The 115 workers evacuated from a North Sea platform following a subsea structural inspection are being flown back to shore.
EnQuest said it took people off the Thistle Platform as a precautionary measure on Monday evening following an inspection relating to a support element on a redundant subsea storage tank.
The workers were transferred to the nearby Dunlin installation by helicopter on Monday evening and the Thistle Platform was safely shut down.
Union leaders voiced concerns the rapid evacuation could have been prompted by a “serious problem” on the platform.
A helicopter and two rescue aircraft from the Norwegian offshore sector were involved in the evacuation, the Coastguard said.
The Thistle Platform is located around 125 miles north east of Shetland.
EnQuest said the 115 workers are safe and well and were in the process of being flown onshore on Tuesday.
Bob Davenport, North Sea managing director at EnQuest, said: “The safety of our people is our absolute priority.
“Our offshore installation manager took proactive action to transfer everyone from the platform as a precaution following yesterday’s inspection.
“This was carried out safely and quickly, with plans then made for their onward travel home.”
He added: “Further inspection work will be conducted and the platform will remain shut down until that has concluded and any necessary remedial action undertaken.
“I’d like to thank everyone involved for their support including the team onboard Dunlin, employees, contractors and the Maritime Coastguard Agency.”
The Coastguard said it was made aware of the incident at around 6.30pm on Monday.
The RMT union said the incident raised questions about the inspection regime.
Mick Cash, general secretary for the union, said: “This is an unprecedented event and the speed with which the installation was evacuated suggests there is a serious problem.
“RMT has had concerns for some time about the impact of cost cutting and we would hope the Health and Safety Executive (HSE) will seek to establish whether a robust inspection regime existed and if not, why not?
“The sudden decision to evacuate would indicate that any inspection regime which did exist failed to identify an issue until it became critical.”
He added: “We hope this event can be used as a learning exercise for the industry as there are a number of steel jacket installations of a comparable age to Thistle still operating in the North Sea.
“For the workers, their families and all affected by these events we need to know what is being done to remedy the problem and what the future holds for the workforce.
“We expect all workers to remain on full pay and for extensive consultations to commence on what has happened.”
An HSE spokesman said: “HSE is aware of the evacuation on the EnQuest Thistle platform and making further inquiries.”