Research On Influence Of Transverse Welds Of Steel Pipe Pile On The Stress Of Pile During Pile Driving

There are abundant oil and gas resources and wind energy resources in the ocean.With the continuous development of oil and gas resources and wind energy resources,more and more offshore oil drilling platforms and wind turbines have been built.With the advantages of high strength,fast construction,convenient manufacture and transportation,and small environmental impact,steel pipe pile foundation has become the most commonly used pile foundation form for offshore oil development platform and offshore wind power generation.In the process of steel pipe pile driving,the impact load generated by the driving equipment hammering the top of the steel pipe pile propagates in the form of stress wave in the steel pipe pile.After thousands of hammering,the steel pipe pile penetrates into the design soil layer.In this process,the steel pipe pile bears the alternating stress of tension and pressure.The existence of transverse weld will affect the propagation process of pile driving stress wave,and then affect the stress of pile body.At present,the steel pipe pile in marine engineering is more and more developing towards the direction of large diameter and super long.There are many transverse welds in the pile body.The existence of these welds has an important influence on the peak stress of the pile body,the stress distribution of the pile body and the number of stress cycles of the pile body.It is of great significance to study the influence of the transverse weld on the pile stress in the process of driving in the offshore engineering,to understand the stress distribution of the pile,to evaluate the safety and stability of the pile,and to study and define the fatigue damage of the pile in the later stage.The following are the main research contents and conclusions of this paper:(1)Based on the principle of stress wave,this paper introduces the propagation characteristics of stress wave in pile during driving,establishes a simplified hammer pile soil dynamic response model,and derives the calculation formulas of displacement,velocity,stress,maximum tensile stress and maximum compressive stress of any element of pile at any time,as well as the propagation coefficient of stress wave in continuous variable section and weld joint.In order to reflect the influence of weld on stress wave and pile stress,the pile weld model is introduced into the hammer pile soil dynamic response model.Based on the principle of rain flow counting method,the program of stress cycle counting is developed by using MATLAB.The equivalent stress amplitude and the corresponding cycle times of the pile are obtained by processing the stress time history curve of the pile under the influence of the weld.(2)Combined with a domestic offshore wind power pile foundation project,taking five test piles in the project as the research object,the F-V curve and the maximum stress value of pile body section of the five test piles are obtained by using the pile driving simulation program.And then the numerical simulation results are compared with the measured results.Based on the analysis results,the numerical model is modified,and then the feasibility of the pile driving simulation program is verified.(3)Based on the modified numerical simulation driving model,the influence of the number and width of transverse welds on the tensile and compressive stresses of the five piles was studied under the conditions of 25 m,40 m and 50 m respectively.According to the simulation results,the maximum compressive stress of the pile is usually near the top of the pile,and the number of transverse welds has an obvious effect on the compressive stress of the pile.With the increase of the number of welds,the compressive stress of the pile body decreases,the tensile stress of the pile body near and above the mud surface increases,and the tensile stress of the pile body near and below the mud surface decreases.With the increase of the weld width,the maximum tensile stress and the maximum compressive stress of the pile body decrease.The relationship between the weld width and the maximum stress of the pile body is logarithmic.(4)Based on the stress cycle counting method,the equivalent stress amplitude and the number of stress cycles of the pile with different weld number and width are obtained.The results show that the steel pipe pile is prone to high cycle stress fatigue in the process of pile driving.With the increase of the number and width of the transverse weld of the pile body,the total number of stress cycles of the pile body increases.With the increase of the depth of the steel pipe pile into the soil,the number of stress cycles of the pile body increases as a power function.