Optimization of Offshore Steel Jackets: Review and Proposal of a New Formulation for Time-Dependent Constraints
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ORIGINAL PAPER
Optimization of Offshore Steel Jackets: Review and Proposal of a New Formulation for Time‑Dependent Constraints Iván Couceiro1 · José París1 · Fermín Navarrina1 · Raquel Guizán1 · Ignasi Colominas1 Received: 30 January 2019 / Accepted: 30 April 2019 © CIMNE, Barcelona, Spain 2019
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. Keywords Structural optimization · Structural dynamics · Offshore jackets · Fatigue
1 Introduction Wind energy has been one of the main and most succesfull strategies over time in the quest for renewable massive power resources . In the last decade, the interest of the industry has turned from the traditional inland concept to the new offshore wind turbine (OWT) farms that are being designed and installed at present. As a general rule, OWTs are meant to increase the efficiency of power extraction by taking advantage of the stronger and steadier wind regimes in open waters, and they can be much bigger than their inland counterparts. Obviously, this severe separation between generation and consumption, along with the inclemency of the enviromental conditions and the difficulty of the installation itself, entails a number of drawbacks and significant engineering challenges. Without any doubt, the first and most restricting factor is the need to build substructures * Iván Couceiro [email protected] 1
Group of Numerical Methods in Engineering, Universidade da Coruña, Campus de Elviña, 15071 A Coruña, Spain
that could support the wind turbine towers over the sea level in locations where the depth can be as large as the size of the whole system, including the blade
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