Australian operator Woodside Petroleum is progressing work to decommission its Enfield oilfield in Block WA-28-L, off the coast of Western Australia.

Woodside this month submitted a new environment plan with Australia’s offshore regulator — the National Offshore Petroleum Safety & Environmental Management Authority (Nopsema) — related to additional decommissioning work.

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The environment plan covers removal of infrastructure above the mudline, including manifolds, flowlines, umbilicals, mooring chains, and spools.

The operator is also proposing to leave in-situ a total of 10 anchors, each including a section of mooring chain which are completely buried below the seabed.

The company also noted in its environment plan that it could potentially leave partial manifold suction piles in situ, if it is determined the risk and impact of fully removing them is not “as low as reasonably practicable”.

The plug and abandonment of the wells at the Enfield field and the decommissioning of the riser turret mooring are already subject to separate environment plans Woodside has previously submitted with the regulator.

The latest environment plan submitted on 6 January also covers ongoing management of the Enfield subsea infrastructure until decommissioning activities are complete, including inspection, maintenance and repair.

Woodside anticipates the decommissioning work under the latest environment plan to start sometime between 2022 and 2024, with the entire programme to take a cumulative period of 12 months to complete.

Decommissioning work under the spotlight

Nopsema hit Woodside with compliance action last year to ensure the “safe and responsible decommissioning” of infrastructure left at Enfield.

The regulator initiated the compliance action due to Woodside being unable to comply with an originally approved plan to remove the equipment for onshore disposal, finding that the equipment’s poor condition meant it was no longer feasible to comply with the original plan without “unacceptable risks to safety and the environment”.

At the time, Nopsema also stated that it was investigating possible breaches of the Offshore Petroleum and Greenhouse Gas Storage Act 2006, with the investigation particularly focused on possible breaches of the law relating to requirements to maintain property in good condition and repair.

The Enfield field started producing oil in 2006 via a network of subsea wells tied back to the Nganhurra floating production, storage and offloading vessel. Oil from the Enfield reservoir was produced through six horizontal production wells and two deviated production wells, and supported by eight water injection wells and two gas injection wells.

The field has reached the end of its economic life, with the 18 wells shut-in in the fourth quarter of 2018.

Block WA-28-L is owned by operator Woodside (60%) and Mitsui E&P (40%).