WO2013131574A1

WO2013131574A1 - Subsea processing

Info

Publication number: WO2013131574A1
Application number: PCT/EP2012/054054
Authority: WO
Inventors: Janiche BEEDER
Original assignee: Statoil Petroleum As
Priority date: 2012-03-08
Filing date: 2012-03-08
Publication date: 2013-09-12

Abstract

There is described subsea cooling apparatus and a method of cooling a fluid subsea. In an embodiment, a well stream fluid is received in a conduit. An outer surface of the conduit may be exposed to the sea upon deployment of the apparatus subsea, for transfer of heat from said fluid to said region of the sea across the cooling surface. Sea water may then be electrolysed in said region of the sea to produce an electrolysis product to electrolytically protect said cooling surface,with the cooling surface forming an electrode for performing the electrolysis.

Description

Subsea processing

Technical field The present invention relates to subsea processing, and in particular to subsea cooling of a fluid.

Background In subsea fluid processing, for example in the oil and gas production industry, it can be necessary to compress a well stream in order to ensure sufficient levels of production from the well. Where wells are located subsea and remote distances from other facilities, it can be desirable to compress the well stream at a location near the well head to help transport well stream fluids onward to a surface facility.

For this purpose, compressors can be installed subsea to compress the well stream, in particular the gas phase. This requires some pre-processing of the well stream in order to meet compressor operational requirements. Typically, pre-processing begins at an upstream end with a cooler receiving the well stream direct from the well head. Cooling can be necessary for example to compensate for an increase in temperature which is caused subsequently due to compression. The cooler may handle the well stream flow through multiple cooling pipes immersed in sea water. The sea water, typically at a temperature of between around -1 to 6 degrees Celsius at the sea bed, acts as a cooling medium for cooling the well stream, thereby cooling the well stream significantly from a typical temperature of around 30-75 degrees Celsius to around 20 degrees Celsius. The cooler needs to maintain its efficiency over time to stay within the operational requirements for the compressor.

A prior art compression system is shown schematically in Figure 1. A well stream is fed into the system 8 through a flow line 10, is passed through a cooler 12, and into a gas-liquid separator 16. The separator 16 separates gas and liquid phases of the well stream. The separated gas is passed into a compressor 18 (often several compressors), driven by a compressor drive motor 19, causing compression of the gas. Separated liquid is drained from the bottom of the separator and is fed downstream with the assistance of pump 20 (driven by pump motor 21). The liquid is combined with the compressed gas from the compressor and the combined well stream is transported, at an elevated pressure differential and flow rate, to a host facility via an export pipeline 22. Figure 2 shows an example of a prior art subsea compression station 30 where a compression system such as that described above in relation to Figure 1 may be implemented. The compression station has a robust frame structure 32 for installation on the seabed. The components of the system are typically designed to be provided in separate modules 6a-6d that can be changed out in order to facilitate maintenance.

However, changing out modules for maintenance is a manual process which can be a burden. This is a particular problem associated with the cooler modules. The coolers can be relatively large structures comprising multiple cooling tubes. The tubes are typically bent back on each other to reduce space and to maximise surface area to ensure good cooling efficiency. However, fouling such as build up of biological growth or scale on outer surfaces of the cooling tubes where seawater absorbs heat can reduce the cooling efficiency significantly. This means that the cooling module may need to be removed and replaced quite frequently in order to ensure good efficiency. In addition, mechanical scraping to remove biological growth is often required and the tubular nature of the coolers means that cooling surfaces can be difficult to access for cleaning such that the cleaning task can be particularly onerous and time consuming.

With oil and gas fields now moving into compression phases, and increasingly being developed in deep sea and remote areas, the basic option of manual removal and scraping of cooling modules is thought to be prohibitive in terms of cost, logistics, and time. The industry in this area of technology has for a number of years been looking for a suitable way for dealing with this issue, but suitable solutions have not been forthcoming. Summary of the invention

According to a first aspect of the invention, there is provided subsea cooling apparatus for use in cooling a fluid under a sea, the apparatus comprising:

a conduit for containing a flow of said fluid to be cooled; a cooling surface across which heat from said fluid is transferred to the sea, in use; and

at least one electrode for electrolysing seawater to produce an electrolysis product at the cooling surface for protecting said surface.

According to a second aspect of the invention, there is provided a subsea processing facility containing the subsea cooling apparatus of the first aspect of the invention.

According to a third aspect of the invention, there is provided a method of cooling a fluid subsea using the subsea cooling apparatus of the first aspect of the invention, the method comprising the steps of:

immersing said cooling apparatus in a sea;

receiving a fluid through said conduit; and

using said at least one electrode to electrolyse sea water of said sea to produce an electrolysis product at the cooling surface to protect said surface.

