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A subsea drilling method and system for controlling the drilling fluid pressure, where drilling fluid is pumped down into the borehole through a drill string and returned back through the annulus between the drill string and the well bore. The drilling fluid pressure is controlled by draining drilli

A subsea drilling method and system for controlling the drilling fluid pressure, where drilling fluid is pumped down into the borehole through a drill string and returned back through the annulus between the drill string and the well bore. The drilling fluid pressure is controlled by draining drilling fluid out of the drilling riser (8) or BOP (6) at a level between the seabed and the sea water in order to adjust the hydrostatic head of drilling fluid. The drained drilling fluid and gas is separated in a subsea separator (28) where the gas is vented to surface through a vent line (39), and the fluid is pumped to surface via pump (40).

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1. A system for drilling subsea wells from a Mobile Offshore Drilling Unit (MODU), comprising: a marine drilling riser arranged from the MODU to a seabed located Blow Out Preventer (BOP);a drill string arranged from the MODU through the marine drilling riser and BOP and further down a wellbore, an a

1. A system for drilling subsea wells from a Mobile Offshore Drilling Unit (MODU), comprising: a marine drilling riser arranged from the MODU to a seabed located Blow Out Preventer (BOP);a drill string arranged from the MODU through the marine drilling riser and BOP and further down a wellbore, an annulus being formed between the drill string and the drilling riser, and between the drill string and the wellbore, said annulus being filled with drilling mud to a low mud level where an interface is formed between the drilling mud and either gas or liquid that extends in the annulus above the drilling mud;at least one closing devise arranged either in the marine drilling riser, integral with the BOP, or in a Lower Marine Riser Package (LMRP), the closing device being able to close the annulus;at least one mud return outlet and mud conduit fluidly connected to the annulus either at a lower part of the marine drilling riser or below the drilling riser, said mud return outlet and mud conduit being connected to the annulus at a level below the low mud level and below said closing device, said mud return outlet and mud conduit being adapted to enable drilling mud to flow from the annulus to a subsea lift pump that via piping or conduits can pump the drilling mud to a location above sea level, said mud conduit including a choke line, a subsea choke valve being provided in said choke line, such that a choked flow of mud can be directed to the subsea lift pump via said gas separator;a well pressure regulator that is able to dynamically maintain a constant pressure in the annulus, said well pressure regulator being fluidly connected to the mud conduit, said well pressure regulator also including a pressure regulating conduit extending upwards from the seabed, or from a level near seabed level, through the sea to a level above sea level, adjustment of a height in the pressure regulating conduit of an interface between drilling mud and a gas or liquid being able to adjust and regulate the pressure in the annulus, said well pressure regulator including a gas separator, said pressure regulating conduit extending upward from said gas separator through the sea; andat least one pipe and one valve are provided that can by-pass said gas separator and connect the choke line directly to the subsea lift pump. 2. The drilling system according to claim 1, wherein said pipe includes a shared conduit, said shared conduit being one of: a part of a booster line, a part of a choke line, a part of a kill line, and the annulus of the marine drilling riser, said shared conduit being operatively connected to function as said pipe whenever said well pressure regulator is in operation. 3. A subsea drilling system comprising: a marine drilling riser extending from above sea level to a well borehole;a drill string arranged through the marine drilling riser and into the well borehole, an annulus being formed between said drill string and said borehole, and between said drill string and said marine drilling riser, said marine drilling riser and said drill string being configured so that drilling fluid can be pumped down into the borehole through the drill string and returned back through the annulus and out of the marine drilling riser to a subsea pump through a pump outlet located at a riser level between the seabed and the sea surface;a subsea located Blow Out Preventer (BOP) that can be closed to seal off the annulus within the borehole; anda separate line through which drilling fluids are diverted from below the closed BOP to above the BOP by diverting the drilling fluids directly from a pressure reduction device (subsea choke valve) to the subsea pump via a valve bypassing the marine drilling riser;said subsea pump being connected to a conduit that is fluidly connected to a mud processing plant on a MODU above sea level. 4. The subsea drilling system according to claim 3, wherein a liquid having a liquid density that is lower than a density of the drilling fluid is located in the marine drilling riser above the drilling fluid, an interface between said drilling fluid and said liquid being located below sea level. 