Controlled buckles in subsea pipelines exposed to expansion forces can be triggered by various methods such as snake-lay, installing sleepers/berms, or the residual curvature method (Statoil patent, 2002). The latter (RCM) is relatively new but is gaining popularity and has now been successfully applied to four pipeline projects. During installation, short sections of residual curvature in the vertical direction are introduced to the pipeline, and these introduce a rotationally destabilising effect. Different as-installed configurations may result: the residual curvature section may rotate over into the horizontal plane on the seabed; or it may remain vertical. If it remains vertical, self-weight can cause the pipe to slump down onto the seabed and become straightened.
When applying the RCM, it is preferable for the pipeline to rotate approximately 90° during installation, for the purposes of reducing the critical buckling force and avoiding the introduction of artificial free-spans at the residual curvature sections. Therefore it is important to analyse the rotation behaviour at the design stage. Rotational fixedness at the lay-vessel and resistance from soil friction act to restrain the pipe, but experience from, for example Statoil's Skuld Pipeline Project, indicates that the residual curvature sections tend to rotate. Recent analysis work on rotation during installation of the Johan Sverdrup in-field pipelines is presented. The shallower depth reduced the tendency to rotation compared to reference projects, and the analysis results were used to guide installation settings to assure a robust rotation response during lay.
Subsea pipelines may twist during installation from a lay vessel due to mechanisms that generate torque. This leads to rotation of the pipe cross section. The torque can be generated by, for example route curves, or the weight of inline structures. The rotation phenomenon may be characterised as an instability, as in the case of top-heavy structures which only generate a torque when their centre of gravity becomes laterally displaced from the axis of the pipe. Another type of rotational instability is that caused by plastic hogging bending in the pipeline. This can originate in, for example, a mild constant plastic straining on an S-lay barge's stinger, or a purposeful manipulation of the straightener settings during reel-lay. Since the suspended pipe between the lay vessel and the touchdown point (TDP) is dominated by sagging, there can be a net decrease in potential energy if the plastic bending becomes rotated to better match the imposed sagging bending (see Bynum & Havik, 1981).
