Sebastian Schafhirt

Support Structure Optimization for Offshore Wind Turbines With a Genetic Algorithm

This study considers the use of a genetic algorithm for the structural design optimization of support structures for offshore wind turbines. Member diameters, thicknesses and locations of nodes are jointly optimized. Analysis of each design is performed with a complete wind turbine simulation, for a load case in the time domain. Structural assessment is in terms of fatigue damage, evaluated for each joint using the hot-spot stress approach. This defines performance constraints. Designs are optimized with respect to their weight. The approach has been tested with the modified 4-legged UpWind jacket from the OC4 project. The weight is quickly reduced, convergence slows after about 100 iterations, and few changes occur after 250 iterations. Interestingly, the fatigue constraint is not active for any member, and it is the validity of stress concentration factors that determines the best design, which utilizes less than 90 percent of the available fatigue lifetime. These results of the preliminary study using the genetic algorithm demonstrate that automatic optimization of wind turbine support structures is feasible under consideration of the simplified load approach. Even for complex, multi-member structures such as the considered jacket a weight reduction was achieved.Copyright

How to Detect Local Out-of-Plane Vibrations in Jacket Support Structures for Offshore Wind Turbines

There has been an ongoing debate whether local out-of-plane vibrations of braces exist in jacket support structures for wind turbines. The issue has been raised with the sequential analysis of offshore wind turbines, where increased fatigue damage for bracings was observed. Local vibration modes, excited by rotor harmonics, were detected as a reason for it. A methodology to remove global motion of the jacket from the displacements of the central joint in a brace is presented and the amplitude of local out-of-plane displacements is analyzed, using an integrated wind turbine simulation based on a flexible multibody solver. Finally, the impact on fatigue damage is calculated. Results indicate that the extent of local vibrations is much less than previously thought or predicted in other studies.Copyright

Comparison of different approaches to load calculation for the OWEC Quattropod jacket support structure

Accurate load simulations are necessary in order to design cost-efficient support structures for offshore wind turbines. Due to software limitations and confidentiality issues, support structures are often designed with sequential analyses, where simplified wind turbine and support structure models replace more detailed models. The differences with an integrated analysis are studied here for a commercial OWEC Quattropod. Integrated analysis seems to generally predict less damage than sequential analysis, decreasing by 30-70 percent in two power production cases with small waves. Additionally it was found that using a different realization of the wave forces for the retrieval run in sequential analysis leads to an increase of predicted damage, which can be explained as the effect of applying two independent wave force series at the same time. The midsection of the detailed support structure model used shell elements. Additional analyses for a model with an equivalent beam model of the midsection showed only small differences, mostly overpredicting damage by a few percent. Such models can therefore be used for relatively accurate analysis, if carefully calibrated.