Study of typhoon impacts on the foundation design of offshore wind turbines in Taiwan

Taiwan is hit by an annual average of three to four typhoons with different intensities and routes. To consider the impacts of typhoon events on offshore substructure design, the historical metocean data for storm event-based frequency analysis are generally applied. However, whether a supertyphoon passes directly over an offshore wind farm has a great influence on the results of frequency analysis. Therefore, the American Bureau of Shipping has recently increased the return period of hydrodynamic loads from 50 to 100 years to address the higher variation in the severity of extreme storms in typhoon-prone offshore regions. In this paper, to assess the impacts of the increase in the return period (from 50 to 100 years) on the design of offshore substructures, case studies of different types of substructures (i.e. monopile, three-leg jacket and four-leg jacket) with different water depths were conducted for the offshore wind farms off the coast of Changhua County in central Taiwan. The quantitative differences in foundation design are presented by way of a comparison of substructure weights, overturning moments and pile lengths. The preliminary results of the case studies show that the monopile design is more obviously dominated by earthquakes than by typhoons and the jacket-type foundation is more significantly dominated by typhoons.