Three men died last night and a search was under way for five others missing after an oil rig support boat capsized in the North Sea.

There were fears that the missing five were trapped in the hull of the upturned boat, potentially submerged in very cold water with limited oxygen.

When the Norwegian vessel overturned off Shetland ten crew were rescued quickly but the coastguard confirmed late last night that three of them had died. The remaining seven were airlifted to the Gilbert Bain Hospital in Lerwick.

Five men, all Norwegian, were still missing hours after the Bourbon Dolphin overturned close to the rig, 75 miles northwest of Shetland, shortly after 5pm.

The Maritime and Coastguard Agency said that sea temperatures were 5C (41F), which would be like being immersed in “a fridge full of water”. A spokesman, Mark Clark, said: “If these guys are trapped under the hull in water we need to get to them. The cold shock and immersion in such temperatures would limit life expectancy. Unless they have got into a dry part of the hull, they need rescuing. We don’t know if they are wearing immersion suits.

“We have searched all around the vessel and they are not there. If people are under the hull we don’t know how much air they have or whether they have managed to struggle into their survival suits.”

The rig operators, Transocean, said that the boat, which was less than a year old, was handling an anchor from a drilling rig when it capsized.

Eight of the 15 people on board were initially recovered and another two were winched by helicopter from the water at about 7pm.

Navy divers searched for the missing crew throughout the night while a coastguard helicopter search continued at first light this morning.

Mr Clark said that a diving operations support vessel was being used to try to locate the men.

“The Viking has an ROV [remotely-operated vehicle] so we can get under the hull but it is getting dark and that is going to hamper our operation. We cannot see anyone therefore we can only assume they are under the hull. We’ve issued a mayday call for any vessels in the area to come to us immediately because we need divers.”

It was not known whether the missing crew members had been wearing survival suits or lifejackets at the time of the capsizing.

A coastguard helicopter and a North Sea Rescue helicopter were scrambled to the scene and an RAF Nimrod was providing overhead cover.

Three vessels were also assisting with the search. The 2,900-tonne Norwegian-registered vessel delivers supplies to oil rigs and supports ROVs. It was commissioned only last October.

The vessel, which had changed crew in Lerwick on March 30, had been working in the Rosebank oil field, operated by Chevron. Weather conditions were good, with clear visibility and moderate seas. In a statement, Transocean said all 99 crew members of the oil rig were accounted for. They were being airlifted to the mainland last night.

The condition of the seven people plucked to safety was not known last night. There will be a Marine Accident Investigation Branch inquiry.

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