WO2010005312A1

WO2010005312A1 - Method for controlling a subsea cyclone separator

Info

Publication number: WO2010005312A1
Application number: PCT/NO2009/000230
Authority: WO
Inventors: Steinar ØYULVSTAD; Geir Inge Olsen
Original assignee: Aker Subsea As
Priority date: 2008-07-10
Filing date: 2009-06-19
Publication date: 2010-01-14
Also published as: NO332541B1; NO20083073L

Abstract

A method for controlling a subsea cyclone separator (5) in a subsea processing system for subsea separation is described. The cyclone separator (5) is adapted to separate hydrocarbons from water. This comprises directing a predominantly water containing fluid into the inlet (4) of the cyclone separator (5), extracting a flow containing the substantial part of the hydrocarbons from an overflow (6) of the cyclone separator (5) and mainly water from an underflow (7) of the cyclone separator (5). The overflow (6) and the underflow (7) are individually pressurized by pumps (8, 9), the ratio of the pump rates of the pumps (8, 9) determining the flow-split of the cyclone separator (5). The individually pressurizing pumps (8, 9) are powered by a common motor (10) and the ratio of the pump rates is set to correspond with a target pressure ratio corresponding to target flow-split of the cyclone separator (5).

Description

Method for controlling a subsea cyclone separator

The present invention relates to a method for managing the underflow and overflow streams from a cyclone separator in a subsea processing environment.

The conditions in a subsea processing environment are very different from those experienced in a topside process system or a land based process system.

Subsea production systems

The last two decades subsea production systems have been widely applied for producing oil and gas from deepwater fields where floating production units like platforms and FPSO's are the most economical and technically feasible option. Subsea Xmas-trees (valve trees) and transport flow lines are applied to transport the gas and oil from the wellhead and to the receiving platform. The production system is often optimized with regard to the plateau production rate, which often takes place in the field's early lifetime. In late field lifetime decreasing reservoir pressure and increasing liquid content, mostly due to water production, will restrict production of hydrocarbons. In addition the production facilities may not be able to handle the produced water without costly modifications. When further production is not economical or feasible the production will be discontinued.

Subsea pumps and subsea separation systems

One way to prolong the field lifetime and to increase the accumulated production volume of gas and oil is to avoid the bottlenecks introduced with the increased water production. By separating water from the hydrocarbon flow in a separation system located on the sea bottom, the pressure drop in the downstream flow lines and risers can be reduced and there is no need for increased water handling capacity at the topside unit. The separated water from the subsea separation system can be re-injected into a reservoir for storage, or re-injected back into the reservoir and used as pressure support in the reservoir for the ongoing gas/oil production. Water injection in hydrocarbon reservoirs

Water injection in hydrocarbon reservoirs is often associated with strict requirements. One advantage of re-injecting the produced water back into the hydrocarbon reservoir is that the danger of polluting the reservoir with for example oxygen and bacteria is low. However, in order not to plug the reservoir, the amount of solids particles and droplets of oil in the produced water should be minimized.

Removal of sand from the produced water is mostly done by letting the solid particles settle in the separator. The settled sand can later be removed by a batch wise operation and disposed into the hydrocarbon pipeline for transport to a topside receiving facility. However, the smallest sand particles will not settle in the separator, and application of sand cyclones may be required to remove these from the produced water. In the cyclones the higher density sand particles are drained through the cyclone underflow and stored for later disposal or re-injected directly into the hydrocarbon flow. The solids free water will leave the cyclone through the overflow outlet.

Similar to the smaller sand particles, smaller oil droplets will also follow the water outlet in the separator. Also here cyclones can be applied to remove this oil from the produced water in a polishing step.

Liquid-liquid cyclones

Liquid-liquid cyclones are widely used in the oil and gas industry. This type of cyclones is a process device used to separate liquids of different density. Cyclonic separation is used for purifying water by removing oil droplets. These liquid-liquid cyclones are sometimes called de-oilers.

A cyclone separator will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining the operating characteristics.

A cyclone has two exits on the axis: the smaller on the bottom (underflow or reject) and a larger at the top (overflow or accept). The underflow is generally the denser or thicker fraction, while the overflow is the lighter or more fluid fraction. In a typical liquid- liquid cyclone separator applied in a subsea process system, the lighter phase will be the oil phase, which will constitute the overflow, while the denser phase will be the water phase, which will constitute the underflow.

Internally, centrifugal force is countered by the resistance of the liquid, with the effect that larger or denser droplets are transported to the wall for eventual exit at the reject side (underflow) with a limited amount of liquid, whilst the finer, or less dense droplets, remain in the liquid and exit at the overflow side through a tube extending slightly into the body of the cyclone at the centre.

