Our client needed an assessment of the viability of re-rating the pressure of a riser on one of their North Sea platforms. The riser was originally rated to 97 barg, but the client planned to re-route an existing well with a potential shut in pressure of 250 barg through the riser. The riser was protected by a safety integrity level (SIL) 3 rated subsea protection system that had failed a barrier valve leak test.
We were asked to determine whether the riser could be re-rated to a higher pressure. This involved reviewing previous assessments, the original design documents and supporting data to identify options for re-rating along with the justifications and mitigations for each.
We investigated several options for re-routing the existing well, providing an analysis of the benefits and challenges of each option.
The first step was to review the available data to determine the maximum pressure rating of each of the components in the system, such as flanges and emergency shutdown valves (ESDVs). Next, a wall thickness calculation to PD8010 was performed to find the theoretical maximum allowable pressure of the riser.
Following this, several options for further study were suggested in order to determine the true maximum allowable pressure. These included code variations/deviations, obtaining original material certificates for the riser and a more detailed finite element analysis. Other options discussed included the use of an adjacent (unused) riser, or a retrofit riser.
We were able to make a variety of recommendations for how our client could proceed with re-rating the riser, ranging from integrity assessment and providing advice on code deviations, to FEA or a new riser design. Each option was presented with a full discussion, allowing the client to evaluate each option and decide on how they wished to proceed.