WO2004029501A1

WO2004029501A1 - An insulated pipe and a mehtod for manufacturing an insulated pipe

Info

Publication number: WO2004029501A1
Application number: PCT/DK2003/000624
Authority: WO
Inventors: Tom B. Jakobsen; Kim Schacht
Original assignee: Løgstør Rør A/S
Priority date: 2002-09-25
Filing date: 2003-09-24
Publication date: 2004-04-08
Also published as: EP1543269A1; AU2003266488A1

Abstract

The invention relates to an insulated pipe intended for advancing media, said media having temperatures lower than the ambient temperature, said pipe comprising at least one jacket pipe, at least one medium pipe and at least one insulating layer situated between the jacket pipe and the medium pipe, wherein the pipe is intended for the jacket pipe and the medium pipe being, prior to operation, interconnected and secured in a pre-tensioned sate. In case of submarine gas lines for the transport of liquid gas, the temperature of which very low, such pipe-in-pipe pipelines are still not known, since the safety level concerning the operation of the line is insufficient; the prior art insulating materials and methods not having proved to be applicable. Novel and characterizing aspects of the pipe according to the invention comprise that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the media pipe are only partially or essentially not transferred to the insulating layer, and that the insulating layer comprises at least one elastic absorbing layer, said absorbing layer being located at the interior side of the insulating layer.

Description

AN INSULATED PIPE AND A METHOD FOR MANUFACTURING AN INSULATED PIPE

The invention relates to an insulated pipe intended for advancing media, said media having temperatures lower than the ambient temperature, said pipe comprising at least one jacket pipe, at least one medium pipe and at least one insulating layer situated between the jacket pipe and the medium pipe, wherein the pipe is intended for the jacket pipe and the medium pipe being, prior to operation, interconnected and secured in a pre-tensioned state.

The invention further relates to a method of manufacturing a pipeline, said method comprising interconnecting and pre-tensioning of a number of insulated pipes intended for advancing media, said media having temperatures lower than the ambient temperature, said pipes each comprising at least one jacket pipe, at least one medium pipe and at least one insulating layer situated between the jacket pipe and the medium pipe. The invention also relates to a use of a pipeline manufactured by this method.

The invention can be used eg for the transport of medium having a relatively low temperature. The term "low temperature" is intended to designate a temperature lower than -100°C, including eg the transport of liquid ethylene (- 104°C), liquid natural gas (-163°C), liquid nitrogen (-196°C), and liquid oxygen (183°C).

Miscellaneous tests made by Sintef Energi in Norway substantiate that it is possible to use pre-insulated pipes with PE jacket for transporting low temperature media. The medium-conveying internal pipe as such, the medium pipe, is a steel pipe around which an insulating layer is arranged, of eg foamed polyurethane (PUR) and a polyethylene (PE) jacket on top of the insulation. Pipes of this type are typically designated 'single-pipe pipes'. However, as the case may be, single-pipe pipes are inconvenient in use due to the thermal contraction of the medium pipe. The configuration of a single- pipe pipeline for submarine use is made even more complex by the necessity of a weight coating of the PE jacket to obtain enough weight to keep the pipeline at the seabed, while simultaneously compensation is to be made for the thermal contraction.

In case of ordinary pipelines advancing hot media, much focus is on minimizing deployment costs. For this reason it is an advantage to deploy single pipes, since steel welding is required on the medium pipe only. The jacket pipe joints are performed by injection of solid polyurethane in a wrap of eg PE, following which the line is ready for deployment in the water.

Typically, single-pipe systems are limited with regard to deployment depth; the PE jacket and PUR foam not being able to resist the ambient pressure in case of high medium-pipe temperatures, without being deformed, since the heat softens the polymeric materials.

For long, efforts have been made to deploy submarine gas lines; the situation typically being that where the gas is, it is not an option for the gas tankers to berth and load the gas. It has been financially cumbersome to establish a pier on these typically low-water areas in order for the tankers to berth. Instead it is an option to establish a submarine gas line for a fraction of the costs, provided the line system and in particularly the insulation system work.

