A barge unit includes LNG receptacle and a first power generation apparatus including a first combustion engine that is operated at least partly by LNG boil-off gas and a first generator that is driven by the first combustion engine. A tug unit includes a propulsion system and a second power generation apparatus for providing power to the propulsion system. In a first operating mode of the barge and tug units the tug unit is connected to the barge unit for propelling the barge unit and the propulsion system of the tug unit is powered at least partly by the first power generation apparatus.