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Abstract
Severe foundation scour may occur around monopile foundations of offshore wind turbines due to currents and waves. The so-called p-y curves method is suggested in the existing design recommendations to determine the behavior of monopiles unprotected against scour and the reduction of effective soil stress is accounted for by the extreme scour depth. This conservative design approach does not consider the geometry of the scour hole and the effect of pile diameter on the soil resistance. An underestimated foundation stiffness would be obtained, thereby influencing the predicted overall response of the support structure of an offshore wind turbine. In this study, we calculated the load-deformation response and foundation stiffness of a monopile when scour occurred. The influence of pile diameter on the initial modulus of subgrade reaction, and the modification of the ultimate soil resistance of a monopile subject to scour are evaluated. The commercial software BLADED was used to simulate the dynamic response of the reference offshore wind turbine with monopile unprotected against scour at Chang-Bin offshore wind farm in Taiwan Strait. The results showed that when the p-y curve suggested by existing design regulation was used to calculate the load-deformation response, the foundation stiffness was underestimated where the scour depth was greater than the pile diameter, but the foundation stiffness was overestimated when the scour depth was less than the pile diameter.
Highlights
A monopile is a steel pipe pile with a diameter D of approximately 4–8 m and an embedded pile length L of approximately 30–50 m
In this study, study, regarding the initial modulus of subgrade reaction and the ultimate soil resistance, we regarding the initial modulus of subgrade reaction nh and the ultimate soil resistance pu, we compared compared various modification methods proposed by various researchers and modified the initial various modification methods proposed by various researchers and modified the initial stiffness of stiffness of - curve according to Sørensen et al [11]
To calculate the deformation response of large-diameter monopile foundation with scour, the approach of Sørensen [11] who proposed to modify the initial modulus of subgrade reaction and Lin et al [25] who proposed to modify the ultimate soil resistance are combined in this study
Summary
A monopile is a steel pipe pile with a diameter D of approximately 4–8 m and an embedded pile length L of approximately 30–50 m. The ratio of embedded pile length L to diameter D for the support structure of the monopile is between 4 and 8 [2], and monopile design is controlled by the lateral loading and moment. According to DNV [17] and DNV-GL [7], for the foundation of a monopile, scour depth may reach approximately 1 to 1.5 times as large as pile diameter. If a monopile foundation design is unprotected against scour, scour will decrease the embedded pile length, increase the foundation lateral deformation [18,19], reduce the foundation stiffness, and increase the natural frequency of the support structure [20,21,22]. Typical p-y curves constructed according to the results of field tests do not consider the geometric shapes of scour holes caused by local scour. Foundation scour must be considered when designing an offshore wind farm in Taiwan; the foundation stiffness of an offshore turbine in its lifetime can meet the requirements of its original design to ensure the safe operation of wind turbines
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