WO2011161472A1

WO2011161472A1 - Passive thermal management system for liquid pipelines

Info

Publication number: WO2011161472A1
Application number: PCT/GB2011/051202
Authority: WO
Inventors: Kipp B. Carlisle; Gary Gladysz
Original assignee: Trellborg Offshore Uk Limited
Priority date: 2010-06-26
Filing date: 2011-06-24
Publication date: 2011-12-29

Abstract

A passive thermal management system especially adapted for pipelines that transport a liquid having a freezing point that is higher than the temperature of the sun-ounding environment. The system includes a phase-change layer that changes from solid state to a liquid state at a temperature intermediate the temperatures of the material freezing point and the environment. If flow is interrupted, heat transfers from the phase-change layer to the stagnant transported liquid to increase the time from the interruption until the stagnant transported material reaches its freezing temperature and thereafter solidifies.

Description

PASSIVE THERMAL MANAGEMENT SYSTEM FOR LIQUID

PI PELIN ES

Field of the Invention

The present invention relates to management of fluid temperature in a pipeline. It is particularly suited to use with pipelines located in a cold

environment. Specifically, but not exclusively, it is applicable to underwater pipelines for transporting a liquid, such as crude oil, from a source, such as a submerged wellhead.

In the following discussion a "pipeline" means an assemblage of components for conveying a liquid from a source to a destination through a series of devices including, but not limited to, pipes, couplings, valves and control devices. The purpose of such pipelines is to convey the liquid, such as crude oil, from a source device, such as an undersea wellhead, to a destination, such as a shore-based distribution center. The use of such pipelines for conveying crude oil introduces unique problems because it is imperative that the pipeline maintain the crude oil in a liquid state, if the temperature of the crude oil in the pipeline were to fall below a "hydrate formation temperature" of about 25°C, the crude oil could solidify and block flow.

More specifically and as known, typically crude oil emerges from an undersea wellhead in the range of 50°C to 90°C while the sea water temperature is significantly lower. At depths of thousands of feet the seawater temperature is about 0°C. During the dynamic conditions that exist during normal flow from the wellhead through the pipeline, the steady-state heat loss to the sea water is often insufficient to reduce the crude oil temperature to its hydrate formation temperature.

However, it is sometimes necessary to interrupt flow for either planned maintenance purposes or unplanned difficulties. If the pipeline has no means for reducing heat loss to the sea water, the crude oil rapidly loses heat to the surrounding environment and the crude oil temperature rapidly falls below a hydrate formation temperature. I this occurs, the crude oil solidifies before a maintenance operation can be completed, making it extremely difficult, if not impossible, to restart flow through the pipeline. There have been a number of proposals for reducing the rate of heat loss from crude oil in a pipeline. For example, the assignee of this invention provides Eccoflex® PT7000 flexible syntactic tape for such applications. A pipe is wrapped with multiple layers of the tape. Each layer comprises multiple abutting tapes applied along helical paths. Such a structure is adapted for incorporation into flexible pipelines. Adding such insulation provides a passive system that extends available maintenance time when crude oil flow is interrupted.

However, for a given sized pipe, increasing the number of layers also increases the overall diameter of the pipe. As known, when such pipe is flexible pipe, it is shipped on reels. Increasing the overall pipe diameter with insulation decreases the length of the pipe that can be stored on a single reel. A pipeline may comprise multiple pipe lengths with expensive interconnections. Any increase in the overall pipe diameter can dramatically increase the pipeline costs by increasing the number of interconnections.

An "active" system includes a jacket around the pipe and an external heat source for maintaining the pipe and the crude oil above the hydrate formation temperature. The heat source may be electricity for heating elements embedded in the jacket or a heated liquid that is distributed through the jacket by pumping. As will be apparent, however, such active systems are expensive to manufacture and expensive to operate.

Summary

Therefore, it is an object of this invention to provide a passive thermal management system that delays cooling during an interruption of liquid flow in a pipeline.

An additional or alternative object of this invention is to provide a passive thermal management system which, for a given overall pipe size, extends the time for the liquid in the pipeline to reach its freezing temperature.

