Abstract
The meteorological mast (met mast) for the Taiwan Power Company’s offshore wind farm is located in Taiwan Strait near Changhua County. The p–y curve method recommended in the current offshore foundation design codes does not account for the local scour around the pile foundation; it overestimates the lateral pile deformation and underestimates the foundation stiffness. This paper presents a method to correct the initial modulus of subgrade reaction and modify the ultimate lateral resistance caused by the local scour. The natural frequency of the met mast structure is also determined by a numerical model and verified with the measured data in situ. A comprehensive parameter study is performed to analyze the effect of scour on the dynamic responses of the met mast. Two types of foundation model, a coupled-springs foundation model and a distributed-springs foundation model, are considered in the dynamic analysis of the met mast. The results demonstrate that using a distributed-springs foundation model provides a relatively accurate estimate of the natural frequencies of the met mast structure. Furthermore, the scour exerted significant effects on certain modes of the vibration responses. The natural frequencies of the met mast structure can be reduced by approximately 14% due to scour, particularly in the horizontal bending modes. This paper also provides a preliminary strategy for structural monitoring and analysis to detect scour damage on offshore wind turbines with monopile foundations.
Highlights
The dynamic behavior of the foundation must be considered in the design procedure of structures required to withstand dynamic loads
If a coupled-springs foundation model is used to simulate the monopile foundation in the dynamic response analysis of the met mast, the changes in the initial stiffness of the p–y curves caused by scour exert a significant effect on the natural frequency of the met mast structure
When a distributed-springs foundation model is used in the dynamic response analysis of the met mast, the natural frequency of the met mast structure calculated by the numerical model is not sensitive to the changes in the initial stiffness of the p–y curves
Summary
The dynamic behavior of the foundation must be considered in the design procedure of structures required to withstand dynamic loads. The natural frequency of an offshore wind turbine support structure is affected by its geometry, weight distribution, and foundation stiffness. When an offshore wind turbine is in operation and experiences typical wind and wave loads, Det Norske Veritas (DNV) [1] recommends that the shear strain γ of the soil surrounding the foundation must be less than 10−3 , such that the foundation stiffness can be calculated using the initial slope of the load–deformation curve [5,6,7]. Numerous researchers have proposed corrections to the p–y curves to analyze the load–deformation response in large-diameter monopile foundations [11,12,13,14]. This paper show the feasibility of structure monitoring and analysis method to detect scour around the monopile foundations of offshore wind turbines
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