Patent ID: 9322232
Filing Date: 2016-04-26
CPC Classification: E21B

Claim Text:
1. A method for inhibiting an explosive atmosphere in a wellbore drilling system including a riser connected to a wellbore above a top thereof, the riser having a fluid outlet below a surface of a body of water in which the wellbore is drilled, the fluid outlet connected to a subsea pump to return drilling fluid to a drilling platform on the water surface, the pump operated to maintain a selected drilling fluid level in the riser or a selected wellbore pressure, a space in the riser above the drilling fluid level filled with air, the method comprising: pumping drilling fluid into a drill string extending from the drilling platform, through the riser, into the wellbore; introducing fluid from a position above the drilling fluid level into an annular volume of the space in the riser above the drilling fluid level situated between the riser and the drill string extending therethrough, a rate of introducing the fluid selected to generate sufficient downward velocity of fluid in the riser so as to substantially stop upward movement of gas in the riser, thereby inhibiting an explosive atmosphere in the space in the riser above the drilling fluid level; and operating the subsea pump during the introducing fluid to remove fluid from the riser outlet at a rate selected to maintain the fluid level or the selected wellbore pressure and maintain the downward velocity of fluid in the riser.