Transocean , owner of the world's largest offshore drilling fleet, has put its Middle East rig assets up for sale, in a deal likely to raise $1 billion for the Switzerland-based company, three sources said.

U.S. private equity firms Lime Rock Partners and Castle Harlan are the main bidders, one of the sources said speaking on condition of anonymity as the matter was not public.

"In all, there are about 30 rigs for sale and the equity cheque is about $1 billion," one source said.

"A sale process was initiated a while back but their weren't a lot of local players involved. It's mainly U.S. private equity firms," a second source said.

A Transocean spokesman declined to comment on what he said was "market speculation."

In March, Transocean Chief Executive Steven Newman said the group expected to sell older rigs this year worth between $500 million and $1 billion as it steps up its effort to upgrade its fleet.

In 2011, Transocean ended up selling five shallow-water jack-up rigs and a swamp barge for a total of $163 million.

The company owned the rig destroyed in the 2010 Gulf of Mexico oil spill, while BP owned a majority of the Macondo well where a blowout led to the largest offshore oil spill in U.S. history.

Transocean took a $1 billion charge related to the 2010 Gulf of Mexico oil spill in February.

Source: Reuters