An offshore union claims oil company Shell Expro has found more than 20 leaking pipework patches on a single North Sea platform.
All pipework repairs are being checked
It comes after the firm ordered urgent checks on all its North Sea platforms following the deaths of two workers.
The OILC union said the Brent Charlie platform has been patched in 47 places, 21 of which were leaking, with seven leaking hydrocarbons.
Last week Sean McCue, 22, of Kennoway in Fife, and Keith Moncrieff, 45, of Invergowrie, Tayside, were overcome by gas while working on patched pipework in a leg of the Brent Bravo platform.
The company said a number of factors led to the tragedy, which were unlikely to combine again.
But all platform managers have now been ordered to re-check the integrity of pipework repairs and certain valves.
Greg Hill, production director of Shell EP Europe, told BBC Radio Scotland that the platform was safe.
He said: "Patches are an industry standard way of repairing pipework, we monitor all of our patches through a patch register and none of the patches on Charlie are safety critical - if they were critical they would be dealt with.
"We have standards around how much these patches can leak and none of these patches are at risk."
The offshore union OILC said it had been raising concerns about patched pipework on Shell platforms for more than three years and had been given countless reassurances about their safety.
General secretary Jake Molloy said it was clear assurances were unfounded.
Mr Molloy is still calling on the Health and Safety Executive (HSE) to be "far more proactive" in ensuring industry maintenance procedures are followed.
The HSE confirmed it had served a prohibition order on Shell Expro's Brent Bravo installation.
The company said it had also brought forward a planned maintenance shutdown on the Brent Charlie platform to carry out checks and repairs, but Mr Molloy said his members still had concerns.
"Shell's announcement only reinforces the views of workers, which has been their for three years, that these are inherent risks and dangers which they have major concerns about," he said.
The UK Offshore Operators Association said patches were used as standard across the industry.