Assessing the importance of the slow drift motion of floating wind turbine platforms

As offshore wind turbines are installed in deeper water, interest is growing in floating wind turbines because, among other reasons, they may become cheaper than fixed-bottom turbines at greater depths. When analysing floating wind turbines, linear diffraction theory is commonly used to model the hydrodynamic loads on the platform. While it well known that slow drift motion due to second-order loads can be important for other floating offshore platforms, it has not yet been established how important such effects are for floating wind turbines. In this paper we aim to give a general result by developing approximate closed-form expressions to estimate the second-order slow drift motion of platforms of different sizes. The values are benchmarked against a typical calculation of the slow-drift response of a platform. The results show that floating wind turbines, which tend to have smaller dimensions than other floating structures, may be expected to show smaller slow-drift motions.