According to a fourth aspect of the invention, there is provided a method of controlling a subsea processing facility, the method comprising the steps of:

providing subsea, the subsea processing facility of the second aspect of the invention; and

transmitting a control signal to the facility for operating the electrode so as to electrolyse sea water and protect the cooling surface.

According to a fifth aspect of the invention, there is provided a computer program comprising computer readable instructions for use in performing the methods of the third or fourth aspects of the invention.

According to a fifth aspect of the invention, there is provided computer apparatus comprising a processor for reading and executing a computer program comprising computer readable instructions so as to perform the methods of the third or fourth aspects of the invention.

Each of the above aspects may include further features as defined in the claims or elsewhere herein in relation to any aspect of the invention, in any suitable combination. In this way, there is proposed a technique which can, at least in certain embodiments, prevent growth and scaling, and maintain effective heat transfer of a subsea cooler. In at least some embodiments, chlorine is developed by electrolysis of sea water at the actual surface of the cooler to thereby prevent growth and scaling and maintain efficient heat transfer. The surface on which chlorine is developed due to the electrolysis can thereby be protected against fouling.

The invention may help to eliminate or reduce the need for mechanical removal, cleaning and replacement of cooling modules in subsea compression stations. Prevention of fouling can be carried out automatically, and/or by control remotely at appropriate times, for example via control systems of the compression station.

Drawings and specific description There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 is a simplified schematic representation of a subsea compression system according to the prior art;

Figure 2 is a schematic representation of a prior art subsea compression station incorporating the system of Figure 1 ;

Figure 3 is a perspective view of a cooler module including a cooler connected to an electrical circuit according to an embodiment of the invention;

Figure 4 is a cross-sectional representation of a cooler tube of the cooler of Figure 3, protected by an electrolytic treatment of an outer surface; Figure 5 is a graph of experimental results showing the effect of biofouling on heat transfer efficiency with and without electrolysis and polarisation of seawater;

Figure 6 is a histogram of experimental results showing bacteria quantities before and after electrolysis of seawater; Figure 7 is a graph of experimental results showing the effect on heat transfer efficiency with periodic electrolytic treatment for titanium and DSA coated titanium; and

Figure 8 is a block diagram of a computer apparatus for controlling activation of the electrolytic treatment for the cooler of Figure 3.

Turning firstly to Figure 3, there is shown a subsea cooler module 112 (constituting subsea cooing apparatus) for a subsea processing facility such as a compression station. The subsea compression station may take a form similar to that described above in relation to Figures 1 and 2, but is adapted somewhat to accommodate the module 1 12, as detailed further below.

The cooler module 112 has a subsea frame 1 14 to which a cooler 1 16 is mounted.

The cooler 1 16 takes the form of a tube-type cooler, provided with at least one cooler tube 118 having an inlet (not shown) and an outlet (not shown) providing a conduit for flow of a hydrocarbon fluid therethrough. The hydrocarbon fluid passes through the cooler tube 118 and is cooled naturally by transfer of heat across outer cooling surfaces 1 19 of the tube walls to the sea 160 in which the cooler is immersed.

Typically, the fluid temperature will be in a range between around 60 and 100 degrees Celsius, whilst the sea temperature at the seabed may be in a range between around -

1 and 6 degrees Celsius.

As indicated in Figure 3, the cooler tube 118 is coiled throughout the frame volume to form a bundle of tube segments where long segments 120 are arranged in parallel and are connected to each other through tight bends 122 at joining ends of the long segments 120. This provides a high surface area for facilitating heat transfer. In particular variants, the cooler 116 may have a plurality of tubes 118 each carrying a separate portion of a well stream and each independently coiled and bent back on each other through a volume of the frame 1 14.

The cooler module 112 is deployed on the sea bed using the sea 160 as a cooling medium. In this environment, the cooler 1 16 is susceptible to fouling (where sea water has locally absorbed heat due to the transfer of heat from the well stream). In order to protect susceptible surfaces of the cooler from fouling, the sea water at the surface of the cooler is electrolysed using the cooling tube 1 18 of the cooler as an electrode, in this example as an anode. The cooling tube 118 is connected to an electrical circuit 140 which drives the electrolysis process. The cooler module may be provided with a connector 142 for connecting the cooling tube to the circuit. A further, sea electrode 144 is placed in the region of sea 160 spaced away from the tube 118. For example, the sea electrode 144 could be mounted at a distance of about 15 to 30 cm from the cooling tube 1 18. The sea electrode 144 is connected to the circuit to act as a cathode in the electrolysis process. The sea electrode 144 may be mounted to the cooler or to the frame of the cooler module in order to position it appropriately. The sea electrode 144 may take the form of an elongate, cylindrical and/or tubular electrode. It is also arranged sufficiently far away not to interfere with the operation of any nearby cathodic protection systems, and/or may be or isolated or insulated from such systems.

