Keywords

1 Introduction

Fatigue analysis is a key part of the quality and safety control of steel structure foundation [1,2,3]. At present, classification societies of various countries use the method of fatigue cumulative damage analysis based on S–N curve to evaluate Marine engineering structures, and carry out the fatigue check of Marine pile foundation [4, 5].

As an essential part of the seaport’s structure, the steel pipe piles are subject to environmental loads and long working loads. As wave is a cyclic load with random changes in environmental loads, the internal stress of steel pipe piles will change under such alternating loads, and fatigue damage of steel pipe piles will be caused as the number of cycles increases [6, 7]. In particular, steel pipe piles are prone to fatigue failure when subjected to large bending moments, so it is crucial to study the fatigue response of steel pipe piles under wave load in combination with realistic parameters from field engineering.

2 Project Overview

A pier is located in deep offshore water without cover and 222 steel pipe piles made of Q345B are under construction. The construction area is characterized by frequent typhoons and strong monsoon winds. Refer to the “Steel Structure Design Standard” (GB50017-2017) [8], when the material is Q345B, the wall thickness is 16 mm < φ < 40 mm, the design value of yield stress at the mud surface of steel pipe pile is 335 MPa, and the minimum limit tensile strength is 470 MPa.

Under the reciprocal action of wind and wave, steel pipe piles are prone to induce alternating stresses in the local structure [9, 10]. The pile foundation constructed in a wharf has a total pile length of 56 m and a pile diameter of 1.2 m, which belongs to the steel pipe with super length to diameter ratio. The length of the pile below the designed low water level is 49 m. The wind load on the pile foundation is much smaller than the wave load, so only the effect of the wave load on the pile foundation is considered.

3 Natural Conditions and Calculation of Pile Foundation Load

Due to the uncertainty of waves, it is difficult to obtain the structural stress response under the action of random waves, and it is also difficult to obtain the maximum and minimum stress under the action of random waves. In this paper, conditions with greater risk of causing alternating loads are selected according to experience or trial calculation, and the maximum wave elements in the monsoon period and the wave elements in the H4% typhoon period, which occur once every five years, are selected for calculation. Due to the large ratio of wave height to water depth, wave load has strong nonlinearity, so the wave load on pile foundation can be calculated using Stokes fifth-order wave theory and Morison equation [11], as shown in Table 1, and soil parameters are shown in Table 2.

Table 1 Wave load on pile foundation
Full size table
Table 2 Soil layer material parameters
Full size table

4 Natural Conditions and Calculation of Pile Foundation Load

The contact properties of the pile-soil interface were defined in the form of Mohr–Coulomb friction function to simulate the shear transfer and relative displacement between pile-soil. The master–slave contact algorithm was adopted, and the pile with large stiffness was selected as the master surface and the soil surface as the slave surface. The finite element calculation model established between the steel pipe pile and soil was established by using 6-hedral and 8-node linear scaling integral solid elements. The load-pile-soil ternary fatigue analysis model combined with soil constitutive model is established. In order to eliminate the influence of boundary on the calculation results, the test results show that the influence of boundary on the calculation results can be basically eliminated when the soil base is fully constrained, the lateral displacement is constrained, and the soil diameter of the straight pile is 10 times of the pile diameter.

In order to verify the reliability of the calculated results of the finite element model, two single pile foundations with displacement monitoring data were taken as an example to verify the reliability of the finite element model. As for the external force on the pile foundation, the wave load calculated by the wave parameters at that time is applied, and the pile foundation of the wharf without pile stabilization measures is simulated. The horizontal displacement of the pile foundation in the harsh Marine environment is shown in Fig. 1.

Fig. 1
Two diagrams display steel pipe pile with the minimum values of U 1, U 2 in the foundation and maximum values at the top of the pipe. Beside each diagram is a line graph of numerical simulation and measured fitting plotted in elevation versus horizontal displacement of pile body.

Model verification

Full size image

5 Numerical Simulation Analysis of Fatigue Response of Steel Pipe Pile

When fatigue analysis of steel pipe piles is performed with finite element software, the load type is loaded using the load ratio method, and the stress lifetime of a single S–N curve of a steel pipe pile material is corrected using Soderberg theory. Fatigue life analysis and safety factor analysis of steel pipe piles under 1 × 109 cyclic load revealed the fatigue damage law of steel pipe piles under wave load.

Under different sea conditions, the fatigue damage degree of steel pipe pile is different, especially the stress concentration is easy at the weld, which aggravates the fatigue damage. In addition, the pile foundation soil parameters also have some influence on the fatigue damage of steel pipe piles. In this paper, the fatigue analysis and calculation conditions of steel pipe piles are determined by comprehensively considering the sea condition, pile weld and soil layer parameters, as shown in Table 3.

