The invention relates to a subsea multiphase flow mixing apparatus, and an associated method, that includes a flow mixer having an inlet for a multiphase flow and an adjustable gas/ liquid outlet. Background of the invention

It is a common practice within the field of subsea fluid handling to allow the well flow from subsea wells to enter a flow mixer in order to mix or homogenize the well flow or production flow. This is normally performed in order to avoid gas/liquid slug flow and to provide stable operating conditions for the multiphase pump, which multiphase pump is arranged downstream of the flow mixer. The flow mixer breaks the energy of the slug flow, smoothes any fluctuations in the flow, and acts as a sand trap. A slug flow is normally referred to as a multiphase fluid flow regime characterized by a series of liquid plugs (slugs) separated by relatively large gas pockets. In vertical flow, the bubble is an axially symmetrical bullet shape that occupies almost the entire cross-sectional area of a tubing. The resulting flow alternates between high-liquid composition and high-gas composition.

A conventional subsea flow mixer is designed as an accumulator having a fixed flow restriction on the liquid outlet. The flow area of the restriction is set based on the expected well flow profile, e.g. production flow, and should prevent complete draining of the liquid during a gas slug, and overfilling during a liquid slug. The slug dampening effect of the flow mixer is dependent on the flow area of the restriction and the size and geometry of the flow mixer vessel.

A conservatively designed flow mixer, e.g. designed for the worst combination of nominal flow and slug during the life of the field, would result in a flow mixer having a physical size that is impractical for integration in a manifold or pump module. If the flow mixer is made smaller, the effective operating range is narrowed, and replacement may be required at some stage. Intervention costs relating to retrieval and re-installation of subsea modules, manifolds in particular, are significant.

From document US 5035842 it is known to feed a non-homogenous mixture of liquid and gas into a vessel to form a body of gas above a pool of liquid. Liquid is fed from the pool through a discharge pipe containing a constriction forming a venturi. Gas is drawn from the gas body through a pipe extending through the liquid pool into the discharge pipe to effect mixing of the liquid and the gas in the venturi. Perforations in the discharge pipe adjust the amounts of gas and liquid leaving the vessel to maintain both liquid and gas within the vessel.

In document US 5135684 it is disclosed a multiphase process mixing and measuring system. A liquid is supplied to a vessel to form a pool from which it discharges through a venturi. A supply pipe or pipes convey other liquids and/or gases from separate sources or from above the liquid pool into the venturi for mixing with the liquid. The supply pipes can extend through the pool and be perforated to tend to maintain the level of the pool. Associated with the venturi are pressure sensors for measuring flow and densitometer permitting mass flow rate measurements of gas and liquid phases. The apparatus can be incorporated in a cartridge for reception in a receptacle at a subsea installation.

An object of the invention is to adjust the flow of a gas and liquid in a mixing apparatus in situ, e.g. subsea, without retrieving the apparatus to the surface.

Another object of the invention is to be able to increase the liquid drainage area as part of a contingency plan to flush out sand and debris from the flow mixer.

Summary of the invention

The invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention.

The invention concerns an apparatus for mixing at least a first fluid phase having a first density and a second fluid phase having a second density. The apparatus comprises; at least one container, the container comprising at least one inlet for a multiphase flow and at least one outlet at a lower axial end of the container, a hollow flow regulating device axially arranged within the container,

wherein a first end of the flow regulating device is arranged in a distance from the outlet providing a drainage gap between the flow regulating device and the outlet, which drainage gap has a drainage area. The flow regulating device comprises a number of perforations along the axial length thereof and a discharge means in a first end, which discharge means opens towards the outlet. The flow regulating device is connected to a position adjustment device, the position adjustment device is arranged to move the flow regulating device in the axial direction, thereby adjusting the drainage area of the drainage gap. The flow regulating device is movable. In a preferred embodiment the first fluid phase is a liquid, while the second fluid phase is a gas.

In another embodiment, there may be arranged a first inlet, e.g. a liquid inlet, and a second inlet, e.g. a gas inlet, instead of one multiphase flow inlet. There might be arranged one, two or a number of perforations along the axial length of the flow regulating device, the perforations extending along the circumference of the flow regulating device. The perforations might have any diameter that allows the liquid or gas to flow through them. A restriction in the number of perforations will slow down the liquid flow inside the container.

Dependent on the multiphase well flow or production flow mixture entering the container, the drainage gap may be adjusted according to the well flow mixture. Moving the flow regulation device away from the outlet, will result in that a larger amount of liquid will flow out of the container. Correspondingly, moving the flow regulating device towards the outlet, a larger amount of gas will flow out of the container. Another application of the invention might be to flush out sand or debris trapped at the outlet in the container. The sand or debris can be flushed by moving the flow regulation device away from the outlet, allowing the sand or debris to flush through the outlet.

The liquid, which due to gravity tend to collect in the lower part of the container closest to the outlet, draws along gas through the outlet and creates a gas/liquid mixture. This is due to a pressure difference between the inside of the container and downstream the outlet outside the container. The pressure difference might be created by a narrowing, e.g a venturi, by a pump, or similar means well known to a person skilled in the art. The gas is drawn from the gas phase, i.e. the gas is normally in the upper part of the container, through the flow regulating device extending through the liquid into the discharge means to effect mixing of the liquid and the gas through the outlet. In an embodiment of the apparatus, the position adjustment device may be connected to a second end of the flow regulating device.

In another embodiment the multiphase flow separates in at least the first fluid phase and the second fluid phase in the container, the inlet and outlet being arranged such that the fluid phase having the largest density separates at the lower axial end closest to the outlet.

