GB2517271A

GB2517271A - Subsea heat exchanger

Info

Publication number: GB2517271A
Application number: GB1410666.0A
Authority: GB
Inventors: Ashish Jain; Espen Hauge; Claudia Martins Da Silva
Original assignee: Vetco Gray Scandinavia AS
Current assignee: Vetco Gray Scandinavia AS
Priority date: 2013-06-18
Filing date: 2014-06-16
Publication date: 2015-02-18
Also published as: NO336863B1; US20140367067A1; GB201410666D0; NO20130852A1; AU2014203171A1; BR102014015502A2

Abstract

A subsea heat exchanger comprising a bundle 1 of tubes having at least one tube winding 2 arranged to operate submerged in water and effective for guiding a fluid F to be cooled by surrounding water in contact with the tube. A plurality of nozzles 3 is distributed in spatial relation to the perimeter O, wherein the nozzles are effective for discharge of jets J of water impinging on the tubes. The nozzles are oriented to induce in the ambient water volume a displacement R that passes the perimeter O at a plurality of locations and directions. The nozzles may be supported by riser pipes 4 extending from a manifold 5, the nozzle discharge is controllable through a variable speed motor (VSD) motor 6, a seawater pump 7 and a pressure regulating device 11. The nozzles may be operable in consecutive order, and are ejectors operating in accordance with Bernoullis principle.

Description

Subsea heat exchanger

TECHNICAL FIELD

The present invention relates to forced convection heat exchangers for subsea use.

BACKGROUND AND PRIOR ART

In the recovery and production of gas and oil from subsea wells heat exchangers are frequently required, e.g. to control the temperature in the production fluid or in cooling media used in production equipment.

Subsea heat exchangers are often based on natural convection to seawater, and may he rated as passive or forced convection coolers. Basically, a passive convection cooler comprises a series of tubes which are exposed to seawater that is allowed to circulate freely between the tubes. Passive convection coolers are usually bulky and heavy structures and suffer from uncontrollable operation parameters such as variations in seawater currents and temperature, resulting in little or no control over the cooling process.

A forced convection cooler typically comprises a bundle of tubes enclosed by an outer shell or duct which is associated with a driven pump or propeller that generates a forced flow of water/seawater through the duct. Examples of forced convection coolers may be found e.g. in W02010/002272A1, WO2012/141599A1 or W02013/004277A1. Ducted, forced convection coolers provide enhanced control over the temperature of the target fluid but may still suffer from biological fouling and deposition of material on heat exchanger tubes.

SUMMARY OF THE INVENTION

The present invention aims at providing a forced convection heat exchanger with improved control over the cooling process.

Another aim of the present invention is to provide a forced convection heat exchanger which is less susceptible of biological or particulate fouling of heat exchanger tubes.

Yet another aim of the present invention is to provide efficient cooling by forced convection in a heat exchanger having compact design.

In a heat exchanger according to the present invention, a turbulent flow of water is created across the heat exchanger tubes by means of a plurality of nozzles arranged to discharge jets of water towards the tubes.

More precisely, a subsea heat exchanger is provided comprising a bundle of tubes, comprising at least one tube winding arranged to operate submerged in water and effective for guiding a fluid to be cooled by surrounding water in contact with the tube, the bundle of tubes having a longitudinal extension and a perimeter, and a plurality of nozzles distributed in spatial relation to the perimeter, wherein the nozzles are effective for discharge of jets of water impinging on the tubes, the nozzles oriented to induce in the ambient water volume a displacement that passes the perimeter at a plurality of locations and directions.

The nozzles may be configured to generate turbulent displacement of water near the tubes.

A turbulent flow can be achieved when, as preferred, nozzles are arranged at an angle in a plane transversally to a longitudinal extension of the tube bundle to discharge jets of water at a direction from about tangential to a 900 angle of impact with the perimeter of the bundle of tubes.

In this respect, the invention can be realized in different embodiments and configurations with respect to the disposition of nozzles in relation to the bundle of tubes.

In one embodiment the nozzles are arranged radially outside the bundle of tubes.

More precisely, the nozzles can be arranged radially outside the circular perimeter of a helical bundle of tubes, the nozzles mouthing inwards towards a center of the bundle of tubes. In this embodiment the nozzles may be directed to generate, in seawater surrounding the heat exchanger, an inwardly directed displacement which ranges from substantially tangential to the perimeter or substantially radial with respect to the center of the helical bundle of tubes, as seen in a radial plane of the heat exchanger.

