Keywords

1 Introduction

Fatigue analysis is a key link of steel structure foundation. In the fatigue assessment of offshore engineering structures, the fatigue cumulative damage analysis method based on S–N curve and Miner linear cumulative damage theory is commonly used [1]. Currently, most of the fatigue assessment methods adopted by various classification societies are based on S–N curve to check the fatigue of offshore pile foundations [2].

As an important component of offshore wind turbine foundation, steel pipe piles are subject to random wind, wave, current and working load for a long time. The wind, wave, current load is a kind of variable load. Under the action of alternating load, the stress in the structural material will change with time, resulting in fatigue damage of the steel pipe pile [3]. In particular, the waves caused by typhoon are characterized by stages, large energy and long period. Therefore, it is very valuable to establish a load-pile-soil ternary fatigue analysis model in combination with the field engineering application of finite element method to analyze the fatigue response of pile foundation structures during the whole typhoon period.

2 Project Overview and Natural Conditions

An offshore wind power project is located in unshielded offshore deep-water area, which is characterized by frequent typhoons and strong monsoon wind. When the typhoon comes, the pile foundation that has been constructed will easily lead to the fracture of the steel pipe pile under the reciprocating action of wind, wave and current. The elevation of pile head above the water surface is small, and the wind load is much smaller than the wave load. Therefore, only wave action is considered.

The wave load changes continuously in a typhoon cycle. In order to study the fatigue damage of steel pipe piles in a typhoon cycle, it is necessary to analyze it in stages. For offshore wind farms, a typhoon cycle can be divided into three periods: approaching period, landing period and far away period. Take typhoon “Lianhua” as an example. In the afternoon of July 8, “Lianhua” opened close to the east coast of Guangdong and became a typhoon at 20:00. The maximum wind force near the center was 12. At 12:15 on July 9, “Lianhua” landed in the coastal area of Jiadong Town, Lufeng City, Guangdong Province. At the time of landing, the maximum wind force near the center was 12, and then quickly weakened to a tropical depression. According to the typhoon period, the wave information with a wave height of more than 4 m was statistically analyzed, as shown in Fig. 1 and Table 1.

Fig. 1
A line graph plots wave height versus time. The line plotted for wave height depicts a gradual, increasing trend that reaches its peak at 8.30 and starts decreasing thereafter.

Wave Height of Typhoon “Lianhua”

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Table 1 Wave Information of Typhoon “Lianhua”
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3 Calculation of Pile Foundation Load and Establishment of Finite Element Model

As the ratio of wave height to water depth is large, the wave load has strong nonlinearity, so the wave load on pile foundation calculated by Stokes fifth order wave theory and Morison equation [4] is shown in Table 2.

Table 2 Wave load on pile foundation
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The finite element model is used to model the pile and foundation. The solid element is used for the pile foundation and soil. The contact treatment is used for the internal and external sides of the pile foundation and the soil. As shown in Fig. 2, See Tables 3 and 4 for the material parameters of the pile foundation and soil respectively.

Fig. 2
Two 3 D finite element models of the pile with the surrounding soil are generated using software, with the orientation of the models indicated.

Pile soil finite element model diagram

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Table 3 Pile foundation material parameters
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Table 4 Soil layer material parameters
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The contact property is defined by the pile soil surface to simulate the shear transfer and relative displacement between the pile and soil. The master–slave contact algorithm is used to select the pile with high stiffness as the main control surface, the soil surface as the subordinate surface, the pile soil contact is in the form of Mohr Coulomb friction penalty function, and the sliding friction coefficient of the interface is selected. The finite element model of steel pipe pile and soil is established by using 6-faceted 8-node linear reduced integral solid element. Through trial calculation, the soil diameter of the vertical pile is 10 times of the pile diameter, which can basically eliminate the influence of the boundary on the results. The soil bottom is fully restrained, and the lateral displacement is restrained.

The steel pipe pile is made of Q345B, which belongs to low alloy steel. Refer to the Standard for Design of Steel Structures (GB50017-2017) [5]. When the material is Q345B and the wall thickness is 16 mm ≪ 40 mm, the design value of the yield stress of the steel pipe pile at the mud surface is 335 MPa, and the minimum ultimate tensile strength is 470 MPa. As the steel is low alloy steel, the double diagonal model (as shown in Fig. 3) is used for numerical modeling in this paper, where the secondary strengthening stiffness of the model is about one percent of the initial stiffness.

Fig. 3
A line graph plots sigma versus epsilon. The line plotted for f y depicts an increasing trend with angles theta and theta prime indicated.

Double diagonal model

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When fatigue analysis of steel pipe pile is carried out through finite element software, the historical data after interpolation is used for loading, and Soderberg theory is used to correct the stress life of steel pipe pile materials with only a single S–N curve.

4 Fatigue Analysis Results

According to the accumulation of fatigue damage of steel pipe piles in different periods in a typhoon cycle, it is found that after 17.6 equal load histories, steel pipe piles will experience fatigue damage within the range of 1.3 m below the sand and mud surface to 2.8 m above the mud surface, resulting in pile foundation failure. The minimum safety factor of pile foundation is 1.84, which means that after the wave load in typhoon period is expanded to 1.84 times, The steel pipe pile is subject to fatigue failure after a typhoon cycle of cyclic loading, and the results are shown in Fig. 4.

Fig. 4
Two 3 D spectral models of steel pipe piles generated using software for fatigue life and safety factors are displayed. Different shades represent the stress distribution.

Cloud Chart of Fatigue Analysis of Steel Pipe Piles in the Period of Typhoon “Lianhua”

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If only the wave height during the landing period of typhoon “Lianhua” is increased by 1 m, that is, from 7.6 to 8.6 m, and other parameters remain unchanged, the wave load on pile foundation is shown in Table 5.

Table 5 Wave load on pile foundation after wave height increases 1 m during landing
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After the wave height of typhoon “Lianhua” increased by 1 m during the landing period, the fatigue analysis results are shown in Fig. 5. The service life of the pile foundation decreases rapidly, and the fatigue damage range becomes larger. After 2.8 load histories of the pile foundation, the steel pipe pile will have fatigue failure within the range of 1.6 m below the sand and mud surface to 3.7 m above the mud surface, resulting in the failure of the pile foundation. The minimum safety factor of pile foundation is 1.22, which means that after the wave load in typhoon period is expanded to 1.22 times, the steel pipe pile will undergo fatigue failure under the cyclic load of a typhoon period.

Fig. 5
Two 3 D spectral models of steel pipe piles with a wave height condition of one meter generated using software for fatigue life and safety factors are displayed. The stress distribution is minimal at the center of the piles.

Cloud chart for fatigue analysis of steel pipe pile after wave height increase of 1 m during landing

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5 Conclusion

This paper based on a project example, by combining the relevant specification, structural mechanics, and finite element method, established the pile soil load—ternary fatigue analysis model, and the typhoon period all pile structure fatigue response analysis, pointed out that the law of the pile foundation damage, in the case of other parameters constant, slightly increase the typhoon wave height, will cause the service life of pile foundation, And the range of fatigue damage is larger. The fatigue response analysis of pile foundation structure during the whole cycle of typhoon is of great significance to the optimization of pile stabilization measures and the improvement of the service life of pile foundation.