Design optimization of the offshore wind turbine support structure is an expensive task; due to the highly-constrained, non-convexand non-linear nature of the design problem. This report presents an analytical gradient-based method to solve this problem in anefficient and effective way. The design sensitivities of the objective and constraint functions are evaluated analytically while theoptimization of the structure is performed, subject to sizing, eigenfrequency, extreme load and fatigue load constraints. A case studyis carried out for the OC4 jacket substructure to evaluate the method. Results shows that an optimal jacket design with 52 percentstructural 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 differenceapproximations. It can avoid numerical artifacts that typically occur in the analysis of extreme load constraint sensitivities.
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