# Deformation Analysis of Large Diameter Monopiles of Offshore Wind Turbines under Scour
## 1 Introduction  
In offshore wind power projects, the foundation design accounts for about 25% of the total cost [1], and the foundation serviceability is of importance for the normal operation of wind turbines. Large diameter monopile (Figure 1) is widely used in practical engineering, with its advantages of superior economy and reliability [2].  
Under the environmental loading from waves and currents, the displacement of monopile can not exceed 0.5\({}^{\circ}\) of rotation at the mudline, or another similar value suggested by the turbine manufacturers, so it is necessary to study the deformation characteristics of large diameter monopile under horizontal loads. Owing to the harsh environmental conditions and high cost, the field test of large diameter monopile is difficult to carry out, hence the numerical analysis is more commonly used. [PERSON] et al. [3]pointed out that the finite element method is effective for the assessment of lateral loaded monopile and the conventional p-y curve method derived from field tests of a small diameter is not suitable for large diameter monopile. [PERSON] et al. [PERSON] _et al._ (2018) evaluated the effect of pile diameter on p-y curves. [PERSON] et al. [PERSON] _et al._ (2018) proposed a p-y curve construction method for large diameter monopile, and takes the roughness of the pile-soil interface into account. [PERSON] et al. [PERSON] _et al._ (2018) studied the cyclic effects on the p-y curve by centrifugal model test. Under the coupled waves and currents, local sour occurs around the pile foundation, which affects the static and dynamic response of monopile [PERSON] _et al._ (2018); [PERSON] _et al._ (2018); [PERSON] _et al._ (2018). Therefore, the scour effect must be considered in the deformation analysis. [PERSON] et al. [PERSON] _et al._ (2018) and [PERSON] et al. [PERSON] _et al._ (2018) reduced the buried depth of pile foundation by removing a certain depth of soil layer (Figure 2). [PERSON] et al. [PERSON] _et al._ (2018) considered the scour pit features and introduced a wedge-shaped failure mechanism to express the influence of local scour.  
It can be seen that, for the analysis of large diameter monopile of offshore wind turbines, horizontal deformation characteristic is of significance, and appropriate methods should be selected. The p-y curve method is widely used and it is crucial to obtain the p-y curve suitable for large diameter monopile. Secondly, as for local scour, both the scour characteristic and scour effect consideration method lack the support of field data, as they only took the maximum scour depth into consideration instead of the scour pattern, and whether they are applicable to large diameter monopile remains uncertain. To quantitatively summarize the scour characteristics and the effect of large diameter monopile is helpful for offshore wind power construction and subsequent load and displacement analysis. In addition, few studies have focused on the displacement sensitivity of monopile under scour.  
Therefore, this paper introduces the p-y curve method and the iterative procedure of finite difference method, and verifies the accuracy of the horizontal displacement analysis frame by model test and field test. Then, on the basis of the scour monitoring data from Dafeng offshore wind farm  
Figure 1: Generally accepted concept of large diameter monopile.  
Figure 2: Embedment reduction diagram.  
in Jiangsu Province, we conclude the scour characteristics of large diameter monopile and propose a method of considering the scour effect applicable to large diameter monopile. Finally, by the proposed method, we extract p-y curves of monopile considering the scour effect through ABAQUS, and analyze the deformation characteristics of large diameter monopile under scour. The sensitivity of pile head displacement under scour is studied by analyzing the relationship among scour depth, pile head displacement, and pile parameters, and the concept of scouring sensitivity is proposed, which may provide ideas for practical engineering.