Working conditions of casing pipes in drilling rigs can significantly influence the initiation and development of damage in the material, and therefore also the safe service of the entire system. In this work, an integrity assessment of a pipe with initial defect (machined surface crack) is presented. The position of this defect is on the external surface; unlike transport pipes, where internal surface is often endangered due to the contact with the fluid, casing pipes are also often exposed to damages at the external surface. A pipe segment exposed to internal pressure is examined experimentally and numerically, using the finite element method. Experimental setup included tracking of crack mouth opening displacement (CMOD) values, as well as J integral. Criteria for pipe failure are determined on the finite element (FE) models of the pipe; fracture initiation and plastic collapse are considered as failure mechanisms. Several 3D models with different crack sizes are evaluated. 2D plane strain models are also examined, to determine the applicability limits of this simplified approach. Integrity assessment criteria for the analysed geometries are discussed. Assessment of fracture resistance of the pipeline material is also considered in this work. Besides the standard SENB specimens, Ring specimens cut from the pipe are tested, and the results are compared. Both specimen geometries are modelled using local approach to fracture, by application of the micromechanical Complete Gurson model (CGM), developed by Z.L. Zhang. It is shown that the Ring specimens have similar fracture conditions under bending load as SENB specimens. Since they are much simpler to fabricate from the pipe than standard specimens, it is concluded that they can be used for assessment of fracture of the pipes with axial cracks.
COBISS.SI-ID: 18871062
According to standard Standard BS 7448: Part 2: 1997, “the tensile properties of the region in which the crack tip lies are required, at the fracture toughness test temperature for calculation of fracture toughness and qualification assessment” is difficult to completed in heterogeneous microstructure of fusion welded joint. Arc welds are heterogeneous as the thermal cycle during welding gives rise to local variations in microstructure. Applied load at the crack tip in heterogeneous microstructure causes additional effect on crack driving force. Both phenomena (heterogeneous microstructure and crack driving force) have effect on structure integrity of welded joint. The goal of paper is analyzed an effect of heterogeneous microstructure on global mechanical properties of mis-match welded joint, in order to determine lower shelf of mechanical properties for structure integrity assessment. The results are going to be analyses by using weakest link concept and Weibull statistical distribution.
COBISS.SI-ID: 19061782
Structural integrity assessment of the pipelines requires fracture toughness in order to predict the loading capacity of pipeline or resistance to the initiation and crack growth. The thin-walled structures as pipelines are usually unsuitable for the standard testing of fracture toughness. Particularly, it is very difficult to perform the fracture toughness testing in the case of longitudinal surface cracks. In order to find an alternative technique for the measurement of fracture toughness of the already delivered pipeline segment, the new pipe-ring notched bend specimen (PRS) has been proposed. In this paper the differences in fracture behavior between the standard single edge notch bending (SENB) and non-standard pipe-ring notched bend (PRS) specimens are discussed. To avoid the uneven fatigue crack front as a consequence of complex fatigue loading which caused the different fracture behavior, the standard single edge notched bend specimens has been used for comparison to ring specimens. The stress-strain conditions at the crack tip were analyzed by finite element modelling which is not the part of content in this paper. The fracture toughness was measured by testing of both types of specimens. The critical crack tip opening displacement was determined as a crack tip surface strain-relaxation by using stereo-optical grading method and compliance method. The comparison between CTOD-R curves of standard and non-standard specimens shows comparable similarity in fracture behavior, especially in the stable crack initiation, but shows significant difference during ductile tearing. It seems that the use of new pipe-ring notched specimens is promising for low cost and simple testing of fracture toughness.
COBISS.SI-ID: 19004694