It is possible that Shell were erring on the side of caution which is to be commended

Jake Molloy OILC union

A third helicopter from a neighbouring Norwegian rig was also asked to help with the operation, a spokesman for Aberdeen Coastguard said.

The workers were taken to six other Shell rigs in the North Sea, while 53 "essential personnel" remained on the platform to wait for the arrival of engineers.

The transfer operation was completed by 0200GMT on Wednesday.

Water tanks

A Shell spokesman said hydrogen sulphate (H�S) gas, which gave off a distinctive "rotten eggs" smell, appeared to have seeped from the platform's legs.

"Each of the legs has water tanks for ballast," he went on. "Any stagnant water will build up the gas."

It was possible that the workers would return to the installation later on Wednesday or Thursday morning once the all-clear had been given by the investigation team.

Brent Charlie, pictured in stormy seas

Jake Molloy, leader of the OILC oil workers' union, said hydrogen sulphate was a "highly toxic gas".

Mr Molloy said he did not know the quantities involved but added: "It is possible that Shell were erring on the side of caution which is to be commended."

Aberdeen Coastguard Ross Greenhill said: "We had to transfer 102 people which is a substantial amount for two aircraft to handle.

"There were around eight or nine trips.

"I would say it was fairly routine.

"Certainly something that we have done on a few occasions before, so it didn't prove to be problematic in any way. And for the air crew it is what they are there for."