Jack-ups are used for offshore wind turbine installation (OWTI). To assess whether a jack-up can safely operate in a desired geographic location with known seismic activity, an earthquake response analysis is required for structural verification. This analysis is generally carried out according to the codes of the International Organization for Standardization (ISO). The prescribed procedure is called extreme level earthquake (ELE) screening and is performed using response spectrum analysis (RSA). This method is intended to be conservative and should produce higher utilization results than more detailed assessments. With offshore wind farm development moving into regions with greater seismic activity, more detailed assessments are required to demonstrate safe operation in those geographic locations.

The aim of this research was to improve the accuracy of earthquake screening of OWTI jack-ups in order to increase their geographic operability. A new ELE-screening procedure was proposed that uses time-history analysis (THA) and spectrum matched acceleration time history records (THR). For the development and benchmarking of the new method, an earthquake analysis was performed using both the new ELE-screening procedure with THA and the existing ELE-screening procedure with RSA. The analysis was performed for an OWTI jack-up based on the GJ-3750C located in a region offshore Japan. The RSA simulations were performed using Minifem; a new tool was developed in OpenSees for the THA simulations. To assess the effect of various parameters, simulations were run with different soil-structure connections, levels of damping, and design response spectra. The performance is evaluated using the global maximum action effects; limited to the forces and moments at the lower guide and footing. A soil-structure interaction analysis was performed on an equivalent single degree of freedom system to validate the soil-structure interface model, and the use of free field ground motions. A procedure and accompanying tools were developed for acceleration time history record selection and modification. The resulting spectrum matched THR are used for seismic excitation in the THA simulations.

The simulation results showed a reduction in the magnitude of calculated action effects when using the new ELE-screening procedure. A reduction of 10 to 20 percent in the global maximum shear force and moment was observed in the simulations best describing the jack-up and site-specific soil conditions. A small reduction of the global maximum normal force was also observed. The new ELE-screening procedure with THA can be used to demonstrate compliance with earthquake performance requirements when a jack-up does not satisfy the ELE-screening assessment criteria using RSA. Since safe operation can be demonstrated for more areas, the geographic operability of OWTI jack-ups is increased.