Transocean Ltd (RIGN.VX) (RIG.N), the world's largest offshore drilling contractor, raised its 2011 tax rate guidance for the second time due to a shift in the regional mix of its working rigs.

Chief Financial Officer Ricardo Rosa on Thursday said the tax rate would be 21 percent to 23 percent, up from a previous estimate of 19 percent to 21 percent. The company also raised the estimate by 2 percentage points three months ago.

Shares of Transocean were down 6 percent in morning trading on the New York Stock Exchange on Thursday, a day after the company reported weaker-than-expected second-quarter profit, hit by higher costs caused partly by maintenance activity. [ID:nN1E7721R3]

The company now expects operating and maintenance expenses of $5.6 billion to $5.8 billion this year, compared with $5.4 billion to $5.7 billion before, due mainly to shipyard work, wage increases and other inflationary pressures, including the weaker U.S. dollar.

But Rosa expects the effects of maintenance activity to diminish in the quarters ahead.

"Looking into 2012, we currently expect revenue losses from out-of-service days to be lower when compared to 2011," he said.