Effect of Sea State Realizations on the Geotechnical Design of OWT Foundations

All relevant standards and design guidelines for Offshore Wind Turbines (OWTs) require the assessment of the effect of cyclic actions on the soil strength and stress-strain behavior. Basis for this assessment is typically a design storm with a duration of 35hrs and a 50-year return period. The corresponding sea state, defined by the wind speed, wave height and wave period, is site and project specific. In order to assess the actual loads on the OWT foundation, the sea state is realized as an irregular process in a time domain analysis using random seed files. In practice, several analyses of the same sea state but with different random seed files are performed in conjunction, and the final foundation load set is determined by averaging the loads from the different analyses. Depending on the OWT analysis code and seed files used, the variations of the calculated loads can be considerable. This paper presents a study where the effect of the sea state realizations on the cyclic soil strength and subsequent foundation response is investigated. For the example of a monopile-supported OWT at a generic site in the North Sea, this effect is demonstrated by comparing the peak rotation at an Ultimate Limit State (ULS) load case, which was found to vary by up to 50%. This study demonstrates the need to define Design Load Cases (DLCs) addressing the particular needs for the geotechnical design. The DLCs defined in IEC or DNVGL appear to be insufficient and are not considering enough the implications for the foundation design.