Numerical Analysis Of Cracks In Submarine Suspension Pipelines Under Pipe-soil Interaction

As an important part of offshore oil and gas field development,submarine pipelines bear the important function of oil and gas transportation,and also the "lifeline" of offshore oil and gas fields.Due to the complicated topography of the seabed,uneven terrain and the scouring effect of the current,some buried pipes have been suspended.The appearance of the suspended pipeline changes the bearing form and stress state of the original pipeline,and the effect of a series of marine environment,such as the current,the collision of the ocean falling objects,the friction of the fishing net and so on,makes the probability of the failure of the submarine pipeline significantly increase.In many defects that cause pipeline failure,the proportion of crack defects is the largest,which is very dangerous for the circumferential cracks of suspension pipelines.Therefore,the research on the circumferential crack of suspended pipeline is carried out in this paper.On the basis of fracture mechanics,the boundary of different pipe supports is considered,and a calculation model of pipelines with cracks is established.The effects of soil properties,current velocity,crack shape ratio,relative depth and length of suspended span on the ductile properties of cracked pipes were studied by numerical simulation.According to the characteristics of the ABAQUS software,the stress intensity factor of the surface crack is solved by the circumference integral,and the stress intensity factor of the circumferential penetrating crack is studied by the extended finite element method(XFEM)?First,the limiting suspension length of the pipeline is calculated by controlling the induced vortex induced vibration.The ultimate suspension length of the pipe with a surface ring crack and unstatic expansion is compared.It is obtained that the pipe contains a surface crack but has a very high bearing capacity under static action.The length of the limit suspension is greater than that of the trigger vortex.When the pipe has penetrated cracks,when the crack half angle is greater than 6 degrees,the limit length of the pipeline is less than the length of the triggered vortex induced vibration.When the pipe is simply supported,the characteristics of the surface crack are analyzed,and the crack toughness is obtained with different crack shape ratio and relative depth,and the critical shape ratio of the crack under different relative depths is found.The variation of the ductile crack toughness under the sea current,internal pressure and different suspension length is also studied,in which the different suspension length and the internal pressure of the pipe have great influence on the toughness of the crack.The fatigue threshold of the surface crack is also studied.It is concluded that the surface crack will crack and expand under the joint action of the sea current and the pressure load range 6-7MPa.When the pipe boundaries are in the pipeline,the distribution of the moment distribution and the size difference between the intact pipeline,the simple pipe,the fixed support boundary and the spring boundary are compared.The crack toughness of pipes under simply supported,clamped edges and spring boundaries is analyzed.The relative percentagesbetween different boundaries are obtained.The influence of different soil mass on stress intensity factors of pipe cracks is calculated.Under the spring boundary,the crack toughness at different positions of the pipe is calculated,and the stress intensity factor of the crack in the pipe is not changed much in the range of 2m in the middle span of the pipe.It shows that the existence of cracks in this interval is very dangerous.The variation of stress intensity factors at various ends of the pipeline at the end of the spring boundary is analyzed,and the dangerous location of cracks is found.And when one end is the boundary of the spring boundary is simply supported boundary,the change law of the stress intensity factor of the crack in all parts of the pipe and the dangerous position of the existence of the crack are found.The penetration crack of pipeline is also analyzed in detail?Because of the nonlinear model of pipe-soil contact,there are some limitations,and it has not risen to the national standard and standard.In the interaction of two cracks,the distance between the two different shapes of cracks is about 40mm,and the circumferential crack angle is about 5 degrees.