Patent ID: 8322439
Filing Date: 2012-12-04
Classification: E21B

Abstract:
1. A method for maintaining a desired pressure in a subsea well under both static and dynamic operating conditions, the subsea well being coupled to a drilling unit with a riser, said drilling unit having a hollow drill string that can be lowered into the well and filled with a drilling fluid flowing out of the bottom of the drillstring and into the well, said riser surrounding the drill string through which the drilling fluid can flow out of the well, thereby forming a drilling fluid column in the riser above the well, and a subsea drilling fluid outlet through which the drilling fluid can be pumped from the riser by a subsea pump so as to cause a height of the drilling fluid column to be below the sea surface but above the subsea drilling fluid outlet, the method comprising: measuring a pressure in the well near the bottom of the drill string; based on the measured pressure and a known density of the drilling fluid, calculating a height of the drilling fluid column in the riser that will cause a pressure in the well at a specified depth between the bottom of the drillstring and the subsea outlet to be equal to the desired pressure; determining an actual height of the drilling fluid column; comparing the calculated height of the drilling fluid column with the actual height of the drilling fluid column; calculating a height increase or reduction of the drilling fluid column in the riser required to obtain the desired pressure; and adjusting the height of the drilling fluid column according to the calculated height increase or reduction, while keeping the desired pressure substantially constant at the specified depth both in static and in dynamic operating conditions.