The feasibility of a new concept for drilling and lining of a well has been investigated. The patented concept is based on the use of a dual channel drill string, where one of the channels is used for the return flow from the well. A sliding piston in the annular downhole section of the well, is used for force and pressure control. The concept is suited for drilling with a liner, which optionally can be expanded in place in the same run. The concept has special advantages for improved downhole pressure control, and for safe and efficient drilling of reservoirs with challenging pressure zones.The concept is also expected to provide solutions for cost efficient drilling, especially for sub-sea wells.

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