A 15-year-old boy and his father are feared to be among five crewmen missing after an anchor handling tug capsized in the Atlantic, killing three people.
The coastguard said it would be a "miracle" if the five were found alive.
Ten crew members were found after Thursday's incident, about 86 miles west of the Shetland coast. Seven are in hospital, but three others died.
Divers had been searching the Norwegian Bourbon Dolphin in the hope the missing crew could be trapped inside.
The teenager is understood to be unaccounted for, along with his father, after the vessel overturned at about 1750 BST on Thursday.
Representatives from vessel owners Bourbon Norway and family members are expected to arrive in Shetland on a chartered flight from Norway on Friday.
An offshore union leader has said he believed the boat could have been caused to capsize by an anchor slipping up the side of the vessel.
Jake Malloy, general secretary of the Offshore Industry Liaison Committee, said the strain on the ship could have been enough to make it capsize.
Mr Malloy said: "There should be anchor stops to prevent slippage but it appears from the lads talking to one another that perhaps the anchor has gone up the side and that this has put strain on the vessel and perhaps caused it to sink.
Norwegian-registered tug supply vessel
Max draught: 6.5m
Deadweight (max): 2,500 t
Gross/net tonnage: 2,974t/892t
Speed: 17.5 knots (trial)
Capacity: 35 personnel
Source: Bourbon Offshore
"The fact that there may be people inside the vessel and they had an inability to prepare themselves for a rapid capsize tells you they have gone down very, very quickly."
Searches by a coastguard helicopter had been stood down until first light but Royal Navy divers continued to search throughout the night.
The search and rescue operation changed to a search and recovery operation on Friday afternoon, with the Royal Navy dive surveying the vessel using an undersea remote operated vehicle.
A spokeswoman for the coastguard said: "We are not really expecting to find survivors now.
"It would be a miracle if they had survived all night in five degrees, freezing water. It's very sad."
The coastguard said that the ship had been still attached to the Transocean Rather drilling unit by an anchor chain.
As a result, non-essential staff were removed to Shetland as a precaution.
Jim Sinclair, watch commander at Shetland Coastguard, said four helicopters and between six and seven surface craft had been involved in the operation.
He said the divers conducted a survey and risk assessment of the ship using a remotely operated vessel (ROV).
Mr Sinclair said: "It's a very complex and delicate operation as the vessel is actually upside down and stability is uncertain so they want to make sure of their own safety before they enter the hull.
"She was connected and that's one of the reasons that the Transocean Rather was down-manned just in case the vessel came up towards the rig on the tide change and damaged it."
Grampian Police said they would begin an investigation into the incident once the search and rescue operation was completed.
The ship, which is less than one year old, had been working in the vicinity of the Rosebank oilfield with a crew of 15.
Two helicopters have been involved in the search west of the Shetlands
RAF spokesman Michael Mulford told the BBC that if the crew had managed to find somewhere to keep themselves alive inside the Bourbon Dolphin the divers had been hopeful of locating them.
Trond Myklebust, manager director of Bourbon Norway, said heat seeking cameras and diving support crew were assisting the operation.
He added: "The vessel is more or less brand new. It has been doing this operation since it was delivered.
"This is a common task and a common job in the whole area and it's absolutely incredible what has happened. We cannot understand it or believe it because this should not happen."
He said he could not speculate on what had caused the accident but said the vessel was carrying out an anchor handling operation with the rig and was connected to it.
Rig operator Transocean said the 99 crew members of the Transocean Rather had been accounted for.
"As a precautionary measure, Transocean has taken the decision to down-man the rig of any non-essential personnel," it said in a statement."
The Bourbon Dolphin had a number of roles in the North Sea, including anchor handling and towing, the installation of subsea construction blocks and operations involving remote vehicles.