Subsea choke and riser pressure equalization system

Abstract

Method and apparatus for controlling down hole pressure during an offshore drilling operation is disclosed. The method and apparatus are particularly useful for drilling in deep water from an offshore surface facility which includes a drilling vessel stationed above an ocean floor drilling site in which well head equipment is embedded. A marine riser is connected to the well head equipment and extends to the drilling vessel. A drill string enclosed within the riser extends from the drilling vessel through the well head equipment and into a borehole beneath the ocean floor. Choke and kill conduit lines are connected in fluid communication with the borehole and extend from the well head equipment to the drilling vessel. According to the invention, a subsea choke valve is connected in fluid communication with the choke conduit line and with the interior of the riser whereby fluid or gas may be discharged at a controlled rate from the choke line directly into the riser annulus. Closure of the choke valve is controlled from the surface in order to maintain the down hole pressure exerted by the gases and fluids of the subterranean formation substantially in equilibrium with the hydrostatic head of the column of drilling mud contained within the riser annulus. In a preferred embodiment, the subsea choke valve apparatus mounted on the lowermost riser section which is coupled to the well head equipment.