Experiment And Safety Assessment On Pipes With Circumferential Surface Crack Repaired By Sleeve Filled With Epoxy Resin

Oil and gas pipelines across the country formed a large pipeline network system.With the increase of the service life,the oil and gas pipelines show various defects such as welding defects,material defects,and corrosion.Pipeline repair has become an indispensable and regular work in the process of oil and gas transportation,every year need to spend a lot of manpower and material resources.The crack mouth opening displacement(CMOD)is an accepted fracture parameter for surface cracks.After the pipeline is repaired,the pipe surface cracks are covered by the repaired material,and the CMOD is difficult to measure and can not evaluate the repair effect.However,strain can be measured by strain gage before and after repair.Therefore,this paper intends to investigate the strain characteristics in the vicinity of the crack,establish the relationship between the strain change trend and the CMOD change trend,and evaluate the repair effect of epoxy sleeve on the circumferential surface crack by strain trend.First of all,the internal pressure and bending experiment of pipe with circumferential short crack is carried out,the strain characteristics in the vicinity of the crack are studied.The results show that axial strain of the middle point of crack changed from positive to negative and keeps decreasing constantly with the increase of bending moment.Then,the internal pressure and bending experiment of pipe with the same depth crack whose angle is 90 in circumferential direction is carried out.The linear variable differential transformer was used to measure the CMOD,also,the load condition of crack initiation is monitored dynamically by testing the voltage at both sides of the crack.Through analyzing the test data,it is verified that axial strain of the middle point of crack keeps decreasing constantly with the increase of bending moment is not accidental.The relations between CMOD and axial strain in the middle of crack are studied,the results show that both CMOD and axial strain of the middle point of crack have a linear relationship.In order to further study the relationship of CMOD and axial strain of the middle point of crack,the internal pressure and bending experiment of pipe with the same depth crack whose angle is 120 in circumferential direction is carried out,the relationship of CMOD and axial strain of the middle point of crack is studied once again,the results also show that both CMOD and axial strain of the middle point of crack have a linear relationship.Then,the relationship between the trend of strain change and the trend of CMOD change is established.At last,the axial strains of the middle point of crack before and after repairing are compared by pipeline repairing experiment.The results verify that using axial strain change trend of the middle point of crack evaluation the repairing effect of the pipes with circumferential surface crack repaired by epoxy sleeve is feasible.In order to study on the security under combined loads of internal pressure and bending moment,on the basis of BS7910,the safety evaluation is executed by using specimen with crack of base metal in test 1.The allowable load of pipe with circumferential surface crack was known,and the safety margin of every assessment point is calculated.According to the axial strain at the middle of the crack obtained in test 1,the safety margin of every assessment point is also calculated,Safety margin obtained by the two methods were compared.