Abstract
This paper presents a methodology to rectify non-rotated Residual Curvature (RC) sections in order to ensure the pipeline behaviour during start up does not jeoperdise the pipeline integrity.
Since 2012, the Contractor has successfully installed more than 100 RC sections within a large range of cross section geometries and water depths.
During installation of a pipe-in-pipe with RC sections an intermediate survey showed that while the majority of the RC sections had landed as expected, three RC sections had landed on the seabed with no observable torsional rotation or Horizontal Out Of Straightness (HOOS), i.e. in an upright position.
The expansion and buckling design for the pipeline depended on the RC sections developing into lateral buckles and it was challenging to confidently demonstrate that the pipeline could be started up and kept within the bending moment and stress limits as defined by the relevant code (DNV-ST-F101).
Engineering was carried out to select and optimise a mitigation method to fully ensure the relevant RC sections would develop into lateral buckles during start-up and ensure bending moment and stress were within acceptance criteria.
The method selected used buoyancy elements attached to the RC sections prior to and throughout the start-up. The buoyancy elements could be safely removed after start-up. The mitigation method was applied offshore in an efficient manner and the observed pipe behaviour matched very well with the predictive analysis results.
With a long track record of installing RC sections, the Contractor has developed efficient and realistic tools to predict torsional rotation and HOOS, but experience shows that there is still some level of uncertainty. A field-proven mitigation method that can be applied as contingency is now available to mitigate this uncertainty.
With this field-proven mitigation method in hand, the confidence in the RC method as a cost efficient and robust solution for expansion and buckling management is further increased.
