The union representing offshore oil and gas workers is campaigning for WorkSafe WA to be cleared out "from top to bottom" over its refusal to prosecute companies involved in a serious incident off the state's north.

Today marks three years since several workers narrowly avoided being crushed by a swinging platform during the decommissioning of Santos's Sinbad platform, near Varanus Island, about 1,200 kilometres north of Perth.

The workers were cutting through the platform's main leg when the topside, which was rigged to a crane on a vessel, unexpectedly moved and detached from the supporting platform and swung over the workers. 

The crane operator quickly manoeuvred the topside away from the workers, lowering it into the water to control its motion.

The workers disembarked from the main leg to a designated crew safety vessel, without incurring any injuries. 

The decommissioning involved Santos-contracted Dutch company Fugro to remove the Sinbad platform, Swiss-based Allseas which  supplied the crane-equipped vessel, and rope-access technicians from the then-Singapore-listed AusGroup.

In a statement, WorkSafe WA said since the date of the incident, the state's work health and safety laws and penalties had been modernised.

"WorkSafe has been in regular contact with Santos since 2021 to ensure that the company has implemented improvements to its decommissioning procedures to prevent similar incidents occurring," a spokesperson said.

Four-page incident report 

The Department of Energy, Mines, Industry Regulation and Safety (DEMIRS) released its findings last week. 

The reasons for the decision included that no one had been injured, the time that had passed since the incident, and that the cost of prosecution may be disproportionate to the maximum possible $100,000 penalty. 

In a statement, a Santos spokesperson said the safety of workers and the environment was a top priority in the conduct of all its activities, including those of contractors. 

"Santos took the Sinbad incident very seriously," the spokesperson said. 

"And took the opportunity to learn from that campaign, working with regulators and contractors to strengthen safety management and assurance processes around contractor-led activities." 

Offshore Alliance coordinator Zach Duncalfe said while its member had since returned to work, he was left shaken by the incident for years. 

"Obviously, for the union and workers in the industry, clearly there has been an injury," he said. 

"Because we've had a member who's been on workers compensation for two whole years, because of the incident."

A video showing the out-of-control platform attached to the bobbing crane went viral on social media. 

Mr Duncalfe said the decision meant companies could stall for time and potentially evade prosecution for safety breaches.

"Companies in the industry make sure that there's a complex arrangement of who's doing the work with sub-contracting arrangements," he said. 

"It's always going to be expensive to prosecute and therefore, they can get away with whatever they want."