Change in Fatigue and Extreme Loading when Moving Wind Farms Offshore

Measurements of loads, wind climate and turbulence, power and other parameters have been conducted for several years on the 5MW Vindeby Offshore Wind Farm. The wind farm is located 2-3 km off the coast of one of the Danish islands in the south of the Baltic Sea and consists of 11 wind turbines each of 450 kW, arranged in two rows with 5 and 6 machines, respectively. Online measurements of fatigue loading (rain-flow counting) have been gathered, resulting in approx. 13,000 sets of valid ½ hour statistics (mean, standard deviation, maximum and minimum); in the case of fatigue the concept of equivalent load has been applied to store results in a compact format. The good data quality and the (obvious) homogeneous terrain surface (water) facilitate detailed analyses of both pure offshore effects and the combined effect of being offshore and having increased loads due to wake effects. Thus, in the paper extreme loads and fatigue loads are analyzed and compared to loads onshore. The results may possibly be applied when re-designing wind turbines from inland climate to the climate of offshore locations in identical climate regions. The work was financed by the European Union under contract JOU2-CT93-0350.