Upscaling the TetraSpar

Abstract

During the past decade, the offshore wind energy industry evolved to bigger turbines, going into deeper waters and farther offshore. As bottom-fixed wind turbines are limited to shallow water depths, floating wind structures are the next frontier to unlock the vast potential of wind energy. Despite many techno-economic challenges, several full-scale floating wind structures have been successfully deployed and have shown the potential for floating offshore wind. One project near completion is the 3.6 MW TetraSpar demonstrator developed by Stiesdal Offshore Technologies, Shell and Innogy. With its tetrahedral shaped base and suspended counterweight keel, developed with the focus on ease of fabrication and installation, this spar concept is expected to offer a competitive package for floating wind using future, larger 10 MW+ wind turbines. The goal of this research is to investigate the capability of the TetraSpar platform to accommodate significantly larger wind turbines and to identify challenges in an early stage of development. Since technology upscaling of floating wind substructures has not been done before, this thesis first develops a novel design methodology for upscaling and then is applies it to the TetraSpar as case study. This work builds on academic efforts thus far but focusses on the key design drivers in for upscaling of floating wind, namely the fundamental equilibria in vertical and rotational direction: the structure’s weight is equal to its buoyancy, and the restoring moment equals the maximum overturning moment by wind. Specific emphasis is put on correctly capturing these equilibria, as they generally apply for floating wind substructure technologies, including the TetraSpar. First a design basis is created with functional requirements and design criteria for floating wind structures in general, and specifics to the TetraSpar. Also, key specifications of future 10 MW, 15 MW and 20 MW wind turbine types are explored. Secondly, a model is developed for upscaling based on physical modelling of hydrodynamic stability (water and waves) and aerodynamic thrust (wind). Based on these inputs, the substructure is upscaled using the future turbine type wind thrusts. The model employs an algorithm to find a new equilibrium design point and computes key properties for upscaled substructures. The resulting design concepts are then evaluated for first order wave-structure interactions using a diffraction/radiation solver (WAMIT). Key evaluation aspects are free-floating hydrodynamics, including hydrodynamic coefficients, wave forces and response amplitude operators. Fourth, selected structural elements of the upscaled design concepts are evaluated for structural strength. The fifth and final step assesses the extent to which the now evaluated upscaled design concepts still meet on the functional requirements and design criteria. The thesis concludes that the developed, first-order design methodology is suitable to explore upscaled design concepts of floating offshore wind turbines. By computing an estimation of the physical dimensions and behaviour of the substructure, this allows for the evaluation of the technical and economic feasibility. Key findings of the physical modelling are the linear trends for structure mass over power rating of the wind turbine, sensitivities in design choices for maximum allowable heel angle due to wind, and keel draught for the TetraSpar specifically. Compared to other technologies, it is found that found that the TetraSpar concept offers a relatively lightweight platform for future wind turbines up to 15 – 20 MW. No fundamental technical showstoppers are identified for upscaling, but it is found that as the structure progresses to larger wind turbines, aspects like in-port water depth, physical dimensions of structural elements, and installability of the TetraSpar at sea will become more challenging. It is expected that at some stage in the development towards large-scale floating wind structures, trade-offs will have to be made to arrive at an improved design. For this, the methodology developed for this thesis can be applied, for example by exploring a lighter, wider TetraSpar design with more slender structural elements. Furthermore, it is recommended to further investigate the mooring design, fabrication capacity and deployment procedures of larger floating wind substructures in general, and upscaled TetraSpar designs in particular.