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.

Bush called on Congress on Wednesday to end a long-standing federal ban on offshore drilling and open the Arctic National Wildlife Refuge for oil exploration, arguing that the steps were needed to lower gas 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 have raised prices since last year by as much as $100 million a vessel to about half a billion dollars.

“The crunch on rigs is everywhere,” said Alberto Guimaraes, a senior executive at Petrobras, the Brazilian oil company that has discovered some of the most promising offshore oil but has been unable to get at it. “Almost 100 percent of the oil companies are constrained in their investment program because there is no rig available,” he said.

As a result, drilling costs for some of the newest deepwater rigs in the Gulf of Mexico — the nation’s top source of domestic oil and natural gas supplies — have reached about $600,000 a day, compared with $150,000 a day in 2002.

These record prices have spurred a new wave of drill-ship construction. This boom could lead to renewed offshore oil exploration that would eventually bring more supplies to the oil market, and push down prices.

Already, 16 new drill ships are scheduled to be delivered to oil companies this year — more than double the number delivered over the last six years combined. In fact, 75 ultra-deepwater rigs should be delivered from 2008 to 2011, according to ODS-Petrodata, a firm that tracks drilling rigs.

Most new orders for drill ships have gone to Asian shipyards. Companies in Singapore and China have benefited, but South Korea’s big three shipbuilders — Samsung Heavy Industries, Daewoo Shipbuilding and Marine Engineering and Hyundai Heavy Industries — have gotten the bulk of orders for the most complex and expensive types of vessels.

“The market for offshore exploration is now the hottest sector in the global shipbuilding industry,” said Lee Jae-kyu, shipbuilding analyst at Mirae Asset Securities in Seoul.