Groot-Ammers — Paradigm Flow Services has completed a fully topsides based decommissioning intervention on the 24" 253b export pipeline from Perenco UK's Trent 43/24 platform in the Southern North Sea — the largest diameter deployment of its patented Flexi-Coil® composite coiled tubing system to date.
The project, which was executed in November 2025, flushed the pipeline to 8.0 ppm Oil in Water in under three days of operations, without the need for any vessel or subsea intervention.
The technical challenge
The 253b pipeline runs 1 km from the Trent platform to the Cygnus Wye Manifold, with 330° of cumulative bends, and was tied into the producing Cygnus A gas line to the onshore Bacton Terminal. Topsides pigging facilities on Trent had already been removed during prior phases of the decommissioning programme, and the line was estimated to contain 150 m³ of residual condensate inside a 262 m³ total volume.
Conventional options were either ruled out or carried significant cost and risk. A topsides-launched isolation tool could not be used because pigging facilities were no longer available and the Wye manifold valves were closed. A subsea hot tap and flush campaign would have required vessel time, specialist isolation equipment, and dive support through the winter North Sea, at a cost estimated in the millions and with the corresponding HSE exposure.
The engineering approach
Flexi-Coil® is a miniaturised coiled tubing system that uses a neutrally buoyant composite coil rather than traditional steel. Flexi-Coil® is deployed from a compact topsides spread, comprising of an injector, BOP, stripper, control panel, spooler frame, drum, and power pack which is connected to a high-pressure pumping system for the circulation of fluid through a multi-nozzle bottomhole assembly.
Due to its low-density composite materials, Flexi-Coil® is neutrally buoyant in seawater, which assists in achieving deployment distances of up to 6 km from the insertion point, while navigating pipeline geometries involving over 1,000 cumulative degrees of bends. Its inherent centralising tendency within the pipeline minimises wall contact throughout
the deployment, reducing frictional resistance and effectively mitigating capstan-induced drag forces.
Due to this being the largest line worked on to date, a feasibility study was conducted to support decision making by Perenco management. The study included offshore site surveys, Flexi-Coil® reach simulation, equipment layouts, ancillary services, operations methodology, and contingency planning.
Returns were routed back to topsides and into a disposal well, closing the flushing loop without any subsea handling of pipeline fluids.
The result
Oil in Water samples were taken at regular intervals through the flushing operation before reaching a final value of 8.0 ppm which was significantly below the 30 ppm threshold agreed with the Offshore Petroleum Regulator for Environment and Decommissioning (OPRED), with the cleanliness target met in under three days. The pipeline was left in a state suitable for cutting and abandonment, with the rest of the decommissioning programme able to proceed.
Wider relevance for the offshore sector
The Trent campaign extends Flexi-Coil®'s validated envelope into export-class infrastructure, which is the segment where the cost and risk of vessel-led alternatives are highest. It follows earlier reference work that included the decommissioning of two subsea flowlines that were flushed to 30 ppm and isolated through 450° of bends to a distance of almost 900 m using a thermally swelling cement pumped through the coil, providing a permanent barrier.
Across these deployments, the underlying value proposition is consistent. By moving pipeline cleaning, isolation, and decommissioning flushing from the seabed to the topsides, operators can execute scopes that would otherwise depend on diving, ROV support, hot tapping, and vessel management.
The use of the Paradigm Flexi-Coil® system to complete the flushing of the Trent Export pipeline offered several benefits to Perenco by reducing the technical complexity, addressing safety concerns and environmental risks related to other methods, eliminating weather constraints and greatly improving the overall cost efficiency of operations.
