The present disclosure relates generally to subsea cooling of fluids. More particularly, the present disclosure relates cooling of fluids in a subsea environment using a heat exchanger with free convection on the ambient seawater side.
Subsea compression is a relatively recent technology developed to enhance the lifetime of existing fields. Installation of subsea compression systems enables balancing of the reservoir depletion over time. The production plateau is extended while also increasing total recovery of fields. These systems face substantial challenges both from a technical and commercial point of view. In the absence of any upstream processing on the well stream, the production fluid flow is multiphase containing mainly gas, but also condensate, water and sand. Additionally, the equipment faces various subsea constraints including availability, flexibility and robustness.
Subsea process cooling is an important aspect for subsea applications. Standard heat exchangers or coolers with forced convection on ambient side requires additional equipment, such as sea water circulation pumps, guiding piping, and/or shells to force the coolant fluid towards the cooling area. Circulation pumps rely on a power supply as well as control systems. Subsea constraints make reliability and operation of forced convection equipment both challenging and expensive.
A free convection velocity field generated by buoyancy force is usually much lower than velocity intensity that can be obtained in forced convection. Therefore, corresponding heat transfer is less efficient with free convection when compared to forced convection. Since the heat transfer performance is inherently lower, the exchange surface needs to be increased in order to maintain sufficiently high cooling.
According to some embodiments a heat exchanger system is described for transferring heat between a first fluid and a surrounding ambient second fluid. The system includes: an inlet configured to accept the first fluid; an outlet configured to expel the first fluid; a plurality of vertically oriented parallel conduits positioned between the inlet and the outlet configured to carry the first fluid therein, the conduits each having an exterior surface that is exposed to the surrounding second fluid when the system is submersed in the second fluid, wherein heat is transferred between the first fluid flowing through the conduits and the second fluid flowing a vertical direction along the exterior surfaces of and parallel to the conduits by free convection; and at least one deflector fixedly mounted exterior to the conduits configured to impart non-vertical momentum in the flowing second fluid thereby enhancing heat transfer between the first fluid and the second fluid.
According to some embodiments, the conduits are tubular pipes grouped into one or more groups, with the tubular pipes of each group being arranged symmetrically about a central axis of the group. The deflectors can be a number of horizontally oriented disks (such as two, three, four or five) for each group of tubular pipes. The group of pipes and the horizontally oriented disks can each have a large central opening configured to allow free passage of the second fluid therethrough, and the horizontally oriented disks can be configured to force the second fluid into and out of the central opening of the group of pipes, thereby enhancing heat transfer.
According to some embodiments, deflectors can be non-horizontally oriented structures that are asymmetric with respect to the central axis of the group. The structure can be configured to impart momentum in a tangential direction with respect to the central axis of the group. According to some embodiments, the non-horizontally oriented structures can be helical in shape.
According to some embodiments, the conduits can be tubular pipes arranged into a rectangular pattern of columns and rows. The deflectors can be one or more horizontally arranged baffles or, according to some embodiments, they can be non-horizontally arranged.
According to some embodiments, no powered equipment is used to force the second fluid to flow past the conduits. According to some embodiments, the coolant second fluid is seawater, and the first fluid includes hydrocarbon gas produced from one or more wellbores penetrating a subterranean rock formation.
According to some embodiments, a method is described for transferring heat between a first fluid and an ambient second fluid surrounding a plurality of conduits through which the first fluid flows. The method includes: exposing exterior surfaces of the plurality of conduits to the surrounding second fluid; and flowing the first fluid through the plurality of conduits, wherein the plurality of conduits are vertically oriented and parallel to each other, and heat is transferred between the first fluid and the second fluid flowing in a vertical direction along the exterior surfaces of an parallel to the conduits by free convection and wherein at least one stationary deflector is fixedly mounted exterior to the conduits configured to impart non-vertical momentum in the flowing second fluid thereby enhancing heat transfer between the first fluid and the second fluid. According to some embodiments, the flowing includes pumping the first fluid through the plurality of pipes. According to some embodiments, heat is transferred from the first fluid to the second fluid thereby cooling the first fluid. According to some other embodiments, heat is transferred from the second fluid to the first fluid thereby heating the first fluid.
