Australia’s offshore safety regulator has issued rig owner Diamond Offshore with a prohibition notice after a worker was injured on the semi-submersible Ocean Monarch during operations in the Bass Strait.

Australia’s National Offshore Petroleum Safety & Environmental Management Authority (Nopsema) revealed late last week it had pursued enforcement action against Diamond after a worker was "seriously" injured when equipment failed during an operation on 17 September to replace a drill line.

In the prohibition notice the regulator noted a wire rope snake was being used during the operation to replace the two-inch drill line on the draw-works - a winch assembly capable of exerting tension to the drilling line during normal drilling operations.

A wire rope snake is used to temporarily join the ends of the old and new drilling line, with Nopsema noting that the working load of the wire rope snake is substantially lower than that of the drilling line.

In the prohibition notice, a Nopsema inspector said the tension applied by the draw-works was not monitored or reliably controlled to within the safe working limits of the snake as the old and new line were threaded through the crown block and travelling block sheaves.

“As a result, friction or snagging of the drilling line during spooling operations can lead to rapid increases in line tension, leading to failure at the weak point, i.e. the wire rope snake,” the inspector said in their report.

“During this incident, the tension applied by the draw works exceeded the breaking strength of the snake causing the destruction of the snake. The drill line fell to the drill floor, impacting a member of the workforce, and resulted in significant injuries. The equipment falling from height had a potential for fatal injuries.”

Nopsema has ordered Diamond to not carry out the operation on the rig again until it has implemented “adequate systems and work practices” to ensure the safety of workers.

The semi-submersible Ocean Monarch had most recently been used to drill the successful Annie-1 exploration well off Victoria for Cooper Energy.

It was revealed last month, prior to incident outlined in the Nopsema report, that the abandonment of the well had been delayed due to the failure of two of the eight mooring chains on the semisub.

With the cause of the failure expected to take more than a month to analyse, Cooper elected to defer a second exploration well which was due to be drilled by the Ocean Monarch on the Elanora prospect to "a future campaign".

The Ocean Monarch is currently contracted in Australia to Cooper and US giant ExxonMobil until January 2020, after which it will head to Myanmar to commence work for Posco Daewoo under a firm seven well contract, with options for five additional wells.