Structural Optimization and Comparison Study of Different Configurations of Offshore Wind Turbine Jacket Substructures

Abstract

[Abstract] In this work, the design of a three-legged jacket is presented, taking into account the susceptibility to the directionality of the loads in comparison with a four-legged jacket. For this purpose, two jackets with different orientations with respect to the incoming environmental loads were modelled: the G30, which has two legs facing the main incoming environmental actions, and the G90 jacket, which only has one. In addition, mathematical optimization is proposed to minimize the weight of the structure using a combination of size and shape optimization techniques. The solution to the proposed optimization problem is based on the application of sequential linear programming, which is then solved using the simplex method. In this study, a first-order sensitivity analysis was reformulated to extend its applicability to a three-legged configuration. This sensitivity analysis provides the derivatives of the coordinates of the structure nodes with respect to the design variables required for the shape optimization. The proposed formulation allows optimized designs with a weight reduction of around 40% relative to the UpWind jacket. In the G30 jacket, there was an increase in fatigue damage from the cyclic loads. The G90 jacket and optimal four-legged jacket present design limitations conditioned by the effects of compression stresses and buckling. The differences between the G30 and G90 models highlight the sensitivity of the structural response to the directionality of the loads, which can constitute a limiting factor in the cost reduction of three-legged jackets. An interesting result of this research is that the added shape design variables for the jacket footprint allowed a significant improvement of the optimized designs.