Oil field decommissioning
Exmouth Sub-basin, Australia
Woodside Energy (60%) and Mitsui & Co (40%)
Enfield oil field is situated in production licence WA-28-L, 52km north-west of Exmouth, offshore Western Australia.
Crude oil production from the field started in July 2006 and ceased in November 2018, following the end of its economic life.
The decommissioning programme for the field’s subsea infrastructure was granted approval by the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) in April 2022.
The permit to decommission the operations at the field, along with the Nganhurra floating, production, storage and offloading (FPSO) vessel, was granted in February 2021.
The decommissioning activities are expected to commence in 2022 and be completed by 2024.
The Enfield oil field lies in the Commonwealth waters of the Exmouth sub-basin, approximately 38km north of North West Cape.
The operational area for the subsea infrastructure decommissioning operation covers a radius of 1,500m at water depths ranging between 400m and 600m.
Enfield oil field infrastructure details
The field produced oil through subsea wells tied back to the Nganhurra FPSO. The infrastructure at the field includes six horizontal production wells, two deviated production wells, eight water injection wells and two gas injection wells.
The Nganhurra FPSO was built in 2006 and produced first oil in June 2006. It has a production capacity of 100,000 barrels of oil per day and produced more than 80 million barrels of oil before the field’s production ceased.
Enfield oil field decommissioning details
The field is being decommissioned in phases. The decommissioning activities include disconnection of the Nganhurra FPSO, plugging and abandonment of wells, removal of the FPSO riser turret mooring (RTM), and flushing and removal of the riser, umbilicals, mooring lines and buoyancy modules.
The wells were shut-in and depressurised in the fourth quarter of 2018. They will be plugged and abandoned, following which the Xmas trees, flowbases and wellheads will be removed.
Woodside originally planned to repurpose the FPSO RTM as a deep-water integrated artificial reef (IAR) installed in 150m of water, approximately 16km off the North West Cape. However, the plan to repurpose the RTM was dropped, and a revised environmental plan for management of the RTM was submitted to NOPSEMA.
The RTM is now planned to be removed from the field by lifting the structure in one piece onto a barge and transporting it for either recycling or future reuse.
Subsea infrastructure removal
The removal of the subsea equipment and infrastructure above the mudline is being undertaken as part of the Enfield Subsea Infrastructure Decommissioning plan.
The subsea infrastructure at the field includes four manifolds, four manifold foundation suction piles, seven flexible flowlines and risers, eight umbilicals, six jumpers, 15 rigid well tie-in spools, nine drag anchors and mooring lines, and one debris anchor and mooring line.
Nganhurra FPSO decommissioning
The Nganhurra FPSO was disconnected from the field in December 2018. It is currently lying outside Labuan, Malaysia.
Woodside Energy and Mitsui signed an exclusive purchase agreement for the FPSO with Yinson, an energy infrastructure and technology company, until June 2023, with an option to extend the agreement until December 2023.
In July 2022, Yinson signed an agreement with bp Exploration (Angola), a subsidiary of oil and gas company BP, for use of the Nganhurra FPSO at the latter’s Palas, Astrea and Juno Oil Fields (PAJ Project) located in Block 31, offshore Angola.
Heerema Marine Contractors, an offshore construction and engineering company based in the Netherlands, was contracted for the engineering, preparation, removal and transport of the Nganhurra FPSO RTM in July 2022.
DOF Subsea Australia, a subsidiary of DOF Group that provides subsea engineering and marine services, was contracted to provide project management, engineering, fabrication and decommissioning services in March 2022.
The scope of this contract includes the recovery of the subsea XTrees and flowbases, as well as associated spool sections, wellhead disconnection and retrieval of 18 temporary guide bases. DOF Subsea’s MPSV Skandi Hercules multi-purpose vessel will be used to perform the decommissioning activities.
Vertech Group, a provider of specialist access, inspection and integrity services, provided marine class inspectors, non-destructive testing (NDT) inspectors and lifting engineers to confirm the integrity of the FPSO structures before they are disconnected from the field.