In an embodiment the container converges as an abutted cone at the outlet. The abutted cone may have a linear-shape, curve-shape, funnel-shape or throat-shape.

In an embodiment is the diameter of the flow regulating device substantially equal to the diameter of the outlet. In another embodiment the position adjustment device comprises an external interface arranged on the outside of the container.

In an embodiment the external interface is provided to be manipulated by the means of a ROV manipulator, a torque tool, or an actuator wired to a subsea control system. The external interface might in the form of a screw, bolt or any other interface suitable for manipulation by one of said means for manipulation. By manipulating the external interface, the position adjustment device is activated and the movable flow regulating device is moved in the axial direction such that the drainage gap, and thus the drainage area, between the lower axial end of the flow regulating device and the outlet, is modified. Dependent on the mixture of the multiphase flow through the inlet, the drainage gap may be adjusted according to the mixture of the multiphase flow. In an embodiment the apparatus may include measuring means, measuring the flow rates of the components in the multiphase flow, and, dependent on the measured flow rates, one may adjust the drainage area by moving the flow regulating device in the axial direction thereof allowing more or less gas or liquid to flow through the outlet.

The invention also relates to a method of mixing at least a first fluid phase having a first density and a second fluid phase having a second density. The method comprising the steps of;

- providing at least one container, the container comprising at least one inlet for a multiphase flow and at least one outlet at a lower axial end of the container,

- arranging a hollow flow regulating device axially within the container, a first end of the flow regulating device is arranged in a distance from the outlet providing a drainage gap between the flow regulating device and the outlet, which drainage gap has a drainage area, the flow regulating device comprising a number of perforations along the axial length thereof and a discharge means in a first end, which discharge means opens towards the outlet,

- connecting the flow regulating device to a position adjustment device,

- adjusting the drainage area of the drainage gap by manipulating the position adjustment device. The invention will now be described in non-limiting embodiments and with reference to the attached drawings, wherein;

Brief description of the drawings

Figure 1 shows an example of a mixing apparatus according to prior art.

Figure 2 shows an embodiment of the mixing apparatus according to the present invention.

Detailed description of a preferential embodiment

Figure 1 shows an example of a mixing apparatus according to prior art, where the mixing apparatus is exemplified as a container 1. The container 1 has an inlet 2 for a multiphase flow. The multiphase flow comprises a mixture of at least a first fluid phase having a first density, e.g. a liquid, and a second fluid phase having a second density, e.g. a gas. There is arranged an outlet 3 at the lower axial end of the container 1. The liquid level inside the container 1 is shown as a gas-liquid interface 10. A hollow flow regulating device 4 is axially arranged inside the container 1 , connected at an upper part of the inner surface of the container 1 , and extending in a fixed distance towards the outlet 3. The flow regulating device 4 opens towards the outlet 3 through discharge means 7. Further, the flow regulating device 4 is provided with perforations 6 along its circumference, which perforations 6 extend along the axial length of the flow regulating device 4. A drainage gap 5 forms a fixed drainage area between the lower axial end of the flow regulating device 4 and the outlet 3. Multiphase flow entering through inlet 2 will, due to gravity, separate in a gas phase and a liquid phase inside the container 1 , shown by the gas-liquid interface 10. The gas flows through the perforations 6 to the inside of the flow regulating device 4. The liquid, which due to gravity separates in the lower part of the container 1 closest to the outlet 3, draws out gas through the discharge means 7 and the mixed gas-liquid flow flows through the outlet 3 as a homogenous flow. This is due to a pressure difference between the inside of the container 1 and downstream the outlet 3. The pressure difference might be created by a narrowing of the flow area, e.g a venturi (not shown), or by a pump creating a suction pressure.

Figure 2 shows an embodiment of the mixing apparatus according to the present invention. Similar to figure 1 there is shown a container 1 having an inlet 2 for a multiphase flow. An outlet 3 is arranged at the lower axial end of the container 1. The liquid level inside the container 1 is shown as a gas- liquid interface 10. A hollow flow regulating device 4 is axially arranged inside the container 1 , connected to an adjustment device 8 in the upper axial end of the container, which adjustment device 8 comprises a flexible arrangement 1 1 and an external interface 9. The flow regulating device 4 opens towards the outlet 3 through discharge means 7. Further, the flow regulating device 4 is provided with perforations 6 along its circumference, which perforations 6 extend along the axial length of the flow regulating device 4. The external interface 9 is arranged on the outside of the container 1 and can be manipulated from the outside. The external interface 9 might be in the form of a screw, bolt or any other interface suitable for manipulation by a ROV manipulator, a torque tool or an actuator wired to a subsea control system (not shown). By manipulating the external interface 9 of the position adjustment device 8, the position adjustment device 8 causes the flow regulating device 4 to move in the axial direction such that the drainage gap 5, and thus the drainage area, between the lower axial end of the flow regulating device 4 and the outlet 3, is modified. Dependent on the mixture of the multiphase flow through the inlet 2, the drainage gap 5 may be adjusted according to the multiphase flow mixture. Moving the flow regulation device 4 away from the outlet 3, will lead to that a larger amount of liquid will flow out of the container 1. Correspondingly, moving the flow regulating device 4 towards the outlet 3, a larger amount of gas will flow out of the container 3. In cases where sand or debris has gathered at the outlet 3, the flow regulating device 4 can be moved away from the outlet 3, allowing sand or debris to be flushed out through the outlet 3.

By the use of the arrangement as described herein, one is able to adjust the amount of liquid and/or gas flowing out from the flow mixer through the outlet, and thus minimize slug flow. The invention is herein described in non-limiting embodiments. A person skilled in the art will understand that there may be made alterations and modifications to the embodiments that are within the scope of the invention as described in the attached claims.