Optimization of Offshore Steel Jackets: Review and Proposal of a New Formulation for Time-Dependent Constraints

Abstract

In this work, a methodology for the optimization of offshore wind turbine substructures (jacket type) is presented. The use of a coupled model allows for capturing the full dynamic interaction between all the elements. The structural analysis is carried out in the time-domain. A non-linear integration scheme is necessarily applied due to the effects of the rotation of the blades. Environmental actions, such as wind and waves, are considered as loading conditions. Fatigue damage at the welds of the joints is taken into account. The objective function to be minimized is the weight of the structure. The shape and sizing optimization model is stated in terms of two variables that define the overall shape of the jacket, along with the dimensions of the cross-sections of the structural members. The model is subjected to Ultimate Limit Stress (ULS), Fatigue Limit State and natural frequency constraints. The time-dependent ULS constraints are efficiently treated by means of a new formulation that is based on constraint aggregation. Fatigue accumulation during the whole design life of the structure is accurately assessed, without the need for excessively costly numerical simulations. The optimization problem is solved by means of Sequential Linear Programming, what requires a full first order sensitivity analysis to be performed. The efficiency, reliability and robustness of the proposed methodology is demonstrated by optimizing a real jacket design.