As President Bush calls for repealing a ban on drilling off most of the coast of the United States, a shortage of ships used for deep-water offshore drilling promises to impede any rapid turnaround in oil exploration and supply.

In recent years, this global shortage of drill-ships has created a critical bottleneck, frustrating energy company executives and constraining their ability to exploit known reserves or find new ones. Slow growth in oil supplies, at a time of soaring demand, has been a major factor in the spike of oil and gasoline prices.

Mr. Bush called on Congress Wednesday to end a longstanding federal ban on offshore drilling and open the Arctic National Wildlife Refuge for oil exploration, arguing that the steps were needed to lower gasoline prices and bolster national security. But even as oil trades at more than $135 a barrel  up from $68 a year ago  the world’s existing drill-ships are booked solid for the next five years. Some oil companies have been forced to postpone exploration while waiting for a drilling rig, executives and analysts said.

Demand is so high that shipbuilders, the biggest of whom are in Asia, have raised prices since last year by as much as $100 million a vessel to about half a billion dollars.