According to some embodiments, a heat exchanger system is described that includes: an inlet and an outlet for a first fluid, and a heat exchanger between the inlet and the outlet where the first fluid circulates. The heat exchanger includes at least one deflector to guide the flow of an ambient second fluid. According to some embodiments, the deflector guides the flow of the second fluid to transfer the vertical momentum from the gravity induced free convection flow of the second fluid to horizontal velocity. According to some embodiments, the deflector is shaped depending upon the heat exchanger design configuration. In one example, the heat exchanger comprises tubular pipes and the at least one deflector includes shapes surrounding the pipes. According to some embodiments, the deflectors are shaped as a horizontal disk, staggered plates or helical screw-like shapes.
According to some other embodiments, a method is described for exchanging heat between a first and a second fluid using a free convection velocity field to create a form of non-powered “forced” convection in the heat exchanger without the use of a pump or other powered equipment.
According to some other embodiments, a method is described for exchanging heat between a first and a second fluid that includes: increasing flow turbulences of the second fluid around the heat exchanger wherein the first fluid is circulated. In one example, the method includes increasing the velocity field, the turbulence level and the flow mixing of the second fluid. In another aspect the method includes breaking the thermal layer of the second fluid by creating transverse flow and unsteady drag effects within the second fluid.
According to some embodiments, the method includes increasing the momentum of the ambient fluid around the heat exchanger. In one embodiment, increasing the momentum around the heat exchanger includes increasing the amount of the second fluid participating in.
According to some embodiments, the method includes breaking boundary layers of the second fluid so that layers of the second fluid remote from the heat exchanger are dragged towards the heat exchanger. It is known that free convection patterns generate a thermal boundary layer that tends to act as insulation which offsets the heat transfer increase due to the vertical momentum increase of the second fluid.
These together with other aspects, features, and advantages of the present disclosure, as well as the various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. The above aspects and advantages are neither exhaustive nor individually or jointly critical to the spirit or practice of the disclosure. Other aspects, features, and advantages of the present disclosure will become readily apparent to those skilled in the art from the following description of exemplary embodiments in combination with the accompanying drawings. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.
To assist those of ordinary skill in the relevant art in making and using the subject matter hereof, reference is made to the appended drawings, in which like reference numerals refer to similar elements:
It should be understood that the drawings are not to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details that are not necessary for an understanding of the disclosed method and apparatus, or that would render other details difficult to perceive may have been omitted. It should be understood that this disclosure is not limited to the particular embodiments illustrated herein.
Some embodiments will now be described with reference to the figures. Like elements in the various figures may be referenced with like numbers for consistency. In the following description, numerous details are set forth to provide an understanding of various embodiments and/or features. However, it will be understood by those skilled in the art that some embodiments may be practiced without many of these details and that numerous variations or modifications from the described embodiments are possible. As used here, the terms “above” and “below”, “up” and “down”, “upper” and “lower”, “upwardly” and “downwardly”, “upstream and downstream”, and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe certain embodiments. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship, as appropriate.
It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first object or step could be termed a second object or step, and, similarly, a second object or step could be termed a first object or step, without departing from the scope of the invention. The first object or step and the second object or step are both objects or steps, respectively, but they are not to be considered the same object or step.
The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” depending on the context. Similarly, the phrase “if it is determined” or “if [a stated condition or event] is detected” may be construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event],” depending on the context.
In the specification and appended claims, the terms/phrases “connect”, “connection”, “connected”, “in connection with”, and “connecting” are used to mean “in direct connection with” or “in connection with via one or more elements”, and the term “set” may mean “one element” or “more than one element”. Further, the terms “couple”, “coupling”, “coupled”, “coupled together”, and “coupled with” are used to mean “directly coupled together” or “coupled together via one or more elements”.
In the following detailed description of some embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized, and structural changes may be made without departing from the scope of the invention.
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Further, like reference numbers and designations in the various drawings indicate like elements.
The following abbreviations and relations shall be used herein:
Note that although many embodiments are described herein as being used in an exemplary application of subsea compression systems, the methods and structures described herein are equally applicable to many other types of heat exchangers. According to some embodiments, the heat exchanger system components described herein are used for nuclear power generation cooling (or heating) applications. According to some other embodiments, the heat exchanger components described herein are used for heating and/or cooling applications in chemical processing applications.
