2. Passage 19 Throughout her working life， a ship — like a motorcar — must be serviced regularly if she is to remain efficient. After a period at sea she must be overall checked and any necessary repairs must be made. There are also instances when a ship must be converted from being one kind of carrier to another. Most of today‘s oil， for example， is carried by huge super-tankers. The smaller tankers， many of which are still in good condition， can be converted into bulk carriers(散货船) for the transport of sugar， coal or ore. Conversion or repair work is carried out in a dry dock， a huge hole cut into the ground and lined with concrete. At one end are large gates separating the dock from the river or sea. When these gates are opened， water pours into the dock and fills it. A ship can then enter the dock， where it is carefully fastened in an upright position. When the ship is positioned inside the dock， the gates are closed and the water is pumped out. The vessel slowly settles on to wooden blocks. Wooden supporters are put in position between the ship and the sides of the dock to prevent her from rolling over. Great care is taken to make sure that the ship rests evenly on the keel(龙骨)blocks to prevent damage being done. As soon as the ship is firmly positioned， repair or conversion work can begin on her body or superstructure. Many smaller tankers have been converted into bulk carriers because( ).