LONDON (Bloomberg) — Bermuda-based Seadrill Ltd. raised its stake in Scorpion Offshore Ltd. to 50.11 percent and made a mandatory offer for the remaining shares at 40.50 kroner apiece, valuing the company at 3.6 billion kroner ($558 million).

Seadrill, an oil-rig operator controlled by billionaire John Fredriksen, plans to make the offer this week and will have an acceptance period of six weeks, the company said yesterday in a statement. It raised an earlier offer of 36 kroner a share to fend off a bid form Ensco Plc.

"We decided to increase the offer price after a thorough evaluation of the situation," chief executive officer Alf Thorkildsen said in the statement. The acquisition "will increase our earnings and strengthen our position as the leading operator of modern high quality drilling equipment."

Ensco Plc said at the end of last week it was not able to complete its partial tender offer for Scorpion after it didn't secure a sufficient number of shares. Ensco, backed by Scorpion, raised its offer to 40 kroner a share from 39.5 kroner a share.