EXCHANGER

TECHNICAL FIELD OF THE INVENTION The present invention refers to a subsea heat exchanger arrangement comprising a heat dissipating device arranged submerged in seawater, wherein the seawater is set in motion to enhance heat transfer by forced convection. The present invention further refers to a method for supplying forced convection to a heat dissipating device arranged for transfer of heat from a hydrocarbon production process medium to the surrounding seawater.

BACKGROUND AND PRIOR ART

In the offshore production of hydrocarbon products heat exchangers find different uses for cooling purposes or for the purpose of heat recovery. For example, in terms of facilitating recovery of hydrocarbon products and transport of production fluid heat exchangers can be used to regulate the temperature in recovered hydrocarbons. In terms of securing stable and efficient operation of subsea production equipment, such as pumps and compressors, heat exchangers can be used to regulate the temperature in lubrication and barrier fluids. For the purpose of explaining the invention, the different fluid media which can be involved in hydrocarbon production processes are commonly referred to as process media in this disclosure.

Transfer of heat from a process medium to the surrounding seawater can take place in heat exchanger equipment according to natural convection or forced convection principles. Forced convection heat exchangers for use subsea are previously known in the art. One example is found in WO 2008/147219 disclosing a subsea cooling unit wherein a number of convection coils are exposed to seawater inside of a funnel that is open to seawater in its top and bottom, having a fan or impeller in the bottom of the funnel driven to create a rising flow of seawater through the funnel. Other examples can be found in WO 2013/004277 and WO 2010/002272, both disclosing convection coils arranged within a sealed enclosure through which seawater can be forced to pass by means of pumps.

SUMMARY OF THE INVENTION

In search for more efficient solutions the present invention provides an alternative to the known heat exchangers for use subsea.

An object of the present invention is thus to present a heat exchanger arrangement and a method for subsea use that supplies forced convection through a non-complex and reliable design, resulting in an inexpensive and yet efficient heat transfer from a process medium to the surrounding seawater. Other objects include reduced fouling of heat exchanger pipes through sediments and marine growth e.g., as well as reduced power requirement for supplying the forced convection.

At least a first object is met in a heat exchanger arrangement wherein a heat dissipating device is exposed to seawater within a tank, comprising a tank wall extending from a tank bottom region to a tank top open to seawater, an outlet for seawater arranged in the bottom region of the tank, a recirculation pipe system connecting the outlet with at least one inlet opening formed through the tank wall, and a pump arranged in the recirculation pipe system, wherein the at least one inlet opening for the recirculation pipe system is arranged substantially tangential with respect to the tank wall.

A heat exchanger arrangement thus configured permits the practice of a method for improving heat dissipating efficiency in a subsea heat exchanger, the method thus comprising: submerging a heat dissipating device in a defined volume of seawater; creating a downward motion in the defined seawater volume by extraction of seawater from a bottom region of the defined volume of seawater, and generating a vortex motion in the defined volume of seawater through recirculation of a portion of the extracted seawater. Accordingly, the vortex motion is generated by means of a pump applying tangential velocity to the recirculation water upon return to the defined seawater volume. By extraction of water from a bottom region of the tank, the pump simultaneously applies vertical velocity and downward motion to the volume of seawater in the tank. According to one embodiment of the heat exchanger arrangement, the recirculation pipe system can be arranged to open into the tank via two or more inlet openings equally spaced about the circumference of the tank. In this way, the tangential velocity can be maintained and a vortex motion be ensured for the full inner circumference of the tank. The recirculation pipe system may further be arranged to open into the tank via two or more inlet openings spaced at two or more levels between the top and the bottom region of the tank. The vortex motion can in this way be maintained or even accelerated towards the bottom region of the tank.

In the heat exchanger arrangement a first valve or flow orifice is integrated in the recirculation pipe system and effective for ejecting a portion of the recirculation water into the ambient seawater outside the tank. More precisely, the first valve or flow orifice may advantageously be arranged to eject an amount of 25-75 %, and at least an amount of 40-60 % or about 50 % of recirculation water into the ambient seawater outside the tank. The method analogously comprises a step of ejecting part of the recirculation water into the ambient sea, wherein preferably an amount of 25-75 %, and at least an amount of 40-60 % or about 50 % of the recirculation water is ejected while an equal amount of ambient seawater is allowed into the tank via the open top of the tank.

