Pipeline stability assessment using PRCI Level 2 and Level 3 analysis tools

The client, as part of their FEED, wanted to assess the stability of a subsea gas transmission line for both thick wall steel and concrete coated steel pipeline options and to determine the minimum wall thickness for the pipe and concrete coating.

The benefit to the client was to provide the pipeline sizing information to allow an assessment of the most cost effective way of installing the pipeline whilst ensuring its stability during operation.

• The work required stability calculations based on PRCI Level 2 and Level 3 for both thick wall steel and concrete coated steel alternatives to determine minimum wall thicknesses for the pipeline
• Sensitivity analyses were carried out on the soil and metocean data to identify any conservatism in the results of wall thicknesses

The results of the study provided the dimensional information required. It also identified that a reduction in conservatism in the matocean data provided to the actual conditions affecting the pipeline allow for a reduction in required wall thickness whilst providing the same level stability. In addition the work demonstrated the value of carrying out dynamic analyses (PRCI Level 3) where stability is determined by current induced rather than wave induced hydrodynamic forces. It was shown that pipeline lateral displacements in excess of 20m could occur if the pipeline was not made heavy enough to resist the strong current loads.