Wellbore annular pressure control system and method using gas lift in drilling fluid return line

A method comprising:

• pumping drilling fluid through a drill string extended into a wellbore extending below a bottom of a body of water, out the bottom of the drill string, and into the wellbore annulus;
• discharging fluid from the wellbore annulus and into a riser disposed above the top of the wellbore, the riser extending to the surface of the body of water,
• discharging fluid from the riser into a discharge conduit disposed below the surface of the body of water, the discharge conduit including therein a controllable fluid choke, a fluid return line coupled to an outlet of the controllable fluid choke and extending to the surface of the body of water;
• pumping gas under pressure into the return line at a selected depth below the surface of the body of water;
• and
• operating the controllable fluid choke to maintain a selected hydrostatic pressure in the riser at a selected distance below the surface of the body of water.

A method comprising:

• pumping drilling fluid through a drill string extended into a wellbore extending below the bottom of a body of water, out the bottom of the drill string, and into the wellbore annulus;
• discharging fluid from the wellbore annulus into a riser disposed above the top of the wellbore and into a discharge conduit, the discharge conduit including a fluid choke and a fluid return line coupled to an outlet of the fluid choke and extending to the water surface;
• pumping gas under pressure into the return line at a selected depth below the water surface;
• and
• controlling a rate at which the gas is pumped into the return line to maintain a level of fluid in the riser at a selected distance below the surface of the body of water.