5. A subsea drilling system comprising: a marine drilling riser extending from above sea level to a well borehole;a drill string arranged through the marine drilling riser and into the well borehole, an annulus being formed between said drill string and said borehole, and between said drill string and said marine drilling riser, said marine drilling riser and said drill string being configured so that drilling fluid can be pumped down into the borehole through the drill string and returned back through the annulus and out of the marine drilling riser to a subsea pump through a pump outlet located at a riser level between the seabed and the sea surface;a subsea located Blow Out Preventer (BOP) that can be closed to seal off the annulus within the borehole; anda separate line through which drilling fluids are diverted from below the closed BOP to above the BOP via at least one pressure reduction device (subsea choke valve) and into the riser through a mud inlet located at a riser level that is higher than the pump outlet;said subsea pump being connected to a conduit that is fluidly connected to a mud processing plant on a MODU above sea level;a gas separating section of the marine drilling riser located between the pump outlet and the mud inlet having a larger diameter than the marine drilling riser below and above said gas separating section, so that the velocity of the drilling fluid within said gas separating section is reduced, thereby allowing any gas entrapped in the drilling fluid to separate from the drilling fluid while the drilling fluid flows through the gas separating section. 6. The subsea drilling system according to claim 3, wherein a continuous circulation system is used. 7. The subsea drilling system according to claim 3, wherein additional fluid is supplied upstream of the choke valve to improve performance of the pressure control system. 8. A subsea drilling system comprising a marine drilling riser extending from above sea level to a well borehole;a drill string arranged through the marine drilling riser and into the well borehole, an annulus being formed between said drill string and said borehole, and between said drill string and said marine drilling riser, said marine drilling riser and said drill string being configured so that drilling fluid can be pumped down into the borehole through the drill string and returned back through the annulus and out of the marine drilling riser to a subsea pump through a pump outlet located at a riser level between the seabed and the sea surface;a subsea located Blow Out Preventer (BOP) that can be closed to seal off the annulus within the borehole; anda separate line through which drilling fluids are diverted from below the closed BOP to above the BOP by diverting the drilling fluids directly from a pressure reduction device (subsea choke valve) to the subsea pump via a valve bypassing the marine drilling riser;said subsea pump being connected to a conduit that is fluidly connected to a mud processing plant on a MODU above sea level, andadditional drilling fluid being supplied upstream of the subsea pump to avoid settling of drill cuttings in the drilling riser above the BOP. 9. A subsea drilling system for controlling drilling fluid pressure within a well borehole, said drilling system comprising: a drill string;a marine drilling riser;a system for circulating drilling fluid by pumping it down into the borehole through the drill string and returning it back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and the marine drilling riser; anda well pressure regulation system that can regulate a pressure of the drilling fluid in the borehole by draining drilling fluid out of the marine drilling riser or out of a blowout preventer (BOP) at a level between the seabed and the sea surface, thereby adjusting a hydrostatic head of drilling fluid in the annulus within the drilling riser;said well pressure regulation system further comprising a gas separator in fluid communication with the annulus via a fluid return line, and a gas vent line from the gas separator to the sea surface located upstream of a liquid line to the sea surface;said well pressure regulation system further comprising a subsea pump coupled to the liquid line downstream of the gas vent line, said subsea pump being adapted to pump drilling fluid to the sea surface. 10. A subsea drilling system for controlling drilling fluid pressure within a well borehole, said drilling system comprising: a drill string;a marine drilling riser;a system for circulating drilling fluid by pumping it down into the borehole through the drill string and returning it back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and the marine drilling riser; anda well pressure regulation system that can regulate a pressure of the drilling fluid in the borehole by draining drilling fluid out of the marine drilling riser or out of a blowout preventor preventer (BOP) at a level between the seabed and the sea surface, thereby adjusting a hydrostatic head of drilling fluid in the annulus within the drilling riser;said well pressure control regulation system further comprising a gas separator in fluid communication with the annulus via a fluid return line, and a gas vent line from the gas separator to the sea surface located upstream of a liquid line to the sea surface;wherein the vent line is either a separate conduit line, a choke line, a kill line, or a riser booster line. 11. The system according to claim 9, wherein the fluid return line is connected