Abstract
Most of the offshore wind turbines (OWT) recently installed in Europe, China and North America are in shallow water. However, unlocking the full potential of OWT lies in deeper waters. Jacket substructures have presented themselves as a reliable foundation concept for transitional water depth. This study focuses on the structural static and dynamic analysis of the traditional jacket substructures (with X and K bracing) and the recently patented three-legged twisted jackets (with a twisted angle of 30 and 60 degrees) for deployment in transitional water (beyond 60 m). To facilitate comparison, the dimensions of all the jackets remain the same, while, the geometric configurations are distinct. Static analysis was implemented to better understand the global load bearing behaviour of the jackets. First, the global displacement patterns at the tower top are compared. The individual reactions at mud-line were investigated, followed by the evaluation of the maximum von Mises stress. Subsequently, this research went on to investigate the effect of dynamic loading. In this dynamic analysis, three main critical points were considered, including the wave point (67 m), the platform and the tower top. A modal analysis was performed to compute the mode shapes and natural frequencies for all the jackets. The first five modes of all the jackets were also checked against the results available for the OC4 project. A similar analytical approach was adopted for the structural design of monopile or tripod foundations for offshore wind turbines. The results showed that in the static analysis both the traditional jackets and the twisted jackets were safe under the provided load combination. The twisted jacket proved to possess excellent structural behaviour compared to the traditional four-legged jackets, while maintaining the merits of lower material usage with fewer nodes. Analysing the von Mises stress revealed that the maximum stress occurred at the transition piece and close to the working platform. The modal analysis results of the jackets demonstrated that the twisted jackets (30 and 60 degrees) with the first natural frequency of 0.29 and 0.31 Hz fell under the soft-stiff design category whereas the traditional four-legged jackets were classified as stiff-stiff designs. The discovered structural performance of OWTs equipped with various jacket foundations contributes to the preliminary structural selection and optimal design of foundations of OWTs to be installed in transitional water.
Highlights
In recent years, there has been tremendous development in Offshore Wind Turbine (OWT) farm designs; both the water depth and the distance to shore are anticipated to keep increasing in the few decades
Jacket foundations have been used as a suitable substitute for other foundation structures in transitional water depth in many parts of the world, and this trend will continue to grow in markets where monopile is not yet consolidated
With lower dependency on soil compared to other structures forms, jacket foundations are preferable for installation under soft soil conditions because the structure is less susceptible to lateral loadings that might cause a pile foundation to tilt
Summary
There has been tremendous development in Offshore Wind Turbine (OWT) farm designs; both the water depth and the distance to shore are anticipated to keep increasing in the few decades. An important engineering challenge faced today is the development of lightweight offshore wind structures with minimal structural failure. This is the firm priority for many researchers in the field. Jacket foundations perform well in the transitional water depth mainly because of their lighter structural mass and greater structural stiffness. Jacket foundations have been used for two other noticeable deep water development projects, i.e. Beatrice (45 m) and Alpha Ventus (30 m), supporting large 5MW turbines [4]. The newly patented iJacket design is expected to provide a 25-30 percent weight reduction in steel
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