A Subsea System Comprising a Preconditioning Unit and Pressure Boosting Device and Method of Operating the Preconditioning Unit

Abstract

A subsea system (1) connected to a subsea well (4) for boosting a process fluid flowing out of the well, comprising: —a preconditioning arrangement (2) connectable to a process fluid line from a well, wherein the preconditioning arrangement comprises at least one sensor for measuring temperature and one sensor for measuring pressure of the process fluid—means for estimating density of the process fluid based on measured temperature and pressure, —a cooler system (20, 21) comprising at least a first cooler for cooling the process fluid wherein the subsea system further comprises: —a pressure boosting device (3) arranged downstream of the preconditioning arrangement (2), the pressure boosting device having an operational window dictating operational parameter in terms of maximum and minimum allowable density of the process fluid entering the pressure boosting device (3).

Claims

1. A subsea system connected to a subsea well for boosting a process fluid flowing out of the well, the subsea system comprising: a preconditioning arrangement connectable to a process fluid line from the well, the preconditioning arrangement comprising: at least one sensor for measuring a temperature of the process fluid and at least one sensor for measuring a pressure of the process fluid; means for estimating a density of the process fluid based on the measured temperature and pressure; and a cooler system comprising at least a first cooler for cooling the process fluid, the first cooler comprising a bypass line for guiding a portion of the process fluid therethrough, and the bypass line comprising a control valve for varying the amount of process fluid flowing therethrough and a temperature control unit for measuring a temperature of the process fluid in the bypass line; a pressure boosting device arranged downstream of the preconditioning arrangement and comprising an inlet for receiving a process fluid with at least 30 volume percentage of CO2 at operational subsea conditions and an outlet for discharge of pressurized process fluid, the pressure boosting device having an operational window dictating operational parameters in terms of maximum and minimum allowable density of the process fluid entering the pressure boosting device; wherein the preconditioning arrangement is configured to ensure that the process fluid is within the operational window of the pressure boosting device before entering the pressure boosting device.

2. The subsea system according to claim 1, wherein the operational window has at least maximum and minimum operational parameters of the pressure and temperature of the process fluid.

3. The subsea system according to claim 1, further comprising a recirculation loop connected downstream of the pressure boosting device and upstream of the preconditioning arrangement.

4. The subsea system according to claim 1, wherein the cooler system comprises a second cooler arranged in series or parallel with the first cooler.

5. The subsea system according to claim 4, wherein the cooler system comprises a third cooler which is arranged in parallel with the first and second coolers.

6. The subsea system according to claim 4, wherein the cooler system comprises at least one flow control device for directing flow through at least one of the first and second coolers.

7. The subsea system according to claim 4, wherein at least one of the first and second coolers comprises a recirculation loop for recirculating process fluid back into an inlet of the cooler.

8. The subsea system according to claim 4, wherein the first and second coolers have a different cooling capacity.

9. The subsea system according to claim 4, wherein the first cooler comprises a chemical injection line.

10. The subsea system according to claim 1, further comprising a recirculation loop connected downstream of the pressure boosting device and upstream of the preconditioning arrangement, wherein the recirculation loop comprises a pump recirculation valve which is connected to a temperature transmitter measuring temperature of the process fluid downstream of the first cooler, and wherein the pump recirculation valve is controlled by the temperature transmitter.

11. A method of operating a subsea system, the subsea system comprising: a pressure boosting device comprising an inlet for receiving a process fluid with at least 30 volume percentage of CO2 at operational subsea conditions and an outlet for discharge of pressurized process fluid, the pressure boosting device having an operational window dictating operational parameters in terms of a maximum and minimum allowable density of the process fluid entering the pressure boosting device; a preconditioning arrangement positioned upstream of the inlet of the pressure boosting device and being connectable to a process fluid line from a well, the preconditioning arrangement comprises at least one sensor for measuring a temperature of the process fluid and at least one sensor for measuring a pressure of the process fluid; a cooler system comprising at least a first cooler; wherein the method comprises the steps of: measuring parameters of the process fluid entering the preconditioning arrangement using the sensors; determining whether any of the parameters are outside an operational window of the pressure boosting device; determining whether any action is required by the preconditioning arrangement in order for the density of the process fluid to be within the operational window of the pressure boosting device; and when any required actions are taken in order for the density of the process fluid to be within the operational window of the pressure boosting device, allowing the process fluid to enter the pressure boosting device, thereby ensuring that the process fluid is within the operational window of the pressure boosting device before entering the pressure boosting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] FIG. 1A is a setup of a subsea system according to the invention;

