NEW YORK ( TheStreet) -- Greenlight Capital, the hedge fund firm run by David Einhorn, has acquired a 5% stake in offshore driller Ensco ( ESV), according to a filing made with the Securities and Exchange Commission after the market close on Monday.

The stakes disclosed by hedge fund managers are often used as way of measuring undervalued plays in the market. Ensco has been among the notable offshore drilling stock losers since the BP oil spill crisis began in the Gulf of Mexico on April 20.

Ensco shares have fallen from a 52-week high above $52 reached shortly after the BP oil spill, to a low of $33 in June. On Monday, Ensco shares closed at $40.53.

Shares of Ensco were up 1.5% in the after-hours session on Monday.

On Monday, the Obama administration introduced its revised offshore drilling ban for deepwater projects, with indications that with certain safety improvement, the government would lift the ban ahead of the six-month duration for which the ban was scheduled before a judge overturned the Obama policy.

BP ( BP) also had one of its biggest rally days since the beginning of the oil spill crisis, up 8% as its improved cap was put on place over the leaking well, and there was hope that within a day or two the leak might be contained.

Offshore driller Transocean ( RIG) had the biggest gain of any of the offshore drillers on Monday, as it too benefited from the positive news turns in the BP oil spill story. Transocean shares were up more than 4% on Monday.

-- Written by Eric Rosenbaum from New York.

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