Research On Fatigue Life Prediction Of Oil Derrick Steel Structures With Semi-rigid Joints

With huge derrick structure enjoying multiple members and complex force,oil rig derrick steel structure provides support and protective supporting structure for the whole oil rig system during the process of oil drilling.While once the structure breaks out and capsize,the outcomes will be beyond our imagination.In the long service process,the derrick shall bear dead load as dead weight and alternative loading by which inner stress of the structure continuously changes in turn,easily resulting in fatigue failure of derrick structure.As the key hinge of beam-column joint, shearing force,axial force and torque,the carry-over moment of derrick with different link way and linking ability directly impact fatigue property of joints.However,when making research on oil derricks,traditional researches always consider joints of various moments as completely rigid,which is not in accordance with actual situations of oil derricks and errors of calculated outcome will occur. Fully considering the semi-rigid character of linking joints,the paper carried out a systematic research on fatigue life issue of huge oil derrick steel structure.According to actual working conditions of oil rig derrick,the paper systematically illustrates fatigue theory of oil derrick steel structure and confirms its fatigue type is mechanical and high-frequent vibration fatigue by adopting safe life design method to proceed the fatigue analysis of derricks and nominal-stress approach to measure fatigue lifetime of derricks;Based on systematically explained semi-rigid jointing character, the paper establishes mathematics model,putting forward the main research method of joint semi-rigid bending moment-corner characters.The paper respectively uses rotational springs element method and calculation component method and combines joint characters of derricks to deduce,calculate,analyze and explain the rigidity of semi-rigid joints of derricks.It finds out that independent spring method is suitable for simulating semi-rigidity of joints in the whole structure. Taking JJ315/45-K oil rig derrick as examples,the paper draws curves of bending moment-corner relationship by building the model of derrick beam-column joints and stimulating bending moment-corner characters. The paper also systematically studies the interactional relationship of joint rigidity,extreme bending moments and extreme bearing capacity by changing section thickness of beam-column joints,height and size and material types. It turns out that the thickness,height of beam flange and material property impose a rather big impact on joint rotary characters,while the thickness of column flange and column web will has less influence on joint rotary characters.The paper establishes theoretical calculation on the fatigue lifetime of semi-rigid joint derrick and compares the calculated results with the results of rigid joints in the same condition,then finding out that the calculated outcomes will be more in accordance with practice when making use of semi-rigid joints to deal with joints of derrick steel structure and to carry out fatigue analysis. In the meantime,when using finite element method to calculate the fatigue lifetime of oil derricks with semi-rigid joints,it shares the same results as semi-rigid theory.The research demonstrates that current fatigue theory can be applied to the analysis of semi-rigid joints of huge steel structure and semi-rigid characters of joints can also not be neglected when studying fatigue lifetime problem of derrick structure.Based on the fatigue theoretical research of joint semi-rigidity,the paper studies the bearing capacity and lifetime issue of huge steel structure as oil derricks,which enjoys significant theoretical meaning and applied value.