Creating a local residual curvature during S-lay pipeline installation by lowering the stinger

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Abstract

Marine pipelines are often operated at high internal pressure and temperature. This loading condition results in the development of axial compressive forces that can cause the pipeline to buckle globally. Global buckling occurs when significant lateral motion is present in the pipeline and excessive feed-in occurs at that specific location, which in turn forms into a sharp curve that can initiate destructive structural failure. Research has been conducted addressing the control of buckle development. The "buckle initiation" techniques were invented to mitigate the uncontrolled buckling of the pipelines on the seabed. These techniques involve the creation of less stiff sections in the pipeline (imperfections), so that buckling occurs in these locations. The most common buckle initiation techniques used in S-lay installation so far, are the "snake lay" method, the "artificial vertical out-of-straightness" and the "distributed buoyancy" method. However, these techniques require the addition of subsea structures on the seafloor or larger pipeline length, which can increase the pipelaying cost dramatically. A beneficial buckle initiation technique is the "residual curvature" method. The residual curvature method (RCM) principle is based on creating intermittent residual curvature sections in the pipeline so that buckling can be initiated at these locations. The curvature sections are created by adjusting the settings of the already existing installation equipment. So far, this method is only used in reel-lay installation. It is particularly urgent to examine if the local residual curvature method can successfully be applied in pipelines laid by S-lay vessels, since S-lay is considered the most common and frequently used technique due to its adequacy on different water depths and pipe diameters. The scope of this master thesis is to assess the feasibility of creating local residual curvatures in the pipeline by lowering the stinger during S-lay. The assessment is accomplished by simulating numerically the pipelaying process and the creation of the residual curvature, by analysing the behaviour of the pipeline while being lowered (in particular, looking at its twist/rotational behaviour) and by verifying if the alterations to the normal pipelaying procedure still respect the integrity of the pipeline and installation equipment.