104-1 2010 American Institute of Physics



Roddier et al.

J. Renewable Sustainable Energy 2, 033104 2010

and semisubmersible/hybrid systems. A barge-type support structure has been studied1 but is not included in this discussion due to its signicant angular motions that hinder its commercial development. In general terms, spar type has better heave performance than semisubmersibles due to its deep draft and reduced vertical wave-exciting forces, but it has more pitch and roll motions since the water plane area contribution to stability is reduced. TLPs have very good heave and angular motions, but the complexity and cost of the mooring installation, the change in tendon tension due to tidal variations, and the structural frequency coupling between the mast and the mooring system are three major hurdles for such systems. When comparing oater types, wave and wind-induced motions are not the only elements of performance to consider. Economics play a signicant role. It is, therefore, important to carefully study the fabrication, installation, commissioning, and ease of access for maintenance methodologies.2,3 Even though there have been a few visionary papers on the topic of oating wind turbines, signicant research and development efforts only started at the turn of this century.4 In the U.S., researchers from NREL and MIT started a signicant R&D effort5 with the development of coupled hydroaerotools,68 while model test campaigns were performed at Marintek in Norway on a spar hull,9 the rst version of the HyWind spar concept. The use of a semisubmersible hull as a oating foundation was proposed independently by Fulton et al.10 and Zambrano et al.11 The latter papers proposed design was a MiniFloat hull, the predecessor of the presented WindFloat design.12 Over the past few years, academic interest in oating foundations for offshore wind turbines has reached industry, and a signicant amount of funding has been allocated to prototype development. Leading the effort, shown in Fig. 1 from top left to bottom right, are the Statoil NorskHydro Hywind spar, top left, the Blue H TLP recent prototype top right, the SWAY spar/TLP hybrid bottom left, and the Force Technology WindSea semi submersible bottom right. The WindFloat hull is semisubmersible tted with heave plates. Extensive technical qualication of the hull has been performed over the past 5 years by Marine Innovation & Technology. Multiple studies have been performed on the MiniFloatthe trademark of the original hull nameand are published in permanent literature.1315. These include model tests, hydrodynamic and structural studies, along with specic tasks based on oil and gas and other industry requirements. The work described herein is based on the learning from those previous studies. The WindFloat system described in this paper aims at enabling oating offshore wind technology by providing both technical and economical solutions. Its intent is to provide acceptable static and dynamic motions for the operation of large wind turbines while limiting expensive offshore installation and maintenance procedures.1618 The challenges associated with design and operations of oating wind turbines are signicant. A oater supporting a large payload wind turbine and nacelle with large aerodynamic loads high above the water surface challenges basic naval architecture principles due to the raised center of gravity and large overturning moment. The static and dynamic stability criteria are difcult to achieve especially in the context of offshore wind energy production where economics requires the hull weight to be minimal.19,20 The following fundamental aspects must be addressed to design such system: 1 The inuence of the turbine on the oater and 2 the inuence of the oater motions on the turbine performance. A large body of work has been published on the hydrodynamics of oating platforms; see Refs. 21 and 22 for comprehensive overviews. Hydrodynamics of a minimal oating platform with similar substructure was discussed by Cermelli and Roddier.23 Wind loads on oating structures discussed in the above references are normally computed using a simple relation between the apparent wind speed and loading based on empirical drag coefcients or results from wind-tunnel tests. In the case of a oating offshore wind turbine, wind load components generated by the turbine and their effects on platform motion are signicant and may lead to coupling effects, which cannot be accounted for using conventional methods. The following methodology is applied in this paper, with increasing level of renement of the coupling effects between the wind turbine and platform motion. In the rst step, consisting of global sizing of the oater, coupling between the turbine and oater is accounted for using the


WindFloat: Offshore oating wind

J. Renewable Sustainable Energy 2, 033104 2010

FIG. 1. HyWind spar, blue H tension leg, SWAY tension leg/spar, and WindSea semisubmersible.

following approximation: The wind thrust is determined by assuming that the base of the turbine is xed and it is applied as force and overturning moment at the base of the mast. This approach is further described in Ref. 11. The second step involves time-domain simulations of the hydrodynamic response of the platform using TIMEFLOAT software. The software was modied to compute wind turbine loads based on an equivalent drag model, which provides suitable wind thrust at the hub, and also generates aerodynamic damping. Gyroscopic effects due to the gyration of the rotor coupled with platform rotations are also included. This model is relatively simple to implement numerically, and could also be adapted to an experimental setup in order to verify the platform motion predictions during wave tank testing of a small-scale model. Results obtained at the UC Berkeley ship-model testing facility are presented. This model does not account for turbine exibilities and the various control systems installed on large wind turbines, which have the ability to pitch the rotor blades resulting in variable thrust and torque, in order to keep the rotor speed constant and the tower stable, despite variable wind velocities. In the third and most advanced step, the aeroservoelastic calculation software FAST developed at the Nati