Another additional or alternative object of this invention is to provide a passive thermal management system for a crude oil pipeline which, for a given overall pipe size, extends the time from the onset of an rntemiption to the time the crude oil in the pipeline reaches the hydrate formation temperature and eventually solidifies.

Still another additional or alternative objection of this invention is to provide a passive thermal management system for a crude oil pipeline which, for a

7 given time to reach the hydrate formation temperature, allows the overall pipe diameter to be reduced.

In accordance with one aspect of this invention a thermal management system is adapted for application to at. least, a portion of a pipeline for transported material having a first freezing point in a location where the pipeline is in an environment with a temperature below the first freezing point. The system comprises at least one layer of a phase-change material having a second freezing point greater than the transported material's freezing point. During norma! operations, heat transfers from the material in its liquid state to the material in the phase-change layer. When How is interrupted, heat transfers from the phase- change material to the now stagnant transported material in the pipeline thereby- reducing heat loss from the transported material.

In accordance with another aspect of the present invention, there is a thermal management system for application to at least a portion of a pipeline for conveying a fluid which is at a higher temperature than the pipeline's

environment, the thermal management system comprising a phase change material able to undergo a reversible transition between first and second phases so that in use, while fluid flows through the pipeline, tire phase change material is maintained in the first phase by heat from the fluid, and so that when fluid flow through the pipeline subsequently ceases, cooling causes the phase change material to undergo a transition to the second phase and so to transfer heat to the fluid.

The phase change material is invested with heat, including the latent heat associated with the phase change, if fluid flow is interrupted, heat is lost to the environment and both the fluid and the phase change material will tend to cool. As the phase change material reaches its phase change temperature, it. releases the latent heat associated with the phase change, tending to sustain fluid temperature. In this way cooling is delayed.

The thermal management system may he formed as a wrappable tape or layer for application to the pipeline. This provides a convenient means of applying the phase change material to the pipeline. The thermal management system may comprise a composite material in which the phase change material is contained in a matrix of polymeric material. Phase change materials are typically effective if encapsulated in small regions and this may be achieved, in accordance with a preferred aspect of the invention, by containing the phase change niaterial in hollow microballoons, microspheres or macrospheres, or in other hollow bodies. The hollow bodies may be set in a polymeric matrix. Such materials are sometimes known as ''syntactic⁵' materials. Preferably the polymeric material is suitable for casting, moulding, injection moulding or extrusion.

In the preferred embodiment of the invention, the phase change material is solid in the first phase and liquid in the second phase. In principle, however, the relevant phase change could be from one solid phase to another, or from a liquid to a vapour phase. Where the thermal management system is used in a pipeline for crude oif, the temperature at which the phase transition takes place is preferably above the hydrate formation temperature of the crude oil and below the temperature of the crude oil as it emerges from the wellhead.

In accordance with still a further aspect of the present invention, there is a pipeline for conveying a fluid through an environment which is cooler than the fluid, the pipeline comprising a passage for the fluid and a phase change material which is in thermal contact with the passage so that the phase change material is able io exchange heat with the fluid in operation, the phase change material being able to undergo a reversible change of phase, so that while fluid flows through the pipehne heat transferred from the fluid to the phase change material maintains it in a first phase, and when fluid flow through the pipeline subsequently ceases cooling of the phase change material causes it to undergo a transition to a second phase and so to transfer heat to the fluid, thereby delaying cooling of the fluid.

Preferably the pipeline further comprises an insulating layer, the phase change material being between the pipeline passage and the insulating layer.

In accordance with yet another aspect of the present Invention, there is a method of managing fluid temperature in a pipeline, the method comprising providing the pipeline with a phase change .material which is in thermal contact with a fluid passage in the pipeline and which is able to undergo a transition between first and second phases at a phase transition temperature, placing the pipeline in an environment below the phase transition temperature, passing fluid through the pipeline, the fluid being at a temperature above the phase transition temperature so that while the fluid flows heat from the fluid maintains the phase change material in the first phase, and so that following cessation of .fluid flow cooling causes the phase change material to undergo the transition to the second phase and to emit heat to the fluid in the pipeline.