It is inherent in the design of cyclone that only one of the two phased from the cyclone can be the purified phase, most often this would be the denser water phase. A clean water phase as underflow from the cyclone is only achieved by allowing the rate of overflow from the cyclone to be larger than the rate of oil entering the cyclone, i.e. the oil in the overflow will contain a fraction of water. This flow-split ratio is controlled by adjusting the pressure ratio between inlet and overflow and inlet and underflow.

As used herein, the term "a cyclone separator" also includes as device with several cyclones arranged for parallel operation.

The pressure loss through cyclone separator may often require that both the overflow and the underflow is be pressurized. In a typical subsea configuration, the overflow containing the lighter phase will be pressurized for transport back into the hydrocarbon flow.

Subsea pumps Subsea pumps have become increasingly popular for increasing production from subsea wells. These pumps may either be inserted directly in the well flow, often in the form of a multiphase pump, or be a part of a subsea process system.

If the liquid is separated from the gas, typically rotor dynamic pumps, like a centrifugal based pump, may be applied to boost the liquid through a dedicated flow line system to the topside receiving unit. Centrifugal based pumps may also be applied to re-inject separated produced water back into the reservoir. Since some gas may flash out from the liquid between a separator and the pump due to pressure losses, a centrifugal based pump must tolerate some gas. Tests have shown that some gas (up to 10- 15%) can be present in the fluid without too much performance degradation.

Subsea pumps are subject to very strict requirements for due to the call for zero emission of produced fluid to the surroundings. Moreover the use of electrical motors, power connection systems and pressure housing add complexity and vulnerability to the equipment. Another challenge is that the pumps and power supply to the pumps must be very reliable since a failure will require a very costly repair operation and will cause a substantial delay before production can be resumed. Even if a simpler type of motor is used, like a hydraulically powered motor, repair would still be very costly. A hydraulic motor is not preferred due to potential leakage of hydraulic fluids. Consequently, an electric motor is usually used to operate pumps and other types of rotating equipment. Due to the above difficulties it is a desire to be depending on as few items as possible for obtaining production from the well. It is therefore a desire to have as few pumps and motors as possible installed, especially if the operations of the pumps are dependent on each other so that failure in one pump or its motor will cause shut down of the other pump.

The present invention devises a way to reduce the complexity of a pump system by applying one motor to simultaneously power two or more pumps. This will especially be favourable if the pumps must be operated together and where both flows are proportional to each other. This will in most circumstances be the case in the above described liquid-liquid separator cyclone system. The pumps may be connected to the motor through the same axel, or by use of a gear transmission system allowing speed difference between the two pumps. The two pumps may also be located inside the same pump housing and share the same rotor. This means that two chambers are separated by a mechanical seal and have separate inlets and outlets.

A subsea process system is most often associated with one stage of separation followed by pressurizing of the oil phase, the gas phase or the water phase or any combination of the separated fluid phases. Unlike topside systems or a land base process system, in a subsea environment the option of routing utility fluid streams to a low pressure part of the process system is limited. Reject flow from a cyclone separator, which normally will be choked to a low pressure system, will in a subsea system need to be pressurized to the hydrocarbon fluid pressure.

Subsea processing is also characterized by high reliability requirements for process equipment, due to the costs associated with maintenance. It is therefore an incentive in a subsea application to utilized one rotation equipment for several duties in order to enhance the overall system availability.

The present invention therefore makes use of two pumps powered by the same motor to pump the overflow of a liquid-liquid cyclone to one location and to pump the underflow of the same liquid-liquid cyclone to a different location.

The pressure drop through a separator cyclone is generally proportional to the square root of the flow. This applies to both the underflow and the overflow. Therefore a pressure ratio across the cyclone separator can be sustained within a suitable range with variation of the pumps rotational speed. Further optimization of the pressure ratio can be achieved by applying flow regulators located on either of the pumps inlets or on either of the pumps outlets, to regulate the relationship between the overflow and the underflow of the liquid-liquid cyclone.

The two pumps may share the same axel as the motor. The pumps may operate on the same speed or on different speeds, if a transmission is applied.

The pumps can alternatively be sized to have a displacement ration adapted to achieve separation in the separator cyclone for all operational conditions.

The two pumps may share the same housing and possibly also rotor, where mechanical seals and separate inlets and outlets separate the two flows.

The present invention has the advantages of reducing the number of vulnerable component, reducing the need for maintenance, reducing the complexity of the system by reducing the number of motors, power supply and control cables and variable speed controllers topsides, increasing accessibility to the various components, and providing for a full synchronous running of the pumps with reduced need for a complicated electronic control system.

