One way to control lateral buckling in the operation phase for High Pressure High Temperature (HPHT) pipelines is by deliberately introducing residual curvature sections at intervals along the pipeline by adjusting the straightener settings of the pipelay tower, as described in a patent held by Statoil . This method has been applied with reel-lay installation for a number of shallow water pipelines in Europe (Statoil’s Skuld project and Total’s Edradour project).
The paper presents the benefits as well as the feasibility of the use of Residual Curvature Method (RCM) to control lateral buckling for deep water applications which involves high top tension in the overbend and high pressure and twist of the RC section in the sagbend. The study cases consider the application of the method for pipelines in 1850m water depth which are pushing the pipe top tension close to the limit of the capacity of the tensioners of Heerema Marine Contractor’s (HMC) Reel-lay vessel the Aegir. There are some challenges of the application of the residual curve method for deep water pipelines. Due to high top tension, some potential issues are investigated during lowering of the curved section from the straightener, passing the tensioners and through the J-lay tower into the water to the seabed. Detailed analyses have been performed to check the interaction of the residual curved pipe section against the tensioners (the effect of the squeeze load on the RC section) and to assess the maximum bending moment generated when the residual curved section is under high top tension below the tensioners against the Load Controlled Condition (LCC) for local buckling bending moment limit. Another consideration is the increase of hydrostatic pressure in deep water which could limit the allowable bending moment in the sagbend when lowering the curved sections to the seabed. Discussions are presented to the feasibility of the concept including the proposed ways of mitigation for the aforementioned potential issues.
The paper will also show an improved prediction of pipe twist/roll by comparing a published analytical 2D plane solution against the 3D FEA model prediction. The improved prediction, which considers the out of plane bending component of the pipe catenary, results in an increase of pipe twist in the sagbend section. This reduces the bending moment in the residual curved section when entering the sagbend and increases the probability to roll the curved section over to the horizontal plane on the seabed.