Design and analysis for a steel braceless semi-submersible hull for supporting a 5-MW horizontal axis wind turbine

Abstract

This thesis deals with design and analysis of steel semisubmersible hulls for supporting MW-level horizontal axis wind turbines. The thesis address the following four topics: 1) conceptual design methods, 2) conceptual design of a steel braceless hull for supporting a reference wind turbine (denoted as 5-MW-CSC), 3) development, verification and validation of numerical approaches for analyzing global structural responses of structural components of semi-submersible hulls in wind and waves, and 4) case studies related to numerical simulations and experimental measurements for load and load effects on semi-submersible wind turbines. Simplified design procedure, criteria and design check approaches for conceptual design with respect to safety have been systematically presented and discussed based on publicly accessible publications and the author’s experience and practice in the past six years. The 5-MW-CSC is developed based on the simplified design procedure, criteria and design check approaches. Numerical analysis shows that the 5-MW-CSC has very good intact stability, well designed natural periods and modes, moderate rigid-body motions in extreme environmental conditions and a reasonable structural design. The structural design of the 5-MW-CSC is checked by using simplified ULS and FLS design checks. Two time-domain approaches, which can be easily implemented in various state-of-theart computer codes to extend their capabilities to analyze sectional forces and moments in structural components of generic and specific floaters subject to environmental loads from wind and waves, were developed by the author. The developed approaches focus on modeling of inertia and external loads on the floaters and mapping of the loads in finite element model of the floaters. The floaters are considered as an assemblage of several structural components. The conventional hybrid frequency-time domain approach is extended to model the external loads on and inertia loads of each structural component. Limitations of the developed time-domain approaches and future work for solving these limitations are discussed. The developed approach for generic floaters were verified and validated by comparing with simulated responses given by other reference numerical models and measurements from a 1:30 scaled model test campaign using the ReaTHM® testing approach to overcome the limitations of conventional model test approaches. The verification and validation consist of five comparisons. Objectives and expected results of the five comparisons are illustrated. In general, the comparisons agree with the expectations while possible reasons for the deviations are thoroughly and quantitatively analyzed. Effect of non-linear wave excitation loads, drag forces, each load component, and steady wind and wave loads induced by changes of the mean wetted body surface on rigid-body motions and sectional bending moments in five specified cross-sections on the hull of the 5-MW-CSC were analyzed by comparing the measurements of the model test campaign and carrying out numerical sensitivity study. These analyses shed more light on features of the loads and load effect on and critical structural components of the hull of the 5-MWCSC, and critical environmental conditions for the 5-MW-CSC with respect to fatigue damage and extreme load effects. The obtained understanding was used to simplify complexity of numerical models of the 5-MW-CSC to reduce computational cost of the design checks, and is helpful for reducing design conditions required by ULS and FLS design checks and structural optimization. Experience acquired from design and analysis of the 5-MW-CSC and development of the time-domain approaches will promote development of novel and cost efficient designs of semi-submersible wind turbines; while the 5-MW-CSC and developed approaches can be used as reference to validate other computer codes for analyzing global responses of floating wind turbines.