Introduction:Offshore wind farm development is gaining traction in seismically active countries such as Taiwan and Japan. Such offshore wind structures must be designed to withstand extreme loading resulting from earthquakes.To determine the design loads, coupled analysis is performed in aeroelastic software, where the support structure is either modelled explicitly (the “integrated” design approach) or included as a superelement. For monopile support structures, these calculations are typically performed using an integrated design approach, which naturally allows for the inclusion of earthquake-induced loading. For jacket support structures, the superelement approach remains preferred, primarily due to the desire to separate design responsibility for different parts of the structure and to protect intellectual property. Typically jacket superelements are derived without information about the rigid body motion required when considering earthquakes. Recent method development has allowed earthquake effects to be included as part of the superelement as a pre-determined time history of applied loads.This study intends to demonstrate that including the earthquake excitation in a superelement approach gives equivalent load results to the integrated design approach. Showing that the two approaches agree will help to build confidence in the use of superelements including seismic effects.Methodology:Using a 4-leg jacket support structure with a generic 7MW turbine on top, Sesam and Bladed are used to perform the calculations for the two methods:·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; For integrated analysis: the support structure is modelled in Sesam and sent to Bladed. The wind turbine is modelled in Bladed and structural analysis including supporting structure, wind turbine, seismic accelerations, wave and wind loading is performed in Bladed.·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; For superelement analysis: the support structure, seismic accelerations and wave loads are modelled in Sesam. These are transferred to Bladed as a superelement and corresponding loads. The wind turbine is modelled in Bladed and the wave and seismic loads are included in the superelement. The interface loads are sent to Sesam, where the jacket internal loads are calculated.Models of increasing complexity are defined to demonstrate alignment between the two approaches. Each was simulated in the time domain for 200s.1.&nbsp;&nbsp;&nbsp; Support structure only, without any external loading except for earthquake loads2.&nbsp;&nbsp;&nbsp; Support structure only, with earthquake and wave loads3.&nbsp;&nbsp;&nbsp;&nbsp;Support structure and wind turbine, with earthquake, wave and wind loadsThe following environmental conditions have been considered:·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Waves: Regular and irregular·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Wind: 50m/s turbulent wind, TI=12%·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Seismic: 30s earthquake at t=150s·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Turbine condition: ParkedFor each model, various comparisons are performed:·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Deflection, velocity and accelerations at tower top, interface node and in the jacket·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Loads at tower top, interface node and in the jacketResults:The results from case (1) with earthquake loads only show an almost exact agreement between the superelement and integrated design approaches for the studied variables. This demonstrates that the approach to modelling earthquakes is equivalent in the two methods.The results from case (2) with earthquakes and waves loads together show some discrepancy during the period of earthquake excitation. This suggests that the coupling between hydrodynamic forces and earthquake motion is not completely equivalent in the two approaches. This could be due to how the earthquake motion is taken into account during hydrodynamic load calculation in both methods.Case (3) with wind, wave and earthquakes shows a good agreement, suggesting that the differences in modelling of wave-earthquake interaction are less significant when the turbine is included.Conclusion:It has been demonstrated that the superelement and integrated methods for modelling earthquake loading are equivalent in terms of the motions and loads at key points of the support structure. This makes it possible for wind turbine manufacturers and foundation designers to perform a seismic analysis using either one of the methods, thereby being able to benefit from the technical and organizational benefits of either method.Further work is required to understand the earthquake-hydrodynamics coupling which is not yet demonstrated to be completely equivalent between the two approaches.

A Comparison of Time Domain Seismic Analysis Methods for Offshore Wind Turbine Structures: Superelement Approach Versus Integrated Approach

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/IOWTC2021/volume/84768

ASME 2021 International Offshore Wind Technical Conference (IOWTC2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.