When the deployment depth exceeds between 50 and 120 m, pipe-in-pipe solutions are often chosen to be able to resist the external pressure. 'Pipe-in- pipe' means that, instead of the PE jacket, a steel pipe is used; ie two steel pipes are used, an inner one and an outer one. However, the outer one can be provided with some kind of anti-corrosion coating, eg a PE, PP or epoxy coating. It is also known that when long, straight pipe lengths are to be deployed wherein there is no room for compensation elements in the form of loop expansion joints, compensators or bends, pipe-in-pipe solutions can be used, whereby the medium pipe is heated and extended without the jacket pipe undergoing same. When the extension has reached the specified pre-heating level, the two pipes are joined by welding by means of fixations, so-called bulkheads that connect jacket pipe and medium pipe to each other. This is known eg from pipes for the transport of remote heating, where the medium temperature exceeds the ambient temperature, and where it may be necessary to pre-tension the pre-insulated system in order to reduce the axial tensions. In particular when pipes are to be deployed in the soil, and compensation is to be made for the thermal expansion of the pipe, it may occur that pre-heating is applied to reduce the tensions in critical areas of the pipe system.

In case of submarine gas lines for the transport of liquid gas, the temperature of which is very low, such pipe-in-pipe pipelines used as pre-insulated pipe elements are still not known, since the safety level concerning the operation of the line is insufficient; the prior art insulating materials and methods not having proved to be applicable.

Testing of pre-tensioned pipe-in-pipe pipes produced in accordance with known methods has shown that the low temperature causes the PUR foam to crack where it abuts on the medium pipe, since the thermal contraction of the PUR foam considerably exceeds the contraction of the steel pipe. The coefficient of thermal expansion is approximately five to six times higher for PUR than for the steel types conventionally used for medium pipes, eg steel with 9% of Ni in the alloy. This leads to reduced longevity of the insulation and hence of the pipeline and poorer insulating capacity and hence increased loss of cold. For the sake of convenience, a number of pipe-in-pipe pipes can be assembled to form one assembled pipe line prior to deployment of same. It would also be convenient not to perform the pre-tensioning until a number of pipes are assembled. However, such procedure entails excessive displacement of medium pipe in relation to jacket pipe, which may damage the insulation. This is due to the fact that the PUR foam will, due to its location between medium pipe and jacket pipe and during usual manufacturing process, adhere to the surfaces of the pipes. The adhesion can be destroyed in an uncontrollable process and be torn apart, which also leads to reduced longevity of the insulation and deteriorated insulating capacity. This problem is relevant in particular when low temperatures of the medium that is to be transported in the pipe line are concerned, where the difference in temperature between the medium pipe and the jacket pipe is very large in operation, with an ensuing need for a correspondingly larger pre-tensioning.

It is an object of the invention to provide a pipe-in-pipe pipe which is suitable for transporting media with very low temperatures. It is yet a further object of the invention to provide pipes to which a large pre-tensioning can be imparted, such that the tension level is kept suitably low when very cold media are transported. It is yet a further object of the invention to provide a method of manufacturing a pipeline from pipes having these properties.

Novel and characterizing aspects of the pipe according to the invention comprise that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the medium pipe are only partially or essentially not transferred to the insulating layer; and that the insulating layer comprises at least one elastic absorbing layer, said absorbing layer being located at the interior side of the insulating layer. By the at least one insulating layer being arranged in such a manner that thermal contractions and expansions of the medium pipe are only partially or essentially not transferred to the insulating layer, it is obtained that no adhesion occurs between the medium pipe and the insulating layer. Thereby it is obtained that the latter is not destroyed due to displacement of the pipes relative to each other. Thereby the longevity and insulating capacity are increased. Simultaneously the advantage is accomplished that pre-tensioning is not lost. A part of the pre-tensioning may be lost due to the desired displacement being unobtainable due to friction that withholds the displacement. This is of significance in particular if it is desired to pre-tension throughout long lengths. Pre-tensioning throughout long lengths enables savings, eg on bulkheads. The insulating layer comprising at least one elastic absorbing layer, said absorbing layer being located at the inner side of the insulating layer, it is obtained that the major part of the insulating layer can be of a good and convenient insulating material, eg PUR foam that would otherwise be broken due to excessive thermal contraction in case of low temperatures. The elastic absorbing layer is made of a material that is not destroyed by thermal contraction in case of the low temperatures in question, such as eg mineral wool.

The at least one insulating layer can moreover be arranged in such a manner that thermal contractions and expansions of the jacket pipe can only partially or essentially not be transferred to the insulating layer. Thereby displacements between the jacket pipe and the insulating layer cannot destroy the insulation either.

