Dynamic SSI of Monopile-Supported Offshore Wind Turbines

Abstract

Offshore wind turbine (OWT) are new type of structures with no track record of long-term performance. The structures are also dynamically sensitive as the natural frequency of these structures are very close (15–20%) to the forcing frequency acting on them. The foundations for OWT are subjected to a combination of millions of cycles of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics and therefore the long-term performance of OWT depends on the soil behaviour and soil–structure interaction. This paper has two aims: (a) Discuss the soil–structure interaction in a monopile-supported wind turbine as observed in scaled model tests of different types of offshore wind turbine foundations and limited field observations; (b) Present result from a series of element test of soil to study the cyclic behaviour of soil in terms of change of shear modulus change and accumulation of strain. Typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long-term performance prediction of OWT foundations.