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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 t

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 has 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.

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1. A method for maintaining a desired pressure in a subsea well under both static and dynamic operating conditions, the subsea well being coupled to a drilling unit with a riser, said drilling unit having a hollow drill string that can be lowered into the well and filled with a drilling fluid flowin

1. A method for maintaining a desired pressure in a subsea well under both static and dynamic operating conditions, the subsea well being coupled to a drilling unit with a riser, said drilling unit having a hollow drill string that can be lowered into the well and filled with a drilling fluid flowing out of the bottom of the drillstring and into the well, said riser surrounding the drill string through which the drilling fluid can flow out of the well, thereby forming a drilling fluid column in the riser above the well, and a subsea drilling fluid outlet through which the drilling fluid can be pumped from the riser by a subsea pump so as to cause a height of the drilling fluid column to be below the sea surface but above the subsea drilling fluid outlet, the method comprising: measuring a pressure in the well near the bottom of the drill string;based on the measured pressure and a known density of the drilling fluid, calculating a height of the drilling fluid column in the riser that will cause a pressure in the well at a specified depth between the bottom of the drillstring and the subsea outlet to be equal to the desired pressure;determining an actual height of the drilling fluid column;comparing the calculated height of the drilling fluid column with the actual height of the drilling fluid column;calculating a height increase or reduction of the drilling fluid column in the riser required to obtain the desired pressure; andadjusting the height of the drilling fluid column according to the calculated height increase or reduction, while keeping the desired pressure substantially constant at the specified depth both in static and in dynamic operating conditions. 2. The method of claim 1, wherein the desired pressure is lower than a fracture pressure in the well at the specified depth and higher than a pore pressure in the well at the specified depth. 3. The method of claim 1, further comprising determining an actual height of the drilling fluid column in the riser using sensors to monitor the height of the drilling fluid column in the riser, the sensors being connected to a regulating device that controls an operating speed of the subsea pump. 4. The method of claim 1, further comprising preventing free gas from entering into the subsea variable speed pump from the riser by including a U-shaped section in a return flow line between the subsea drilling fluid outlet and the subsea variable speed pump. 5. The method of claim 4, further comprising adjusting a height of the U-shaped section by varying a height of the subsea variable speed pump below the sea surface. 6. The method of claim 1, further comprising separating gas flowing out of the well from the drilling fluid by: closing a “blow-out preventer” (BOP) or a rotating diverter element at an upper end of the riser; andremoving the gas from the riser through a gas outlet located below the BOP and above the column of drilling fluid, thereby using the riser as a gas separator. 7. The method of claim 6, wherein removing the gas from the riser includes using a compressor to draw the gas from the subsea gas outlet, thereby reducing a pressure of the gas in the riser to below atmospheric pressure. 8. A method for maintaining a desired pressure in a subsea well under both static and dynamic operating conditions, the subsea well being coupled to a drilling unit with a riser, said drilling unit having a hollow drill string that can be lowered into the well and filled with a drilling fluid flowing out of the bottom of the drillstring and into the well, said riser surrounding the drill string through which the drilling fluid can flow out of the well, thereby forming a drilling fluid column in the riser above the well, and a subsea drilling fluid outlet through which the drilling fluid can be pumped from the riser by a subsea pump so as to cause a height of the drilling fluid column to be below the sea surface but above the subsea drilling fluid outlet, the method comprising: determining a height of the drilling fluid column in the riser;calculating a pressure in the well at a specified depth between the bottom of the well and the subsea outlet using the height of the drilling fluid column, based on known properties of the drilling fluid, known environmental conditions, and a known configuration and dynamic status of the drilling apparatus; andadjusting the height of the column of drilling fluid in the riser until the pressure in the well at the specified depth is substantially equal to the desired pressure, and so that the pressure at the specified depth is constant both in static and in dynamic operating conditions. 