Heat exchanger design. The cooling principle for heat exchangers is to transfer heat from one fluid (the cooled fluid) to another (the coolant fluid). Heat exchangers are commonly designed using a forced convection heat transfer principle for both the cooled and the coolant fluids. This is due to the higher heat transfer rate that can be obtained using forced convection versus free convection. As used herein, the term “free convection” refers to a mechanism or type of heat transport in which the fluid motion is not generated by any external power source (e.g. pump, fan, suction device, etc.), but rather only by density differences in the fluid occurring due to temperature gradients. Note that although several embodiments are described herein with respect to the application of cooling, the methods and structures described herein are equally applicable to heating applications. In such cases the heat is transferred from the ambient fluid (the heating fluid) to the fluid flowing through the pipes (the heated fluid).
The subsea environment is complex and aggressive. For example, routine maintenance, inspection and cleaning possibilities are both limited and challenging. In the case of forced convection, difficulties are further increased by the nature and the multiplicity of the equipment (pumps for example) used to generate forced convection on the ambient side (the coolant fluid). Conventional forced-forced convection (i.e. both the cooled fluid and coolant fluid are pumped) heat exchanger technology is hence not well suited for subsea applications. It has been found that a passive design based on free convection on the ambient side (the coolant fluid) is often more appropriate to face to challenges of the subsea.
According to some embodiments, a heat exchanger system is designed both to give a large turndown on thermal performance for operation flexibility and, for subsea applications, to handle flow assurance issues like sand accumulation, hydrates formation, wax deposition, etc.
According to some embodiments, the free convection velocity field is used to create a form of non-powered “forced” convection on the ambient side (coolant fluid). According to some embodiments, of described heat exchange systems, the heat exchanger includes one or more external shapes to guide the coolant fluid flow and transfer the vertical momentum of the freely convecting coolant fluid from the gravity field to generate horizontal velocity. Various shapes can be used to both generate either radial velocity and circumferential velocity with respect to the longitudinal axis of the pipes.
In this particular example, group 210 is made up of 33 vertically oriented pipes 200 that are symmetrically arranged in two concentric rings about a central axis 310 such that there is a large central open space within group 210. As can be seen, each of the five horizontal baffles 230, 232, 234, 236 and 238 is a disk-shaped piece mounted horizontally (i.e. perpendicular to the vertically oriented pipes 200).
In order to evaluate the proposed heat exchanger system performance and to compare it with empirical results for simple geometries, the heat exchanger part of the heat exchanger system and a single pipe were run in parallel. The single pipe had the same characteristics (diameter, length, thickness, material . . . ) as the cooler pipes. The cooling pipe length was 4.3 meters. In an example test, a slipstream of dry nitrogen was provided from an existing compressor discharge and routed through the test pipes. Flow/pressure through the pipes was controlled by means of the chokes on the compressor outlet and control valves downstream test pipes. The test operating conditions, taken in this example test, are listed in Table 1 for a flow of process fluid to be cooled in a subsea environment:
The process flow was measured by means of Coriolis mass flow meters. Temperature was measured upstream and downstream the test objects. Pressure was measured upstream the test objects. Six individual temperature measurements were made in the seawater along the test objects to determine ambient temperature and to check any temperature layering in the pit. The head loss across the bundle of pipes (or group of pipes) was in addition measured using a differential pressure sensor.
Using pressure and temperature measurements at the test pipes inlet, the gas thermodynamic properties (density and heat capacity) are calculated and hence the amount of heat removed to the process fluid (the cooled fluid) passing through the pipes was obtained.
The thermal performances of the two test objects (the group of cooler pipes and the single cooler pipe) were characterized for different mass flow amounts. The global heat transfer coefficient U can be calculated according to the following equations.
The heat transfer is defined by:
Q=UAΔT
with A the object area and ΔT the temperature difference between the process gas and the ambient water.