to the annulus above the BOP. 12. A subsea drilling system for controlling drilling fluid pressure within a well borehole, said drilling system comprising: a drill string;a marine drilling riser;a system for circulating drilling fluid by pumping it down into the borehole through the drill string and returning it back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and the marine drilling riser; anda well pressure regulation system that can regulate a pressure of the drilling fluid in the borehole by draining drilling fluid out of the marine drilling riser or out of a blowout preventer (BOP) at a level between the seabed and the sea surface, thereby adjusting a hydrostatic head of drilling fluid in the annulus within the drilling riser;said well pressure regulation system further comprising a gas separator in fluid communication with the annulus via a fluid return line, and a gas vent line from the gas separator to the sea surface located upstream of a liquid line to the sea surface;wherein the fluid return line is connected to the annulus via a choke line from below the BOP. 13. The system according to claim 9, wherein the gas separator is an integral part of the marine drilling riser. 14. The system of claim 9, wherein the gas separator is located outside of the marine drilling riser. 15. A subsea drilling method, comprising: pumping drilling fluid down into a borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and drilling riser surrounding the drill string above the seabed;draining drilling fluid out of the drilling riser at a level between the seabed and the sea surface through a pump outlet to a subsea mud lift pump that is fluidly connected to a mud processing plant above the sea surface, thereby creating a drilling fluid interface below the sea surface between the drilling fluid in the annulus within the drilling riser and either gas or liquid extending in the annulus above the drilling fluid,regulating a height of the drilling fluid interface thereby controlling and regulating a pressure of the drilling fluid in the annulus within the wellbore; andwhen necessary, closing a subsea located Blow Out Preventer (BOP) to seal off the annulus between the drill string and the bore hole, and diverting the drilling fluid from below the BOP in a separate bypass line to a bypass inlet above the BOP into the marine drilling riser at a higher level compared to the pump outlet level;wherein the drilling fluid in the riser between the bypass inlet and the pump outlet flows downwards with a velocity lower than a rising velocity of a less dense gas that is mixed with the drilling fluid, thereby resulting in gravity type separation of the gas from the drilling fluid and a net upwards rising velocity of the gas as bubbles within the downward flowing drilling fluid. 16. A subsea drilling method according to claim 15, where said bypass line connects the wellbore annulus below the closed BOP to a bypass inlet to the marine drilling riser located above a level of the pump outlet, and wherein said bypass line contains at least one pressure reduction device (subsea choke valve) that can regulate an amount of drilling fluid flowing into the drilling riser. 17. The subsea drilling method according to claim 15, wherein the drilling fluid from below the BOP is diverted from a choke valve directly via a valve and piping to the subsea lift pump. 18. The subsea drilling method according to claim 15, wherein said bypass line connects the wellbore annulus below the closed BOP to a bypass inlet to the drilling riser located above a level of the pump outlet, and the drilling fluid in the riser between the bypass inlet and the pump outlet flows downwards with a velocity lower than a rising velocity of a less dense gas that is mixed with the drilling fluid, thereby resulting in gravity type separation of the gas from the drilling fluid and a net upwards rising velocity of the gas as bubbles within the downward flowing drilling fluid. 19. The subsea drilling method according to claim 15, wherein the separated gas is vented via the drilling riser and via a diverter system to the atmosphere. 20. The subsea drilling method according to claim 15, wherein a fluid with a fluid density lower than a density of the drilling fluid extends in the annulus within the drilling riser above the drilling fluid. 21. A subsea drilling method, comprising: pumping drilling fluid down into a borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and drilling riser surrounding the drill string above the seabed;draining drilling fluid out of the drilling riser at a level between the seabed and the sea surface through a pump outlet to a subsea mud lift pump that is fluidly connected to a mud processing plant above the sea surface, thereby creating a drilling fluid interface below the sea surface between the drilling fluid in the annulus within the drilling riser and either gas or liquid extending in the annulus above the drilling fluid,regulating a height of the drilling fluid interface thereby controlling and regulating a pressure of the drilling fluid in the annulus within the wellbore; andwhen necessary, closing a subsea located Blow Out Preventer (BOP) to seal off the annulus between the drill string and the bore hole, and diverting the drilling fluid from below the BOP in a separate bypass line to a bypass inlet above the BOP into the marine drilling riser at a higher level compared to the pump outlet level;wherein a gas separating section of the drilling riser between the pump outlet and the bypass inlet has a larger diameter compared to the marine drilling riser above and below the gas separating section, thereby reducing a downward fluid velocity of the drilling fluid in the gas separating section and allowing gas mixed with the drilling fluid to separate from the drilling fluid. 