Table 3 Fatigue analysis of steel pipe piles to calculate working conditions
Full size table

The fatigue analysis of the pile foundations under wave load during monsoon and typhoon periods, without taking into account the welds, is shown in Figs. 2, 3. The analysis of the fatigue response of the pile foundations under wave load during monsoon and typhoon periods, taking into account the welds, is shown in Figs. 4, 5. The analysis of the fatigue response of the steel pipe piles at the maximum wave load during the typhoon is shown in Fig. 6, after taking into account the welds and increasing the riprap by 5m.

Fig. 2
Two diagrams display, a, fatigue life and, b, safety factor of the pile foundation. In a, the fatigue life is evenly distributed across the steel pipe pile. In b, the middle part has a near minimum value going to the maximum value on both ends.

Monsoon fatigue analysis diagram

Full size image
Fig. 3
Two diagrams display, a, fatigue life and, b, safety factor of the pile foundation. In a, the middle part has minimum fatigue life and maximum fatigue life near the middle going to both ends. In b, the middle part has a near minimum value going to the maximum value on both ends.

Typhoon fatigue analysis diagram

Full size image
Fig. 4
Two diagrams display, a, fatigue life and, b, safety factor of the pile foundation. In a, the fatigue life is evenly distributed across the steel pipe pile. In b, the middle part has a near minimum value going to the maximum value on both ends.

Considering the fatigue analysis diagram of the weld seam during the monsoon period

Full size image
Fig. 5
Two diagrams display, a, fatigue life and, b, safety factor of the pile foundation. In a, the middle part has an average fatigue life and maximum fatigue life near the middle going to both ends. In b, the middle part has a near minimum value going to the maximum value on both ends.

Considering the fatigue analysis diagram of the weld during typhoon period

Full size image
Fig. 6
Two diagrams display a, fatigue life and, b, safety factor of the steel pipe piles. In a, the middle part has an average fatigue life and maximum fatigue life across. In b, the middle part has a near minimum value going to the maximum value on both ends.

Fatigue analysis diagram for typhoon period considering weld seam and adding 5m riprap

Full size image

The finite element numerical analysis of fatigue response of steel pipe pile shows that there is no fatigue damage in the whole steel pipe pile during the monsoon period after 1 × 106 cyclic loading. When the influence of the weld is considered, the minimum safety factor is reduced from 2.44 to 2.16, which indicates that after the wave load increases to 2.16 times during the monsoon period, the steel pipe piles with welded seams will suffer fatigue failure within the range of 3m below the sand surface to 10m above the mud surface after 1 × 109 cycle load.

When the influence of weld is not considered, fatigue failure occurs within the range of 1.8 m below the mud surface to 3.2 m above the mud surface after 3 × 105 cyclic loading. When considering the influence of welding seam, fatigue damage occurs in the welding area after the steel pipe pile is subjected to 1.7105 cyclic load, and the minimum safety factor decreases from 0.81 to 0.72, indicating that after the wave load is reduced to 0.72 times during the typhoon period, after 1 × 109 cyclic loading, steel pipe piles with welded seams will suffer fatigue failure within the range of 1.7 m below the sand surface to 3.6 m above the sand surface, resulting in pile foundation instability failure.

The numerical simulation results of fatigue response analysis of steel pipe pile after 5 m thick ripolite is added above the original seabed mud surface show that, considering the influence of weld seam, steel pipe pile fatigue failure occurs in the welding area after 9.4 × 105 cyclic load during typhoon period, and the minimum safety factor is 0.99, which means that after the wave load is reduced to 0.99 times during typhoon period, the steel pipe pile fatigue failure occurs in the welding area. After 1 × 109 cyclic loading, fatigue failure of steel pipe piles with welded seams will occur within the range of 7.8 m below the riprap surface to 10.7 m above the riprap surface, resulting in pile foundation failure. The results show that the fatigue resistance of the pile foundation can be increased after the riprap stabilizing pile treatment. However, it is difficult to carry out underwater construction with 5m thick underwater rubble. When conditions are poor, the quality of construction may not be guaranteed and it is difficult to form artificial foundation sections.

6 Conclusions

By establishing a load-pile-soil tridimensional fatigue analysis model and analyzing the effects of sea conditions, pile foundation welds and soil layer parameters on the fatigue damage of steel pipe piles, the following conclusions can be drawn:

  1. (1)

    Regardless of whether the factor of weld is considered, the wave load of the wave height below 3.5 m during the monsoon period will not cause fatigue damage of steel pipe piles with pile diameter of 1.2 m and wall thickness of 20 mm.

  2. (2)

    Considering the weld during typhoon will cause the fatigue life of steel pipe piles to decrease rapidly, the range of fatigue damage will increase, and the starting position of fatigue damage will shift to the weld.

  3. (3)

    The fatigue life of steel pipe pile can be increased after the riprap stabilization pile treatment, the fatigue damage range is larger, and the fatigue resistance of steel pipe pile is improved as a whole.