The Piranema unit was built in 2007 and is operating under charter to Petrobras SA near Brazil. The charter runs through March 2018 with up to 11 one-year extension options.
The Piranema deal is part of Teekay's previously-announced plan to buy three such units from Sevan. The other two will be bought if they get charter contracts for at least three years.
The partnership also said it has agreed to sell 7.1 million common units to a group of institutional investors. It said it expects proceeds of about $170 million. The partnership said it will put the money toward buying the Piranema unit as well as to partially pay for its purchase of four new shuttle tankers to be delivered in mid-2013.
The partnership said it expects the Piranema deal and the private placement of common units to happen during the fourth quarter of this year.
Bermuda-based Teekay Offshore Partners LP is a master limited partnership formed by Teekay Corp. It provides marine transportation and oil and production and storage services to the oil industry.
Teekay Offshore Partners shares rose 86 cents, or 3.3 percent, to $27.25 in morning trading. Teekay Corp. shares were up 28 cents to $28.43.