The invention will now be described in detail referring to two different embodiments, where

Figure 1 shows a simple layout of a subsea separator system in a first embodiment, including the use of the present invention, and

Figure 2 shows a more sophisticated system for subsea separation in a second embodiment, including the use of the present invention.

In figure 1 is shown a well flow 1, which contains a mixture of water and oil (it may also to a certain degree contain solids in the form of sand and gas, but for simplicity we will treat the well flow as water and oil only).

The well flow is led to a gravitational separator 2. In the gravitational separator 2 the well fluid is allowed to settle so that oil floats upward and water sinks down. By the time the flow reaches the outlet end of the separator 2, most of these components will have separated into two layers, oil on top and water at the bottom. The oil phase (which to some degree still may contain a fraction of water) is released to an oil export line 3, which extends to a topside facility or to shore.

The water phase is led through a line 4 to a liquid-liquid cyclone separator 5. The function of the cyclone separator 5 will be well known to the person of skill, so a detailed explanation of how it works is not necessary. The purpose of the cyclone separator is to remove as much oil from the water as reasonably possible. If the water is re-injected it should be more or less free from oil. If it contains too much oil, small oil droplets will gather into larger droplets, which may clog formation pores. Eventually, this clogging may prevent water from entering the formation and finally the formation cannot be used for water injection. If the water is to be disposed of by other means it will nevertheless be of advantage that it contains as little oil as possible, so that the need for treatment of the water is reduced. The cyclone separator 5 has an overflow 6, where the lighter phase exits and an underflow 7, where the heavier phase exits. The most of the oil will exit through the overflow and most of the water through the underflow. In order to ensure that the underflow contains substantially no oil it is necessary to accept that the overflow contains a substantial amount of water. The water content may be as much as 50% or more.

The performance of cyclone separators is very much depending on the pressure, and hence outflows, at the overflow and underflow outlets. The pressure ratio of a cyclone separator is defined as follows:

Where P₁ is the pressure at the inflow into the separator, Po is the pressure at the overflow and Pu is the pressure at the underflow.

The flow-split of a cyclone separator is defines as follows:

Q₁

Where Qo is the flow rate at the overflow and Q₁ is the flow rate at the inflow.

To adjust the performance of the cyclone separator 5 and have as little oil as possible in the underflow (water phase) the pressure ratio is adjusted by an underflow pump 8 and an overflow pump 9. The ratio of the pump rates of these two pumps 8, 9 determines the flow-split of the cyclone separator, i.e. the higher the pump rate of the overflow pump 9 is relative to the underflow pump 8; the more water will be entrained in the oil. The relative pump rate of the pumps 8, 9 is therefore adapted to a ratio where the cyclone separation removes virtually all the oil from the water phase, but still limits the water content of the overflow to a reasonable amount. This will ensure that practically all the oil produced by the well is recovered and the water, which is to be injected or otherwise disposed of, is more or less pure water. The obvious solution to this would be to use separate pumps driven by separate motors, which then can be easily individually adjusted to set the desired flow-split. In the present invention the pumps will be driven by one motor 10 only. This means that the pumps 8, 9 will have to have a pre-set pump rate ratio, which corresponds to the desired flow-split. The overflow will be substantially less than the underflow, so the overflow pump 9 has a correspondingly smaller capacity than the underflow pump. This may be achieved, e.g., by designing the overflow pump with a smaller displacement, gearing the overflow pump to a lower rotational speed or a combination of these.

The water from the underflow pump 9 is preferably fed to a line 11 for re-injection into a ground formation or alternatively released to the sea, provided the purity requirements at the location are fulfilled. It is also an alternative to export the water to an external facility for disposal or final treatment.

The overflow is channelled via a line 12 to the export line 3 and mixed with the oil from the gravitational separator 2.

The line 12 may include a flow regulator 13, which may be used to limit the flow from the overflow pump 9 if the pre-set pump rate ratio of the pumps 8, 9 is not accurately enough selected or of the water cut from the gravitational separator 2 changes over time. Limiting the flow from the overflow pump 9 will increase the back pressure at the overflow and hence change the flow-split of the cyclone separator 5. The flow regulator 13 may initially be set to a specific choke setting, so that the flow from the overflow pump may be adjusted both up and down. The flow regulator may also be placed before the inlet of the overflow pump 9 or before inlet or after the outlet of the underflow pump 8. It is also conceivable to have a flow regulator both in the flow path of the overflow pump 9 and the underflow pump 8. This will increase the adjustment options.

