Operational Programme Education and Lifelong Learning Continuing Education Programme for updating Knowledge of University Graduates: Modern Development in Offshore Structures AUTh TUC 8.5 Structural Optimization Georgios E. Stavroulakis Professor, Technical University of Crete, Department of Production Engineering and Management Email gestavr@dpem.tuc.gr
four legged and three legged jacket substructures. 5 MW Turbine Offshore wind turbine structure
A total of 6 variables, i.e., one outer diameter ratio and five outer diameter-to-thickness ratios, were used as design variables. Structural optimization
four legged and three legged jacket substructures Member component Outer diameter D [m] Thickness t [mm] Outer diameter-tothickness ratio D/t X- and mud braces Leg crossing transition piece 0,8 20 40 1,2 40 30 Leg at 2nd to 4th level Leg at lowest level 1,2 35 34,5 1,2 50 24 pile 2,082 60 34,7
Structural optimization three main design drivers that are studied in this paper are the load cases, loading directionality and wind-wave-misalignment.
Loading cases The load cases selected for this study are adapted from the wind-wave coupled load case set 5.X in the OC4 project, which accounts for a fully flexible offshore wind turbine (OWT) (Popko, et al, 2012). LC 5.7 is classified as DLC 1.2 in IEC 61400-3, and performs fatigue assessment on the wind turbine system in power production. The simulation will be conducted under wind conditions that lie between the cut-in and cut-out speed, with the normal turbulence model (NTM) and coupled with a codirectional irregular normal sea-state (IEC, 2009).
Loading cases Secondly, we investigated the dynamic response of the axisymmetric jacket substructures when wind-wave loadings with different incident angles were applied to both three-legged and four-legged designs. The three-legged substructure is a triad (C3) with rotational symmetry of 120 degrees, while the fourlegged substructure is a tetrad (C4) with rotational symmetry of 90 degrees. During the simulations, codirectional coupled wind-wave loads under LC 5.7 (i.e., with zerodegree misalignment) were used, while the incident angle was varied in
Thirdly, the wind-wave misalignment study compares the sensitivity of the structural response to misaligned windwave loadings. The misalignment angles were varied from 0 degrees to 180 degrees in steps of 30 degrees, repeated for each incident angle Structural optimization
Optimization task the material mass requirement, while attaining comparable static, dynamic and eigenanalysis performances with respect to the reference model. The dynamic performances were gauged by the fatigue limit state (FLS) and ultimate limit state (ULS) assessments of the structures.
Structural optimization details The support structure should be designed to exhibit natural frequencies that do not fall into the range of excitation frequencies imposed by wind and wave loading. This is important in order to avoid resonance which is able to create large dynamic responses of the structures, and thereby reduces their fatigue lifetimes.
Schematic diagram illustrating the wind-wave excitation frequency zone and the allowable frequency range for structural natural frequencies excitation by the wind, driven by the rotor rotational frequencies (1P) and blade-passing frequencies (3P), is to be avoided