GB2365942A

GB2365942A - Heat-insulated steel pipe for deep-sea pipelines and method for producing the same

Info

Publication number: GB2365942A
Application number: GB0127948A
Authority: GB
Inventors: Allan Boye Hansen; Eirik Simonsen
Original assignee: THERMOTITE AS
Current assignee: THERMOTITE AS
Priority date: 1999-05-26
Filing date: 2000-05-23
Publication date: 2002-02-27
Anticipated expiration: 2020-05-23
Also published as: BR0011544B1; GB2365942B; AU5962900A; NO20015682L; AR024081A1; GB0127948D0; BR0011544A; NO20015682D0; WO2000073694A1

Abstract

The invention relates to a conduit pipe (1) made of steel having a wall thickness enabling said pipe to be laid in the deep sea and an external heat-insulating envelope made of aluminum foam (2) whose outer surface has no pores. The invention also relates to a method for producing a heat-insulated steel conduit pipe having a wall thickness enabling said pipe to be laid in the deep sea, whereby the surface of the steel pipe is cleaned and the cleaned surface of the steel pipe is the covered with a heat-insulating layer made of aluminum foam, wherein possible pores opening outward in the envelope are closed.

Description

Steel pipe with heat insulation for deep-sea pipelines and method of

producing it

Description

The invention relates to a steel pipe with heat insulation for deep-sea pipelines and to a method of producing it.

In the course of shifting the activities of the offshore industry to greater water depths (for example a depth of over 1500 m), and with the increasing importance of long pipelines at these water depths, the requirements imposed on the thermal insulation of the steel pipes used for this have increased considerably. Not only must considerable requirements with regard to the thermal properties of this insulation be fulfilled, but the high mechanical stresses due to the -water pressure at these depths must also be coped with. Conventional thermal insulation based on foamed plastics (e.g. polypropylene) is not sufficient for this purpose. Due to the compression processes occurring as a result of the water pressure, the foamed plastic structure changes in an inadmissible manner.

To a certain extent, these adverse phenomena can be countered by foaming the plastic used for the coating to a smaller degree, so that this plastic retains a higher density. This certainly improves the mechanical properties but at the same time impairs the capacity of the thermal insulation. To ensure sufficient heat insulation, it is therefore necessary to correspondingly enlarge the respective layer thicknesses of the insulation. Of course, this in turn increases the costs of the heat insulation layer.

The object of the present invention is to propose a steel pipe with heat insulation which is suitable for laying in deep-sea regions, in particular for water depths of over 1500 m. In addition, a method of producing such a steel pipe is to be proposed.

According to the invention, the heat-insulated steel pipe, the wall thickness of which is designed for the static pressure of deep-sea laying, has a layer consisting of an aluminum foam instead of the layer consisting of a foamed plastic which has been customary to date, and this layer consisting of an aluminum foam completely surrounds the steel pipe on the outside in a uniform thickness, this thickness, for the desired thermal insulation, being sufficient for the respective application. The thickness of the layer is typically around at least 3 mm, preferably within a range of 20-60 mm. However, it may also be markedly above this, for example around 80-100 mm. Such a layer of aluminum foam likewise has very good values with regard to the thermal insulation, but, compared with a plastic foam, has markedly better mechanical stability, which can withstand the water pressure even at greater water depths. The mechanical stability is all the greater, the finer the pores of the aluminum foam are and the higher the density of the foam is. However, at a higher foam density, the heat insulation property of the aluminum foam is reduced. An expedient average pore size is within a range of about 0.03-4 mm, in particular within a range of 0.3-2 mm. The density of the aluminum foam is expediently about 10-60% of the density of the solid aluminum material corresponding to the material used. If the aluminum foam has an openpored structure, care must be taken to ensure that its outer surface is tightly closed. This may be effected, for example, by applying a preferably thermoplastic outer protective layer which, delle'nding on the application, is applied in different thicknesses. This prevents water from penetrating into the pores of the aluminum foam and accordingly impairing the thermal insulation. In addition, the task of such additional plastic insulation is to protect the aluminum layer from inadmissible deformation caused by external shock effect. A particular advantage of the aluminum foam layer consists in the fact that this layer is metallic and therefore has good electrical conductivity. This enables a pipeline consisting of the heatinsulated steel pipes according to the invention to be provided with cathodic protection in an especially simple manner.