According to a convenient embodiment it is an option, between the insulating layer and the medium pipe, to arrange a friction-reducing layer, including layers of polymeric material, grease or wax. This is an inexpensive solution which is readily exercised. According to a further convenient embodiment it is also an option, between the insulating layer and the jacket pipe, to arrange a friction-reducing layer, including layers of polymeric material, grease or wax.

According to a preferred embodiment the insulating layer may comprise polyurethane which is a good and applicable insulating material.

According to yet a preferred embodiment the polyurethane can be of a type that has a compression strength above 0.1 MPa.

According to yet a preferred embodiment the insulating layer may comprise polyurethane of a type having a density of more than 50 kg per cubic meter.

According to yet a preferred embodiment the elastic absorbing layer may be mineral wool.

Preferably the absorbing layer may have a thickness of between 5% and 30% of the overall thickness of the insulating layer. The thickness must be sufficient for absorbing the thermal contraction of the remaining part of the insulating layer thereby avoiding destruction thereof.

According to yet a preferred embodiment the interconnection of the medium pipe and the jacket pipe may be accomplished by use of bulkheads. Other joints are also possible, eg spacer blocks that are secured by welding between jacket pipe and medium pipe.

According to yet a preferred embodiment the pipe can be configured with the interconnection of the medium pipe and the jacket pipe being performed in a state in which the jacket pipe had a relatively higher temperature than the medium pipe such that, when the medium pipe and the jacket pipe have essentially the same temperature, there is a state of essentially axial tensile stress within the jacket pipe and a state of essentially axial compressive stress within the medium pipe, where transport of a medium in the medium pipe, said medium having a lower temperature than that of the surroundings, results in complete or full relief of the state of compressive stress reigning within the medium pipe. Thereby it is ensured that the pipe is suitable for transporting very low-temperature media. If steel-types are selected having a high strength level it is not always necessary to subject the system to pre- tensioning in relation to the selected temperature of operation.

Novel and characterizing aspects of the method according to the invention comprise that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the medium pipe will only partially or essentially not be transferred to the insulating layer; and that the insulating layer is provided with at least one elastic absorbing layer, said absorbing layer being arranged at the interior side of the insulating layer; and:

- that the pipes are successively interconnected by full welding of medium pipe and jacket pipe in an end-to-end relationship, until an assembled pipe line is accomplished that has two free ends; - that the medium pipe is cooled or the jacket pipe is heated;

- that jacket pipe and medium pipe are, at the free ends of the pipeline, interconnected by bulkhead.

Thereby it is accomplished that a pipeline can be manufactured as pipe-in- pipe pipes that are suitable for transporting media having very low temperatures and whereto a very high degree of pre-tensioning can be imparted, such that the stress level is kept suitably low during transport of very cold media without the insulating layer being destroyed. It is also obtained that the displacement of the jacket pipe and the medium pipe is distributed expediently, it being now an option that the displacement takes place in two directions. The method of manufacturing a pipeline can, according to a convenient embodiment, comprise:

- that jacket pipe and medium pipe are, at the one free end of the pipeline, coupled to a bulkhead;

- before the medium pipe is cooled or that the jacket pipe is heated;

- and that the medium pipe and the jacket pipe are interconnected at the other free end by means of a bulkhead after the medium pipe has been cooled or the jacket pipe has been heated.

In this manner the positioning of the first bulkhead becomes very precise, thereby improving the length tolerance of the pipeline.

According to yet a preferred embodiment the method may comprise that: - following successive interconnection of the pipes by full welding of medium pipe and jacket pipe in an end-to-end relationship, for forming an assembled pipeline with two free ends;

- and following interconnection of jacket pipe and medium pipe at the free end of the pipe line by means of a bulkhead; - a bulkhead is mounted at the other free end of the pipe line that is, in the first instance, only connected to the medium pipe, said bulkhead being of the type wherein the diameter of the outer jacket exceeds that of the jacket pipe; and that the medium pipe is cooled;

- following which the medium pipe and the jacket pipe are interconnected at the other free end by interconnection of the outer jacket of bulkhead with the jacket pipe.

Thereby a safe and reliable pre-tensioning of the pipeline is accomplished; and that comparatively long lengths of the pipeline can be pre-tensioned at a time. A preferred use of a pipeline manufactured by a method according to the above may comprise that the pipeline is used for submarine transport of gas, said gas having a temperature lower than minus 50°C.