22. The subsea drilling method according to claim 15, wherein a continuous circulation system is used in combination with a circulation/drilling method. 23. A subsea drilling method, comprising: pumping drilling fluid down into a borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and drilling riser surrounding the drill string above the seabed;draining drilling fluid out of the drilling riser at a level between the seabed and the sea surface through a pump outlet to a subsea mud lift pump that is fluidly connected to a mud processing plant above the sea surface, thereby creating a drilling fluid interface below the sea surface between the drilling fluid in the annulus within the drilling riser and either gas or liquid extending in the annulus above the drilling fluid,regulating a height of the drilling fluid interface thereby controlling and regulating a pressure of the drilling fluid in the annulus within the wellbore; andwhen necessary, closing a subsea located Blow Out Preventer (BOP) to seal off the annulus between the drill string and the bore hole, and diverting the drilling fluid from below the BOP in a separate bypass line to above the BOP into the marine drilling riser at a higher level compared to the pump outlet level;wherein additional fluid other than the drilling fluid supplied through the drill string is supplied into the wellbore upstream of a choke valve, thereby improving the regulation of the pressure of the drilling fluid in the annulus within the wellbore. 24. A subsea drilling method, comprising: pumping drilling fluid down into a borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and drilling riser surrounding the drill string above the seabed;draining drilling fluid out of the drilling riser at a level between the seabed and the sea surface through a pump outlet to a subsea mud lift pump that is fluidly connected to a mud processing plant above the sea surface, thereby creating a drilling fluid interface below the sea surface between the drilling fluid in the annulus within the drilling riser and either gas or liquid extending in the annulus above the drilling fluid,regulating a height of the drilling fluid interface thereby controlling and regulating a pressure of the drilling fluid in the annulus within the wellbore; andwhen necessary, closing a subsea located Blow Out Preventer (BOP) to seal off the annulus between the drill string and the bore hole, and diverting the drilling fluid from below the BOP in a separate bypass line to above the BOP into the marine drilling riser at a higher level compared to the pump outlet level;wherein additional fluid is supplied through a booster line upstream of the subsea lift pump to avoid settling of formation particles from the drilling fluid. 25. A subsea drilling method, comprising: pumping drilling fluid down into a borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore, and between the drill string and drilling riser surrounding the drill string above the seabed;draining drilling fluid out of the drilling riser at a level between the seabed and the sea surface through a pump outlet to a subsea mud lift pump that is fluidly connected to a mud processing plant above the sea surface, thereby creating a drilling fluid interface below the sea surface between the drilling fluid in the annulus within the drilling riser and either gas or liquid extending in the annulus above the drilling fluid,regulating a height of the drilling fluid interface thereby controlling and regulating a pressure of the drilling fluid in the annulus within the wellbore; andwhen necessary, closing a subsea located Blow Out Preventer (BOP) to seal off the annulus between the drill string and the bore hole, and diverting the drilling fluid from below the BOP in a separate bypass line to above the BOP into the marine drilling riser at a higher level compared to the pump outlet level;wherein at least one additional fluid is supplied upstream of the subsea lift pump to avoid settling of drill cutting in the drilling riser above the BOP. 26. The subsea drilling method according to claim 25, wherein gas escaping from a submarine formation into the borehole is transported out of the borehole in the annulus between the drill string and the borehole and separated from the drilling fluid within the drilling riser, which is kept open to the atmosphere above the sea surface. 27. The subsea drilling method according to claim 26, wherein a combined hydrostatic and dynamic pressure at any one particular depth in the wellbore is kept constant during a drilling process by regulation of the height of the drilling fluid interface in the annulus within the drilling riser. 