Figure 2 shows a second embodiment of the present invention, in which a sand removal unit 14 is also included. This system also comprises an option for water recirculation. The basic function of this embodiment is the same as for the embodiment of figure 1 and will not be repeated. Although some sand is removed in the gravitational separator 2, as indicated by reference numeral 15, the flow 4 from the gravitational separator 2 will still contain some sand. It is preferred to remove this substantially before the flow enters the cyclone separator 5. Consequently a de-sander unit 14 containing a multiple of cyclones is inserted in the line 4. The overflow 16 from the de-sander unit 14 basically contains liquid only. Sand (entrained in liquid) exits through line 17 to the export line 3 in order to be removed from the oil in a later stage.

A water re-circulation line 20 branches off from the re-injection line 11 and leads back to the inlet of the gravitational separator 2. A flow regulator 21 is included in the line 20. The line 20 branches off into a line 18, which leads to the de-sander unit 14. A valve 19 is included in the line 18. A valve 22 is included in the line 20 between the branch off point to the line 18 and the gravitational separator 2.

At start-up of the system, it is set in re-circulation mode. This means that at least some of the water from the underflow pump 8 is re -directed back into line 20 and through an open valve 22 to the gravitational separator 2. When the circulation has reached a desired flow rate and is stable, valve 22 is closed so that the separated water is led into the re-injection line 11. Re-circulation is also used if the inflow to the cyclone separator via line 16 is so low, and consequently the target pump rate is so low, that the motor is not capable of running at a correspondingly low speed. In such a case a portion of the water is returned to the inlet of the cyclone separator 5, via the de-sander 14 as shown or directly via a line not shown. This way the inflow to the cyclone separator 5 will be increased and the motor is allowed to run on a higher speed.

From time to time the de-sander unit 14 has to be emptied of sand that has settled at the bottom of the unit. In order to do so, the valve 19 is opened to allow water from the underflow pump 8 to flow into the de-sander unit 14. This water flushes the sand into the line 17 and into the export line 3. The efficiency of the flushing may be regulated by the flow regulator 21 in the line 20.

It is evident that the well flow also may contain some gas, either liquefied or in gas phase. The system of the present invention is capable of handling this gas up to a maximum percentage, depending on the type of hydro-cyclone separator and pumps used. The gas will largely be separated out from the flow together with the oil.

Although the cyclone separator is shown above as constituting an additional separation step after a gravitational separator, it is conceivable that the cyclone separator can be used in a first separation step or as the only separation step.

Claims

C l a i m s

1.

Method for controlling a subsea cyclone separator (5) in a subsea processing system for subsea separation, said cyclone separator (5) being adapted to separate hydrocarbons from water, comprising directing a predominantly water containing fluid into the inlet

(4) of the cyclone separator (5), extracting a flow containing the substantial part of the hydrocarbons from an overflow (6) of the cyclone separator (5) and mainly water from an underflow (7) of the cyclone separator (5), the overflow (6) and the underflow (7) being individually pressurized by pumps (8, 9), the ratio of the pump rates of the pumps (8, 9) determining the flow-split of the cyclone separator (5), characteri sed in that the individually pressurizing pumps (8, 9) are powered by a common motor (10) and that the ratio of the pump rates is set to correspond with a target pressure ratio corresponding to target flow-split of the cyclone separator (5).

2.

Method according to claim 1, characteri sed in that connecting the pumps

(5) to the motor (10) though the same axel or through a gear transmission either between the pumps or between pumps and the motor.

3

Method according to claim 1 or 2, characteri sed in that the overflow rate is adjusted by a flow restriction device (13) in the overflow (6).

4.

Method according to claim 1,2 or 3, characteri sed in that part of the pressurised underflow (7) is re-circulated to upstream of the cyclone separator (5).

5.

Method according to any of the preceding claims, characterized in that predominantly water containing fluid is routed to the cyclone separator (5) from a gravitational separator (2).

6.

Method according to any of the preceding claims, characterized in that the predominantly water containing fluid is routed to the cyclone separator (5) from a de- sanderunit (14).

7.

Method according to claim 6, characterized in that the de-sander unit (14) is flushed with water from the underflow of the cyclone separator (5).

8.

Method according to any of the preceding claims, characterized in that the water from the underflow (7) of the cyclone separator (5) is re-injected into an underground formation.

9.

Method according to any of the preceding claims 1 -7, characterized in that the water from the underflow (7) of the cyclone separator (5) is released to the sea.

10. Method according to any of the preceding claims, c h ar a ct e r i s e d i n that at least two cyclone separators (5) are connected in a serial configuration, and that the separators are controlled and pressurized by pumps powered by the same motor.