Oil company BP says it will buy back $8 billion in shares after completing a complex deal for TNK-BP.

Friday's announcement comes after BP completed the sale of its 50 percent interest in Russia's third-largest oil producer, TNK-BP.

BP invested $8 billion in cash, shares and assets when it formed TNK-BP in 2003 and received a total of $19 billion in dividends over the next decade. BP sold its interest in TNK-BP to Rosneft and then reinvested in Rosneft shares. BP now holds a 19.75 percent interest in Rosneft.

BP Chairman Carl-Henric Svanberg says the company expects their stake in Rosneft to generate long-term gains, but that the buyback also gives our shareholders benefits now.

The buy-back is expected to take between 12 and 18 months to complete.