The incident started at about 21:30 GMT on Sunday when the 70m (230ft) tug slowed to check a problem with the tow and was struck on the port side by the Moto Vessel Emsstrom.
The report also highlighted the lack of an accepted international industry standard for tugtow line emergency release systems, and asked risk management organisation Lloyd's Register to develop a standard to ensure tow lines can be released effectively in an emergency.