Santos Limited (Santos) is the registered titleholder for production licences WA-54-L, WA-26-L and WA-27-L, located in water depths of 130-160m on the NWS approximately 150km offshore from Dampier. Santos has been producing via the Mutineer Exeter Development in these permit areas since 2005. Production occurs from a series of subsea wells linked by subsea pipelines via a disconnectable turret mooring (DTM) to a Floating Production Storage and Offloading unit (FPSO).
Operation of the facility is undertaken under the Mutineer Exeter Development Field Operations (Operations) Environment Plan (EP), accepted by the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) in 2014. Operations described in the Operations EP include onboard processing of production fluids (and associated discharges), ongoing vessel based inspection and intervention activities, as well as periodic disconnection of the FPSO.
Production from the field is declining, and Santos now intends to cease (shut in) production from the remaining wells in April 2018, with the FPSO scheduled to depart in Q2 2018. The subsea infrastructure will remain in place and the development transition to a ‘cessation’ phase that will continue until field decommissioning occurs. A Cessation of Production EP to cover the inspection/maintenance activities that will occur during the cessation phase will be submitted to NOPSEMA prior the departure of the FPSO. To reduce the potential for hydrocarbon leaks from flowlines and umbilicals after production is shut in, Santos plans to use seawater to flush all remaining well fluids from the system back to the FPSO prior to its departure. The subsea infrastructure will then be filled with treated seawater so that the subsea infrastructure will be left safely ‘preserved’ on the seabed. Flushing/preservation activities will take approximately 7 – 10 days. All flushing fluids will be captured onboard the FPSO and processed via the production system. All recovered hydrocarbons added to the crude inventory. Flushing fluids containing preservation chemicals and residual hydrocarbons will settle in the slops tanks prior to discharge at an oil in water content <30ppm OIW.
Although similar in nature to the maintenance and intervention activities that have been undertaken during production, the specifics of the flushing/preservation activities and the presence of preserved infrastructure remaining on the seabed are not considered by Santos to be adequately described in the existing Operations EP. Accordingly, Santos has prepared an Addendum to the Operations EP to describe these activities in detail, to evaluate the associated impacts/risks, and document the controls that will be implemented to ensure all impacts and risks are managed to ALARP and acceptable levels.