In operation, the electrical circuit 140 provides power from a power supply 146 to the tube 118 and to the sea electrode 144. These are electrically charged respectively to form the anode and cathode for the circuit. The sea water is ionized and negative chloride ions present in the sea water is electrolyzed by polarization of the tube 1 18 acting as the anode. At the outer surface 1 19 of the tube 118, chlorine is produced as seen with further reference to Figure 4. The chlorine on the outer surface provides a protective barrier or layer 130 of chlorine surrounding the outer surface 119 of the tube 1 18, protecting the surface from fouling and bio-growth. The chlorine acts as a biocide locally on the surface 119. The chemical reactions which occur at the outer surface 1 19 of the tube 118 (anode) and at the separate electrode 144 (cathode) are shown in the following: 2CI^"→ Cl₂ + 2e^"; and (1)

H₂0 0₂ + 2H⁺ + 2e^" (2)

These are well known in the art of electrolysis in relation to electrolysis of salt water. In Figure 5, the effect of electrolysis on heat transfer efficiency over time from a test experiment is shown in the graph 200. A plot of results from a reference experiment without electrolysis of seawater is shown in curve 202. The efficiency of heat transfer reduces over time. A plot of results with electrolytic treatment once per week, for a 30 minute period, is shown in curve 204, and it can be seen that the efficiency of the cooling tubes is maintained at near the original levels. Figure 6 shows the biocidal effect of the electrolytic treatment in histogram 300. The histogram shows, in column 302, the quantity of bacteria per cm² of metal surface before electrolysing and that afterwards, in column 304. It can be seen that bacteria are absent after treatment has taken place.

The tube 1 18 is formed from a corrosion resistant metal or alloy. In this example the tube 1 18 is formed titanium, and by way of connection to the circuit, the tube 1 18 as a whole constitutes the anode of the electrolytic circuit. Preferably, the titanium cooler tube 118 is coated with a dimensionally stable anode (DSA) coating. For example, the DSA coating comprises a catalytic oxide layer comprising one or more noble metals for example selected from the group comprising iridium, ruthenium, platinum, rhodium, and tantalum. The use of a DSA coating increases the effectiveness of the protection against fouling considerably. Other DSA coated metals could be used for forming the electrodes. The cathode (sea electrode 144) may be coated in the same way as the anode (tube 1 18).

It will be appreciated that a plurality of electrodes 144 may be used in forming a cathode for the circuit 140. For example, electrodes 144 may be placed at multiple positions in and around segments of the tubes. The electrode 144 may be mounted between adjacent tube segments for example equidistant from respective outer surface portions of the adjacent segments. Similarly, where there is a plurality of tubes, each tube may form an anode. Thus, in particular variants, a plurality of electrolytic circuits similar to the circuit 140 may be provided, to protect the tube 118 in particular sub- regions of the volume.

In Figure 7, there is shown a graph 400 of experimental results showing the effectiveness of treatment over time using titanium and DSA coated titanium electrode tubes in sea water, with a 30 minute treatment once per week. The x-axis specifies the date in 10 day intervals. A plot of results with titanium (non-DSA coated) is provided in curve 402, whilst that of DSA coated titanium is provided in curve 404. It shows that the use of DSA coated titanium has a better and more consistent effect on heat transfer efficiency, and that the tested treatment frequency and duration is suitable for keeping the heat transfer efficiency close to original levels. Much greater reduction and variation in heat transfer efficiency over time is experienced with titanium as seen in curve 402

Sea water electrolysis as described above can therefore be performed intermittently, i.e. providing an intermittent electrolytic treatment of the cooling surface 119, in order to maintain cooler performance. This minimises power consumption and limits environmental effects from producing chlorine. It is found that the protective effect will continue after the power circuit is switched off, for example up to around 2 to 4 weeks. Thus, a treatment may be repeated with a corresponding frequency. The frequency of treatment may be fixed, or could be variable.

