MOSCOW (Reuters) - Russia's biggest oilfield services company Eurasia Drilling Co (EDC) (>> Eurasia Drilling Co Ltd) reported a 12-percent fall in first-quarter revenues on Thursday on a weaker rouble and changes in its clients.

Eurasia Drilling has reduced its cooperation with Russia's top oil producer Rosneft (>> Rosneft' NK OAO), which plans to develop its own drilling services.

It said Rosneft's share of EDC's total metres drilled fell to 15 percent from 20 percent a year earlier. Total drilling output was down 12 percent to 1.2 million metres.

"The first quarter was a challenging one as approximately one-third of our fleet was being moved which resulted in a lower EBITDA margin," Richard Anderson, EDC's chief financial officer, said in a statement.

Top line revenue fell to $697 million while the company's earnings before interest, taxes, depreciation and amortisation (EBITDA) margin declined to 18.8 percent from 22.6 percent a year earlier.

EDC had expected 80 percent of its onshore drilling rigs currently used by state oil company Rosneft would be reallocated to other customers this year, while the remaining 20 percent would continue to work for the state-own oil producer for the full year.

EDC accounts for 29 percent of the total number of metres drilled in Russia and its largest clients are Lukoil (>> Neftyanaya kompaniya LUKOIL OAO) and Rosneft. It said its total market share in Russia was stable last year.

(Reporting by Vladimir Soldatkin; Editing by Lidia Kelly and Jason Neely)