The term "phase change material" refers herein to any material (solid, liquid, or gas) that goes through a reversible transition that is end thermic on heating and exothermic when cooling through said transition. The phase transition may be preferentially from a solid state to another solid state, as in a crystal structure change, or from solid to liquid, bat could also entail a transition to and from solid to gas, liquid to gas. liquid 1 to liquid 2, gas 1 to gas 2. etc., so long as cooling of the material through the transition temperature from the second phase to the first phase is an exothermic process.

By virtue of the invention, it is possible to provide transient thermal cooldown performance equivalent to a conventional pipeline using thinner insulation layer thickness. Alternatively, improved cooling performance can be achieved without change of insulation layer thickness compared with a conventional pipeline.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drasvings in which:

FIG. 1 is a perspective view of a portion of an insulated pipe used in underse pipelines that incorporates a passive thermal management system of this invention;

FIG. 2 is a simplified cross section of a pipe incorporating a phase-change layer in a passive thermal management system, of this invention;

FIG. 3 is graphical analysis that depicts the impact of the use of the passive thermal management system shown in FIG. 2;

FIG. 4 is a table that lists properties of one phase-change material that is useful in this invention; and

FIGS. 5A and 5B are cross-sections of two possible embodiments of a phase-change layer shown in FIG, 2.

FIG. 1 depicts a portion of a pipe 10 useful in undersea pipelines in which different layers are exposed for purposes of explanation. The pipe 10 includes an interlocked carcass that forms a central flexible rnetal pipe 1 1 , typically of steel An internal pressure sheath 12 surrounds the pipe 1 1 to contain the fluid transferred within the pipe 1 3.

interlocked pressure armor 13 provides compressive strength to the pipe 10, while inner and outer tensile armor layers 14 and 1 5 provide tensile support. An outer sheath 1 6 provides abrasion protection and a seawater seal for the entire pipe 10. A pipe 10. particularly as a component of an undersea pipeline, often includes one or more layers of insulating material intermediate the outer tensile armor layer 15 and the outer sheath 16. FIG. 1 depicts such an insulating layer 1 7. i this embodiment an insulation layer is formed by wrapping strips of insulation atop the outer tensile armor layer 15 along helical paths. Successive layers are formed in the same fashion by laying the strips in different hands and/or with different pitches. Still other methods can be used to provide such an insulating layer. The foregoing elements are conventional components of an undersea pipe. in accordance with this invention, however, the pipe 10, as shown in FIG. 1, is enhanced by adding a phase-change layer comprising a phase-change material as shown in FIG. 2, Such a phase-change layer is disposed between the outer tensile armor 1 5 and the insulating layer 17. If no insulating layer were included, the phase-change would be intermediate the outer tensile armor layer 15 and the outer sheath 16.

FIG. 2 depicts a pipe assembly 11 A that is a simplified cross section of a pipe that incorporates a phase-change layer 20 incorporating this invention. In FIG. 2 the pipe assembly 1 1 A represents the assemblage of the pipe 1 1 in FIG. 1 along with its pressure sheath 1 2, pressure armor 1 3 and inner and outer tensile armor layers 14 and 15 in cross section. The pipe assembly 1 1 A is surrounded by a phase-change layer 20 and is filled with crude oil 21. The phase-change layer 20 surrounds the pipe 1 1 A. An outer layer 22 includes the represents any outer insulation, such as insulating layer 17 and the outer sheath 16 in FIG. 1.

During normal operation heat transfers from the crude oil 21 to the phase- change layer 20 with some heat transfer from the phase-change layer 20 to the sea water 23 through the outer insulating layer 22. The phase change material initially is a solid, but it has a lower freezing/melting temperature than the crude oil temperature. As the heat transfers from the crude oil 21 into the phase-change layer 20, the temperature of the phase change material rises and eventually it melts. Consequently, the phase change material "stores" the heat attributable to the increased temperature and to the latent heat of melting for the phase change material. The outer layer 22 continues to reduce heat transfer to the surrounding sea water 23, but the pipeline is essentially in a state of thermal equilibrium.

