Structural analysis and design of semi-submersible platform for floating offshore wind turbines

Abstract

To solve the issues of energy shortages and environmental pollution, renewable energy is increasingly exploited. As one of the most promising new energy resources, wind energy is actively developed and utilized around the world. To increase the use of offshore wind, there is a need to develop floating platforms to support wind turbines in deep water. However, the cost of floating platform needs to be reduced sharply to be competitive with the traditional energy sources and fixed wind turbines. Moreover, the design code for floating wind turbines (FWTs) is primarily based on offshore ships, oil & gas industry, which still needs to be assessed and evaluated. To both ensure the safety and avoid overdesign of offshore wind, the structural behaviour and the evaluation of the design guideline of FWTs need to be studied. Because of the combined actions of the aerodynamic load, hydrodynamic load and tension of mooring lines, the structural behaviour of a FWT becomes extremely complicated in the marine environment. Previous studies for FWTs mainly focused on the dynamic response due to the environmental load, while the structural performance has not been well addressed. In this context, the aim of this thesis is to improve understanding of the nonlinear structural behaviour of a semi-submersible platform for FWTs, and compare and evaluate the relevant design codes.

Firstly, the structural behaviour of a semi-submersible platform (SSP) for an offshore wind turbine is studied. A novel one-way coupled fluid structure interaction simulation that combines hydrodynamic and structural analysis is undertaken, with a focus on structural nonlinearity – especially the geometrical nonlinearity. The analysis is divided into four steps. First, the hydrodynamic response is simulated in the frequency domain using ANSYS AQWA to generate the wave pressures acting on the floating platform. Second, the wave pressures are transferred from ANSYS AQWA to ANSYS Mechanical to study the structural performance. Third, linear elastic analysis is carried out to identify the critical load cases. Finally, geometrically and materially nonlinear analysis is adopted to investigate the failure modes and corresponding critical failure locations in comparison with the linear elastic analysis. The effects of azimuthal angles of environmental loads, wave-wind misalignment and boundary conditions on the overall structural performance are also investigated.

Secondly, the structural design of the main component of a floating platform for a FWT are carried out by hand calculations based on the worst load case from the global structural analysis. The prevailing design guidelines, the DNVGL (Det Norske Veritas and Germanischer Lloyd), and two versions of EN1993-1-6 (2007 and the latest, 2017) have been adopted for carrying out the structural design. The design process of a shell structure based on the three codes is presented in detail. The unstiffened and stiffened cylindrical shells are designed separately, and a cost-effective design for stiffened cylindrical shell is proposed to reduce the levelized cost of energy.

Lastly, but not least, the buckling behaviour and ultimate strength of cylindrical shells under combined axial compression and bending are investigated by means of the finite element method (FEM), in conjunction with the design codes for reference. Starting from the geometry of the main component of a floating platform, a series of FE models considering geometrical and material nonlinearities with a wide geometric range and different types of initial geometric imperfections are simulated. The effects of geometric imperfection profiles and amplitudes on the ultimate strength of the unstiffened cylindrical shells are examined. The typical buckling modes due to the different initial imperfections are discussed. Moreover, a stiffened cylinder is proposed as a case study and compared with an unstiffened cylinder in respect of buckling behaviour and effects of initial imperfections. The accuracy of three design codes for plain shell structures, the DNVGL and EN1993-1-6 (2007 and 2017 versions), are evaluated using the FEM results. Then, a reliability analysis is adopted to propose modified partial safety factors γ_M to ensure that these design codes possess the required level of safety for offshore structures.