Subsea flowline development for a field on the Norwegian Continental Shelf comprises design of HP/HT flowlines for oil and gas transport from subsea manifolds. Flowline engineering faces several challenges related to flowlines crossing very uneven seabed. Among them is choosing an expansion design philosophy that minimizes the need for continuous survey and intervention work updates. Control of buckling behavior is ensured by use of rock berms. The standard design of the rock dumps according to  is based on buckle sharing criterion for axial friction capacity, which aims to control initiation of buckles. However, fulfilling the buckle sharing criterion alone does not provide sufficient control of pipeline behavior through the different operational conditions. In addition to buckle sharing criterion fulfillment , anchoring rock berms shall also ensure that the point of zero axial displacement is inside the berm for all operational conditions. This will give control over feed-in lengths and counter pipeline walking between sections. Criteria for ARBs are established, covering post buckle and shutdown conditions in addition to buckle sharing. Unstable buckle configuration during shutdown/start-up cycles is defined as buckle walking. Redistribution of feed-in between buckles is frequently observed as the cause of buckle walking. Use of uplift cover is avoided or minimized in order to eliminate extra axial friction and the uncertainty around such friction, and thus to guarantee that the sectioning by anchoring rock berms (ARBs) is working. Within each section between ARBs the axial force in the system is held at a minimum level by controlled buckling. The combination of isolated pipeline sections with minimum axial restraint within the section provides control over unstable buckling behavior. Thus the risk of unexpected buckles is minimized. This is particularly important for uneven seabed. 3D global buckling analyses are performed by ANSYS with upper bound, best estimate and lower bound design parameters for friction in accordance with  and capacity control for local buckling of pipeline in accordance with .
Expansion Design Philosophy to Prevent Buckle Walking at Very Uneven Seabed
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Igland, RT, Kvittem, MI, & Vysochinskiy, D. "Expansion Design Philosophy to Prevent Buckle Walking at Very Uneven Seabed." Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering. Volume 3: Pipeline and Riser Technology. Honolulu, Hawaii, USA. May 31–June 5, 2009. pp. 243-248. ASME. https://doi.org/10.1115/OMAE2009-79363
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