Arrow 24 represents this heat transfer.

When the flow stops, the heat stored in the phase-change layer 20 transfers to the now stagnant crude oil 21 and to the sea water 23 as shown by arrows 25 and 26, respectively. The rate of transfer to the sea water 23 is dependent upon the insulating properties of the outer layer 22. However, the rate of heat transfer to the crude oi! 21 will increase as crude oil temperature begins to fall. Moreover, given the relative heat conductivities along the paths represented by arrows 25 and 26, most of the heat in the phase-change layer 20 transfers to the adjacent crude oil 21 thereby increasing the interval before the crude oil 21 reaches the hydrate formation temperature.

Simulations predict that the use of a phase-change layer, such as the phase- change layer 20, provides valuable options for pipeline construction and operation, Referring to FIG. 3, graphs 30, 31 and 32 present the results of these simulations. Graph 30 presents crude oil temperature as a function of time beginning with the interruption in a pipe wherein the only insulation is provided by multiple layers of Eceoilex® PT7000 .flexible syntactic tape. The multiple layers provide a 56mm thick insulating structure. Graph 31 presents the same information assuming the addition of a phase-change layer 20 with a radial thickness of 7mm and the outer layer 22 thickness reduced to 49 mm of insulation. Graph 32 represents the same information assuming the additio of a phase- change layer 20 having the same radial thickness and a 42mm reduction of the outer layer 22 thickness.

Each simulation assumes a crude oil temperature in the pipeline prior to interruption of 90°C and a seawater temperature of 5°C. The phase-change layer 20 is assumed to include a phase-change material having the following

characteristics:

1. Melting point: 57°C

2. Latent Heat of Fusion: 179 kJ/kg

3. Specific heat capacity: 2.5 kJ/kgK

4. Thermal Conductivity: 0.2 W/rnK

5. Density: 770 kg/m"

6. Volume expansion: <10%

Still referring to FIG. 3, graph 30 indicates that a simulation of a prior art system maintains the crude oil temperature about the hydrate formation temperature for approximately thirty-four (34) hours. With the second simulation and as shown by graph 31. the interval increases to about fifty-five (55) hours. When the number of syntactic layers is reduced as described with respect to the third simulation, graph 32 predicts that an interval of thirty-seven (37) hours will expire between the time the flow is interrupted and the time the stagnant crude oil reaches the hydrate formation temperature. However, the thickness of the outer layer 22 reduces by 42mm for an overall pipehne diameter reduction of 70mm, accounting for the addition of the phase-change layer 20. Where such a time interval is acceptable, the resulting pipe diameter reduction both provides greater pipe flexibility and allows more pipe to be stored on a single reel for shipment. Consequently a pipeline may require fewer connections between pipes on separate reels with attendant cost reductions.

For undersea applications, the phase change material should have certain characteristics, namely:

1. A freezing temperature above 25 °C and below 140°C, preferably below 90°C, and still more preferably in the range between 30°C and 60°C;

2. A high specific heat capacity;

3. A high latent heat of fusion;

4. A minimal volume expansion;

5. Low heat conductivity;

6. Chemical inertness; and

7. Environmental neutrality.

The table of FIG. 4 depicts a commercially available material that satisfies some or all of these requirements.

There are a variety of constructions for implementing a phase-change layer 20. FIGS. 5A and 5B depict two implementations. FIG. 5A depicts a syntactic tape 50 comprising a thermoplastic matrix 51 that binds a plurality of

microballoons 52. Each niieroballoon 52 contains and encapsulates a phase change materia]. FIG. 5B depicts a tape 53 including an extruded phase change material 54 encapsulated in a sheath 55 of a thermoplastic matrix. Alternatively, a thermoplastic matrix may bind non-encapsulated phase change materials m particle form.

It will become apparent to those skilled in the art that different

configurations will be possible for other pipehne components, such as couplings, valves and control devices. Moreover, it will also be apparent that the use of an intermediate phase-change layer can be applied to any transport system in which an outside environment has a temperature below the melting point of the material being transported such that the material can solidify if its flow is interrupted for any length of time. In other pipeline configurations, the phase-change layer could be situated internally of the pipe or externally of any insulation or even used without any insulation, depending upon the particular application and environment.