In another embodiment the nozzles are arranged inside the bundle of tubes. More precisely, the nozzles can be arranged radially inside a helical bundle of tubes, the nozzles mouthing outwards towards a perimeter of the bundle of tubes. In this embodiment the nozzles may be directed to generate, in seawater within the helical tube bundle, an outwardly directed displacement which may be radial or non-radial with respect to the radial plane of the bundle of tubes.

The angled orientation of nozzles may be utilized to create a rotating movement and displacement of ambient seawater in and about the bundle of tubes.

In each of the above embodiments the nozzles may further be arranged at an inclination in order to generate or support a rising displacement of seawater through the heat exchanger. Thus, alternatively or in addition to the angled orientation of nozzles in radial planes, the nozzles may be arranged at an inclination to the longitudinal axis, in axial plane, to discharge jets of seawater at from about 300 to 900 angle of impact with the perimeter of the bundle of tubes.

The nozzles may be arranged in sets, wherein the nozzles are commonly supplied with seawater from a subsea motor and pump assembly which discharges seawater at elevated pressure and flow rate into a manifold. In preferred embodiments, a manifold supplies seawater to a number of riser pipes extending from the manifold in the longitudinal direction of the bundle of tubes, each riser pipe carrying a set of nozzles, respectively.

Seawater may be supplied to the nozzles by means of a subsea motor and pump assembly. By regulating the output from the motor and pump assembly and/or shutting on/off nozzles by means of valves, an active control of temperature in the target fluid is obtainable. To this purpose, a variable speed drive (VSD)-motor driving the seawater pump permits common control of the nozzles.

The nozzles may alternatively be controllable in common through a pressure regulating device in the water distribution manifold.

A set of nozzles may additionally be controllable separately from other sets of nozzles. For instance, the sets of nozzles may be controlled for intermittent discharge ofjets of water at an alternating schedule and in consecutive order. This embodiment effectively reduces the capacity which is required by the motor and pump assembly.

In one embodiment pulsating jets of water from the nozzles are obtainable by the installation of a flow pulse generator upstream of the nozzles.

The nozzles may be realized as orifices formed on the riser pipes which are supplied seawater via the manifold.

The nozzles may alternatively he realized as Venturi tubes or ejectors operating in accordance with Bernoulli's principle.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be further explained below, by way of example only, with reference to the accompanying, schematic drawings. In the drawings, Fig. 1 is a side view showing a first embodiment of a heat exchanger according to the present invention, Fig. 2 is a top view of the heat exchanger of Fig. 1, Fig. 3 is a top view corresponding to Fig. 2, showing a second embodiment of the heat exchanger, Fig. 4 is a broken away detail on a larger scale showing nozzles arrangement in the heat exchanger, Fig. 5 is a corresponding detail showing alternative arrangement of nozzles in the heat exchanger, Fig. 6 is a side view showing another embodiment of the heat exchanger, and Fig. 7 is a top view showing yet another embodiment of the heat exchanger.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It serves to point out that while the invention is described herein with reference to a vertically oriented heat exchanger open to the ambient sea, the teachings presented may likewise be applied in submerged heat exchangers of horizontal or inclined orientations. Accordingly, any term used in the description to define spatial relations should be understood to include equivalent terms applicable to heat exchangers having main orientations other than the illustrated vertical one.

A forced convection heat exchanger 1 for subsea use utilizes a bundle of tubes, comprising at least one tube winding 2, through which a fluid stream F is advanced undergoing transfer of heat via the tube wall to surrounding seawater SW. The heat exchanger 1 thus operates submerged in seawater, as illustrated in Fig. 1.

The fluid stream F may be a hydrocarbon production fluid stream which requires cooling before reaching downstream located equipment such as pumps or compressors, etc. The heat exchanger 1 is however not limited to the cooling of production fluid but may likewise serve for cooling of other fluid involved in subsea hydrocarbon production, such as coolant, lubricant, or barrier fluid.

The heat exchanger 1 further comprises a plurality of nozzles 3 which are distributed in spatial relation to a perimeter 0 of the bundle of tubes 2. In this context, as used in the disclosure, spatial relation to the perimeter" should be understood to mean that the nozzles 3 are distributed along the perimeter 0, or at least along substantial portions of the perimeter, and externally or internally at a distance from the perimeter 0, as seen in a side view or in a top or bottom view of the heat exchanger.

