NEW YORK (Bloomberg) — Scorpion Offshore Ltd., an offshore oil rig company, recommended shareholders accept an offer by Ensco plc to buy 19 percent of its outstanding shares in an attempt to fend off a takeover by Bermuda-based Seadrill Ltd.

Ensco (Bermuda) Ltd. is making a partial tender offer for Scorpion shares at 39.50 kroner each, London-based Ensco said in a statement. The offer ends May 28. Scorpion has been seeking alternatives to a bid of 36 kroner apiece made by Seadrill.

If the "offer is successful, Ensco or one of its subsidiaries will take such additional steps permitted or required by law to acquire the remaining shares of Scorpion at the same cash price," Scorpion said. "If the partial tender offer is not successful Ensco doesn't intend to take further steps to acquire Scorpion."

Scorpion climbed as much as 3.4 kroner, or 9.1 percent, to 40.9 kroner, the highest intraday price since October 2008. The stock traded at 40.5 kroner as of 1:18 p.m. in Oslo and has gained 66 percent this year, valuing the company at 3.6 billion kroner ($554 million).

Seadrill, the oil-rig owner founded by billionaire John Fredriksen, triggered a mandatory bid for Scorpion last month after raising its stake to 40.1 percent. The offer, announced May 10, was at a 2.7 percent discount to Scorpion's May 7 closing price, valuing the company at $535 million. The offer period ends June 7.

"We have taken note of the fact that another offer has come in and we'll use the time at our disposal to consider our options," Jim Daatland, Seadrill's head of investor relations, said on the phone today. "We're happy with anything from 40 percent to 100 percent" in Scorpion.

The market for jack-ups, which are oil rigs with extendable legs, has strengthened since the end of 2009 and Scorpion said it's optimistic about demand for the "ultra premium" segment of the market.

Scorpion "is a highly respected offshore drilling contractor with an excellent reputation for safety and operational excellence," Dan Rabun, Ensco chief executive officer, said in the statement. "The proposed transaction fits our strategy of high-grading our fleet with newer rigs that will benefit from stronger utilisation and higher day rates."

Scorpion's board is entitled to end the agreement with Ensco or pursue a superior proposal prior to the conclusion of the partial tender offer, for a breakup fee of $13.5 million, the company said. Members of Scorpion's senior management and board and their affiliates, representing about 31 percent of the outstanding shares of the company, have agreed to accept any subsequent voluntary or mandatory offer made by Ensco, should the partial tender be successful, it said in the statement.