Rational Selection of Floater Designs for Offshore Wind Farms Using Power Transfer Functions

Da, Shu (Texas A&M University at Galveston) | Sweetman, Bert (Texas A&M University at Galveston)

OnePetro 

Abstract

Rational design of floating offshore wind turbines requires a trade-off between very stiff structures that support the nacelle in a near-vertical orientation versus less expensive floaters that allow larger angular displacements; the stiffer structures generally cost more but enable greater energy harvest. A computationally friendly method based on power transfer functions is presented in which the total power output from specific designs is computed by convoluting power output transfer functions for the floating wind turbine with site-specific long-term environmental conditions. Environmental conditions characterized by each of four proposed sites are four random variables, representing the wind velocity, significant wave height, peak period of the wave process, and the angle between the wind and waves. One step in the overall method is development of modified two-dimensional wind-power curves, which can be used to show the relative importance of consideration of the wave properties on overall energy harvest.

INTRODUCTION

Wind-energy is increasingly gaining acceptance as an economically viable and environmentally friendly method of energy harvest. At the same time, local permitting and environmental issues associated with considerations such as noise emissions and interruption of the view-scape are becoming critical problems as desirable space for on-shore wind turbines becomes increasingly scarce. European countries, which are relatively densely populated compared with the United States, began siting offshore wind turbines in shallow waters near shore, most notably in Denmark and Germany in the 1990’s. Today, the growth of offshore wind energy in European countries is significant, with projected growth rates of 1700 to 3000 MW per year (Snyder and Kaiser, 2009). In the Far East, China is beginning installation of offshore wind, including the first offshore wind farm in East China Sea, which will which will produce 267 GWh per year for the energy market in Shanghai (Chen, 2011). In very deep waters, the bottom-founded support towers may prove cost-prohibitive, but the cost of floating offshore systems is relatively insensitive to water depth, and may prove to offer a viable way to develop wind energy beyond the sight of land.

The relatively small profit margins and high costs of floating systems make design optimization critically important. Unfortunately, use of computer-based time-domain numerical simulations for long-term prediction of energy harvest in site-specific conditions requires so much elapsed time as to be impractical. For example, direct simulation of ten years of wind turbine performance for one case would require over 70,000 20-minute cases, which would require almost 10 weeks running on a modern personal computer. Here, a new method is presented that enables accurate prediction of energy harvest for a specific design in site-specific wind-wave conditions but that requires substantially less computer time.