Subsea Field Decommissioning Engineering

The Challenge

Australian Commonwealth waters operate on a strict premise: under section 572 of the OPGGS Act, full removal of all structures, equipment and property is the base case, and any deviation must be justified to NOPSEMA case by case. A subsea field with a DSPM, 20 km of flowlines, manifolds and wellheads therefore starts from the assumption that everything comes out.

The North-West Shelf adds its own constraints — cyclone season limiting the workable window, water depth and seabed conditions affecting recovery methods, and flowlines whose burial and stability status directly affect whether recovery is even feasible.

Our Approach

In Commonwealth waters the base case is that everything comes out. Anything else has to be earned with evidence.

VORYX team members delivered the engineering and planning for the programme against the full-removal base case, developing recovery methods per asset type — DSPM disconnection and recovery, flowline recovery and cutting strategy, manifold and wellhead removal, and mattress and protection structure recovery.

Where full removal was not straightforward, the alternative was not asserted but assessed: comparative assessment against safety, technical feasibility, environmental impact, societal impact and cost, structured to meet the evidentiary standard NOPSEMA applies to deviation requests.

Campaign planning was built around the cyclone-constrained weather window, with vessel spread selection, cycle times and downtime modelled for the specific season rather than annual averages.

Standards Applied

• OPGGS Act s.572
• NOPSEMA guidance
• Environment Protection (Sea Dumping) Act
• DNV-ST-N001
• IMO Res. A.672(16)

Operator and project names are anonymised for confidentiality and disclosed on request. Experience described is that of VORYX team members.