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 experience therefore research is required to investigate the effect of the prolonged cyclic loading on the dynamic response of these very dynamic sensitive systems. This paper describes typical experimental results gained from a series of 1-g testing carried out on a wind turbine scale model. Three different soil profiles are considered: dry sand, saturated sand and soft clay. The tests were carried out on wind turbine model supported by two types of foundation, namely monopile and suction caisson. The experimental 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 a densification of the soil next to the pile. However, tests with suction caisson show a more complex behavior. Contrastingly, tests performed in soft clay show decreasing in natural frequency of the turbine with the cycles possibly due to a softening soil behavior.