Abstract: A method for inhibiting an explosive atmosphere in a wellbore drilling system, including a riser (12) connected to a wellbore above a top thereof wherein the riser has a fluid outlet (17) below a surface of a body of water in which the wellbore is drilled, and wherein the fluid outlet is connected to a subsea pump to return drilling fluid to a drilling platform on the water surface and wherein a space in the riser (12) above the drilling fluid level filled with air includes pumping drilling fluid into a drill string (15) extending from the drilling platform into the wellbore. Fluid (27) is introduced proximate an upper end of the riser (12). A rate of introducing the fluid (27) is selected to inhibit an explosive atmosphere in the space in the riser above the drilling fluid level. The subsea pump to remove fluid from the riser outlet is operated at a rate selected to maintain the fluid level or to maintain a selected wellbore pressure.

Abstract: A method for inhibiting an explosive atmosphere in a wellbore drilling system, including a riser (12) connected to a wellbore above a top thereof wherein the riser has a fluid outlet (17) below a surface of a body of water in which the wellbore is drilled, and wherein the fluid outlet is connected to a subsea pump to return drilling fluid to a drilling platform on the water surface and wherein a space in the riser (12) above the drilling fluid level filled with air includes pumping drilling fluid into a drill string (15) extending from the drilling platform into the wellbore. Fluid (27) is introduced proximate an upper end of the riser (12). A rate of introducing the fluid (27) is selected to inhibit an explosive atmosphere in the space in the riser above the drilling fluid level. The subsea pump to remove fluid from the riser outlet is operated at a rate selected to maintain the fluid level or to maintain a selected wellbore pressure.