Structural Integrity Assessment Of X65 Pipeline Including Constraint Effects

In the welded structure of offshore pipeline used to deliver oil and gas, the welded joints are the weakness location. This is mainly because there are inevitable welding defects, residual stresses and inhomogeneous microstructures and poor mechanical properties in the welded joint zone.In this study, the Crack Tip Opening Displacement (CTOD) tests were carried out to investigate the crack resistance behavior of weld metal, HAZ and base metal of X65 pipeline steel. CTOD tests were conducted at 0 degree and performed according to the requirements of the standard of BS7448, Part1: Method for determination of KIc, critical CTOD and critical J values of metallic materials and Part2: Method for determination of KIc, critical CTOD and critical J values of welds in metallic materials, in which a standard TPB (three point bending) shape was accepted. The test results show that the impact and fracture toughness of weld metal are much lower than that of base metal. The results of crack tip opening displacement tests show that the crack resistance of X65 base steel is much better than that of the weld.In this investigation, Structure Integrity Assessment Procedure (SINTAP) was applied as the standard to assessment and we used the procedure which designed by ourselves to assess the API 5L X65 pipeline steel with inner surface flaw. This assessment includes constraint effects. The failure line of level 4 of the weld was derived from the tensile test results. The results of the assessment showed that the assessment point (Lr, Kr) are located within the failure lines of analysis level 4. The results show that the welded joint of the pipeline is safe. The study is of great significance in terms of engineering and economic effectiveness. This study applied the designed procedure which based on SINTAP method to assessment the safety of pipelines structure has laid the foundation of application of SINTAP to pipeline structure assessment.