[0074] FIG. 1B is an example of a cooler system forming part of the subsea system;

[0075] FIG. 2A is an example of a subsea system connected to a well, wherein the subsea system comprises a subsea tree, a preconditioning arrangement and a pressure boosting device;

[0076] FIG. 2B is an example of a subsea system connected to a well, wherein the subsea system comprises a subsea tree, a separation device, a preconditioning arrangement and a pressure boosting device;

[0077] FIG. 3A shows a side-view of a cooler which can form part of the subsea system;

[0078] FIG. 3B shows a top view of a perforated plate of a single cooler;

[0079] FIG. 4 shows a cooler system as illustrated in FIG. 4 in WO 2013/174584 comprising five parallel cooler series, where some of the coolers are provided with a recirculation loop;

[0080] FIG. 5 shows a cooler system as illustrated in FIG. 5 WO 2013/174584, where some of the coolers are provided with a recirculation loop and a bypass loop;

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT

[0081] In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings.

[0082] FIG. 1A is a setup of a subsea system 1 according to the invention. The subsea system 1 as disclosed in FIG. 1 comprises a preconditioning arrangement 2 and a pressure boosting device 3. The pressure boosting device 3 having an operational window dictating operational parameter in terms of maximum and minimum allowable density of the process fluid entering the pressure boosting device, and wherein the preconditioning arrangement ensures that the process fluid is within the operational window of the pressure boosting device before entering the pressure boosting device 3. Other operational parameters such as temperature, pressure and flow of the pressure boosting device may also be limiting factors relevant for the operational window. The preconditioning arrangement 2 ensures that the process fluid entering the pressure boosting device 3 is within the operational window for the pressure boosting device 3 such that the pressure boosting device is not damaged by the process fluid.

[0083] In operation, process fluid from e.g. a well (not shown in FIG. 1A) enters the preconditioning arrangement subsea system 1 via an inlet pipe or process fluid pipe 45. In the preconditioning arrangement 2 the process fluid pipe direct the fluid to the main line 45. An on-off valve 51, direct the fluid to the coolers 20, 21 or bypass the coolers 20, 21. The branch line 50 comprises a first on-off valve 56 and a second on-off valve 57 arranged in series. A first cooler 20 is arranged downstream of the second on-off valve 57 and a second cooler 21 is arranged downstream of the first cooler 20. A temperature transmitter 23 control the temperature in the process fluid after exiting the first cooler 20. The temperature transmitter 23 is connected to a controller controlling a pump recirculation valve 66 arranged in a recirculation line 65 connected downstream of the pressure boosting device 3 via control lines 69. In case of risk of hydrate formation, the controller manipulates the pump recirculation valve 66 to guarantee a minimum temperature by opening the pump recirculation valve 66 as will discussed in greater detail below. A first bypass line 58 is connected between the first and second on-off valves 56, 57 in one end thereof and between the first and second coolers in the other end thereof, thereby bypassing the first cooler 21. The bypass line 58 comprises an operated control valve 22 for guiding the flow in the bypass line 58 to the second cooler 21. The control valve 22 uses the temperature transmitter 23 to control the temperature in the process fluid in the pressure boosting device 3. An on-off valve 61 is arranged in the outlet line 60 of the second cooler 22. The outlet line 60 is connected to the main line 45 downstream of the on-off valve 51 in the main line 45 and upstream of the pressure boosting device 3.

