Subsea choke and riser pressure equalization system

In combination:

• an offshore production facility including a drilling vessel stationed above an ocean floor production site in which well head equipment is embedded;
• a marine riser connected to the well head equipment and extending from the well head equipment to the drilling vessel;
• a drill string enclosed within said riser extending from said drilling vessel through the well head equipment and into a borehole beneath the ocean floor;
• a choke conduit connected in fluid communication with the borehole and extending from the well head equipment to said drilling vessel;
• a choke valve having an inlet port and a discharge port;
• a gate valve connected in tee fluid circuit relation with said choke conduit and having an outlet port connected in fluid communication with the inlet port of said choke valve;
• and,
• a resilient discharge conduit connecting the outlet port of the choke valve in fluid communication with the interior of said marine riser.

A riser mounted, surface controllable subsea choke valve assembly comprising:

• a riser pup joint having pin and box connections at opposite ends for connection to a riser string;
• a section of choke conduit line mounted on said riser pup joint;
• a surface controllable choke valve having an inlet port and a discharge port;
• and,
• means connecting the inlet and discharge ports of said choke valve in fluid communication with the choke conduit line and the interior of said riser pup joint, respectively, for permitting diversion of fluid from said choke conduit line into said riser pup joint.

In a method of drilling a well in a subterranean formation beneath a body of water from a floating surface vessel wherein a drill string passes through a riser pipe which extends from said vessel to a subsea well head and then through a borehole under the body of water and wherein a drilling fluid is introduced into said drill string and is returned through the annulus between said drill string and said riser pipe, and wherein a choke conduit line is connected in fluid communication with said borehole and extends to said surface vessel for relieving the pressure exerted by gases and fluids of the subterranean formation, the improvement comprising the following steps:

• monitoring the down hole pressure of the subterranean fluid;
• opening a surface controllable valve positioned near the bottom of said riser pipe to selectively divert the gas and formation fluids conveyed through the choke conduit into the annulus of said riser pipe in counterflow relation with respect to return flow of said drilling fluid whenever the pressure exerted by the subterranean fluids exceeds a predetermined level;
• and
• varying the closure rate of said choke valve 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 fluid contained within the riser annulus.