In this regard, the cooler module 1 12 may be provided with measurement equipment such as sensors 170 for measuring cooling performance, or for measuring or detecting biological activity or fouling at the surfaces of the tube 118. The circuit may be activated to perform an electrolytic treatment according to the level of fouling, for example upon receipt of a signal from the monitoring equipment. Similarly, an indicator of cooling performance, for example temperature of the fluid at the exit of the cooler, may be measured and the electrolytic treatment may be performed according to the indicator of performance, for example if the temperature deviates from a threshold value by a pre-determined amount. The sensors 170 may include temperature sensors for measuring heat transfer efficiency across the tube surface and/or may include a camera or other equipment.

In certain variants, a particular duration of the electrolytic treatment can be set. Typically, the treatment may last up to 2 hours, but more typically about 30 minutes, as seen in the test results discussed above. For treating bio films and growth, a duration of about 1 to 30 minutes, preferably around 10 to 20 minutes may be sufficient. For treating scaling, the same or a longer duration could be appropriate, for example a duration of around 20 minutes to 2 hours, or more specifically around 30 minutes to 1 hour, may be appropriate.

In order to control activation of the electrolytic treatment, the electrical circuit 140 and the sensors 170 may be connected to a controller, in the form of computer apparatus 150. The circuit 140 and sensors 170 are connected to the computer apparatus 150 via an In/Out device 152 as shown in Figure 3 and Figure 8. The In/Out device is used for sending instructions to activate the circuit 140 and/or the sensors 170 and for receiving data from the circuit and/or the sensors for example measurement data indicative of a condition such as scaling, fouling or cooling performance. A processor 154 is used for generating instructions to be sent to the circuit 140 and the sensors 170. The processor 154 also processes the received data. A computer readable medium in the form of a memory 156 may also be provided for storing information such as collected data and/or a computer program with instructions for operating the circuit according to a condition or with a particular timing, frequency and/or duration, as described above. The computer apparatus 150 may form part of a subsea facility control system (not shown) for a subsea facility. The computer apparatus 150 may in whole or in part be provided separately of the cooler for example remotely. The computer apparatus 150 may have a plurality of In/Out devices, processors, and memory blocks. One or more of these components may communicate over a network, and be located in different locations.

The cooler may be contained in a subsea processing facility along with other processing components such as subsea compressors, pumps, separators etc. These components may be provided on respective removable modules, interconnectable to make up the subsea processing facility, for example to form a subsea compression system.

In an alternative embodiment, the voltage source may be configured such that different electrode potentials may be obtained from the same source. When an increased voltage to the electrodes is applied, a higher electrolytic potential can be obtained. Electrolysis of seawater then results in the formation of oxygen at the anode, which prevents build up of marine growth and scaling, thereby protecting the surface. This can be carried out and controlled in a similar way to that described above in relation to chlorine except that higher voltages are supplied. The electrolytic potential is however not raised so far as to cause corrosion of the titanium electrodes.

It can be noted that the term conduit may include containers, tanks, pipes or other spaces in which the fluid may eventually pass through, even if fluid flow in the conduit there may have temporarily halted. In certain variants, the cooling surface by which heat is transferred from the hydrocarbon fluid to the sea, and which is to be treated, is not a surface of the tube through which the flow of hydrocarbon fluid is conveyed. For example, the cooler may have a separate liquid cooling circuit in heat exchange relationship with that flow of fluid, and the liquid cooling circuit may comprise a tube with a cooling surface facing the sea across which heat is transferred. In this case, the surface of the cooling circuit facing the sea may be treated with electrolysis as described above.

It will be appreciated that anodic protection of a surface as described above includes protection by products of the anode reaction from electrolysis of the sea water. It does not necessarily require the cooling surface itself (e.g. outer surface of tube) to be an anode. Thus, the anode can be another component, for example it could be a separate electrode spaced away from the cooling surface in the region of the sea to be electrolysed. In such embodiments, chlorine produced at the anode may spread and/or be directed such that chlorine is present adjacent to the cooling surface in sufficient amount to protect the cooling surface from marine fouling and growth. In preferred embodiments however, the cooling surface or cooling tube is the anode. Treatment of the cooling surface may also be applied such that another second surface of the cooler can be treated, at the same time, by spreading of chlorine to the second surface.

The term "subsea" can in principle include usage in land locked or partially land locked seas, such as fjords or estuarine channels, in addition to open seas and oceans, provided conditions for operating the described apparatus and methods are met. The terms "subsea" and "under the sea" include use anywhere beneath the sea surface, for example immersed in the sea, on or above the sea bed.

It will be noted that the hydrocarbon fluid to be cooled may comprise fluid from a well.

Various modifications may be made without departing from the scope of the invention herein described.

