Decommissioning Subsea Tie-Back Pipeline with Engineered Gel System in the Norwegian Sector pf the North Sea While Main Facility is Still in Production

Abstract As larger fields decline in production, tie-backs from smaller infields provide a viable economic solution to all parties in maintaining overall field production; however, decommissioning these smaller tie-back fields have their challenges where the main facility is still in production, such was the case for this case study in the North Sea. An integrated customized chemical solution allowed the operator to successfully decommissioned the 8″ pipelines while mitigating the risks of production disruptions. The greatest challenge was integrating the pipeline system hydrocarbon displacement topsides at the platform without impacting ongoing production from several other infield producers at the same time, or export quality after subsequent passage through the platform's process system. A crosslinked gel train was the concept selected for the pipeline system decommissioning from subsea to subsea. Due to restriction on subsea connection, the gel would need to be crosslinked subsea prior to entering the pipeline. It would also require breaking before it reached the subsea choke due to subsea pressure limitations and passage into the well. The trial provided two critical points to the overall operation, the flowrate at which the gel could be displaced into the well, and the required concentration and injection rate for the breaker to reduce the fluid viscosity and break the gel prior to entering the well. The breaking time had to occur just prior to reaching the tree choke and more critically prior to entering the well. The scenario was carefully analyzed and planned, making sure the crosslinker was injected subsea downstream of choke valve, while breaker solution was injected prior entering the well at the receiving end. Through using this customized chemical solution, the operator company did not require a diving vessel, pig launcher-receivers, nor oil tankers, neither affecting the platform production and operations, and at the same time creating a considerable economic benefit. Pipelines were decommissioned successfully using a gel system. This paper will discuss how the collaboration and engineering solutions enabled the decision-making process, leading to the successful execution of this special project. It will also outline how chemical solutions add value to even decommissioning operations allowing cost savings and solving the challenges.