BACKGROUND OF THE INVENTION

The invention relates to a method and system for separating gaseous and liquid components from multiphase well effluents of an underwater well.

Such a method and system are known from US patent

6,537,458, which discloses a separation vessel for separating gaseous from liquid components of a multiphase well effluent mixture produced by an oil and/or gas production well.

A disadvantage of the method and system known from this prior art reference is that it requires a bulky separation vessel, which is expensive and requires regular inspection and/or replacement since it is configured as a large diameter high pressure vessel.

US patent application US2005/0081718 discloses a downhole gas/liquid separator with a co-axial inner and outer conduit assembly which is arranged in a vertical tubing string in a multiphase hydrocarbon fluid

production well.

Disadvantages of the substantially vertical

orientation of the co-axial inner and outer conduits are that the separation performance is low, since no use is made of differences in density between gaseous and liquid components of the well effluents and that it does not inhibit a slug flow regime if the liquid and gaseous components flow in a slug flow regime into the vertical separator .

There is a need to provide a method and system for separating gaseous and liquid components discharged by an underwater well which does not require the use of a bulky separation vessel and which is configured such that requires less frequent inspection and/or replacement since it may be configured as a small diameter pipeline instead of as a large diameter high pressure vessel.

There is also a need to provide a method and system for separating gaseous and liquid components discharged by an underwater well which is configured as a small diameter pipeline and which separates liquid and liquid components with a high efficiency and such that a slug flow regime in the separator and associated flowlines and hydrocarbon fluid processing facilities is inhibited.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method for separating gaseous and liquid components from multiphase well effluents of an underwater well, the method comprising:

- providing a riser or underwater flowline with an upper portion for lifting substantially gaseous components to a gas processing or receiving facility above the water surface ;

- providing a gas-liquid separator comprising an annular liquid collecting chamber which at least partially surrounds a lower portion of the riser or underwater flowline having an at least partially permeable wall, which annular liquid collecting chamber is configured to allow liquid components that pass through the permeable wall to drain to a lower section of the chamber;

- connecting a liquid transportation conduit to the lower section of the liquid collecting chamber, which conduit is provided with a liquid pump assembly that pumps substantially liquid components from the liquid

collecting chamber to a liquid processing or receiving facility at the water surface;

- and wherein: - the lower portion of the riser or underwater flowline comprises an impermeable bottom section and a permeable upper section, which sections extend through the liquid collecting chamber, such that the impermeable bottom section extends through the lower section of the liquid collecting chamber to a level above the predetermined upper level; and

- the permeable upper section of the lower portion of the riser or underwater flowline has an inclined orientation relative to a horizontal plane.

Optionally, the lower portion of the riser comprises an impermeable bottom section and a permeable upper section, which sections extend through the annular liquid collecting chamber, such that the impermeable bottom section extends through the lower section of the liquid collecting chamber to a level above the predetermined upper level .

In accordance with the invention there is further provided a system for separating gaseous and liquid components from multiphase well effluents of an

underwater well, the system comprising:

- a riser or underwater flowline with an upper portion for lifting substantially gaseous components to a gas processing or receiving facility at the water surface; - a gas-liquid separator comprising an annular liquid collecting chamber which at least partially surrounds a lower portion of the riser or underwater flowline having an at least partially permeable wall, which liquid collecting chamber is configured to allow liquid

components that pass through the permeable wall to drain to a lower section of the chamber; and

- a liquid transportation conduit to the lower section of the liquid collecting chamber, which conduit is provided with a liquid pump assembly that is configured to pump substantially liquid components from the liquid

collecting chamber to a liquid processing facility above the water surface;

- and wherein:

- the lower portion of the riser comprises an impermeable bottom section and a permeable upper section, which sections extend through the annular liquid collecting chamber, such that the impermeable bottom section extends through the lower section of the liquid collecting chamber to a level above the predetermined upper level; and

- the permeable upper section of the lower portion of the riser or underwater flowline has an inclined orientation relative to a horizontal plane.

The annular liquid collecting chamber and the

permeable upper section of the lower portion of the riser or other underwater flowline may have an inclined

orientation relative to a horizontal plane, with an angle of inclination of at least 5° and at most 85° relative to a horizontal plane.

These and other features, embodiments and

advantages of the method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of non- limiting embodiments depicted in the accompanying

drawing, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

Fig.l shows a lower portion of a riser which is provided with a gas-liquid separator according to the invention . DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

Fig.l shows a multiphase well effluent flowline 1 at the bottom 2 of a body of water 3, which may be a sea, ocean, f ord, lake or river with a waterdepth of up to several kilometers.

The flowline 1 is connected to a riser 4 having a lower portion 4A which is provided with a gas-liquid separator 5 and a substantially vertical upper portion 4B for transporting substantially gaseous components 15 to gas processing or receiving facilities arranged at a receiving unit such as platform 6, which (floats) at the water surface 7. The receiving unit may be a floating platform, a Tension Leg Platform (TLP) , a Floating

Production, Storage and Offloading (FPSO) unit or bottom supported offshore platform.

The lower portion 4A of the riser 4 comprises an impermeable curved lower section 4A1, which is connected to the downstream end of the multiphase well effluent flowline 1, and an inclined permeable upper section 4A2, which riser sections 4A1 and 4A2 are surrounded by a liquid collecting chamber 8, such that an annular liquid drainage channel 9 is provided between the inner wall of the liquid collecting chamber 8 and the outer walls of the riser sections 4A1 and 4A2.

A liquid transportation conduit 10 is connected to a lower section of the liquid collecting chamber 8, which conduit 10 is provided with a liquid pump assembly 11 that pumps substantially liquid components 12 from a lower section 9A of the annular liquid drainage channel 9 of the liquid collecting chamber 8 to a liquid processing or receiving facility at the receiving unit 6 at the water surface 7. The liquid collecting chamber 8 may be provided with upper and lower liquid level gauges 13 and 14, which monitor the liquid level in the annular liquid drainage channel 9 and which may be connected to liquid pump assembly 11 such that the assembly 11 is switched on when the liquid level in the annular liquid drainage channel 9 has reached the upper liquid level gauge 13 and that the assembly 11 is switched off when the liquid level in the annular liquid drainage channel 9 drops below the lower liquid level gauge 14.

Optionally a liquid recycle conduit 16 interconnects a section of the liquid transportation conduit 10

downstream of the pump assembly 11 with a section of the conduit 10 upstream of the pump assembly 11, and a valve 17 may be arranged in the liquid recycle conduit 16 to fine tune the control of the liquid level in the annular liquid collecting chamber 9A such that the liquid level is maintained between the upper and lower level gauges 13 and 14.

The inclined orientation of the co-axial tubular gas-liquid separator according to the invention has the advantage that the inclination enhances the separation efficiency and reduces slug flow in the riser or other underwater flowline.

Such other underwater flowline may be a catenary flowline or flowline extending from an underwater well or

multiphase oil and/or natural processing facility to an onshore oil and/or natural gas production and/or further processing facility.

If the underwater flowline has a section with an inclined orientation then the co-axial tubular gas-liquid

separator according the invention will be installed in this inclined section such that a compact and efficient oil/gas separator is provided which effectively inhibits slug flow in the separator and associated oil and/or gas production flowlines and other facilities that are connected to the up and downstream ends of the separator.