A study on soil-structure interaction of offshore wind turbine foundations

Abstract

Offshore wind power is one of the most popular renewable sources of energy. However, there are many challenges during the design, construction and operation of offshore wind farms. One of these challenges is the stability of offshore wind turbines. The main loads on the foundations of wind turbines are from the environment (wind and wave) and there are other loads arising due to their operations (known as rotor frequency loads-1P and blade passing loads-2P/3P). All these 4 loads are unique in terms of magnitude, number of cycles and the strain they apply to the supporting soil. Furthermore, due to innovation in turbine technology, the sizes of turbines also increased few folds (3MW to 12MW) in a span of about 5 years and these large turbines need customised foundations. Due to the attractiveness of this new technology and the reduction of LCOE (Levelized Cost of Energy), offshore wind turbines are also sited not only in deeper waters but also in seismic areas and other disaster-prone areas (typhoon and hurricane). Any new foundation must be validated using scaled model tests (i.e. study of Technology Readiness Level) to satisfy the industry requirements. This thesis developed techniques for scaled model testing to study different aspects of long-term performance of foundations. The novel testing methodology and apparatus is based on understanding of the loads on the foundations. The apparatus consists of two eccentrically loaded gears which can be customised to apply cycloid loads. The apparatus can be easily upscaled to study bigger models and is very simple to assemble and operate. The apparatus can also apply millions of cycles of loading of different amplitude and frequency which is representative of a real wind turbine. Results from scaled model tests on few types of foundations are presented and they revealed interesting Soil-Structure Interaction. In a wind turbine system, long term performance is mainly governed by the SSI and this thesis summarised the limited field observations reported in the literature and compared with the laboratory observations. One of the scientific challenges is the prediction of long-term performance of these relatively new and novel technologies. While scaled model tests can identify the physics, this is not a practical tool for routine design as it is difficult to create model tests for each of the sites. As a result, this thesis aimed to link the understanding of SSI to element testing of soil. This will allow to use the recovered sample from offshore wind farm location to carry laboratory tests to obtain design parameter. This thesis proposed a simple method to obtain the strain level in the soil which is beneficial for planning offshore Site Investigation. Offshore wind turbines are currently designed for 25 to 30 tars and the number of cycles of loading are in the range of 100 million. This thesis presented data from element testing of soil where up to 50,000 cycles of loading were applied. The general trends of behaviour were noted, and it was observed that the soil behaviour was attaining a steady state. All the above helped to understand some SSI aspects of offshore wind turbines. Future work is also suggested