Photo: FPSO Stybarrow Venture MV16; Source: MODEC

Australian giant seeking approval for subsea equipment removal

Australian energy player BHP has submitted an environment plan (EP) to the country’s offshore regulator for subsea decommissioning and infrastructure management of a field, located in Commonwealth waters in Northern Carnarvon Basin, offshore Western Australia.

The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) confirmed that BHP submitted this environment plan on 14 April 2022, proposing to remove subsea infrastructure within the Stybarrow field, continue field management of subsea infrastructure, and remove wellheads within production licence WA-32-L.

While the Stybarrow field was still in production, it comprised the MODEC-operated FPSO Stybarrow Venture MV16 – capable of processing 100,000 barrels of liquids per day and a storage capacity in the order of 900,000 barrels – with production, gas injection and water injection wells at four drill centres routed to the disconnectable turret mooring (DTM) via flexible flowlines. During that time, oil products were stabilised and stored for offloading via tanker.

The production at the field started in 2007 and ceased in June 2015. Since then all flowlines and gas lift lines were flushed and filled with treated seawater and production flowlines disconnected, while all production, gas injection and water injection wells were shut-in and capped to await plugging and abandonment.

After the Stybarrow Venture FPSO was disconnected from the DTM and demobilised from the field, the DTM unexpectedly sunk to the seabed at some point between May 2016 and October 2016, where it lies in water depth of approximately 825 metres with risers still attached. Following the DTM sinking, the riser buoyancy modules were removed to eliminate any buoyant risk, according to the EP.

NOPSEMA advised that BHP is planning for the safe and sustainable closure of the Stybarrow development, located in Commonwealth waters around 55 km north-west of Exmouth, in water depths of approximately 810 – 850 metres. The operational area where planned activities will occur encompasses a 1,500-metre radius around the subsea infrastructure and wellhead.

Source: BHP
Source: BHP

As BHP is now planning for progressive decommissioning of the Stybarrow field, the first approvals are being sought for the removal of the subsea equipment and ongoing field management activities until the equipment is removed. The company’s proposal for the Well Plug and Abandonment (P&A) and equipment to remain in situ will be subject to separate future environmental approvals, which are expected to demonstrate that full removal of the equipment may not be practicable and that abandonment in situ yields equal or better environmental outcomes than full removal.

Based on the EP, the scope of the petroleum activities includes the removal of the DTM and associated mooring legs and mooring support buoys; 60 km of flexible lines encompassing the flexible risers; flexible production flowlines; gas and water injection flowlines; 19 km of umbilicals; 10 wellheads and Xmas trees; hydraulic jumpers; a water injection manifold; 5 subsea distribution units; 2 umbilical termination assemblies; and 15 anode skids.

Within the EP, BHP stated that the subsea infrastructure removal activities are expected to be conducted over a period of around six months, with activities required to be completed before 31 March 2025. The firm explained that typically only two project vessels will be within the operational area during removal activities, although, this can be extended to include up to six vessels.

BHP further informed that the proposed petroleum activities do not overlap with any established or proposed marine protected areas, with the closest being the Gascoyne and Ningaloo Australian Marine Parks, located 5 km and 24 km respectively, from the operational area.

When it comes to BHP’s most recent activities, it is worth noting that the Australian player is in the process of merging its oil and gas portfolio with Woodside. To this end, a new executive team was revealed in February 2022.

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This team will lead the new company after the merger is complete, allowing Woodside to take full advantage of energy transition opportunities to foster further growth.