A UK accident report has revealed how a tug lost control of the semi-submersible rig Transocean Winner in severe weather off the Scottish coast last year.
The Maritime Accident Investigation Branch (MAIB) said the passage of the ship and the platform close to the shore left little room for error.
Transocean Winner grounded on 8 August on the Isle of Lewis while under tow by the 3,200-dwt tug ALP Forward (built 2008).
It was heading from Stavanger, Norway, to Valletta in Malta.
The effect of the wind and waves on Transocean Winner led to the loss of ALP Forward’s ability to control the direction and speed of the tug and tow, MAIB said.
After being dragged backwards by the tow for over 24 hours, the tow line, weakened by the repeated sudden loadings, parted and the tug was unable to pick up the emergency towline, it added.
Tug owner ALP Maritime Services has been recommended to review its towing manuals to ensure crews have all necessary information, including tow-specific guidance on the need to consider sea room and lee shores during passage planning.
"Without the necessary information, it was not possible for the master to predict the tug’s inability to hold the rig and change his passage plan in time to seek shelter," MAIB said.
"The planning of a passage so close to the coast left little sea room for the tug and tow to drift. When ALP Forward lost control of the tug and tow, it was very likely that Transocean Winner would have grounded even if the tow line had not parted."
The report said the tow line was in a generally poor condition and there was insufficient slack, leading to repeated sudden loadings.
But it added: "It is quite possible that a new tow line would have also parted under the same circumstances and conditions."
