Riser mounted controllable orifice choke

1. A system, comprising:

a variable orifice choke disposed at a longitudinal position along a riser, the riser connected between a drilling platform and a wellbore, wherein the variable orifice choke is disposed along the riser such that a drill string extends through each of the variable orifice choke and the riser;

a casing placed in the wellbore, creating an annular space between the drill string and the casing;

a well pressure control apparatus connected to the casing;

a lower marine riser package (LMRP) having a lower end and an upper end,

wherein the lower end of the LMRP is connected to the well pressure control apparatus, and the upper end of the LMRP is connected to the riser; and

a control unit in signal communication with the variable orifice choke, the control unit operable to control a flow area of the variable orifice choke such that a selected fluid pressure is maintained in the wellbore,

wherein the variable orifice choke comprises: a housing having a coupling at longitudinal ends thereof for connection between two selected external segments of the riser; and a closure element disposed in a larger diameter portion of the housing, the closure element being operable to adjust a cross sectional flow area through an interior of the housing.

2. The system of claim 1 wherein the closure element comprises an iris type variable flow orifice.

3. The system of claim 1 wherein the closure element comprises an inflatable bladder.

4. The system of claim 1 wherein the coupling at each longitudinal end comprises a mating flange.

5. The system of claim 1 wherein an internal diameter of the closure element is substantially the same as an internal diameter of the riser when the closure element is fully opened.

6. The system of claim 1 further comprising an umbilical line forming signal connection between the variable orifice choke and the control unit.

7. The system of claim 1 wherein a lower end of the riser comprises a lower marine riser package coupled to a well pressure control apparatus, the well pressure control apparatus coupled to an upper end of a casing disposed in the wellbore.

8. A method, comprising:

automatically controlling a cross sectional flow area of a variable orifice choke such that flow of fluid returning to a drilling platform from a wellbore is restricted so as to maintain a selected fluid pressure in the wellbore,

wherein the variable orifice choke is disposed at a longitudinal position along a riser such that a drill string extends through the riser and the variable orifice choke, and

wherein a well pressure control apparatus is connected to a casing placed in the wellbore,

wherein a lower end of a lower marine riser package (LMRP) is connected to the well pressure control apparatus, and an upper end of the LMRP is connected to the riser,

wherein the variable orifice choke comprises: a housing having: a coupling at longitudinal ends thereof for connection between two selected segments of the riser; a larger diameter portion between the couplings; and a remotely operable closure element disposed in the larger diameter portion of the housing.

9. The method of claim 8 , wherein the remotely operable closure element comprises an inflatable bladder.

10. The method of claim 8 , wherein the remotely operable closure element comprises an iris type variable flow orifice.

11. The method of claim 8 , wherein an internal diameter of the variable orifice choke is substantially the same as an internal diameter of the riser when the variable orifice choke is fully opened.

12. The method of claim 8 , wherein the cross-sectional flow area is defined between an interior surface of the remotely operable closure element and an exterior of the drill string.