Arrangement and method for regulating bottom hole pressures when drilling deepwater offshore wells
AssigneeOcean Riser Systems AS
InventorBorre Fossli
Life of this patent
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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 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.
Claims22
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;
- and
- adjusting 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.
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 the 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;
- and
- adjusting 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.
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;
- and
- a 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.
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;
- and
- adjusting 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.
Dependencies are inferred from the claim text.