Investigation On Fracture Toughness Test And Structural Integrity Assessment Of High Strength X80 Pipelines

Pipeline transportation is the most economical and environmentally friendly way for the oil and gas industry.However,due to harsh environments,pipelines may subject to large plastic strains during its laying,installation and operation,which are typically in the range of 1%-3%.Based on the principles of fitness for service,the utilization ratio of the pipeline should be increased as far as possible for the sake of economic costs,while ensuring structural safety.In this study,the determination of fracture toughness of the pipeline steel and the calculation of crack driving force of circumferencial cracks are systematically investigated,aiming to improve the accuracy of fracture assessment.Moreover,a simplified reference stress method is proposed,and the corresponding fracture assessment procedure is introduced in detail.Fracture toughness is highly sensitive to the out-of-plane constraint effect of single edge notch tension specimens,it is found that as specimen thickness increases,the critical fracture toughness decreases significantly until the thickness-to-width ratio equal to 4,beyond which thickness effect becomes relatively weak.Accordingly,a dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint effect into account for the single edge notch tension specimen,while the specimen dimensions for single edge notch tension specimens recommended by DNV-RP-F108 and BSI 8571 deserve second thoughts.The crack tip opening displacement of the single edge notch tension specimen consists of tension displacement and rotation.As the specimen thickness increases,the maximum strain decreases sharply,and the high longitudinal strain zone moves from crack tip region to the uncracked side.It is found that as the specimen thickness increases,the location of the high strain zones changes significantly.For B/W=0.5,two high-strain zones originate from the crack tip;as the B/W ratio increases to 1,a large strain region moves to the uncracked side.A steady state is reached when B/W?3,where the maximum strain is located at a certain distance from the crack plane.The higher out-of-plane constraint leads to bending-dominated deformation in the ligament of single edge notch tension specimens,which reduces the plastic deformation ability and thus the low fracture toughness.This finding is further validated by the variation of stretching zone width against specimen thicknesses.Based on the theoretical background of the modified J-Q theory and J-T_z theory,a modified J-Q_z-M theory is proposed to address the globle bending stress and specimen thickness effect on the crack tip stress field of the single edge notch tension specimens.Numerical results show that Q_z is load-and distance-independent at all considered deformation levels,which is also sensitive to the specimen thickness.The proposed J-Q_z-M theory can be effectively used to quantify crack tip constraint for the investigated single edge notch tension specimens.An improved limit load solution is proposed to determine the reference stress and the crack driving force for pipelines subjected to large plastic deformation.A geometry and material dependent parameter?is used to relate the reference stress directly to the uncracked remote stress of the component for a wide range of crack configuration and material property.An empirical formula for?is provided based on the detailed parametric analyses.Finally,the proposed solution is manifested by comparing with various J estimation schemes in the literature.