When a 17,000-tonne oil rig runs around there are a limited number of options in moving it again.

The Transocean Winner was being towed from Norway to Malta and came loose from the tugboat Alp Forward in the early hours of Monday and crashed into the shore of Lewis.

And although there is no actual oil on board the semi-submersible rig is currently carrying around 300,000 litres of diesel, with the risk to nature thought to be minimal at the moment despite 52 tonnes leaking out.

Salvage experts carried out detailed inspections and risk assessments of the rig on Wednesday and a careful removal plan will be considered after stormy weekend conditions brought the oil rig close to the coastline of Lewis.

The process

Four experts have been drafted in from SMIT Salvage, a Dutch company, to assess the situation. Once safety checks have been made and conditions have been met only then will they attempt to recover the rig.

Currently four large offshore tugs have been assigned to help with the operation: MV Union Bear, MV Olympic Orion (both in Stornoway Port), coastguard ship Herakles (off the coast of Dalmore where the rig is) and rig towing vessel Alp Forward.

Equipment to repair the rig is being mobilised, with a replacement of the tow line, however, there are concerns the coastline rocks could be holding the rig underneath or that rocks are embedded into the other fuel tanks.

Hugh Shaw, secretary of state’s Representative for Maritime Salvage and Intervention, said: “The main option and main objective we’re going for is to refloat the installation, get it towed back out to repair and assess the damage.

“Already we’ve had heavy seas and the rig would have been lifted high onto the rocks. Determining the weight of the rig and the diesel still on board takes a lot of planning and naval architects are prepared for this but there may be engineering works required too.

“The more extreme case is that the rig is unable to float and if we cannot do that then we need a wreck removal crew to dismantle and remove the rig piece by piece from the location.

“I must stress that is not in our plans at the moment because we are trying the best we can to get it away in one piece.

“We have many considerations to take in. If it is safe we have to try and extract the fuel to take some weight off and get the rig floating again – it’s not much but enough to possibly make it come loose. However there may be more difficulties with us being open to the elements and the shallow waters.”

The conditions

With weather closing in around the Western Isles force 6 winds are expected to pick up before the weekend. This escalates because of the exposed coastline and creates higher waves at sea.

This will make it more complicated for any salvage team to get on board to finalise preparations for a tug to take it back from shore and put the assessment phase at risk of setback, further delaying the removal of the rig itself.

Peak tides are expected in around seven to ten days time, with 4.5m to 5m of high tide allowing the tugboat easier access to the rig.

Mr Shaw added: “It’s not a quick fix; we could be talking months away. We don’t want to risk life and we’ve not got any good weather coming because the summer is over.

“Every high water or storm could move it around. It’s inevitable that we are going to get more damage so it’s a case of moving it as quickly as we can.

“There are so many uncertainties and we have to be prepared for the unexpected although there is no concrete plan in front of me now because since Monday we have only had a salvage team on board for three and a half hours.

“We need to get more information on the damage and how much fuel remains intact to help us with the weight distribution, and another issue is there is not currently enough wind to fly the helicopter.

When on a search and rescue operation there are certain limitations but when there is no risk to life the pilot has to abide by regulations.”

The rig itself

Experts are wary of the amount of potential risk to local wildlife if more diesel was to spill without the further complication if oil was on board as well.

As previously stated, another problem facing any salvage operation is the emergency tow line on the rig being caught, therefore making it more difficult to access when a tugboat is on approach to retrieve the rig.

Shown below is the difference between a semi-submersible drilling rig and a drill ship, with the various restraint methods used to keep it from moving when situated at sea.