Innovative subsea pipeline decommissioning during ongoing production

Author: Iain Shepherd, Engineering Manager for Pipeline and Process Solutions at Halliburton.

Overview

A customer in Norway approached Halliburton seeking a decommissioning solution for a pipeline system where pigging was impractical. Decommissioning of tie-back fields have challenges where the main production facility is still in production.

An initial engineering study presented several possible options. The primary challenge involved managing the displacement of hydrocarbons at the platform topsides without affecting ongoing production or export quality from several other infield pipelines. Consequently, handling displaced hydrocarbons at the subsea tree was proposed.

Challenges

A crosslinked SureGL™ pipeline gel decommissioning train was chosen for its viscosity, capable of removing hydrocarbons from the pipeline without using pigs. However, the back pressure generated by the crosslinked gel risked exceeding the design pressure of the pipeline system. It was critical to break the gel before it reached the disposal well choke and formation to ensure the success of the project.

In collaboration with the customer, Halliburton conducted a series of tests to validate the timeline for breaking the gel. This was followed by yard trials, which included simulating the passage of the SureGL™ gel slug through the subsea tree choke and subsequent injection.

The trials identified two key factors: the flow rate the gel could be displaced into the well, and the required concentration and injection rate for the breaker to reduce the fluid viscosity. It was crucial for the gel to break just before reaching the tree choke, and more importantly, before entering the well. To ensure accurate breaking time, the internal volume of the pipeline had to be accurately calculated, considering the potential for wax build-up in the pipe.

Solution

Halliburton successfully decommissioned the 8-in. pipelines using a SureGL™ gel slug from the south tree, displacing the pipeline contents and gel into the north well with a MEG/seawater mix. A high-velocity flush (1.7m³/min) with filtered seawater pushed the MEG/seawater mix into the north well. Subsea samples were collected from the pipeline and analyzed for hydrocarbon content.

A vessel provided the bulk of the pumping services at the south well with integrated onboard pumps. Halliburton managed the gel transfer, crosslinker pumping services, and gel breaker pumping services onboard a second vessel located at the south well.