Assessment of Fracture Integrity for Trawl Impact of the Ormen Lange SFD Pipelines

Abstract

Ormen Lange Southern Field Development (SFD) is part of the phase 2 development of the Ormen Lange gas field located about 120 km offshore the coast of Norway. The SFD includes an 8 slot template, two 16 inch infield flowlines, one 6 5/8 inch MEG line and one umbilical located at about 850 m water depth. Although there are presently no fishing activities at the development area, the pipeline design has included a design case with evaluation of the structural integrity and potential for failure caused by future interaction with fishing gear such as trawl impact/pull-over and hooking. In contrast to the MEG line and the umbilical, which will be trenched and buried along the whole pipeline route, the 16 inch production flowlines will be left exposed on the seabed and may therefore be subjected to interference with trawl equipment in the future. It was therefore decided that pipeline engineering shall document that impact from trawl equipment during operation will not cause detrimental damage to the exposed flowlines, resulting in leakage of hydrocarbons to the environment and/or high cost of repair. In the event of impact from trawl equipment, it is likely that the pipe will be operating and thus be in a state of internal overpressure. Recent research has shown that the effect of internal pressure can be detrimental to the fracture response of pipelines with circumferential flaws subjected to bending or tensile loading. Today’s analytical equations that are the basis for most engineering critical assessments (ECA) are not capable of accounting for the effect of internal pressure when elastic-plastic fracture mechanics is considered. LINKpipe, which is a special purpose finite element program for assessing the fracture integrity of pipelines, is capable of accounting for the effect of internal pressure and was therefore chosen for the fracture integrity assessment. The flowline was analyzed for a range of defect sizes and material stress-strain behaviors. The finite element model was subjected to bending while under internal pressure, and both surface breaking defects and embedded defects have been assessed to ensure that the Ormen Lange SFD flowlines are capable of withstanding impact from trawl equipment during operation. The analyses were used to determine safe operational windows regarding acceptable defect sizes for both surface breaking and embedded defects for the parameters analyzed.