In this paper, 2DoF VIM of a semi-submersible FOWT subjected to current flow is investigated numerically, which could influence its aerodynamic performance but has been rarely studied. Simulation results for a number of flow speeds show that lock-in occurs across a wide range of reduced velocity from 6 to 30. The maximum transverse response amplitude of the platform reaches 0.94 times the column diameter. Drag force increases significantly at high reduced velocity of 30, possibly due to an increase in transverse response frequency. Interactions between vortices shed from upstream and downstream columns are also observed via flow field visualisation.
A recent trend in the offshore wind industry is to install wind turbines farther offshore in deeper water sites in order to exploit more abundant wind resource (Liu et al., 2019b). However, when the water depth of installation sites increases further to over 50 metres, fixed-bottom wind turbines widely adopted nowadays will be too costly to build and install while floating offshore wind turbines (FOWT) can be a more economical alternative. Meanwhile, installing a wind turbine on a floating platform brings about many engineering challenges. Most notably, the aerodynamic performance of an FOWT, e.g. thrust and power, is greatly affected by the six-degree-of-freedom motions of its floating platform, which are induced by environmental loads from wind, wave and current (Liu et al., 2019a; Tian et al., 2020). The majority of existing researches on FOWTs focused on the analysis of coupling effects between the wind turbine and the floating platform under various wind and wave conditions either experimentally (Coulling et al., 2013; Duan et al., 2016) or numerically (Tran and Kim, 2016; Liu et al., 2017b). However, there have been few studies on the motion responses of FOWTs subjected to current flow. Previous researches (Waals et al., 2007; Liang and Tao, 2017) on floating oil and gas platforms show that when placed in current flow, these structures will experience inline and transverse motions induced by periodic vortex shedding, a phenomenon commonly referred to as vortex-induced motion (VIM). As floating platforms of FOWTs mostly inherit the mono- or multi-column design from the offshore oil and gas industry, it is thus reasonable to expect that VIM will also occur for FOWTs subjected to current flow.