Research and application of retriivable devices for casing drilling

　　Based on the device study, key parts strength test, model experiment and entire tool assembly simulating experiment conducted on rig site, three field trials have been performed in diverse areas and drill crews in Henan oilfield in August, 2008. Perfect result has been gained at last by repeated experiments and modifications. The trial result has proven that the retrievable casing drilling device is simple in structure, the design is rational and it can meet the demand of field applicatioin. Using the new designed retrievable casing drilling device can accomplish bit replacement during casing drilling process, reduce bit changing time and save drilling cost.