Recovering Loose Material During Decommissioning of Subsea Tie-Back Pipeline Using Chemical Solutions in the Norwegian Sea: A Case Study.

Abstract The development of the Lavrans and Kristin Q gas and condensate discoveries, high-pressure and high-temperature (HPHT) reservoirs, located in the Norwegian Sea along the north-west coast of Norway; involved the decommissioning of the Kristin R-102 Flowline from the R-template, to use it as single project tied-back to the Kristin semisubmersible platform and transport the production from both fields. This paper reviews the operational sequence and contingencies, even containing chemical solutions, to make this operation possible. During the pigging operations for the flowline decommissioning, several dual-module pigs had their rear module disconnected from the joint assembly, causing connecting cap-screws to be left within the flowline system. The main concern was to ensure no damage was caused by this loose material, that could generate further operational costs and affect the integrity of the system. Due to this incident, additional contingency operations were planned: Internal inspection of riser. Removal of the remaining and last dual-module pig, through open subsea end of flowline. Cap-screw removal from flowline using linear gel slugs. Crosslinked gel plug and foam pigs’ installation. Once the riser of the R-101 Flowline was inspected, it was confirmed that there was no visible damage in it. However, there were high probabilities of having cap-screws loosen also from the last dual module pig and damaging the riser if this was propelled back to Kristin semisubmersible production platform (Kristin Semi); therefore, the decision was to remove the pig through open-end subsea. The R-102 Flowline was cut-off and a subsea cage installed onto the open flowline-end to catch the pig and any cap-screws. Injecting freshwater into R-102 flowline, the last pig was removed and caught into the subsea cage. As suspected, the joint assembly had disconnected on this pig likewise. Together with the pig, eleven cap-screws were recovered. After the removal of the last dual module pig, two foam pigs and linear gel were sent from Kristin platform and pumped out subsea into the cage. No additional cap-screws were found. Therefore, it was decided that a second cleaning run would not be necessary, and the crosslinked gel plug followed by another foam pig from the Kristin semisubmersible production platform was installed as planned. Initially twenty-eight cap-screws were missing after the decommissioning operation, and in total after the contingency operations were performed, nine cap-screws remain unaccounted for. This paper will discuss how the collaboration and engineered solutions enabled the safe execution of contingency actions within the time frame available, before the R-102 Flowline was planned to be connected to the new production flowline on the Kristin Sør Development project. It will also highlight the importance of using a bespoke pig design during engineering phase and implementation of custom pipeline chemical solutions to enhance the decommissioning operations success.