Woodside in the clear to leave certain subsea oil & gas equipment in situ during decom

Australian energy giant Woodside has received a stamp of approval from the country’s offshore regulator for an environment plan (EP), covering decommissioning and dismantling activities at a field off the coast of Australia.

FPSO Griffin Venture; Source: BHP

The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) recently accepted Woodside’s environment plan, proposing to permanently decommission in situ the subsea infrastructure, including up to 11 of the 12 riser turret mooring (RTM) anchors and associated anchor chains, five piled foundation structures and six concrete gravity bases associated with the Griffin field within production license WA-10-L offshore Western Australia.

The Australian player is decommissioning the field, which is located approximately 65 km northwest of Onslow and 94 km northeast of Exmouth, in around 130 m water depth. While in production, the Griffin field comprised the FPSO Griffin Venture with 12 production wells – permanently plugged in 2017 – from the Griffin, Scindian, and Chinook reservoirs routed to the riser turret mooring via flexible and rigid flowlines. Oil products were stabilized and stored for offloading via tanker, while gas products were transported to the shore via the Griffin gas export pipeline (GEP) for domestic sale. The field ceased production in 2009.

In a bid to put its decommissioning plans into action, Woodside handed out a series of decommissioning deals to multiple players – Transocean, TechnipFMC, Heerema, McDermott, Fugro, DOF, and McMahon – for the removal and disposal of subsea infrastructure at several oil and gas fields offshore Western Australia, including Griffin. The Australian giant will remove most of the equipment in the field, including all that contained hydrocarbons during production.

However, the company has undertaken an environmental assessment of the feasible decommissioning options and determined that leaving certain equipment in situ will result in equal or better environmental outcomes than full removal. As a result, this EP provides for leaving up to 11 of 12 RTM anchors and associated 30 m chain bridles, with the chain cut as close as practicable to the seabed and removed.

In addition, five piled foundations, with the exposed part of the piles removed as close as practicable to the seabed, and six mid-depth buoy concrete gravity bases are also included in the list of equipment to be left in situ.

Woodside pointed out: “The equipment proposed to be left in situ would require extensive seabed intervention to remove due to their burial status or substantial weight. None of the equipment proposed to be left in situ was exposed to hydrocarbons during production. The equipment consists almost entirely of steel and concrete.

“Impacts associated with the ongoing presence of the equipment include localised changes in sediment quality from long-term degradation of the equipment and potential displacement or interactions with future trawl fishers. Commercial trawl fishing is not permitted in WA-10-L under existing fisheries management arrangements.”