The 3,600-dwt Fri Ocean (built 2000) got stuck at a speed of 10.5 knots off the Isle of Mull in June.
Bow plating was damaged, resulting in flooding to the bow thruster room.
The crew carried out a temporary repair, and the vessel was refloated within hours.
The Marine Accident Investigation Branch (MAIB) said the second officer fell asleep largely through "lack of stimulation, possibly exacerbated by fatigue", shortly after making a course alteration.
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