Arrangement and method for regulating bottom hole pressures when drilling deepwater offshore wells

Abstract

An arrangement and a method to control and regulate the bottom hole pressure in a well during subsea drilling at deep waters: The method involves adjustment of a liquid/gas interface level in a drilling riser up or down. The arrangement comprises a high pressure drilling riser and a surface BOP at the upper end of the drilling riser. The surface BOP havs a gas bleeding outlet. The riser also comprises a BOP, with a by-pass line. The drilling riser has an outlet at a depth below the water surface, and the outlet is connected to a pumping system with a flow return conduit running back to a drilling vessel/platform.

The invention claimed is: 
     
       1. A drilling system for compensating for changes in equivalent mud circulation density (ECD) or dynamic pressure in an annulus bore in a well resulting from drilling activities during subsea drilling at great water-depths, comprising:
 a high pressure drilling riser extending from a seafloor wellhead to near the surface; 
 a near surface BOP at the upper end of the drilling riser, the near surface BOP having an upper high pressure line; 
 a subsea outlet in communication with the interior of the riser at a point above the seafloor wellhead; 
 a flow return conduit running back to the surface; 
 a pumping system suspended above the seafloor and connecting said subsea outlet to said flow return conduit; 
 a valve adapted to isolate the riser from the pumping system; 
 means for converting changes in pressure in said riser to an equivalent change in height of drilling fluid in the riser; 
 means for adjusting the pump rate of said pumping system according to the difference of the height of drilling fluid in said riser and said equivalent change in height of drilling fluid; thereby adjusting the height of drilling fluid in the drilling riser so as to neutralize the changes in pressure in said annulus bore created by said drilling activities by varying the actual amount of drilling fluid in the riser; and 
 a subsea shut-off device at the sea floor, the shut off device having at least one by-pass line providing communication between the well below the shut-off device and the interior of the riser, the by-pass line containing at least one shut-off valve. 
 
     
     
       2. A system according to  claim 1 , further comprising a gas bleeding outlet connected to a choke line in communication with a high pressure choke and stand pipe manifold on a drilling vessel. 
     
     
       3. A system according to  claim 1 , wherein the pumping system with flow return line is adapted to be launched and run with the riser. 
     
     
       4. A system according to  claim 1 , further comprising a filling line coupled to the riser substantially below sea level and above said subsea outlet, the filling line being adapted for filling the riser with a gas or liquid. 
     
     
       5. A system according to  claim 4 , wherein the gas is an inert gas for displacement of the air above the drilling fluid. 
     
     
       6. A system according to  claim 1 , further comprising a valve in the flow return conduit, and a particle collection box in the flow return line, the valve being adapted for opening and closing the communication between the particle collection box and the flow return conduit. 
     
     
       7. A system according to  claim 6 , wherein the particle collection box is hanging underneath the pumping system and the particle collection box has a re-circulation and jetting means for breaking down particle size to prevent particle build up. 
     
     
       8. A method for compensating for equivalent mud circulation density (ECD) or dynamic pressure increase or decrease in an annulus bore in a well during subsea drilling at great water-depths resulting from drilling activities, comprising the steps:
 maintaining the pressure in the top of a drilling riser extending from a seafloor wellhead to the surface at equal to or lower than atmospheric pressure, said riser configured with a seabed BOP and bypass; 
 monitoring bottom hole pressure in the well for a change in pressure created by drilling activities; 
 converting a the change in pressure in the well created by drilling activities to an equivalent change in height of drilling fluid in the riser; 
 adjusting the pump rate of a drilling fluid return pump suspended above the seafloor and connected at a point above the seafloor wellhead to the drilling riser so as to adjust the height of drilling fluid in the drilling riser by the equivalent change in height of drilling fluid so as to neutralize the change in pressure created by the drilling activities by varying the actual amount of drilling fluid in the riser. 
 