Claims

CLAIMS:

1. Subsea cooling apparatus for use in cooling a fluid under a sea, the apparatus comprising:

a conduit for containing a flow of said fluid to be cooled;

a cooling surface across which heat from said fluid is transferred to the sea, in use; and

2. Subsea cooling apparatus as claimed in claim 1 , wherein said cooling surface forms a first electrode, the first electrode being operative as an anode.

3. Subsea cooling apparatus as claimed in claim 2 having a second electrode arranged to be spaced away from said cooling surface, and operative as a cathode.

4. Subsea cooling apparatus as claimed in claim 3, wherein the second electrode is spaced away from said cooling surface by a distance of at least 10 cm.

5. Subsea cooling apparatus as claimed in claim 3 or 4, wherein the second electrode is spaced away from said cooling surface by a distance of around 15 to 40 cm.

6. Subsea cooling apparatus as claimed in any preceding claim, wherein said cooling surface is an outer surface of said conduit.

7. Subsea cooling apparatus as claimed in any preceding claim, wherein the conduit comprises a tube having a plurality of tube portions which double back on each other.

8. Subsea cooling apparatus as claimed in claim 7, wherein said plurality of tube portions are substantially parallel with each other.

9. Subsea cooling apparatus as claimed in claim 7 or 8, wherein said plurality of tube portions are arranged in layers.

10. Subsea cooling apparatus as claimed in any preceding claim, wherein said conduit is exposed directly to the open sea.

1 1. Subsea cooling apparatus as claimed in any preceding claim, wherein said electrolysis product comprises chlorine.

12. Subsea cooling apparatus as claimed in any preceding claim, wherein the cooling surface or the at least one electrode or both are formed from a metal and have a dimensionally stable anode (DSA) coating.

13. Subsea cooing apparatus as claimed in claim 12, wherein the metal comprises titanium.

14. Subsea cooling apparatus as claimed in any preceding claim, wherein the fluid received through the conduit comprises a hydrocarbon fluid.

15. Subsea cooling apparatus as claimed in any preceding claim, wherein the fluid received through the conduit comprises fluid from a well.

16. Subsea cooling apparatus as claimed in preceding claim including a circuit operable to electrolyse seawater using the at least one electrode, the at least one electrode being coupled to the circuit.

17. Subsea cooling apparatus as claimed in claim 16, wherein the circuit is operable intermittently to electrolyse the sea water in order to electrolytically protect the cooling surface.

18. Subsea cooling apparatus as claimed in claim 17, comprising control means configured to activate the circuit to electrolyse the sea water with a set frequency.

19. Subsea cooling apparatus as claimed in claim 17, comprising control means configured to activate the circuit to electrolyse the sea water according to a detected performance indicator.

20. A subsea processing facility containing the subsea cooling apparatus of any preceding claim.

21. A subsea processing facility as claimed in claim 20, including at least one further component selected from the group consisting of:

a compressor for compressing the fluid;

a separator for separating the fluid into separate fluid phases; and

a pump for hydrocarbon liquids.

22. A subsea processing facility as claimed in claim 21 , including a controller adapted to process control signals for controlling operation of the subsea cooler and said at least one further component.

23. A method of cooling a fluid subsea using a subsea cooling apparatus as claimed in any of claims 1 to 19, the method comprising the steps of:

immersing said cooling apparatus in a sea;

receiving a fluid through said conduit; and

24. A method as claimed in claim 23, wherein the step of electrolysing the sea water is performed intermittently.

25. A method as claimed in claim 24, wherein said step of electrolysing the sea water is performed intermittently with a period of at least 5 days.

26. A method as claimed in any of claims 23 to 25, wherein said step of electrolysing sea water is performed with duration of at least 1 minute.

27. A method as claimed in any of claims 23 to 26, wherein performing the step of electrolysing sea water includes producing chlorine on the outer surface of the conduit.

28. A method as claimed in any of claims 23 to 27, including an additional step of detecting fouling, and performing the step of electrolysing the sea water in dependence upon the detection of fouling.

29. A method as claimed in any of claims 23 to 28, which further includes monitoring a performance indicator of the cooler, and performing the step of electrolysing the sea water in dependence upon the performance indicator.

30. A method as claimed in any of claims 23 to 29, wherein the step of electrolysing sea water is performed at regular time intervals.

31. A method of controlling a subsea processing facility, the method comprising the steps of:

providing subsea, a subsea processing facility as claimed in any one of claims 20 to 23; and

32. A method as claimed in claim 31 , wherein the control signal includes instructions for operating a further component of the facility.

33. A computer program comprising computer readable instructions for use in performing the method of any one of claims 23 to 32.

34. Computer apparatus comprising a processor for reading and executing a computer program comprising computer readable instructions so as to perform the method of any one of claims 23 to 32.