Conceptual analysis on Guyed Monopiles for Offshore Wind Turbine Generators in Deep Water

Abstract

The demand for offshore wind energy has never been higher, yet the number of ideal areas for wind farm installations is decreasing. As a result, larger wind turbines are being placed in deeper waters, which poses opportunities for floating foundations. However, due to the industry's inability to scale up the production of relatively expensive floaters, companies are exploring alternative foundation methods for offshore wind turbine generators in deep water, including the extension of the use of monopiles, which are now being considered for use in water depths up to 60 meters. However, higher loads in deep water require increased stiffness in the monopile structure, which is typically achieved by using larger thicknesses and diameters, resulting in XXL-monopiles. The steel used for these XXL-monopiles increases exponentially for greater water depths, and the designs cross the production limits. This study investigates the use of a guyed monopile as a more favorable alternative to a conventional monopile for deepwater applications with large wind turbine generators in the range of 60-120 meters of water depth and a rated power of 15 MW. The guyed monopile concept involves adding moorings to a conventional monopile to provide additional stiffness at a certain elevation, reducing the required material within the monopile. A numerical model is elaborated where the structure is represented by a one-direction FEM model having two degrees of freedom. The model is supported by springs that represent the soil and mooring system. The soil is modelled by a series of p-y reaction curves. For the mooring stiffness, a nonlinear stiffness approach is used where the axial stiffness and geometry of the mooring are considered. The method is used to determine the maximum stiffness that a mooring system can bring. After that, the mooring system is included in the FEM model. Static and dynamic analyses are performed to determine the response of the structure. The described methodology also implements requirements based on potential resonance (1P and 3P frequencies), ultimate limit state (yield strength, column buckling, and local buckling), and fatigue limit state (fatigue damage due to wind and waves). The static and dynamic responses are validated using Ansys. It is concluded that the moorings can provide enough stiffness to reduce the impact of the acting loads on the structure. A case study is conducted to compare the use of a conventional monopile and a guyed monopile for a foundation based on realistic wind and wave loads. Data of a reference wind turbine and a reference location is taken to find the advantages of a guyed monopile for a realistic case. The study iterates the process for different water depths and varying numbers of moorings. Greater water depths lead to more issues due to resonance and fatigue, which can only be overcome by increasing the diameter and thickness of conventional monopiles. This can also be overcome for guyed monopiles by increasing the mooring stiffness. The results show that the guyed monopile is favored over the conventional monopile in all cases, with up to a 45% reduction in steel required.
The case study's findings are used to identify key parameters in the design of a guyed monopile. The sensitivity of the mooring system stiffness is compared to other key design parameters, including monopile diameter, thickness, and embedded length. The study finds that the moorings have a favourable effect on the system's natural frequency. Also, bending and shear stress may be reduced when moorings are applied.