Many of the North Sea Oil constructions are too heavy or of an awkward shape to be moved on a conventional vessel. When this happens, barges and pontoons are towed by one or more tugs to their destination. Barges are much more than empty boxes, as loading them involves complex shifting of water-ballast between compartments, so that they remain level as the load moves from the shore to the barge and vice versa. Pontoons are generally empty boxes.