The nozzles 3 are effective for discharge of jets of seawater towards the bundle of tubes 2. The nozzles 3 are arranged on riser pipes 4, such that each riser pipe 4 carries a set of nozzles 3. The riser pipes 4 are supplied with seawater via a manifold 5 which distributes seawater at elevated pressure and flow rate generated by a subsea motor 6 and pump 7.

The operation of the motor and pump assembly 6, 7 may be controlled via a variable speed drive (VSD) 8 and a heat exchanger control unit (HXC) 9 which adjusts the supply of seawater to the nozzles 3 in response to the temperature of the target fluid F, detected by a temperature sensor (TS) 10. The HXC and VSD this way adjust the operation of the nozzles in common, in response to a required cooling effect and reduction of temperature in the target fluid F. The operation of the nozzles 3 may additionally or alternatively be controlled through valves 1 1 arranged to permit or cut the flow of water through the riser pipes 4, thus controlling the operation of a set of nozzles 3 separately from the other sets of nozzles.

The valves 11 can be realized as on/off valves, and controlled from the HXC. By opening the valves 11 one at a time in consecutive order, e.g., a pulsating discharge ofjets towards the bundle of tubes 2 is obtainable. This arrangement also reduces considerably the capacity required in the motor and pump which are installed to supply seawater to the nozzles. Pulsating jets may alternatively be generated by means of a pulse generator installed in the supply of seawater upstream of the nozzles (not shown in the drawings).

Operational control of the plurality of nozzles may alternatively be achieved from a pressure regulating device 11' arranged to adjust the flow in the seawater distributing manifold 5, as illustrated in Fig. 1.

In a structurally non-complex way the nozzles may be realized as orifices made through the wall of the riser pipes.

More efficient jets of water may be generated from nozzles made in the form of Venturi tubes or ejectors that operate in accordance with the well-known Bernoulli's principle. A corresponding ejector is shown in Fig. 4, this ejector comprising a nozzle 3 which is installed in a Venturi tube 12 having a diffuser section 13 of increased radius. The nozzle 3 communicates with the riser pipe 4 via a passage 14 through the wall of the riser pipe. A high velocity jet J is discharged from the convergent mouth of the nozzle 3, creating within the tube 12 a low pressure zone that draws in seawater via an open inlet to the tube. The entrained seawater is mixed with the jet in the tube, the mixed flows then being discharged from the discharge end facing the bundle of tubes 2.

The nozzles 3 are oriented for discharge of jets of water that impinge on the heat exchanger tubes 2, inducing in the ambient volume of water a flow or displacement R that passes the perimeter 0 of the heat exchanger tubes 2. The jets of water are split by the tubes and rejoined in zones of turbulent water T on the lee-side of the tubes, substantially as illustrated schematically in Fig. 4. The impinging jets on the windward side of the tubes and the turbulent flow on the lee-side of the tubes both contribute to reduce fouling, such as fouling in the form of particle deposition, scaling and biological growth on the heat exchanger tubes.

Whereas the partial view of Fig. 4 illustrates nozzles 3 which are oriented transversally or at right angles to the perimeter 0, the partial view of Fig. 5 illustrates nozzles which are angled relative to the perimeter, and more precisely, which are inclined upwards at an angle a relative to the perimeter 0. The inclined orientation of nozzles 3 may be used to enhance a rising displacement U of seawater through the bundle of tubes 2, adding to the natural upwards displacement in the form of convection currents due to heat which is absorbed by the ambient water. It is conceived that the angle a may vary from about 300 to 90° in practice.

Returning to the embodiment depicted in Figs. 1 and 2, a heat exchanger design is illustrated wherein a plurality of nozzles 3 is arranged along the circular perimeter of a helical bundle of tubes 2, and radially outside the same. More precisely, a number of riser pipes 4 is distributed about the bundle of tubes, the pipes 4 rising from a circular manifold 5 connecting the riser pipes with the motor and pump assembly 6, 7 (shown in Fig. 1 only). Preferably, the riser pipes may be equally angularly spaced about the bundle of tubes 2 as illustrated.

Each riser pipe 4 carries a set of nozzles 31 to 36 Whereas the nozzles 31 -33 on the left hand side of the drawing in Fig. 2 are oriented in radial directions towards the center C of the tube bundle, the nozzles 34 -36 on the right hand side of the drawing are oriented substantially in tangential directions relative to the perimeter of the bundle of tubes 2. Arranging the nozzles at different orientations in the same heat exchanger as illustrated in Fig. 2 is possible. This option is shown herein for illustrative purposes, and it is assumed that a more frequent practice will involve nozzles which are equally oriented with respect to the perimeter or to the center of the heat exchanger. However, different combinations of number of nozzles and orientations may be conceivable.

