The objective is to determine the severity of the flaws in the deformed pipe and to make the repair/replacement decision. 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 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. Because of the large local deformation, the materials in the deformed pipe region have undergone large local plastic strains i.e. 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. The stress concentration factor can be up to 10 depending on the geometry of the imperfection. These kinds of imperfections change the local geometry of the pipe, and therefore, a stress concentration and local bending stress will be induced. 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. During the installation of offshore pipelines the pipe wall at the 6 o’clock position sees large compressive strain and local buckling may occur.

According to the AGA report, 39% of offshore and 37.7% land based natural gas pipeline failures were caused by outside force. A major threat to the integrity of pipelines is mechanical damage, caused by outside natural forces. Pipelines are relied upon to transport hazardous liquids and gasses over long distances.

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