Although this invention has been disclosed in terms of certain embodiment, it wil l also he apparent that the foregoing and many other modifications can be made to the disclosed apparatus without departing from the invention. ^'Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.

Claims

1 . A thermal management system for application to at least, a portion of a pipeline for conveying a fluid which is at a higher temperature than the pipeline's environment, the thermal management system comprising a phase change material able to undergo a reversible transition between first and second phases so that in use, while fluid flows through the pipeline, the phase change material is maintained in the first phase by heat from the fluid, and so that when fluid flow through the pipeline subsequently ceases, cooling causes the phase change material to undergo a transition to the second phase and so to transfer heat to the fuiid,

2. A thermal management system as claimed in claim 1 formed as a wrappable tape or layer for application to the pipeline.

3. A thermal management system as claimed in claim 1 formed as a composite material in which the phase change material is contained in a matrix of po 1 ym e ic m atenai.

4. A thermal management system as claimed in claim 1 which comprises a porous matrix material through which the phase change material is distributed.

5. A thermal management system as claimed in any preceding claim in which the phase change material is contained in hollow niicroballoons, microspheres or m cro spheres, or in other hollow bodies.

6. A thermal management system as claimed in claim 5 in which the hollow microballoons, microspheres, microspheres or other hollow bodies are set in a po I ymeric rn atrix .

7. A thermal management system as claimed in claim 1 in which the phase- change material is chemically grafted or otherwise bound into or onto a carrier polymer matrix.

8. A thermal management system as claimed in claim 3 or claim 6 in which the polymeric material is suitable for any of casting, moulding, injection moulding and extrusion.

9. A thermal management systern as claimed in any preceding claim in which the phase change material undergoes the phase change at a temperature between 25 and 90°C.

10. A thermal management system as claimed in any of claims 1 to 7 in. which the phase change material undergoes the phase change at a temperature between 30 and 6Q°C.

1 1. A thermal management system as claimed in any preceding claim in which the phase change material is solid in the first phase and liquid in the second phase.

12. A thermal management system as claimed in any preceding claim in which the phase change material comprises one of a wax, a thermoplastic and a salt hydrate.

13. A pipeline provided with a thermal management system as claimed in any preceding ciaim.

14. A pipeline for conveying a fluid through an environment which is cooler than the fluid, the pipeline comprising a passage for the fluid and a phase change materia] which is in thermal contact with the passage so that the phase change material is able to exchange heat with the fluid in operation, the phase change material being able to undergo a reversible change of phase, so that while fluid flows through the pipeline heat transferred from the fluid to the phase change material maintains it in a first phase, and when fluid flow through the pipeline subsequently ceases cooling of the phase change material causes it to undergo a transition to a second phase and so to transfer heat to the fluid, thereby delaying cooling of the fluid.

15. A pipeline as claimed in claim 14 which further comprises an insulating layer, the phase change material being between the passage and the insulating layer.

1 6. A pipeline as claimed in claim 15 in which the phase change material is in a layer between the passage and the insulating layer.

^'1 7. A. method of managing fluid temperature in a pipeline, the method comprising providing the pipeline with a phase change materi l which is in thermal contact with a fluid passage in the pipeline and which is able to undergo a transition between first and second phases at a phase transition temperature, placing the pipeline in an environment below the phase transition temperature, passing fluid through the pipeline, the fluid being at a temperature above the phase transition temperature so that while the fluid flows heat from the fluid maintains the phase change material in the first phase, and so that ollowing cessation of fluid flow cooling causes the phase change material to undergo the transition to the second phase and to emit heat to the fluid in the pipeline.

18. A method of managing fluid temperature in a pipeline as claimed in claim

17, in which the fluid is crude oil having a hydrate formation temperature, and in which the phase transition temperature is higher than the hydrate formation temperature.

1 . A method as claimed in claim 18 in which the phase transition temperature, is lower than a temperature of the crude oil as it emerges from a wellhead.