     
     
       9. A method according to  claim 8 , wherein gas escaping from an underground formation is separated from liquid during offshore drilling, comprising the steps:
 permitting gas and drilling fluid in a drilling riser extending from a seafloor wellhead to the surface to form a gas/liquid interface within the drilling riser; 
 providing a liquid outlet below the gas/liquid interface level and substantially above the seafloor wellhead, said outlet being connected to a pumping system suspended above the seafloor and hence to a return conduit, 
 providing a gas outlet above the gas/liquid interface level, 
 closing a near surface BOP at the upper end of the drilling riser, and 
 pumping liquid out of the drilling riser through the liquid outlet, whereby the drilling riser is acting as a gas separator. 
 
     
     
       10. A method according to  claim 9 , wherein a flow return line between the liquid outlet and the pumping system is adapted to prevent free gas from entering the return conduit by having a U-shaped V-shaped loop acting as a gas-lock. 
     
     
       11. A method according to  claim 10 , where the height of the gas-lock can be adjusted by varying the subsea level of the pumping system. 
     
     
       12. A method according to  claim 9 , wherein the level of the gas/liquid interface between the drilling fluid and the gas in the drilling riser is maintained below sea level so that the pressure in the bottom of the well is lower than the hydrostatic pressure exerted by seawater from sea level. 
     
     
       13. A method according to  claim 12 , wherein the drilling riser comprises sensors for monitoring the height of the gas/liquid interface level in the riser, the sensors being coupled to a regulating means controlling the pump rate of the pumping system and thereby controlling the height of the gas/liquid interface level. 
     
     
       14. A method according to  claim 8 , said change in pressure occurring in said riser by drilling activities comprising a change in pressure created by the drill string being moved up or down in the well. 
     
     
       15. A method according to  claim 8 , said change in pressure in said drilling riser being created by circulation of drilling fluid through the bit. 
     
     
       16. A method for controlling equivalent mud circulation density (ECD) in a well during subsea drilling operations, comprising:
 using a high pressure drilling riser extending from a seafloor wellhead and subsea BOP to the surface, within which there is drilling fluid present, there being no outside kill or choke lines extending from the surface to the subsea BOP, said subsea BOP configured with a bypass; 
 maintaining the pressure in the top of the drilling riser at equal to or lower than atmospheric pressure; 
 monitoring the height of drilling fluid in the riser; 
 monitoring bottom hole pressure in the well for a change in pressure; 
 calculating an equivalent change in height of drilling fluid to the change in pressure; 
 using a drilling fluid pump suspended above the seafloor and connected to the riser substantially above the seafloor wellhead and below the height of drilling fluid, adjusting the height of drilling fluid in the riser by the equivalent change in height of drilling fluid, thereby adjusting the drilling fluid level in the drilling riser so as to reverse the change in the bottom hole pressure. 
 
     
     
       17. A drilling system for controlling equivalent mud circulation density (ECD) in a well resulting from drilling activities during subsea drilling operations, comprising:
 a high pressure drilling riser extending from a seafloor wellhead to the surface and having a surface BOP at the upper end of the drilling riser, the surface BOP having an upper high pressure line, there being no kill or choke lines extending from the surface to the seafloor wellhead; 
 a subsea outlet in communication with the interior of the riser at a point above the seafloor wellhead; 
 a flow return conduit running back to the surface; 
 a pumping system suspended above the seafloor and connecting the subsea outlet to the flow return conduit; 
 a valve adapted to isolate the riser from the pumping system; 
 means for monitoring the height of drilling fluid in the drilling riser; 
 means for sensing a change in pressure in the drilling riser; 
 means for converting the change in pressure in the drilling riser to an equivalent change in height of drilling fluid in the riser; 
 means for adjusting the pump rate of the pumping system according to the difference of the height of drilling fluid in the drilling riser and the equivalent change in height; thereby adjusting the height of drilling fluid in the drilling riser so as to neutralize the change in pressure by varying the actual amount of drilling fluid in the riser; and 
 a subsea shut-off device at the sea floor; the shut off device having at least one by-pass line providing communication between the well below the shut-off device and the interior of the riser, the by-pass line containing at least one shut-off valve or pressure regulating valve.