Research on the Fatigue Life and Fatigue Reliability Analysis of the Vibroseis Baseplate
High precision vibroseis become a key technology of oil and gas exploration due to its high efficiency,environmental protection,safety and low cost.The core signal excitation device of vibroseis is the vibrator and its baseplate.In the processes of seismic wave excitation,the baseplate is not only subjected to the gravity of the entire vibroseis,but also suffered the alternating excitation force.Cracks could be generated in the baseplate,and fracture could even occur due to such a serious loading.This failure reduces the baseplate life significantly,as well as affects the quality and precision of the signal excitation seriously.To solve above problems,the objective of this work is to study the fatigue life and fatigue reliability analysis research of the vibroseis baseplate.The research results will provide the theoretical basis of strengthening baseplate’s fatigue resistance,therefore leading to longer baseplate life,higher quality of the seismic signal excitation,smaller signal distortion,higher the accuracy of oil and gas exploration ultimately.The main research contents and achievements are as follows:(1)On the basis of vibroseis vibrator structure and working principle,the dynamics model of baseplate has been established by using Sallas model and vibrator system dynamics theory during the seismic signal excitation.The analysis results show that the baseplate force amplitude reaches up to 275KN,such a huge baseplate force is an important causing of damage and fractures of baseplate.The macroscopic and microscopic morphology analysis of baseplate fracture have been carried out,the result shows that the baseplate fracture is a typical fatigue failure.The baseplate damage mechanism turns out that the baseplate materials with defects change from elastic deformation to plastic deformation under the cyclic loading,this causes initial cumulative damage crack region.As baseplate damage accumulates continuously,the transgranular fracture of inner tissue will happen.The micro crack becomes macroscopic crack,then expands stably,and finally the baseplate fractures.(2)Since it is difficult to obtain the baseplate stress response law with high accuracy and efficiency,so the orthogonal experiment design has been used to get the parameter optimization combination experiment scheme.Based on this scheme,we carry out the vibrator baseplate dynamic simulation research.The results show that the hart-shape reinforcing plate is the key fatigue part of baseplate,where the stress is highest,the fatigue damage value is largest,and it appears the concave and upturn deformation.Besides,the linear extrapolation method has been adopted to figure out the hot spot stress value.Then the function formula between the baseplate hot spot stress and input parameters are fitted by response surface method.The stress response formula has a very high efficiency.Above research result is the important step to establish baseplate fatigue life prediction model and fatigue reliability analysis model.(3)According to the special working condition of seismic signal excitation,a test platform is set up to develop the fatigue experimental research of baseplate specimen.The crack propagation law has been found out,as well as the S-N curve of baseplate specimen has been fitted by using the fatigue life results.The fitting S-N curve is high in accuracy,for it has high correlation coefficient and low fitting error mean square value.Meanwhile,the baseplate fatigue life prediction models have been established based on the S-N curve method and fracture mechanics method respectively.The effects of macroscopic and microscopic factors on the baseplate fatigue life are revealed through the research of the baseplate fatigue life theory.Further,the two fatigue life theoretical model have been proved to be accurate and efficient by using the fatigue life test value.The research results provide an important theory and experiment foundation to reveal the baseplate fatigue behavior characteristics and improve the fatigue life prediction model.(4)The multifactorial evaluation analysis of the baseplate fatigue reliability analysis methods(S-N curve method and fracture mechanics method)have been carried out by using the fuzzy comprehensive evaluation method.A new fatigue reliability analysis method(fuzzy comprehensive analysis method of fatigue reliability)of baseplate is proposed,which is not only considering the macro-structure and load factors,but also thinking over the micro-material and tissue factors.The baseplate fatigue reliability is figured out to be 0.9759,which means that the fatigue resistance of the baseplate is qualified,but it is still insufficient compared to the fatigue reliability engineering standard(0.99).Therefore,it is necessary to improve the baseplate fatigue reliability by structural design and material selection.(5)The sensitivity analysis of baseplate fatigue reliability to random parameters have been developed by using the Monte-Carlo sensitivity analysis method.The research results show that the mean value and variety of the below cross column diameter D and stress amplitude △σ have the most significant effect on its fatigue reliability.Therefore,the optimization model has been built,the failure probability is the restriction,and the fatigue life is the objective function.Then the random parameters optimization design is carried out based on the fatigue reliability,the comprehensive fatigue reliability of baseplate reaches up to 0.9935,increased by 1.8%.The research results provide scientific theory foundation and engineering guidance to improve the fatigue resistance and safety of the vibroseis baseplate.