Decommission or repurpose?

On the UK Continental Shelf, there are 880 fixed facilities and subsea installations – 23% of which are now subject to decommissioning approvals. That’s expected to hit 25% over the next three years, Lorna Ramsay, head of the offshore decommissioning team at Offshore Petroleum Regulator for Environment and Decommissioning (OPRED), told the Offshore Decommissioning Conference in the UK, in late 2021. If they’re going to be reused, the clock is ticking.

To address the opportunity, the Oil and Gas Authority (OGA) has set up a Decommissioning and Repurposing Taskforce (DaRT). Ian Fozdar, formerly decommissioning manager and now infrastructure repurposing manager at the OGA, told the event that the greatest potential for repurposing is in the trunk lines, which could be used for carbon capture and storage (CCS) or in a hydrogen economy.

CCS, in particular, has been getting a lot of attention, and a number of the proposed CCUS projects in the UK have repurposing elements, says Fozdar. William Dickson, project manager at ENI, is working on the Liverpool Bay CO2 storage project. This is part of the wider HyNet North West project, involving low carbon hydrogen production and distribution with CO2 capture, transport and offshore storage. ENI secured a CO2 appraisal and storage license for the project in 2020 and it was selected as one of two winners of government CCS Cluster funding to progress.

Liverpool Bay CCS

ENI’s side of the project would be repurposing the Hamilton, Hamilton North, and Lennox fields in Liverpool Bay, in the Irish Sea, into CO2 storage sites. All three are expected to come to the end of their lives in coming years. The project would involve reusing infrastructure, including the Douglas production platform. A new pipeline would be built from emitters to Connah’s Quay, the site of a power station that’s fed with gas from ENI’s fields via Point of Ayr gas terminal, which means existing pipelines between Connah’s Quay and the offshore storage sites could also be reused, as well as existing wells, Dickson told the conference, organised by Oil & Gas UK.

Work, including inline inspections, has already been done to check the condition of the existing pipelines, to make sure they’d be fit for service. Dickson points out that the design pressure would be lower than the design pressure at their original installation. The platforms have also been assessed and confirmed fit for reuse, he says.

“On the wellhead platform, all existing redundant equipment will be removed, creating headroom and reduced maintenance,” he says. “New prefabricated modules will be installed, helideck and other services plus heater to manage transient conditions. Douglas will continue as field-wide hub.” Again, redundant equipment will be removed, and new modules installed. There will also be a new 33kV power cable from shore to electrify the facility.

Reusing CMS for CCS

Neptune Energy is working on a reuse case for the Caister Murdoch System in the UK for CO2 storage, Calum MacDonald, lead development engineer at Neptune Energy, told the decommissioning event. Caister Murdoch was a gas collection and gathering system in the southern North Sea, transporting gas from Total, Conoco, and Shell’s Caister, Murdoch, and Schooler fields (respectively) and others to shore in Lincolnshire from 1993 to 2018.

The CMS would provide the transport system for the CO2 to storage sites. As long as the CO2 arrives “at spec”, i.e. dry, and to the right pressure (in a supercritical state), transport through the existing system should go without difficulty, he says. In the future, additional tie-ins could also be added. For processing offshore, a newbuild normally unmanned installation (NUI), powered by renewables with walk-to-work access, would be required, as well as a new flowline, which would include pressure management, potentially via an electric submersible pump, for later in field life. Subsea infrastructure would be all-electric with local power generation, such as from a power buoy, with reservoir management from a vessel to minimize reservoir intervention. However, he says that reuse can have a significant impact on the operating envelope of infrastructure, depending on the materials and impurities in the gas stream. “The biggest impact is moving from low-density gas system to higher phase or dense phase CO2,” he says.

Ian Fozdar said that the greatest potential for repurposing is in the trunk lines, which could be used for carbon capture and storage (CCS) or in a hydrogen economy.

Harbour and Acorn eye opportunities

Harbour Energy also has a license for CO2 storage, in the southern North Sea. Under a plan initially drawn up by Chrysaor, now part of Harbour Energy, this would see depleted gas fields, Victor and Viking, and the LOGGS pipeline reused.

The Acorn project, in Scotland, also has a license for CO2 storage, but lost out on the recent government funding round for CCS cluster projects. This would have seen the reuse of the Goldeneye pipeline from the St Fergus terminal to the Goldeneye field, with control and communications for the well from a nearby platform or using a local control buoy.

Getting the right combination of available assets will be a challenge. According to Fozdar, there are thousands of kilometers of pipeline in place that have opportunities for repurpose, but not all will be suitable, he says. “Out of roughly 3,000 pipelines in the UKCS, a screening evaluation indicates perhaps 100 will have repurposing potential,” he says, and not all of those will be reused. Wells could also be repurposed for CO2 injection, he says. “But having the right geology at the same well locations is pretty unlikely,” says Fozdar. “If it is possible, the wells need to not be leak paths for CO2.”

Will Webster, Energy Policy Manager at Oil & Gas UK, says well decommissioning for CO2 storage guidance is being developed and due to be issued in Q2 this year, while guidance about the reuse of other assets is underway as part of the DaRT group. However, other issues around commercial and financial liability when it comes to reuse and decommissioning will also need to be considered, he says.