A second valve or flow orifice may be installed in the recirculation pipe system and arranged adjustable with respect to the first valve or orifice so as to set the amount of flow that is recirculated to the tank versus the amount of water that is ejected to the surrounding seawater. In one embodiment of the heat exchanger arrangement, the recirculation pipe system comprises a rising pipe section feeding one or more distributor pipes surrounding the tank at one or more levels.

In one embodiment of the heat exchanger arrangement the tank, or at least a lower portion of the tank is funnel-shaped, tapering towards the outlet in the bottom region of the tank.

In this embodiment, the method may include the step of accelerating the downward motion by forcing the seawater through a tapering/converging passage upon extraction.

The tapering portion of the tank wall may be formed as a straight cone, or may in other embodiments be of parabolic shape, or convex or concave.

In one embodiment the outlet from the tank comprises a perforated pipe section that rises upwards from the tank bottom towards the heat dissipating device, in a central region of the tank.

The heat dissipating device can comprise a coiled or a serpentine shaped pipe or series of pipes, as conventional.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be further explained below with reference made to the accompanying drawings, which are schematic and not to scale, and wherein

Fig. 1 is a top view of a heat exchanger arrangement according to the invention,

Fig. 2 is a side view showing an embodiment of the heat exchanger arrangement illustrated in Fig. 1,

Fig. 3 is a simplified schematic view showing an alternative embodiment of the heat exchanger arrangement, Fig. 4 is a corresponding simplified schematic view showing an alternative embodiment of the heat exchanger arrangement, and

Fig. 5 is a corresponding simplified schematic view showing yet an alternative embodiment of the heat exchanger arrangement. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the drawings a subsea heat exchanger arrangement comprises a heat dissipating device 1 submerged in a volume of seawater defined and separated from the ambient sea through the walls and bottom of a tank 2.

The heat dissipating device 1 can be a pack of coiled or serpentine shaped pipes having an inlet 3 and outlet 4 for a hot medium M which is passed through the pipes of the heat dissipating device for cooling by transfer of heat to the seawater in the tank.

The tank 2, which is circular in section, has a tank top 5 open to seawater above the tank. The tank top is defined by an upper rim 5 of a surrounding tank wall 6 that rises from a bottom region 7 to the tank top. In the bottom region of the tank an outlet 8 is formed via which seawater can be extracted from the tank by means of a pump 9, connected to the outlet 8.

The pump 9 is installed in a seawater recirculation pipe system 10 feeding extracted seawater to a distributor pipe 11. More precisely, several distributor pipes 11, 11 ', 11 " etc. arranged at different levels about the tank 2 can this way be supplied extraction water via a rising pipe section 12 forming part of the recirculation pipe system. Each distributor pipe 11 is connected to the seawater volume in the tank via one or more inlet pipes 13, 13', 13" etc., injecting flow through the tank wall 6 via corresponding inlet openings 14, 14', 14" etc. The inlet pipes 13 extend at an angle relative to the tank wall 6 and relative to a radius line from the center of the tank to the inlet opening 14. More precisely, the inlet pipes 13 open substantially tangential with respect to the tank wall. As used herein, the expression "substantially tangential" should be understood as referring to a direction which is in parallel or almost in parallel with a true tangent or an almost true tangent to the circular periphery of the tank.

In result, upon injection into the tank via the inlet openings 14, the recirculation water is introduced in the circumferential direction of the tank, thus generating tangential velocity and vortex motion to a peripheral portion of the volume of seawater in the tank (see the vortex arrows V in Fig. 1). The vortex motion successively propagates into the more central portions of the seawater volume to pass the pipes of the heat dissipating device 1 before entering the outlet 8. A portion of the extraction water, being slightly warmer than the ambient seawater from absorbing heat that is conducted through the pipe walls of the heat dissipating device, is ejected to the ambient sea via a first valve or flow orifice 15 installed in the recirculation pipe system 10.

A second valve or flow orifice 16 is installed in the recirculation pipe system as shown in the figures. The second valve or flow orifice 16 is adjustable with respect to the first valve or flow orifice 15 so as to set the amount of flow that is recirculated to the tank versus the amount of water that is ejected to the surrounding seawater.

