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This document provides an overview of the relevant design parameters for a novel X-rotor shaped windturbine at a representative North Sea site, focusing on the structural design. This includes a definition ofenvironmental conditions, as well as of the scope of the design and how it will be verified. In order to facilitatecomparison with previous design efforts, the site conditions and design choices have been adapted fromwork performed for the DTU 10 MW offshore wind turbine and the UpWind design basis.

The wind industry has realized substantial growth reaching over 500 gigawatts (0.5 terawatts) of installed capacity in 2017 (Global Wind Energy Council 2018) and producing about 5% of global electricity demand in 2016 (Wiser and Bolinger 2018). The levelized cost of energy (LCOE) for wind energy projects both on land and offshore has fallen as a result of substantial innovation over the last several decades. More specifically, equipment, installation, and operation costs have decreased while energy production per turbine has increased (Wiser et al. 2016). At the same time as LCOE has been decreasing, integration challenges in the broader electric system have been successfully addressed in many markets, thereby enabling the level of wind energy generation to grow to more than 10% of electricity consumption in at least eight countries around the world and more than 30% in Portugal and Denmark (Wiser and Bolinger 2018).

This paper presents a comparison of two state-of-the-art codes that are capable of modelling floating vertical axis wind turbines (VAWTs) in fully coupled time-domain simulations, being the HAWC2 by DTU and the SIMO-RIFLEX-AC code by NTNU/MARINTEK. The comparative study focusses on the way aerodynamics, hydrodynamics and structural dynamics are treated for DeepWind’s 5MW Darrieus rotor mounted on a modified OC3 spar platform. The relevant modelling differences are described, followed by an introduction to the spar VAWT concept and selected load cases. Isolation of the aerodynamic model is achieved using an equivalent rigid land-based VAWT in steady wind-only environments. The added complexity in SIMO-RIFLEX-AC’s aerodynamic model has shown to increase aerodynamic torque at tip-speed ratios above 2.5. Differences in the hydrodynamic and structural models were brought forward through fully coupled analyses in turbulent wind and irregular wave climates. It is found that the simplified mooring system in HAWC2 introduces a 2P yaw response (1P in SIMO-RIFLEX-AC), stronger motion coupling in surge-heave and a largely reduced mooring line tension since the dynamics of mooring lines are not considered. Indications are given that a higher tower mode is excited by 4P aerodynamic loading; an effect that is significantly stronger in HAWC2. © 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Design optimization of the offshore wind turbine support structure is an expensive task; due to the highly-constrained, non-convex and non-linear nature of the design problem. This report presents an analytical gradient-based method to solve this problem in an efficient and effective way. The design sensitivities of the objective and constraint functions are evaluated analytically while the optimization of the structure is performed, subject to sizing, eigenfrequency, extreme load and fatigue load constraints. A case study is carried out for the OC4 jacket substructure to evaluate the method. Results shows that an optimal jacket design with 52 percent structural mass reduction is attained in 27 iterations, while satisfying all design constraints under the simplified load cases used. Besides, it is shown that the analytical sensitivity analysis is more accurate and efficient than the often used finite difference approximations. It can avoid numerical artifacts that typically occur in the analysis of extreme load constraint sensitivities.

River, Estuarine and Coastal Dynamics