28. A subsea drilling method for controlling a wellbore annular pressure within a borehole, the method comprising: pumping drilling fluid down into the borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the bore hole, and between the drill string and a drilling riser extending from the sea bed to the sea surface;draining drilling fluid out of the drilling riser or out of a blowout preventer (BOP) from a pump outlet located at a pump outlet level between the seabed and the sea surface, thereby adjusting a height of a hydrostatic head of the drilling fluid in the annulus within the drilling riser, and regulating a pressure of the drilling fluid in the annulus within the wellbore; andseparating gas from the drained drilling fluid in a subsea gas separator whereby the gas is vented to the sea surface through a vent line extending from the gas separator to the sea surface, and the fluid is pumped to the sea surface via a pump discharge line by a subsea lift pump;wherein an annular seal, located above the pump outlet is used to seal the annulus in the event that well fluids enter the borehole, and wherein an influx volume and a corresponding influx displacement height of the well fluids entering the bore hole causes a height of an interface between drilling fluid and gas in the vent line to increase, the increase in height of the interface in the vent line being larger than the influx displacement height the borehole annulus, thereby increasing the pressure of the drilling fluid in the annulus within the wellbore until it is in balance with a formation pressure surrounding the wellbore. 29. The method according to claim 28, wherein the gas separator is located outside of the marine drilling riser and an annular seal, located above an outlet from the drilling riser to the gas separator, is used when necessary to seal the annulus before the flow of drilling fluid through the drill string is stopped, and the level of liquid in the vent line is increased to compensate for a loss in annulus pressure when the flow of drilling fluid through the drill string is stopped. 30. The method according to claim 29, wherein the liquid level in the vent line is reduced when the flow of drilling fluid through the drill string is commenced or increased, thereby maintaining a substantially constant bottom hole pressure. 31. A subsea drilling method for controlling a wellbore annular pressure within a borehole, the method comprising: pumping drilling fluid down into the borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the bore hole, and between the drill string and a drilling riser extending from the sea bed to the sea surface;draining drilling fluid out of the drilling riser or out of a blowout preventer (BOP) from a pump outlet located at a pump outlet level between the seabed and the sea surface, thereby adjusting a height of a hydrostatic head of the drilling fluid in the annulus within the drilling riser, and regulating a pressure of the drilling fluid in the annulus within the wellbore; andseparating gas from the drained drilling fluid in a subsea gas separator whereby the gas is vented to the sea surface through a vent line extending from the gas separator to the sea surface, and the fluid is pumped to the sea surface via a pump discharge line by a subsea lift pump;wherein an annular seal, located above an outlet from the drilling riser to the separator, is used to seal the annulus before the flow through the drill string is stopped, and wherein the subsea lift pump and a hydrostatic head in the pump discharge line are used to compensate for surge and swab pressure. 32. A subsea drilling method for maintaining constant pressure in a borehole of a well during drilling and well circulation, after an influx of formation fluid containing gas into the wellbore annulus has occurred, the method comprising: pumping drilling fluid down into the borehole through a drill string;returning the drilling fluid back through an annulus, said annulus being formed between the drill string and the well bore and between the drill string and a drilling riser extending from the sea bottom to the sea surface;controlling removal of drilling fluid out of the annulus so that a rate of removing the drilling fluid is either more or less than a rate of pumping the drilling fluid into the borehole, the drilling fluid being removed from the annulus through a pump outlet located at a riser level between the seabed and the sea water surface, thereby adjusting a height of an interface between the drilling fluid and a gas extending above the drilling fluid, said gas being exposed to atmospheric pressure, said adjusting of the interface height serving to maintain or regulate the pressure in the borehole;pumping the influx of formation fluid containing gas from an influx depth up the annulus; andreducing the rate of pumping the drilling fluid down the drill string into the borehole to a minimum, while regulating the pressure of the drilling fluid in the borehole to be not less than that of an open hole formation pressure surrounding the borehole by regulating the height of the interface between the drilling fluid and the gas, and letting the influx rise by gravity separation under constant pressure without any other physical interference or regulation. 33. A subsea drilling system according to claim 3, further comprising at least one of a diverter element, a wiper element, and a rotating BOP in the upper part of the riser above a pump outlet containing at least one shut off valve. 34. A subsea drilling method according to claim 15, further comprising using an inert gas to purge the drilling riser. 35. A subsea drilling system according to claim 32, further comprising at least one of a diverter element, a wiper element, and a rotating BOP in the upper part of the riser above a pump outlet containing at least one shut off valve. 36. A subsea drilling method according to claim 32, wherein the drilling riser is configured to enable purging thereof by an inert gas.