The heat transfer Q is directly related to the heat removed to the gas:
Q={dot over (m)}
gas
Cp
gas(Toutlet−Tinlet)
Replacing the temperature difference between the process gas and the ambient water by the Log Mean Temperature Difference (LMTD) as the process gas is not constant all along the cooling pipes, the global heat transfer coefficient is calculated using the following formula:
It should be noted the global range from the tests is quite uncommon due to the wide dimensions and cooling capacities of the studied case (see Table 2: global results). The length scale, the temperature difference and the total cooling load are likely to be outside normal test conditions used to define the empirical correlations. Comparison of Nusselt number based on 3 different approaches which are experimental, analytical and numerical, for such as high Rayleigh number range up to 1013 makes this study very valuable.
The measured performance increase given in the
This relation is explained by the pipes interaction for the cooler group with the baffles. The heat transfer from the pipe's external surfaces to the ambient seawater is driven by free convection. An external flow is generated due to the density variation induced by the temperature increase caused by proximity to the pipes. In the case of the group of cooler pipes, the pipes' closeness causes the seawater flows to interact. The obtained momentum is thus higher than the one obtained with a single pipe. This phenomenon, called chimney effect, is described in further detail infra.
Simulations. The heat transfer from the process gas (the cooled fluid) to the ambient (coolant fluid) observed on the test objects can be decomposed into the 3 following features: (1) internal forced convection between the bulk gas and the internal surfaces of the pipes; (2) conduction across the walls of the pipes; and (3) external free convection between the external surfaces of the pipes and the ambient water.
Based on the physical mechanisms as split above, the global heat transfer coefficient can be defined by the following formula:
The thermal performance difference obtained between the SP (single pipe) and the cooler group of pipes is related to the free convection pattern and intensity on the ambient side. In order to characterize and to understand in detail the phenomena occurring and the complex three-dimensional (3D) flow that develops around the pipes, Computational Fluid Dynamics (CFD) simulations are performed on both the cooler group and the single pipe. The case with a mass flow of 0.22 kg/s has been studied using commercial CFD software.
Numerical method. The geometry simulated consists of three domains: (1) the gas flowing inside the pipes; (2) the solid pipe walls; and (3) the ambient side of the pipes.
The two flows (ambient and gas) are described by the Reynolds Averaged Navier Stokes (RANS) system coupled to the internal energy equation for the ambient side by the buoyancy force based on the Boussinesq assumption:
The momentum source is hence for the water domain:
S
M=−ρrefβ(T−Tref)g
Turbulence is solved using the Shear Stress Transport (SST) model. Both fluids, water and gas, are considered incompressible. In the example simulations, the solid domain material is stainless steel described by its thermal conductivity.
The boundary condition for the top and bottom faces of the ambient domain is set to opening. In the proposed example, the lateral face representing infinite is set to wall with free slip condition and imposed temperature to 12° C. For the gas domain, standard incompressible boundary conditions are used, i.e. mass flow imposed at the inlet (top) and static pressure imposed at the outlet (bottom). The inlet temperature is fixed to 90° C.
Single pipe simulation. As shown in Table 3, the heat transfer for the single pipe is very well simulated. The discrepancy between test data and simulation is only about 5%. The difference between test data and empirical correlation is about 10%, which is also acceptable regarding correlation accuracy. Comparison of the CFD and test results for the single pipe is a step that validates the numerical approach.
Cooler group simulation. Due to the compressed gas flowing inside the pipes, the external pipe wall was warm and thus a vertical flow in the ambient is generated due to free convection. The gas flow continuously delivers heat to the ambient side and the averaged gas temperature drops off from about 90° C. down to about 60° C.
8-1 and 8-2 illustrate flow patterns on the ambient (sea water) side of the cooling group of pipes having horizontal and helically-shaped baffles, according to some embodiments. In the case of the cooler group 210 with horizontal baffles shown in
It has been found that the baffles generate secondary flows of the ambient fluid around the baffles. The seawater is ejected from the area in the center of the group of pipes just below each baffle (positive radial velocity with respect to central axis 310) and then just above each baffle the water is routed back into the central area (negative radial velocity). This transverse flow generated firstly increases the momentum level and by consequence the heat removal is improved. A second aspect is that it increases the turbulence level creating some turbulent structures.
Test results and comparisons of the thermal performance between the single pipe and the cooler group highlighted an increase of the heat transfer for the cooler group. Due to the similarity of the two test objects and the physical mechanisms decomposition of heat transfer, the free convection on the ambient side has been identified as the key phenomena to explain test results deviation.
