An anchor slipping up the side of the Bourbon Dolphin could have caused the vessel to sink, a union leader has suggested.
North sea workers must contend with difficult conditions
Jake Molloy, general secretary of the Offshore Industry Liaison Committee, said the strain on the ship could have been enough to make it capsize.
Mr Molloy said there should be anchor stops on the ship to prevent slippage.
And he said the tug must have capsized "very, very quickly" for crew members to have been trapped inside.
Mr Molloy said: "There are mixed messages and we can't be absolutely sure but at this stage there is some suggestion that perhaps the anchor slipped up the side of the vessel.
"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 but we have got to wait for the marine accident investigation.
"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."
Mr Molloy said crew members underwent "rigorous and extensive" safety training every two or three years which would have prepared them for "almost anything".
He added: "I'm certainly not aware of a similar incident in the 25 years I've been involved with the industry.
"There can be big swelling in the North Atlantic and winds changing variably but this is unheard of.
"We hope obviously for the lads that are missing that the divers are able to locate the. I think that will be determined by their position on the vessel at the time of the capsize."
Richard Crowther, regional operations manager for the Maritime and Coastguard Agency, said he still did not know what caused the tragedy.
Mr Crowther said: "It is unclear what happened. There will be a marine accident investigation, an investigation by the procurator fiscal and the police and a fatal accident inquiry.
"They will report exactly what they felt happened. All we do know is that during anchor handling operations something clearly went very wrong and resulted in this very unusual capsizing of a very big, modern vessel."
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