Long-term performance of offshore wind turbines supported on monopiles and suction caissons

Offshore wind turbines are relatively new technology with limited prior long-term performance experiencetherefore research is required to investigate the effect of the prolonged cyclic loading on the dynamic response of these verydynamic sensitive systems. This paper describes typical experimental results gained from a series of 1-g testing carried out on awind turbine scale model. Three different soil profiles are considered: dry sand, saturated sand and soft clay. The tests werecarried out on wind turbine model supported by two types of foundation, namely monopile and suction caisson. Theexperimental results show that the change in natural frequency of the model is strongly dependent on the soil conditions.Specifically in sandy soils (both dry and saturated) the frequency increases with number of cycles, possibly due to adensification of the soil next to the pile. However, tests with suction caisson show a more complex behavior. Contrastingly, testsperformed in soft clay show decreasing in natural frequency of the turbine with the cycles possibly due to a softening soilbehavior.