Observations on the influence of soil profile on the seismic kinematic bending moments of offshore wind turbine monopiles

Abstract

The installed capacity for generation of electric energy from offshore wind power is growing exponentially. At the end of 2015, there were 3230 offshore wind turbines in Europe, 80% of which are founded on monopiles. The expansion in the number of offshore wind turbines may involve the need of installing new wind farms in sites of poorer soil conditions and increasing seismic risk, which puts additional responsibility on the foundation subsystem, which takes a significant part of the total initial investment when setting up new wind farms. These reasons justify the need for analyzing the influence of soil profile on the seismic kinematic bending moment of offshore wind turbine monopiles. To do this, a Beam-On-Dynamic Winkler approach is used to perform a parametric analysis of the system considering large diameter monopiles, realistic material and geometrical properties for soils and pile, and a large set of soil profiles assumed to be composed by several different strata. Each case is assumed to be subject to ground motions described by the elastic response spectrum given by Eurocode 8 (part I) for the ground type corresponding to each profile. Results are presented in terms of envelopes of bending moments along the monopiles, and the large number of results is synthesized in a set of ready-to-use graphs. It will be shown that, for the monopiles studied (with diameters of 3.5 and 6 meters), the peak kinematic bending moments are not necessarily found in the interfaces between strata, as observed in the case of piles with not so large diameters.