More than 50,000 litres of fuel is believed to have leaked from an oil rig that ran aground on the Western Isles.
The Transocean Winner rig was carrying 280 tonnes of diesel – more than 300,000 litres – when it was blown ashore in severe weather conditions on the western side of the Isle of Lewis early on Monday.
However, no oil has been seen on the surface of the sea and Hugh Shaw, the UK government’s representative for maritime and salvage, said most of the escaped diesel would have evaporated.
He said the “worst-case scenario” is that about 52 tonnes, more than 50,000 litres, of oil has leaked from the two breached tanks which salvage teams discovered on Wednesday.
He said: “We’re not sure if all the oil has been lost from these tanks but we’re probably talking about a maximum of 52 tonnes of diesel oil as the worst-case scenario at this time.
“Any oil lost to the environment is obviously a concern for us but this is extremely low-risk.
“The oil was probably lost from these tanks when the installation came in and grounded itself.
“There is no oil at the moment on the sea surface. The fact that the tanks have been breached, that oil has long gone now, most of it will have evaporated on the Monday morning.”
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He said an air search of the coastline found “no trace of oil” and work would continue with the salvage teams and Transocean to keep the two remaining tanks from being damaged.
A Maritime and Coastguard agency spokeswoman said: “Salvors will determine the quantity of diesel remaining in those tanks during the course of the salvage operation.
“Diesel is a light and non-persistent oil which presents much lower environmental risks than heavy black crude oil.”
A “low level” of pollution was detected by tests on Monday, Western Isles Council said, but no obvious signs of a spill remain at the site.
The semi-submersible rig detached from its tug during towing and the weather prevented the line being reconnected.
No-one was on board the rig when it grounded at Dalmore beach near Carloway.
People are being urged to stay away from the beach as teams continue to work and a temporary 300-metre (980ft) exclusion zone has been set up at sea.
Environmental groups have raised concerns while it is expected to take “some time” to re-float the rig as it came ashore at high tide with a storm surge.
Scotland’s environment secretary Roseanna Cunningham said serious questions need to be asked as to why the rig was being towed during severe weather.
She said: “This whole incident raises serious questions about why this rig was being towed through Scottish waters when such stormy conditions were forecast and the Deputy First Minister has been in direct contact with the UK government about this very point.”
Friends of the Earth Scotland said it could create a “serious problem” for wildlife, tourism and fishing in the area.