In the following the invention will be described in further detail with reference to figures that exemplify embodiments of the invention:

Figure 1 is a cross-sectional view of the pipe wall of a pipe-in-pipe pipe according to the invention. The cross section is taken longitudinally of the main axis of the pipe;

Figure 2a shows a simplified cross section of a pipe at a free end of a pipeline with pipes according to the invention and also shows the relative arrangement of bulkhead prior to pre-tensioning;

Figure 2b shows a simplified cross section of a pipe at a free end of a pipeline with pipes according to the invention and also shows the relative arrangement of bulkhead after pre-tensioning.

Figure 1 shows a pipe-in-pipe pipe 1 comprising a jacket pipe 2, a medium pipe 4 and an insulating layer 6. The insulating layer 6 comprises an elastic absorbing layer 7 arranged to the side that faces towards the medium pipe 4. Between the jacket pipe 2 and the insulating layer 6, a friction-reducing layer 8 may be provided. Between the medium pipe 4 and the insulating layer 6 there may be arranged a friction-reducing layer 9. The main axis of the pipe is designated by 10.

The absorbing layer 7 is arranged as a thin layer of material that is able to absorb a radial and preferably also an axial movement, such as eg mineral wool or corresponding material that is also able to tolerate the low temperature. When the material is also able to tolerate axial movement, it can be avoided to coat the medium pipe 4 with a friction-reducing material. Conveniently the inner side of the jacket pipe 2 can be coated with a PE film, a layer of grease or some other material that is able to ensure that the insulating layer 6 that may be eg PUR foam does not adhere to the jacket pipe.

The length of medium pipe 4 and jacket pipe 2 may be the same with regard to the subsequent mounting of the pipe system. Insulating end sections are mounted, the use of which is known from the manufacture of usual pre- insulated central-heating pipes. For the sake of the weldings that are subsequently to be performed there needs to be a so-called 'cut back' from the end of the pipe. This means that the foam edge is withdrawn from the welding zone such that the insulating layer 6 does not catch fire during the welding procedure.

The length of the jacket pipe 2 can also be shorter than the medium pipe 4, as is known from the manufacture of pre-insulated central-heating pipes. Hereby an open steel sleeve is subsequently to be mounted by welding therein in order to enable subsequent pre-tensioning of the entire pipe system.

Once the pipe 1 has been produced, the pipe lengths can be supplied in the manner in which pipes are usually delivered, viz. as straight elements of a specific length.

Figures 2a and 2b show a free end of a pipeline, ie a number of pipes that are interconnected. At the opposite, free end of the pipeline it is presupposed that jacket pipe 2 and medium pipe 4 are interconnected, thereby enabling transfer of forces there between. On the medium pipe 4 a bulkhead 12 is mounted that comprises a shroud 16, a shroud 14 and a cone 18. The shroud 16 has an internal diameter exceeding the external diameter of the jacket pipe 2.

Depending on steel type (max stress level), cross section and friction it can be calculated how long a pipe length can be established before bulkheads are to be mounted at each respective end. When the medium pipe 4 is subsequently cooled to the correct pre-tensioning temperature, see figure 2b, the shroud 14 is welded to the jacket pipe 12. Alternatively the jacket pipe 2 can be heated with a view to expansion, which would yield the same desired pre-tensioning, viz compressive stresses in the medium pipe and tensile stresses in the jacket pipe. If it is inconvenient to construct a bulkhead 12 having an outer diameter that exceeds the jacket on the pipe, it is also an option to select the same outer diameter as the pipe, whereby the two diameters are caused to be right opposite each other. By this method bulkhead is to be welded to the jacket pipe 2 instead. Since it is not an option to weld on a cold medium pipe 4 it is required, during cooling, to ensure that the medium pipe is long enough for it to be ensured that, when it has contracted to a length that is slightly smaller than needed, the welding zone can subsequently be heated locally, eg with a gas burner, without significantly influencing the contraction of the medium pipe 4. Both methods are feasible options.

The process of pre-tensioning the pipes can also be exercised by keeping both ends of the pipeline free during heating or cooling, following which jacket pipe 2 and medium pipe 4 are interconnected at both ends, eg by mounting of bulkheads.

