* To sell "substantial minority" to unnamed investor
* Buyer to get option to buy majority later
* Seadrill plans to list North Atlantic Drilling in U.S.
OSLO, Aug 15 (Reuters) - Seadrill, the world's biggest gas and oil rig company by market capitalisation, is to sell part of its North Atlantic Drilling subsidiary to take on a strategic partner with expertise on Arctic operations.
The offshore drilling arm of shipping tycoon John Fredriksen's business empire said it will sell a "substantial minority" of North Atlantic Drilling to an unnamed investor at current market prices.
Seadrill will offer an option to increase the stake later, but the buyer would have to pay a premium to acquire an eventual majority, it added.
"The potential strategic partner has a strong foothold in the Russian market, vast experience and competence in Arctic operations, including unique expertise in ice management," Seadrill said.
Seadrill will use part of the proceeds from the deal to buy more shares in North Atlantic Drilling in the open market and also plans to list the subsidiary's shares on a U.S. market immediately after closing the deal.
"The remaining proceeds will be used to fund existing and new growth opportunities within Seadrill's global ultra-deepwater and premium jack-up (rig) businesses and might also be used to support Seadrill's share of further growth in North Atlantic Drilling," it said.
North Atlantic Drilling, which has nine drilling units, has a market capitalisation of about $2.3 billion. Although it is not listed, its shares are traded over the counter in Oslo and were up 3.9 percent on Wednesday.
Seadrill, currency worth $20.8 billion, was trading 0.7 percent lower on Thursday.