A Pipeline End Manifold (PLEM) connected to a decade-old subsea pipeline in Asia Pacific required repair. The operator’s goal was to replace a flange connection and badly corroded sections of both the PLEM and associated pipeline, and then return it to service. The operator’s preferred recovery method was the reverse-lay approach. However, this would only be carried out if the structural integrity of the pipeline could be ensured.
ROSEN Integrity Services was approached to support this project and ensure that the reverse-lay procedure would not cause damage to the pipeline. Therefore, ROSEN integrity specialists put together a feasibility study which would identify any risks to the safety or efficiency of this critical offshore asset.
The first stage of the study involved a series of DNV-OS-F101 code assessments that were conducted to assess collapse due to external pressure, local buckling, and propagation buckling under a maximum allowable set of loads specified by the operator for the reverse-lay procedure. Levels of general wall loss were also assessed. However, some of these scenarios were unacceptable according to code, and therefore further analysis was required.
In the second stage of the study, a series of finite element analyses (FEA) was conducted to generate failure assessment curves for the two worst corrosion features and pipeline sections without corrosion (i.e. defect-free pipe).
The curves show the loads at which local collapse and global collapse would be expected under a range of different combinations of axial force and bending.
The successful reverse-lay analysis provides the operator with confidence that the repair and recommission of this asset can be carried out safely, without risk of damage to the pipeline or the environment. Furthermore, the results of the feasibility study can be incorporated into future integrity assessments to ensure the continued integrity of this critical offshore asset.