Schlumberger Ltd. expects to post a pretax write-down of $800 million for the fourth quarter, related to the oil-services giant’s plan to reduce the size of its WesternGeco marine seismic fleet. The company said in a regulatory filing Tuesday that it plans to retire some older vessels, convert some others and cancel most of its third-party charters owing to expectations that oil-and-gas companies will reduce spending to explore for new sources of energy. As crude prices tumble , a number of big oil companies have been shelving expansion plans and shedding operations with particularly tight profit margins. Schlumberger said the moves will reduce the WesternGeco marine seismic fleet size to nine survey vessels and six source vessels, compared with 15 survey vessels and eight source boats at the end of last year. In October, Schlumberger reported that its third-quarter earnings rose 14% on revenue growth led by its North […]