It will be appreciated that the external location of the nozzles 3' -33 in Fig. 2 will induce, in the ambient water volume, a displacement wherein the main component of direction is radial towards the center of the bundle of tubes 2. It is likewise appreciable that the external location of the tangentially oriented nozzles 34 -36 in Fig. 2 will induce, in the ambient water volume, a displacement comprising a tangential component of direction, which is deflected towards the interior of the heat exchanger by the jet discharged from the adjacent downstream nozzle. In both cases the externally located and inwardly or substantially tangentially opening nozzles 31 to 36 will generate a displacement R of ambient water which passes the perimeter 0 of the tube bundle at a plurality of locations and directions, causing turbulence near the tubes. The displacement of water from the exterior to the interior of the bundle of tubes will further cause an axial, upward displacement and replacement of the volume of water that is surrounded by the tube winding 2. In addition the non-radially or substantially tangentially oriented nozzles 34 -36 will generate a rotating displacement of water surrounding the heat exchanger tubes 2 (i.e., a clockwise rotation according to the setup of Fig. 2).

An inverted design of the heat exchanger is illustrated in Fig. 3. The embodiment of Fig. 3 differs from the previous one in that the riser pipes 4 and nozzles 3 are located along a circular perimeter of a helical bundle of tubes 2 and radially inside the same, the nozzles opening outwardly towards the perimeter of the bundle of tubes. Whereas the nozzles 31 -33 on the right hand side of the drawing are oriented substantially in radial directions from the center C, the nozzles 34 -36 on the left hand side of the drawing are oriented in non-radial directions relative to the center C. Again, arranging the nozzles at different orientations in the same heat exchanger as illustrated in Fig. 3 is possible. This option is shown herein for illustrative purposes, and it is assumed that a more frequent practice will involve nozzles which are equally oriented with respect to the perimeter or to the center of the heat exchanger. However, different combinations of number of nozzles and orientations may be conceivable.

It will be appreciated that the internal location of the nozzles 31 -33 in Fig. 3 will induce, in the ambient water volume surrounded by the tube winding 2, a displacement wherein the main component of direction is radially outwards with respect to the center C. It is likewise appreciable that the internal location of the nozzles 34 -36 in Fig. 3 will induce, in the ambient water volume, a displacement comprising a tangential component of direction.

In both cases the internally located and outwardly, radially or non-radially opening nozzles 31 to 36 will generate in the ambient water volume a displacement R which passes the perimeter 0 at a plurality of locations and directions, causing turbulence near the tubes. The displacement of water from the interior to the exterior of the bundle of tubes will further cause a replacement from below of the volume of water that is displaced from inside of the tube winding 2. In addition, the non-radially oriented nozzles 34 -36 may generate a rotating displacement of water surrounding the heat exchanger tubes 2 (i.e., an anti-clockwise rotation according to the setup of Fig. 3).

The invention is not limited to any specific number of riser pipes 4, number of nozzles 3 or number of turns in a tube winding 2. Other designs beside the circular configuration illustrated in Figs. 1-5 are also possible.

An alternative design is illustrated in Fig. 6, showing in a side view a heat exchanger comprising a flat or substantially flat serpentine tube winding 2. A plurality of nozzles 3 is distributed along major portions of a rectangular perimeter O and externally thereto, the nozzles effecting in operation a displacement R of ambient water which passes the perimeter 0 at a plurality of locations and directions, causing turbulence near the tubes 2.

Yet another alternative design is illustrated in Fig. 7, showing in a top view the uppermost tube layer of a heat exchanger comprising a bundle of tubes including several flattened or substantially flat helical or serpentine tube windings 2. A plurality of nozzles 3 is distributed along major portions of a rectangular perimeter O and externally thereto, the nozzles being arranged in sets on pipes 4 that rise from a manifold 5. It shall be noted that in the top view of Fig. 7 only the uppermost nozzle 3 in each set of nozzles is shown.

Other nozzle designs than the illustrated Venturi tube are feasible, such as a fluidic nozzle designed to generate a self-oscillating jet. Self-oscillating jet nozzles involve no moving parts and require basically no maintenance, which makes them attractive for subsea use and for implementation in the forced convection heat exchanger. The use of nozzles that create self-oscillating jets results in a higher heat transfer coefficient and a bigger coverage area, thus fewer nozzles are required compared to standard jet nozzles.

