Woodside Energy Ltd (Woodside) operates the Enfield field within Production Licence Area WA-28-L. In 2018 the Nganhurra floating production, storage and offtake facility (FPSO) was utilised to flush, isolate and preserve the riser turret mooring (RTM) and the subsea infrastructure, before the FPSO was disconnected and removed from the Enfield field.

The Nganhurra Operations Cessation Environment Plan (EP) was revised and resubmitted on 20 December 2019 (Revision 3) to address alternatives for removal of the Riser Turret Mooring (RTM) from the field for onshore disposal. Following further stakeholder consultation, Woodside elected to withdraw the EP and submit a new revision (Revision 6). The revised EP involves a more comprehensive evaluation of the impacts and risks associated with repurposing the RTM into an Integrated Artificial Reef (IAR), as well as further consideration of the practicability of onshore disposal.

The activities that form the scope of this revised Environment Plan are as follows:
• implementation of an inspection regime during preservation period until all wells are abandoned and subsea infrastructure is decommissioned (which will be subject to a future, separate EP)
• inspection, monitoring, maintenance and repair (IMMR) activities to ensure integrity of subsea infrastructure
• well intervention, if required
• IMMR of the RTM while it remains on station
• disconnection of mooring lines from RTM and lay on seabed (accepted as part of Revision 2)
• removal of RTM from the title area
• towing the RTM to the proposed integrated artificial reef (IAR) site
• placing the RTM on the seabed and undertaking stabilisation and modification (removal of risers and grouting foam, etc.) activities for it to become part of an IAR
• augmenting the RTM by installing additional purpose-built reef modules to complete the IAR.

The RTM removal is planned to be completed between March and April 2021 and is estimated to take 30 days (with potential for a cumulative 90 days when accounting for potential IMMR activities and limiting operations to periods of suitable weather conditions i.e. within cyclone season). Exact commencement and activity duration will be dependent on weather, vessel availability, regulatory approvals and any unforeseen circumstances. If unable to meet this commencement window, the activities will be undertaken between December 2021 and March 2022 (next suitable weather window). Well intervention activities may commence from 2021, depending on vessel/MODU availability and would be undertaken opportunistically. If undertaken, well intervention is estimated to take about 10-20 days per well. If undertaken as a campaign, the cumulative duration is expected to be 18 months (including mobilisation and demobilisation) and may be performed over multiple campaigns.

Towing the RTM outside WA-28-L and installing it on the seabed as an IAR has been included as part of the revised Petroleum Activity Program on the basis that these constitute impacts and risks associated with the petroleum activity.

For the purpose of this Environment Plan, two Operational Areas have been defined to allow impacts and risks to be evaluated separately for activities conducted within WA-28-L and activities associated with towing, installation on the seabed, stabilisation and modification of the RTM on the seabed, and placement of reef modules to form an IAR. The operational areas have been defined as follows:

Operational Area 1 (activities within WA-28-L):
• 1500 m radius around the RTM to allow for IMMR activities and for the disconnected anchor chains to be laid on the seabed
• 4000 m radius around all wells which allows for a moored MODU to undertake well intervention-related petroleum activities
• 500 m area around flowlines to allow for subsea IMMR activities to be undertaken.

Operational Area 2 (impacts and risks from the towing, placement, stabilisation and augmentation of RTM as an IAR):
• 2000 m buffer either side the proposed tow route of the RTM from its current location within WA-28-L to the proposed IAR
• 2000 m buffer surrounding the proposed area for the IAR, except where this comes in proximity to an identified sensitive feature (e.g. Ningaloo World Heritage Property (WHP) and Australian Marine Park (AMP))
• 1700 m distance between the proposed area for the IAR and the Ningaloo WHP and AMP. The IAR will not be installed within this area, however it has been included in Operational Area 2 for vessel activities
• 280 m Ningaloo proximity buffer (including 100 m no-go zone) to ensure no activities occur within the Ningaloo WHP and AMP

The Petroleum Activities Program within Operational Area 1 is in Commonwealth waters in the Exmouth Sub-basin. It is located approximately 38 km north of the North West Cape of Western Australia about 2 km to the east of the Enfield reservoir. The water depth across WA-28-L varies from 200 m in the east to over 2000 m to the west. Water depth is approximately 400 to 600 m within Operational Area 1.

For the Petroleum Activities Program in Operational Area 2 the water depth ranges from 130 to 400 m. The proposed IAR site lies approximately 17 km north of the North West Cape within a vacant petroleum title. The Petroleum Activities Program does not overlap with any established or proposed marine protected areas.