[0084] A recirculation line 65 is connected to the outlet line 64 downstream of the pressure boosting device 3 and the main line 45. An operable pump recirculation valve 66 is arranged in the recirculation line 65 to control minimum flow of the boosting device 3 and minimum temperature in the preconditioning arrangement 2. The pump recirculation valve 66 is connected to temperature transmitter 23 measuring temperature of the process fluid downstream of the first cooler 20 via control lines 69. The pump recirculation valve 66 is controlled by the temperature transmitter 23. If the temperature of the process fluid downstream of the first cooler 20 is low (e.g. due to reduced flow from the well) with the risk of hydrate formation in the cooler(s) 20, 21, the pump recirculation valve 66 opens thereby recirculating process fluid which has been pressurized by the pressure boosting device 3 into the preconditioning arrangement 2. As such, the risk of hydrate formation resulting from reduced flow, and thereby reduced temperature of the process fluid exiting the first cooler, is reduced. I.e. the recirculation loop 65 may be necessary if the process fluid has not reached satisfying temperature at the outlet of the first cooler 20.

[0085] FIG. 1B is an example of a cooler system 4 forming part of the subsea system. the cooler system 4 comprises a connection to a process fluid line 45 or a branch line 50 as disclosed in the subsea system 1 in FIG. 1A. In the cooler system 4 of FIG. 1B a first and second cooler 20, 21 are arranged in series where a passive cooler system is actively controlled by the pneumatically operated valve 22 which can be adjusted in order to adjust the amount of process fluid flowing through the bypass line 58.

[0086] The operational conditions of the disclosed cooler system in terms of cooling capacity is as follows: [0087] 1) operated valve 22 closed: all process fluid flow through first and second coolers 20, 21=maximum cooling capacity, [0088] 2) operated valve 22 fully open and on-off valve 57 closed: all process fluid flows through the bypass line 58 and into the second cooler 21 only=minimum cooling capacity, [0089] 3) operated valve 22 partly open: some process fluid flows through the bypass line 58=medium cooling capacity.

[0090] The amount of process fluid is thus dependent on the active control of the operated valve 22 and how much of the process fluid which flows through the bypass line 58.

[0091] A chemical injection line 68 is connected to the process fluid line 45 upstream of the first cooler 20. Alternatively, the chemical injection line 68 could be connected downstream of the first cooler 20 but upstream of the second cooler 21.

[0092] Fluid exiting the second cooler 21 is typically directed to or towards the pressure boosting device 3 (as shown in FIG. 1A).

[0093] FIG. 2A is an example of a subsea system 1 connected to a well 5, where the subsea system 1 is arranged on a seabed 7 and comprises a subsea tree 6, a preconditioning arrangement 2 and a pressure boosting device 3. The components of the subsea system 1 are fluidly connected to each other via a process fluid line/main line 45.

[0094] FIG. 2B is an example of a subsea system 1 connected to a well 5, wherein the subsea system 1 is arranged on a seabed 7 and comprises a subsea tree 6, a separation device 8, a preconditioning arrangement 2 and a pressure boosting device 3. The separation device 8 serves to separate the process fluid before entering the preconditioning arrangement 2. The components of the subsea system 1 are fluidly connected to each other via a process fluid line/main line 45.

[0095] FIG. 3A shows an example of a single cooler. In the exemplified cooler, the cooler is arranged in a subsea environment. The well flow, i.e. process fluid flow, enters the cooler coil 10 in the upper part. The inflow direction is shown by arrow A. The well flow exits the cooler in a lower part. The outflow direction out of the coil 10 in the cooler is shown by arrow B. Preferably, seawater enters from beneath the cooler (shown by arrow C in the figure) and escapes through the upper part of the cooler, shown by arrow D. On the upper end of the cooler it is arranged a first perforated plate 11 and second perforated plate 13, with perforations 12. The second perforated plate 13 is connected to the walls of the cooler. The first perforated plate 11 is movable and arranged in a parallel plane relative the second perforated plate 13. The movement of the first perforated plate 11 is for example conducted by means of an actuator 14, which actuator 14 is typically of a mechanical, electrical type etc. By arranging the first perforated 11 plate movable relatively the second perforated plate 13, it is possible to adjust the flow of seawater through the cooler, as the cooling of the well flow is driven by natural convection. The well flow, having a high temperature, enters the coil 10 in the cooler at arrow A and is heat-exchanged with seawater that has already been heated by the well flow in the lower part of the cooler. Therefore, the well flow experiences a graduated cooling, i.e. first it is exposed to heated seawater, then it is exposed to cold seawater. The heated seawater will move within the cooler, in this case it arises. Due to the convection, the heated seawater travels to a relatively colder area.