Another group of nozzles, which can be contemplated for use in submerged forced convection heat exchangers, are the nozzles designed to create synthetic jets formed by ambient water through the periodic suction and ejection of fluid out of an orifice to a cavity. A diaphragm which is built into a wall of the cavity is brought into a time periodic motion, e.g. driven by a piezoelectric generator or an electromagnetically driven piston.

Based on the teachings provided herein, other modifications will be possible without departing from the scope of the invention as defined by the accompanying claims.

Claims

CLAIMS1. A subsea heat exchanger comprising: -a bundle (1) of tubes, comprising at least one tube winding (2) arranged to operate submerged in water and effective for guiding a fluid (F) to be cooled by surrounding water in contact with the tube, the bundle of tubes having a longitudinal extension (L) and a perimeter (0), -a plurality of nozzles (3) distributed in spatial relation to the perimeter (0), wherein the nozzles are effective for discharge of jets (j) of water impinging on the tubes, the nozzles oriented to induce in the ambient water volume a displacement (R) that passes the perimeter (0) at a plurality of locations and directions.
2. The heat exchanger of claim 1, wherein the nozzles (3) are configured to generate a turbulent displacement (R) of water near the tubes (2).
3. The heat exchanger of any previous claim, wherein the nozzles (3) are arranged in a plane transversally to the longitudinal extension (L) to discharge jets of water (J) at a direction from about tangential to a 9Ø0 angle of impact with the perimeter (0) of the bundle of tubes.
4. The heat exchanger of claims 1-3, wherein the nozzles (3) are arranged at an inclination in a plane parallel to the longitudinal extension (L) to discharge jets of water (J) at from about 3Ø0 to 90° angle of impact with the perimeter (0) of the bundle of tubes.
5. The heat exchanger of any previous claim, wherein the nozzles (3) are arranged radially outside the perimeter (0) of a helical bundle of tubes, the nozzles mouthing inwards towards a center (C) of the bundle of tubes.
6. The heat exchanger of any of claims 1-5, wherein the nozzles (3) are arranged radially inside a helical bundle of tubes, the nozzles mouthing outwards towards the perimeter (0) of the bundle of tubes.
7. The heat exchanger of any previous claim, further comprising a manifold (5) and a number of riser pipes (4) extending from the manifold in the longitudinal direction (L) of the bundle of tubes, each riser pipe supporting a set of nozzles (3).
8. The heat exchanger of any previous claim, wherein the nozzles (3) are controllable in common by a VSD-motor (6) driving a seawater pump (7).
9. The heat exchanger of any previous claim, wherein the nozzles (3) are controllable in common through a pressure regulating device (1 1') in the water distribution manifold (5).
10. The heat exchanger of any previous claim, wherein a set of nozzles (3) is controllable separately from other sets of nozzles.
1 1. The heat exchanger of claim 9, wherein sets of nozzles are alternatingly operable in consecutive order.
12. The heat exchanger of any previous claim, wherein the nozzles (3) are ejectors (12) operating in accordance with Bernoulli's principle.
13. The heat exchanger of any previous claim, wherein a flow pulse generator is installed upstream of the nozzles.
14. The heat exchanger of claim 1 comprising: -a vertically oriented, non-ducted, spiral wound heat exchanger tubing open to surrounding seawater; -a circular manifold arranged in the lower end of the heat exchanger, the manifold supplied with seawater from a motor and pump assembly; -a number of riser pipes rising vertically from the circular manifold, angularly spaced about the spiral wound heat exchanger tubing, and -a set of nozzles on each riser pipe, the nozzles mouthing radially inwards towards the center of the heat exchanger tubing.
15. The heat exchanger of claim 1 comprising: -a vertically oriented, non-ducted, spiral wound heat exchanger tubing open to surrounding seawater; -a circular manifold arranged in the lower end of the heat exchanger, the manifold supplied with seawater from a motor and pump assembly; -a number of riser pipes rising vertically from the circular manifold, angularly spaced inside the spiral wound heat exchanger tubing, and -a set of nozzles on each riser pipe, the nozzles mouthing radially outwards towards the periphery of the heat exchanger tubing.
16. A subsea heat exchanger, substantially as described herein with reference to Figures 1 and 2, or Figure 3, 4, 5, 6 or 7, of the accompanying drawings.