Tug-assistance is generally not compulsory, but sometimes ordered by the Master in order to ensure a safe approach - especially in Brunsbüttel due to current/tide in River Elbe.
Vessels transitting in traffic group 6 (vessel's dimensions exceeding a length of 200 metres and/or a breadth of 27 metres) have to accept tug-assistance when approaching Brunsbüttel locks from River Elbe.