Oslo (Bloomberg) — North Atlantic Drilling Ltd., the rig-owner controlled by billionaire John Fredriksen, signed a cooperation deal with OAO Rosneft that will make Russia’s biggest oil company one of its largest shareholders.

The accord, signed as the US and the European Union threaten further sanctions against Russia over Ukraine, will see Rosneft employ as many as nine offshore rigs for the equivalent of 35 rig years, the Hamilton, Bermuda-based driller said. Rosneft will buy ‘a significant’ stake and Seadrill Ltd. will remain the biggest owner. Seadrill, 24.5 percent owned by Fredriksen, holds 69.7 percent of North Atlantic.

It was one of several deals between Rosneft and international companies signed at the St. Petersburg International Economic Forum this weekend, less than a month after the US adopted sanctions against the Russian oil firm’s CEO Igor Sechin, a close ally of President Vladimir Putin.

Seadrill and North Atlantic Drilling see “huge possibilities” in the Russian market and aren’t concerned about the current tensions, Seadrill chief financial officer Rune Magnus Lundetrae said from London. “We operate in many parts of the world where there is or can be unrest, so it’s just business as usual for us.

“Right now, there are no challenges related to Rosneft and sanctions.” North Atlantic is already set to drill two wells in 2014 and 2015 in the Arctic Kara Sea for Rosneft’s joint venture with Exxon Mobil Corp. The Russian state-controlled oil producer extended that deal with the biggest US crude producer at the same venue in St. Petersburg, in addition to signing deals with BP Plc and companies from India to Venezuela.

Under the deal with Rosneft, due to last until at least 2022, North Atlantic will also venture into onshore drilling in Russia. The agreement, which is expected to be completed during the second half of the year, may involve asset swaps between the two companies, Sechin said in St. Petersburg May 24.

Rosneft could over time increase its stake in North Atlantic to as much as 50 percent, two people familiar with the agreement said. They declined to be identified because the details of the deal hadn’t been made public yet.

Seadrill will remain the largest shareholder in North Atlantic “in the short and medium term,” Lundetrae said. He declined to comment on the rates that will be charged under the agreement while Sechin said they would be at market levels.

“This is a good deal,” Lundetrae said. “We’ve seen that you don’t make the best deal through traditional tendering processes, where you’re usually measured only on price, but by offering a package.”