Eni’s Woollybutt field ‘a threat to safety’ after piece of equipment emerges on sea surface

Australian offshore regulator NOPSEMA has started an investigation into an incident involving a piece of equipment on Eni’s Woollybutt offshore field, which emerged from the depths of 50 meters subsea to the ocean surface.

According to the regulator, the object is a hazard to vessels in the area and the integrity of the field’s subsea infrastructure, including wells.

On 30 August 2020 Eni notified NOPSEMA of an object floating on the sea surface within WA-25-L that was suspected to be one of the two mid-depth buoys (MDBs) in the Woollybutt field where the water depth is approximately 100m.

Eni confirmed that the object was an MDB on 3 September 2020.

The MDB has dimension 5m x 9m x 8.5m and weighs approximately 40t and was designed to be tethered by chains at a depth of 50 m.

Eni has reported that the MDB is within the gazetted petroleum safety zone, under 24-hour observation and is still connected to the Woollybutt subsea infrastructure.

In the direction issued to Eni and the field partners on 4 September, NOPSEMA said it is of the opinion that the failure of the tethering system and physical presence of an object of this size poses a threat to the safety of vessels in the area and potentially to the integrity of the subsea infrastructure, including the wells.

Following confirmation on 3 September 2020 of the presence of the mid-depth buoy on the surface, NOPSEMA has started an investigation into the circumstances of this incident.

NOPSEMA also previously issued an improvement notice to Eni on 4 May 2020, after an inspection in response to the sinking of the detachable single point mooring in January 2020, requiring Eni to prevent or reduce the risk of a mooring system failure resulting in a mid-depth buoy floating to the surface.

Namely, the regulator found that the sinking of the single point mooring buoy was likely a result of a failure to maintain adequate cathodic protection which protects the buoy from corrosion.

The inspection also identified a number of deficiencies related to the integrity management of the facility’s two mid-depth buoy mooring systems.

The inspection also revealed that he MDBs had a design life of 5 years and fatigue life of 15 years. The Woollybutt field started production in 2003, 17 years ago. Therefore, the original design and fatigue life of the MDBs has been exceeded.

There was no evidence that the MDB’s design and fatigue life had been reassessed and certified as fit for service beyond 2016.

At the time, the inspector warned that, in the event of a mooring system failure resulting in an MDB floating to the surface, this would create a marine vessel collision hazard leading to vessel damage or sinking and could result in injuries to, or loss of life of, vessel personnel.

After this inspection, Eni was given a deadline of 180 days to take action to prevent or reduce the risk.

The Woollybutt field is located approximately 80 km off the northwest coast of Western Australia in Lease Area WA-25-L.

During production, the field development consisted of four subsea wells that produced through subsea wellheads and flexible flowlines to an FPSO vessel.

Production at the field ceased in May 2012 with the FPSO departing from the field in June 2012. The field is now being prepared for decommissioning.

Eni received approval from the Australian regulator for the environment plan for the Woollybutt field decommissioning project in July 2019.

The scope of the EP covered passive field management activities and plug and abandonment activities (including well intervention) on four to seven wells within permit WA-25-L.