Research On The Girth Weld Cracks Inline Inspection And Safety Assessment Of In-service High Strength Pipelines

This dissertation focuses on the current in-service high-strength steel pipeline（HSSP）,for frequent failure of girth weld（GW）,which leads to pollution caused by oil leakage and casualties caused by explosion and fire.Combined with current situation of in-line inspection（ILI）and safety assessment technologies for GW of HSSP at home and abroad,the key technical requirements for safety assurance of welding structure of in-service HSSP are analyzed,and research ideas and methods of weld crack ILI and safety assessment models are proposed.In the aspect of ILI,the applicability of 4^th generation magnetic flux leakage（MFL4）and ultrasonic circumferential crack detection（USCCD）for GW cracks were compared and analyzed by conducting pull-test,real pipeline ILI and excavation verification;the feasibility of electromagnetic eddy current detection for inner surface cracks of GW was discussed through laboratory single probe and probe array related experimental research.In terms of safety assessment,the assessment method for GW defects of in-service HSSP was established based on specific failure assessment curve（FAC）.Finite element simulation,wide plate tensile test and full-scale composite load blasting test under internal pressure and bending load were carried out to verify the applicability of the method.Based on Monte Carlo method,the failure probability of GW structure of in-service HSSP under typical working conditions is quantitatively evaluated,and the influence of probability distribution of defect depth,fracture toughness and yield strength on the structural integrity of pipeline GW is analyzed;a method based on indentation strain is proposed to calculate the residual stress and load of GW of HSSP in service,which provides theoretical support for obtaining external load parameters for safety assessment.The main achievements of this paper include:（1）MFL4 is compared with USCCD.The former is more sensitive to volume GW defects such as incomplete fusion or incomplete penetration,undercut with certain opening width and local wall thickness thinning,but it is difficult to detect small-scale cracks with closed or very narrow opening.USCCD is sensitive to the crack boundary,but at present,the technology is easily disturbed by irregular weld formation,resulting in false reproting.（2）Based on the principle of eddy current testing,an electromagnetic eddy current nondestructive testing equipment test platform was designed.The high-speed eddy current signal acquisition device and defect signal imaging software were developed.When the detection speed reached 3.2m/s,the signal-to-noise ratio（SNR）of surface crack-like defect signal was up to 30d B with good quality of detection signal.Based on the main technical indexes such as lift off value,detection speed and automatic detection signal identifiability,it is confirmed that the electromagnetic eddy current technology is suitable for the internal detection of circumferential surface crack like defects in oil and gas pipelines,and it overcomes the technical limitations of MFL4 and USCCD equipment for the low detection rate of closed or narrow surface crack like defects.（3）The specific FAC for in-service HSSP weld safety assessment method is established,which is verified by finite element method,wide plate tensile test and full-scale composite load blasting test under internal pressure and bending load.Based on a large number of in-service HSSP GW material performance test and statistical analysis of defect excavation non-destructive testing data,the distribution of evaluation parameters such as tensile strength,impact energy,defect length and defect depth are given;Based on the Monte Carlo method,failure probability evaluation of typical conditions are carried out,and quantitative analysis on the parameter sensitivity of safety assessment is realized.（4）In order to solve the problem that it is difficult to obtain the service load for the safety assessment of in-service pipeline GW,an indentation strain method is proposed for the first time to measure the strain in the far end（far away from GW）and the near the weld zone of the in-service HSSP,and the method of subtracting each other can obtain the axial welding residual stress and external axial load of the in-service pipeline GW.The main mechanical properties such as micro fracture toughness,yield strength and tensile strength of in-service pipeline welded joint are obtained by indentation method,which provides theoretical support for comprehensively and accurately evaluating the safety of in-service pipeline girth weld.