By successive interconnection of the pipes 1 to a pipeline, the jacket pipes 2 can be joined both by end-to-end welding, but assembly is also possible by use of semi-pipe sections of steel. The pipe length can be mounted on land and pulled to sea when the pipe length is assembled and pre-tensioned.

Claims

C l a i m s

1. An insulated pipe for advancing media, said media having temperatures lower than the ambient temperature, said pipe comprising at least one jacket pipe, at least one medium pipe and at least one insulating layer situated between the jacket pipe and the medium pipe, wherein the pipe is intended for the jacket pipe and the medium pipe being, prior to operation, interconnected and secured in a pre-tensioned state, characterized in that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the media pipe are only partially or essentially not transferred to the insulating layer, and that the insulating layer comprises at least one elastic absorbing layer, said absorbing layer being located at the interior side of the insulating layer.

2. A pipe according to claim 1 , characterized in that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the jacket pipe are only partially or essentially not transferred to the insulating layer.

3. A pipe according to claim 1 or 2, characterized in that, between the insulating layer and the medium pipe, at least one friction-reducing layer is arranged, including a layer of polymeric material, grease or wax.

4. A pipe according to one or more of claims 1-3, characterized in that, between the insulating layer and the jacket pipe, at least one friction-reducing layer is arranged, including a layer of polymeric material, grease or wax.

5. A pipe according to one or more of claims 1-4, characterized in that the insulating layer comprises polyurethane.

6. A pipe according to claim 5, characterized in that the insulating layer comprises polyurethane of a type that has a compression strength of more than 0.1 N per square millimeter.

7. A pipe according to claim 5 or 6, characterized in that the insulating layer comprises polyurethane of a type that has a density of more than 50 kg per cubic meter.

8. A pipe according to one or more of claims 1-7, characterized in that the elastic absorbing layer is mineral wool.

9. A pipe according to one or more of claims 1-8, characterized in that preferably the absorbing layer has a thickness of between 5% and 30% of the overall thickness of the insulating layer.

10. A pipe according to one or more of claims 1-9, characterized in that the interconnection of the medium pipe and the jacket pipe is accomplished by use of bulkheads.

11. A pipe according to one or ore of claims 1-10, characterized in that the pipe is configured with the interconnection of the medium pipe and the jacket pipe performed in a state, in which the jacket pipe had a comparatively higher temperature than the medium pipe, such that - when the medium pipe and the jacket pipe have essentially the same temperature - there is essentially a state of axial tensile stress within the jacket pipe and essentially a state of axial compressive stress in the medium pipe, wherein transport of a medium within the medium pipe, said medium having a lower temperature than the ambient temperature, results in full or partial relief of the state of compressive stress within the medium pipe.

12. A method of manufacturing a pipeline, said method comprising interconnection and pre-tensioning of a number of insulated pipes intended for advancement of media, said media having temperatures lower than the ambient temperature, said pipe comprising at least one jacket pipe, at least one medium pipe and at least one insulating layer situated between the jacket pipe and the medium pipe, characterised in that the at least one insulating layer is arranged in such a manner that thermal contractions and expansions of the medium pipe will only partially or essentially not be transferred to the insulating layer; and that the insulating layer is provided with at least one elastic absorbing layer, said absorbing layer being arranged at the interior side of the insulating layer; and:

- that the pipes are successively interconnected by full welding of medium pipe and jacket pipe in an end-to-end relationship, until an assembled pipe line is accomplished that has two free ends; - that the medium pipe is cooled or that the jacket pipe is heated;

13. A method according to claim 12, characterized in that the method comprises:

- that jacket pipe and medium pipe are, at the one free end of the pipeline, coupled by means of a bulkhead before the medium pipe is cooled or that the jacket pipe is heated;

14. A method according to claim 13, characterised in that the method comprises: - following successive interconnection of the pipes by full welding of medium pipe and jacket pipe in an end-to-end relationship, for forming an assembled pipeline with two free ends;

- and following interconnection of jacket pipe and medium pipe at the free end of the pipe line by means of a bulkhead;

- a bulkhead is mounted at the other free end of the pipe line that is, in the first instance, only connected to the medium pipe, said bulkhead being of the type wherein the diameter of the outer jacket exceeds that of the jacket pipe; - and that the medium pipe is cooled;

15. Use of a pipeline manufactured by a method according to one or more of claims 12-14, characterized in that the pipeline is used for submarine transport of gas, said gas having a temperature that is lower than minus 50°C.