a method for drilling offshore wells in deep water with greater safety and with fewer casing strings , maintaining a maximum diameter wellbore in the early stages of drilling to facilitate casing operations in and beyond deviated sections , and providing full well control while setting surface casing . this method utilizes a two - riser system for drilling in which the surface casing interval is run and set within a large diameter , light duty riser . thereafter , drilling proceeds with a heavy duty riser .