An amount of the water that is extracted from the tank is in this way ejected to the ambient sea via the first valve or flow orifice 15. The valve or flow orifice 15 may be of fixed or adjustable orifice diameter and dimensioned or controlled to eject from about 25 % to about 75 % of the extracted water to the ambient sea. Preferably at least 40-60 % or about 50 % of the recirculation water is ejected.

In any event, the net volume of recirculated water is less than the volume extracted from the tank, thus making room for an amount of fresh seawater to be ingested via the open top of the tank.

In all cases the extraction of water from the tank to the recirculation system creates a downward motion in the seawater contained in the tank. An accelerated downward motion in the volume of seawater contained in the tank can be achieved if the tank wall or at least a lower portion of the tank wall, or even the bottom of the tank, is shaped like a funnel tapering towards the outlet 8. A tapering tank bottom can be realized as a straight cone as shown in the embodiment of Fig. 2. A tapering tank bottom may alternatively be designed with a convex or parabolic shape, or concave shape as shown in the embodiment of Fig. 3, which otherwise corresponds to the heat arrangement of Fig. 2, except for the installation of the recirculation pipe system 10. More precisely, in the embodiment of Fig. 3 the distributor pipes 11 are integrated in the tank wall whereby the inlet pipes of the previous embodiment can be omitted by forming the inlet openings 14 as tangentially directed mouths or nozzles that open directly in the wall of the distributor pipes.

Another embodiment of the heat exchanger arrangement is shown in Fig. 4. The embodiment of Fig. 4 differs from the previous embodiments with respect to the structure of the outlet 8. More precisely, in the embodiment of Fig. 4 the outlet comprises a pipe section 17 rising upwards towards the heat dissipating device 1 in a central region of the tank. A plurality of holes 18 form a perforated pipe wall via which seawater can be extracted from a range of levels below the top of the tank. In embodiments of the invention, the perforated pipe section 17 may be extended into the heat dissipating device, as illustrated. Still another embodiment of the heat exchanger arrangement is shown in the simplified schematic view of Fig. 5. The embodiment of Fig. 5 corresponds to the previous ones except for the shape of the tank 2, which in this case is shaped as a funnel in its entirety. More precisely, the tank wall 6 is continuously tapering from the rim 5 at the open top of the tank to the bottom region 7, from where the outlet 8 leads to the recirculation pipe system 10. In the embodiment of Fig. 5 the tank wall is arcuate or convex or parabolic in section. As water moves downward inside the tank its converging shape accelerates this movement. Also, losses due to recirculation pattern of water at bottom corner zones can be avoided due to the smoothly converging shape. It should be noted that the purpose with extraction and recirculation of water is to generate tangential velocity and vortex formation about heat exchanger pipes submerged in a defined and limited seawater volume, this way promoting forced convection and enhanced transfer of heat and improved heat exchanger efficiency. The limited volume of seawater about the heat exchanger pipes leads to reduced requirements with respect to pipe diameters and pump rating versus that required to maintain the same seawater velocity in e.g. an open arrangement (i.e. without an enclosing tank).

For purpose of illustration it can be noticed, as learned through calculation and tests, that a pump size of about 100-125 kW can be operated to generate a nozzle velocity of about 10-12 m/s at discharge from each of nine (3x3) 50 mm diameter inlet pipes arranged at a tank having a radius of about 1000-1250 mm. In this setup of the heat exchanger arrangement, the diameter of the outlet from the tank can be in the order of 500 mm, the mass flow through the tank can be in the order of 800-900 kg/s, and the flow rate at exit of the nine inlet pipes can be about 40 1/s, approximately. Naturally, these figures are inserted here as non-limiting examples.

It is commonly known that transfer of heat via the walls of the pipes in a heat exchanger which is submerged in seawater is proportional to the rate of the continuous supply of cold seawater over the heat exchanger pipes and the level of disturbance/turbulence that is created in the water upon passage of the heat exchanger pipes. It will further be appreciated that the tangential velocity and vortex motion applied to the seawater volume in the tank open to seawater additionally prevents fouling of heat exchanger pipes in result of the turbulence that is created in the water around the pipes. It is believed that these conditions are highly satisfied in all disclosed embodiments of the invention as defined by the features and limitations of appending claims.