The coastguard tug Herakles is on her way from Orkney to a grounded oil rig off Lewis.
The semi-
The platform was being towed by the tug Alp Forward during severe weather west of Lewis.
The tug remained connected to the Transocean Winner overnight but was unable to make headway with the tow.
The coastguard emergency towing vessel Herkales was deployed from Kirkwall on Sunday night to assist. She is expected to arrive later on Monday.
At 4.20am on Monday, the master of Alp Forward reported that the tow line had parted and due to the severe weather conditions was unable to reconnect the tow line.
There are no personnel on board the rig and there is no risk to life.
Transocean and ALP Marine have established their emergency response rooms, SMIT salvage has been mobilised to deal with the incident.
Coastguard tug on route to grounded oil rig
8 August 2016