Patent ID: 7185719

Claim:
A drilling system for drilling a bore hole into a subterranean earth formation, the drilling system comprising: a drill string extending into the bore hole, whereby an annular space is formed between the drill sting and the bore hole wall, the drill string including a longitudinal drilling fluid passage having an outlet opening at the lower end part of the drill string; a pump for pumping a drilling fluid from a drilling fluid source through the longitudinal drilling fluid passage into the annular space; a fluid discharge conduit in fluid communication with said annular space for discharging said drilling fluid; a fluid back pressure system in fluid communication with said fluid discharge conduit; said fluid backpressure system comprising a bypass conduit and a three way valve provided between the pump and the longitudinal drilling fluid passage, whereby the pump is in fluid communication with the fluid discharge conduit via the three way valve and the bypass conduit which bypasses at least part of the longitudinal fluid passage.