Investigation On Vertical And Lateral Thermal Buckling Of Subsea Pipes With Imperfection

As the marine technology is explored at high speed,the marine oil and gas in various countries have gradually expanded and radiated from the shallow sea to the deeper sea.According to the requirements of the mining and transportation process,subsea pipes used to transport oil and gas need to withstand the high temperature and high pressure.The temperature rise load,internal and outside pressure bring huge additional axial force for the subsea pipes.Excessive axial force may lead to the buckling failure of subsea pipes easily,and even lead to the leakage of oil and gas,which pollutes the marine ecological environment and threatens seriously the survival of marine organisms and humans.To this end,ensuring the global stability of subsea pipes during their design life cycle has become a key and common technical issue in the offshore engineering fields.Therefore,it is of great significance to study the thermal buckling behavior of the subsea pipes at various stages of the laying,safety evaluation and maintenance.Firstly,this thesis derives the critical load of vertical buckling for the straight pipe based on energy conservation.The reliability of the developed theoretical model is verified by the Hobbs classical solution.And the effects of Coulomb's friction coefficient and buckling waveform on the lateral thermal buckling are analyzed.Secondly,the numerical model of thermal buckling of subsea pipe with buoyancy module is established through the spring element,and the influences of length and coefficient of buoyancy module are analyzed.It is found that increasing the length of buoyancy module can reduce the stress level of buckling deformation.But this method is applicable only when the length of buoyancy module is less than that of the geometric imperfection.Thirdly,the thesis deduces the critical temperature rise of vertical buckling for the subsea pipe with prop imperfection on soft seabed based on energy conservation,and studies the effects of soil resistance coefficient and supporting height on the vertical buckling behavior.The buckling design method of subsea pipe on soft seabed is given,and the reason of the dynamic jump phenomenon of vertical buckling is discussed.The results show that vertical thermal buckling of subsea pipe on soft seabed occurs easier than that of pipe on rigid seabed,but the bending moment of buckling deformation becomes smaller.Therefore,the subsea pipe on the soft seabed should be induced actively to take place the controlled vertical buckling,rather than be prevented from buckling.Lastly,all subsea pipes in engineering have the local initial geometric imperfection due to the installation error or wave scouring.In this thesis,the critical temperature rises of lateral buckling for five kinds of subsea pipe with different initial symmetry imperfections are calculated numerically.The influences of imperfection type and out-of-straightness are discussed.Moreover,a dimensionless parameter is defined to distinguish the different initial imperfections.In addition,a general formula for calculating the critical temperature rise is also proposed based on the dimensional analysis method.This formula predicts successfully the critical temperature rise of asymmetric imperfect subsea pipes,which can be used to guide the engineering design on the thermal buckling of subsea pipes.