Transocean Inc. agreed to buy GlobalSantaFe Corp. for about $17 billion, combining the world’s two largest offshore oil and gas drillers in a market where demand for rigs has never been higher.

The new company will be triple the size of its largest peer by sales, Noble Corp., with 146 rigs and an order backlog of $33 billion. In a statement, Transocean and GlobalSantaFe, both based in Houston, called the transaction a merger, with shareholders getting a combined $15 billion in cash.

The chief executive officer, Robert Long, said the deal will give Transocean a full range of drilling and a global reach. Rig rates are rising as producers press their search for reserves to deeper waters. A rig shortage is hindering exploration, contributing to near-record oil prices.

“Demand is excellent for deepwater rigs,” an analyst with Hodges Capital Management Inc. in Dallas, Michael Breard, said. The Hodges fund includes $730 million in equities and holds shares of both Transocean Inc. and Global-SantaFe Corp. “They’ve got a $33 billion backlog — money is the least of their problems.”

Shares of Transocean had their biggest gain since September 1, rising $5.99, or 5.5%, to $115.96 in New York Stock Exchange composite trading. GlobalSantaFe climbed $3.59, or 4.8%, to $78.33, its biggest increase since May 11.

Rents for deepwater rigs rose 13% from a year earlier to a record June average of $494,957 a day, ODS-Petrodata Inc. reported.

Transocean stockholders will get $33.03 in cash and 0.6996 share of the combined company for each of their shares, the drillers said yesterday in their statement. GlobalSantaFe owners will get $22.46 in cash and 0.4757 share. Shareholders on neither side are getting a premium for their stock.