Pipelines are relied upon to transport hazardous liquids and gasses over long distances. A major threat to the integrity of pipelines is mechanical damage, caused by outside natural forces. According to the AGA report [1], 39% of offshore and 37.7% land based natural gas pipeline failures were caused by outside force. During the installation of offshore pipelines the pipe wall at the 6 o’clock position sees large compressive strain and local buckling may occur. Dents may also occur by impact onto hard objects such as the rollers on the stinger or rocks on the seabed and by anchor impact etc. These kinds of imperfections change the local geometry of the pipe, and therefore, a stress concentration and local bending stress will be induced. The stress concentration factor can be up to 10 depending on the geometry of the imperfection. As a result, the local stresses will be much higher than the design stresses for the pipeline in operation subject to internal pressure and axial strain, and fracture and fatigue capacity of the pipelines with these imperfections will decrease dramatically. Because of the large local deformation, the materials in the deformed pipe region have undergone large local plastic strains i.e. 10–20% plastic deformation. The material properties of the pipe with large plastic strain will be drastically changed, and therefore the fracture resistance of the pipe is expected to be decreased, especially when the damage is located at the seam or girth welds. To assess the criticality of such damage which often can be associated with strain induced flaws in the heavily deformed parent metal and welds, ‘fitness-for-service’ assessment is required. The objective is to determine the severity of the flaws in the deformed pipe and to make the repair/replacement decision. At present there are no definitive assessment guidelines that consider these aspects and how to incorporate the behaviour and fracture capacity of the heavily deformed material. In this paper, a numerical model of typical local imperfections i.e. buckles and wrinkles was established from the in-situ geometry measurements. The local stress distributions of the pipes were analyzed. Based on this stress analyses, the stress concentration around the local imperfections in operation were obtained and the fracture capacity and fatigue life of the pipeline was assessed. The tensile and J R-curve data for deformed pipeline materials were obtained by the DNV Energy laboratory to study the influences of the large plastic strain on the material properties, and the fracture resistance and fatigue crack growth of the pipe. Based on the numerical analysis and test results, a fracture combined fatigue assessment was performed to decide on the mitigation and remediation strategies for the pipeline.
Resetting scheme for plastic strain surface in constitutive modeling of cyclic plasticity