The far larger, second type of heavy lift ship is the ‘semi-submersible‘, equipped with a powerful ballasting system by which tanks are flooded as required, sufficient to submerge the vessel’s cargo area, which can be located beneath the object to be transported – e.g. an oil drilling platform or another ship. Once all is secured in the carriage position, the ballast tanks are pumped dry and the mother vessel – the semi-submersible itself – emerges from the water bearing the weight of the cargo. Cousins of heavy-lift vessels are barge carrying ships, of which there are several designs. In fact barges can be readily compared with containers, in that they are self-contained units capable of being loaded and discharged at the places of origin and destination of their cargoes, being transported between the two by mother conveyances. However, the capacity of a barge is much greater than that of containers and these larger, floating units lend themselves to the carriage of large unit commodities. The barge units are ‘dumb’ (unable to self-propel) but are designed for ease of transport under one of several carrying systems. LASH (‘Lighter Aboard Ship’) and SeeBee both employ a system by which lighters are lifted on and off mother vessels, being collected and distributed along waterways by towing craft. BACO (Barge aboard Catamaran) uses a system of floating barges into the mother ship through large bow doors. The USSR had a particularly extensive LASH and SeeBee system for strategic purposes, with mother ships capable of carrying individual barges of over 1,000 tonnes deadweight. Frequently, a combination of barges and containers can be handled. Despite the obvious attractions of this type of ship for certain trades it has to be said that it has failed to capture popular imagination and is likely to remain an interesting but very niche market.