PREVENTER

FIELD: oil production, particularly to seal well head during oil and gas well drilling. ^ SUBSTANCE: preventer comprises body with central axial channel and upper and lower sealing members with lock chamber arranged in-between. Each sealing member comprises upper and lower hydraulic drives correspondingly. The upper and lower hydraulic drives are adapted to seal pipe string to be lowered in well or lifted from well. Preventer is connected with pumping plant provided with upper and lower pressure regulators. The lock chamber may be divided into upper and low cavities in air-tight manner. Upper sealing member may rotate without lock chamber air-tightness failure. Upper lock chamber cavity may be communicated with upper hydraulic drive. Lower lock chamber cavity may be connected with pumping plant through lower pressure regulator after lower sealing member urging to pipe string under lower hydraulic drive action. Pipe string interior may be communicated with pumping plant through upper pressure regulator. ^ EFFECT: increased operational simplicity and reliability. ^ 2 cl, 2 dwg