Transocean Sedco Forex Inc., already the world's biggest offshore oil-drilling contractor, agreed to buy R&B Falcon Corp. in a $5.3 billion stock deal that will make it the dominant deepwater driller. Under the agreement, approved by the boards of both companies, R&B Falcon stockholders will receive 0.5 shares of newly issued Transocean Sedco shares for each R&B Falcon share. Transocean will also assume approximately $3 billion in R&B Falcon debt, putting a total value on the deal of some $8.3 billion. The combined company would have a market capitalization of about $16.5 billion, almost three times as much as its biggest competitors, and would rank only beneath diversified oilfield services giants Schlumberger Ltd. and Halliburton Co. as a contractor to the global oil and gas industry.