9. The method of claim 8, wherein the desired pressure is lower than a fracture pressure in the well and higher than a pore pressure in the well. 10. The method of claim 8, wherein the known properties of the drilling fluid, known environmental conditions, and known configuration and dynamic status of the drilling apparatus include at least one of: the density of the drilling fluid;the height of the drilling fluid in the riser;a friction pressure caused by circulation of the drilling fluid;a static surface pressure; anda dynamic pressure change caused by pipe movements in the well. 11. The method of claim 8, wherein determining the height of the drilling fluid column in the riser includes using sensors to monitor the height of the drilling fluid column in the riser, the sensors being connected to a regulating device that controls an operating speed of the subsea pump. 12. The method of claim 8, further comprising preventing free gas from entering into the subsea variable speed pump from the riser by including a U-shaped section in a return flow line between the subsea drilling fluid outlet and the subsea variable speed pump. 13. The method of claim 12, further comprising adjusting a height of the U-shaped section by varying a height of the subsea variable speed pump below the sea surface. 14. The method of claim 8, further comprising separating gas flowing out of the well from the drilling fluid by: closing a “blow-out preventer” (BOP) at an upper end of the riser; andremoving the gas from the riser through a gas outlet located below the BOP and above the column of drilling fluid, thereby using the riser as a gas separator. 15. The method of claim 14, wherein removing the gas from the riser includes using a compressor to draw the gas from the subsea gas outlet, thereby reducing a pressure of the gas in the riser to below atmospheric pressure. 16. A drilling system for drilling a subsea well, the drilling system comprising: a hollow drill string through which drilling fluid can flow into the subsea well;a drilling riser surrounding the drill string and extending from a seafloor wellhead of the subsea well to a location at least near the sea surface, the drilling riser being configured to contain a drilling fluid column therein as the drilling fluid flows out from the subsea well and into the riser;a subsea outlet in communication with the drilling riser at a point above the seafloor wellhead but below the sea surface;a pump system connected to the subsea outlet and configured for removing drilling fluid from the drilling fluid column at an adjustable pumping rate sufficient to maintain a drilling fluid column height at a level above the subsea outlet but below the sea surface;at least one level-sensing mechanism configured for measuring the height of the drilling fluid column;a pressure-determining mechanism configured for determining a downhole drilling fluid pressure within the subsea well; anda pump controller configured for comparing the downhole pressure with a desired downhole pressure and adjusting the pumping rate so as to change the drilling fluid column height until the downhole drilling fluid pressure is equal to the desired downhole pressure. 17. The drilling system of claim 16, wherein the drilling riser is configured with a subsea blowout preventer which is connected to the drilling riser near the wellhead. 18. The drilling system of claim 16, wherein the drilling riser is configured with a near-surface blowout preventer having gas vent lines extending to a choke manifold on a surface drilling vessel. 19. The drilling system of claim 16, wherein the pump controller is in communication with the level sensing mechanism, and configured to regulate the pumping rate of the pump system so as to maintain the drilling fluid column height at a calculated drilling fluid column height. 20. A method for controlling a borehole pressure of a subsea well, the method comprising: providing a hollow drill string through which drilling fluid can flow into the subsea well;providing a drilling riser surrounding the drill string and extending from a seafloor wellhead of the subsea well to a location at least near the sea surface, the drilling riser being configured to contain a drilling fluid column therein as the drilling fluid flows out from the subsea well and into the riser;providing a subsea outlet in communication with the drilling riser at a point above the seafloor wellhead but below the sea surface;providing a pump system connected to the subsea outlet and configured for removing drilling fluid from the drilling fluid column at an adjustable pumping rate sufficient to maintain a drilling fluid column height at a level above the subsea outlet but below the sea surface;measuring the height of the drilling fluid column;determining a downhole drilling fluid pressure at a predetermined depth within the subsea well; andadjusting the pumping rate of the pump system, and thereby adjusting the height of the drilling fluid column, so as to maintain the downhole drilling fluid pressure at a desired downhole pressure. 21. The method of claim 20, wherein the desired downhole pressure is above a pore pressure of the subsea well but below a fracture pressure of the subsea well. 22. The method of claim 20, wherein determining the downhole drilling fluid pressure at the predetermined depth within the subsea well includes: measuring a pressure of the drilling fluid column at a level below the height of the drilling fluid column; andcalculating the downhole drilling fluid pressure according to the measured drilling fluid column pressure, the fluid column height, and a known density of the drilling fluid.