The CFD analyses performed on the two objects revealed a complex 3D flow development for the cooler. This 3D flow is responsible for the external HTC increase.
Based on the analysis described supra, a new empirical correlation can be described. According to some embodiments, the heat exchanger is a cooler group with a passive design. That is, no additional powered equipment is used to create forced flow on the ambient side. Nevertheless the interaction between the pipes and baffles is such that free convection flow generated by one pipe tends to act similarly to a forced convection flow source for the neighboring pipes. An expression of the following form can be used describe the mixed convection:
Numixedn=Nufreen±Nuforcedn
It is important to note that analysis and results are dependent of the configuration. Three special cases can be identified: (1) buoyancy induced flow and forced flow parallel with the same direction; (2) buoyancy induced flow and forced flow parallel in opposite directions; and (3) buoyancy induced flow and forced flow perpendicular, such as provided by several embodiments described herein.
When analyzing a new heat exchanger design, challenges can relate to the fact that there can be two different characteristic lengths representative of the combined mechanism. It hence makes it difficult to establish the combination of the characteristic dimensionless numbers for the free and the forced convections. The characteristic length to build the Nusselt number and quantify the free convection intensity is the total vertical pipe length (L) while the one representative of the forced convection in a staggered bank is the pipe diameter (D). For this reason, the formula proposed above is no longer well suited as the Nusselt magnitude based on different length scale strongly deviates.
According to some embodiments, the following formula can be used to correlate mixed convection heat transfer for the cooler external heat transfer:
The Nusselt number for a free convection vertical boundary layer development is:
NuL
The value of 0.31 is appropriate for configurations using two horizontal baffles. Other values, an be used for other baffle configurations such as 5 horizontal baffles and/or helical baffles. The Nusselt number for forced convection in staggered bank is:
Nu
D
=1.13C1C2ReDmPr1/3
where the constant has the following values according to the geometry configuration: C1=0.416, C2=0.75 and m=0.568.
Therefore, the external heat transfer for the cooler corresponds to that of the single pipe in addition to a component representing the pseudo forced convection in the vicinity of the baffle. According to the simulation analysis described supra, in the case of a two-horizontal baffle configuration, only one third of this component is included, as only one third of the total pipe area is affected by this horizontal flow pattern. Other amounts of the component should be included for other configurations and numbers of baffles.
The test results presented in the
Based on these analyses, a new formula for combined free convection and crossflow for a vertical pipe is proposed, supra. The overall heat transfer coefficient obtained from these correlations matches very well with the experimental data.
This described method enables optimization of heat exchanger design and thus improves the heat exchange performances and reducing the weight of the system for similar performances. According to some embodiments, the heat exchange system includes an inlet and an outlet for a first fluid, increases the velocity and the turbulence intensity of a second fluid flow in neighborhood of the heat exchanger with the first fluid. The heat exchange system enables an increase of the heat transfer with the ambient second fluid. According to some embodiments, the heat exchange surface area is substantially reduced for equivalent performance and thus resulting in a substantial decrease in cooling system footprint.
Forced convection causes, as mentioned, higher heat transfer rates than free convection. In some cases, currents and waves will make the cooler acting as a forced convection cooler. These heat transfer rate changes may be challenging when operating a system while trying to maintain a constant cooling performance. It has been found that according to some embodiments, the influence of sea currents and waves on the heat transfer performance can be decreased. The cooling performances will tend to be independent of the horizontal velocity field from the sea current. This benefit can be very useful on the system regulation point of view.
Although many embodiments have been described supra in the context of a heat exchanger system in which parallel pipes are arranged in symmetrical such as shown in
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Further, like reference numbers and designations in the various drawings indicated like elements.
Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.
Number | Date | Country | Kind |
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0905338.0 | Mar 2009 | GB | national |
This patent application claims the benefit of U.S. Prov. Ser. No. 61/705,368 filed Sep. 25, 2012. This application is a continuation-in-part of U.S. patent application Ser. No. 13/259,789 filed Dec. 6, 2011. Both of the above applications are incorporated by reference herein.
Number | Date | Country | |
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61705368 | Sep 2012 | US |
Number | Date | Country | |
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Parent | 13259789 | Dec 2011 | US |
Child | 14035868 | US |