Abstract
A current trend in offshore wind is the quest for exploitation of ever deeper water sites. At depths between 50m and 100m a promising substructure is the column-stabilised semi-submersible floating type. This solution is currently being tested at full scale at the WindFloat and Fukushima Forward demonstrator sites in Portugal and Japan respectively. The semi-sub design class frequently adopts passive motion control devices based on the water entrapment principle, such as heave plates, tanks, and skirts. Whilst effective for small inclinations, these can underperform when the structure is inclined under wind loading. This study examines the alteration of potential hydrodynamics due to wind-induced trim (geometric non-linearity) and its impact on the wind turbine׳s wave response with focus on heave plate performance. Firstly it is shown by using the boundary element approach that wind trim affects wave loading in the ocean wave band between 5s and 15s, and introduces hydrodynamic coupling typical of non-symmetric hulls. These features are incorporated in frequency-domain dynamic response analysis to demonstrate that said effects bear a significant impact on the turbine׳s motion in waves. Accounting of heave plate excursion improves the assessment of the seaworthiness of floating wind turbine concepts, potentially leading to new design constraints.
Highlights
Going afloat is deemed to be one of the principal innovations to impact the offshore wind market in the years, enabling expansion to deep water areas where bathymetry exceeds 50 m (EWEA, 2013)
In order to isolate potential hydrodynamic effects, all terms entering the 6-DoF frequency-domain equations of motion maintain the classic linearisation about the undisplaced, untrimmed position, with the exception of those derived from the BEM calculation
The potential hydrodynamic database is calculated twice: once for zero trim and once for 6.0 ◦, whose actualised mesh is shown in Figure 14; the respective frequency-domain heave and pitch responses of the turbine are reported
Summary
Going afloat is deemed to be one of the principal innovations to impact the offshore wind market in the years, enabling expansion to deep water areas where bathymetry exceeds 50 m (EWEA, 2013). Numerous countries worldwide hold most of their offshore wind potential in deep water, but so far only a handful have hosted utilityscale machine deployment. Statoil of Norway has commissioned the first operating MW-class floating demonstrator, Hywind, in 2009 (Stiesdal, 2009). Portugal has followed by hosting Principle Power’s WindFloat prototype (Figure 1), and more recently two Japanese consortia have successfully installed floating test units off Goto city (Figure 2) and Fukushima (see Hitachi, 2014; Main(e) IC, 2013). All commissioned large scale prototypes adopt turbines of about 2 MW capacity, mounted either on spar or semi-submersible type substructures. The ongoing industrialisation initiatives are seeking to extend the FWT (floating wind turbine) design envelope with the use of tension leg type platforms (see Scott, 2012), vertical axis turbine technology (see IWES, 2014; Nenuphar, 2012), and by increasing turbine size
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