A union leader has called for urgent action to address offshore safety amid fears that somebody could be killed.
Sunday's fire was followed by two other offshore incidents
An investigation has been launched into a fire which broke out on Shell's North Cormorant on Tuesday.
It followed a serious fire on the Thistle Alpha on Sunday. Both platforms are north east of Shetland.
Graham Tran, of the Unite Amicus union, said there had been a series of worrying incidents. Shell said safety was the company's highest priority.
Production was also shut down on the Cormorant Alpha platform after a power failure knocked out two of the four generators on Tuesday.
Lighting and heating was still available and the decision to shut down was taken as a precaution.
Cormorant Alpha also lies north east of Shetland, close to the North Cormorant platform, which went to muster stations while the "small" fire was put out.
Meanwhile, Health and Safety Executive (HSE) inspectors were excepted to fly to Thistle Alpha to begin their investigation into the fire which led to an evacuation there on Sunday.
Petrofac said more staff were being flown ashore while that installation remains shut down.
Mr Tran told BBC Scotland: "This is after the HSE launched a scathing report on the industry and after a major incident on Thistle Alpha. It was a fire, a fire that should not have happened.
"It's unacceptable and it's really putting lives at risk, it's only a matter of time, I believe, before somebody is seriously injured or killed."
Earlier, First Minister Alex Salmond's call for the Scottish Government to handle North Sea offshore safety in Scots waters was dismissed by Westminster.