Oilfield services provider Petrofac Ltd said strong performance at its engineering and construction division had helped increase its order backlog to $20.5 billion at the end of May.The company, however, said it expected pretax costs at its Laggan-Tormore project to increase by about 30 million pounds ($47.35 million) as it began additional completion and pre-commissioning works.The company has incurred about 140 million pounds in costs on the project so far this year. Petrofac has been taking heavy losses on the Shetland Islands project, buckling under the North Sea's hostile environment and the high cost of doing business in the region.The company had said in April it expected a pretax loss of about 130 million pounds in 2015, in addition to the $230 million it recognised in February.Laggan-Tormore, a high-quality oil and gas asset 80 percent owned by French oil major Total, is expected to yield about 93,000 barrels of oil equivalent per day at its peak.