[0096] FIG. 3B shows a top view of an example of the configuration of the first perforated plate 11 being provided with perforations 12. A movement of the first perforated plate 11 relative the second perforated plate 13, controls the flow area through the perforations of the first and second perforated plates, i.e. the convective flow rate, of seawater flowing through the cooler.

[0097] FIG. 4 shows an example of a cooling system to be used with the invention, and in particular shows the cooler system as disclosed in FIG. 4 in WO 2013/174584. The well flow enters the cooler system through inlet pipe 45. The flow direction is shown by arrow A. The flow exits the cooler system through outlet pipe 46. The flow direction is shown by arrow B. In the figure it is shown five branches 30, 31, 32, 33, 34, where the branches are all arranged in parallel with each other. At the inlet of each of the connection series 30, 31, 32, 33, 34 it is arranged a flow control device 36 controlling the inflow into each branch, and into each cooler. The flow control device 36 is typically a three-way valve or other means capable of directing a well flow. Additionally, sensors such as temperature sensor, flow sensors, pressure sensors may be used. The sensors can be arranged at different positions in the cooler system, e.g. one at each cooler, between the coolers, at the inlet of a cooler series or branches etc. Dependent on required cooling capacity, the flow control means 36, arranged at each inlet of a connection series, may direct the flow into one or more of the different series connections. In the exemplified embodiment, series connection 31 is the cooling series that has the largest cooling capacity of the shown series connections, while series connection 33 has the lowest cooling capacity if excluding series connection 34. Connection 34 is a bypass line, allowing the flow to flow through the cooler system bypassing all the coolers.

[0098] FIG. 5 shows a cooler system to be used with the subsea system, and particular shows the cooler system as disclosed in FIG. 5 in WO 2013/174584. In connection with each cooler, it may be arranged a bypass circuit 37, 38 bypassing at least parts of a fluid flow if, for instance, the temperature is above a threshold value. The bypass circuit 37, 38 may be by the form of a one-way flow loop as disclosed by reference numeral 37 or a two-way flow loop as shown by reference numeral 38. The system may in addition include all the features of the embodiment disclosed in FIG. 4.

[0099] The cooler system provides large flexibility with regards to the cooling requirement. Being able to provide a cooler system having different cooling capacities dependent on the cooling need, is advantageous bearing in mind that the hydrate formation temperature and/or flow rates may vary during the lifetime of a field.

[0100] The invention is now explained with reference to non-limiting embodiments. However, a skilled person will understand that there may be made alterations and modifications to the embodiment that are within the scope of the invention as defined in the attached claims.

LIST OF REFERENCES

[0101]

TABLE-US-00001 1 Subsea system 2 Preconditioning arrangement 3 Pressure boosting device 4 Cooler system 5 well 6 Subsea tree 7 Seabed 8 Separation device 10 Cooler coil 11 First perforated plate 12 perforations 13 Second perforated plate 14 actuator 20 First cooler 21 Second cooler 22 operated valve/on-off valve 23 Temperature transmitter 24 Second bypass line 30, 31, 32, branches 33, 34 36 Flow control device 37, 38 Bypass circuit 45 Inlet pipe/process fluid line/main line 46 Outlet pipe 50 Branch line 51 Pressure control valve (main line)/on-off valve 53 Pressure transmitter (main line) 54 Temperature transmitter (main line) 55 Flow transmitter 56 First pressure control valve (branch line)/on-off valve 57 Second pressure control valve (branch line)/on-off valve 58 First bypass line 59 Second bypass line 60 Outlet line 61 Pressure control valve (outlet line)/on-off valve 63 Check valve 64 Outlet line (pressure boosting device) 65 Recirculation line 66 pump recirculation valve 68 Chemical injection line 69 Control lines A, B, Direction of flow C, D