The Transocean Winner was blown ashore on the western side of Lewis in severe weather on 8 August after detaching from its tug en route from Norway to Malta.
A decision when to start the refloating operation will be taken this morning – although a team hope to start the process by Tuesday morning at the latest, the Maritime and Coastguard Agency said.
The rig had 280 tonnes of diesel on board and two of its four fuel tanks were damaged in the incident, resulting in the loss of up to 53,000 litres of fuel – most of which is thought to have evaporated.
By yesterday morning, salvage teams on board had successfully transferred 137 tonnes of diesel fuel from the two intact tanks below the waterline to tanks above sea level.
A Maritime and Coastguard Agency spokeswoman said: “A decision will be made tomorrow morning as to whether the operation to refloat the rig will commence within the next 48 hours.
“It is hoped that a transfer of the recovered oil from the higher tanks within the rig to the supply vessel Olympic Orion should be completed in the next 24 hours. A small quantity of diesel oil will be retained on board to supply the emergency generator and other essential services.
“During the past 24 hours, further equipment for the refloat operation has been placed on the rig. No pollution has been reported. A temporary exclusion zone of 300 metres remains in place.”