The wall thickness of the steel pipe itself must be designed for the static pressure prevailing at great water depths, so that the steel pipe does not collapse. As the person skilled in the art knows, this wall thickness must be all the larger, the larger the diameter is. The diameter (nominal diameter) is expediently at least 100 mm and preferably lies within a range of about 200600 mm. The wall thickness should expediently be at least 8 mm, in particular at least 12 mm, and is typically within a range of about'15-50 mm.

The invention is explained in more detail below with reference to the schematic illustrations shown in the two figures. Figure 1 shows a section through a steel pipe according to the invention, whereas figure 2 reproduces a schematic block diagram of the method according to the invention.

A schematic cross section through a pipe according to the invention is shown in figure 1. The steel pipe is designated by the reference numeral 1. Laid around this steel pipe is the heat insulation layer 2, which consists of foamed aluminum and has a considerably greater thickness than the thickness of the steel pipe 1. The wall thickness of the steel pipe 1, for example at a pipe diameter of 324 mm, is about 16 mm, whereas the layer of aluminum foam is, for example, 60 mm thick. As outermost layer, a mechanical protective layer '3 made of thermoplastic (e.g. polypropylene) and having a thickness of, for example, 8 mm is laid around the pipe and completely encases the heat insulation layer 2. The thickness of the plastic insulation is generally around at least 2 mm and is preferably within a range of 5-80 mm. Such additional plastic insulation advantageously increases the heat insulation value, since plastics are very poor heat conductors as a rule.

The method according to the invention for producing heatinsulated steel. pipes for deep-sea applications can be explained with reference to the scheme in figure 2. The starting point is a steel pipe produced conventionally in a seamless manner or by welding. After its surface has been cleaned, during which cleaning this surface becomes metallically bright, this steel pipe is covered with a layer consisting of a foamed aluminum. The aluminum foam may be applied, for example, by an extrusion process. To close any possible pores of the aluminum foam which open outward, and to ensure external mechanical protection for the aluminum foam layer, it is advisable to subsequently apply, in a manner known per se, a layer consisting of a plastic, preferably a thermoplastic (e.g. polyethylene or polypropylene), to the chilled aluminum foam. After the mechanical protective layer has been cooled down, a thermally insulated steel pipe is obtained, and this thermally insulated steel pipe is protected against mechanical damage due to shocks and withstands the static pressure at great water depths without substantial deformation.

The aluminum foam layer may be produced in different ways, for instance by the extrusion already mentioned. To this end, DE 40 18 360 Cl discloses a method in which a mixture of aluminum powder and a likewise powdery blowing agent (e.g. titanium hydride) is inserted as charge material into an extruding press and is extruded with uniform wall thickness to form a semifinished product. Within the scope of the present invention, provision is made for the semifinished product to be extruded in the form of pipe half-shells. The term "half-shells" serves below to stand for shell elements of any desired size which can be assembled to completely cover a pipe surface over its circumference. Such "half- shells" may also be divided into a plurality of pieces in the axial direction. The blowing agent will still not be decomposed during the extrusion of the half-shells. The halfshells are then laid around the steel pipe to be insulated and are subsequently enclosed, while being oriented coaxially, by an outer mold which permits heating of the half- shells to a temperature at which the aluminum changes into the liquid state and the blowing agent decomposes to form a gas and -foams the aluminum. By cooling, the aluminum foam solidifies to form a firm structure.

Within the scope of the invention, the term "aluminum" is not restricted to pure aluminum but includes any desired aluminum alloys, in particular aluminum magnesium alloys.

An alternative embodiment of the invention provides for the steel pipe to be jacketed directly with extruded aluminum foam. This foam may be produced by a blowing agent, in the form of uniformly distributed fine particles, being put into an aluminum melt, this blowing agent decomposing under the effect of the heat of the melt, -, N as disclosed in principle, for instance, by WO 91/01387. 7-7 ' -_ f o am, for example, as also known in the plastic encasing of stee- pipes, may be applied to the surface of the steel pipe in a tubular manner by an annular die or also by a slit die according to the "wrapping method" and then cooled down.

In another procedure of the invention, a mixture of aluminum and powdery blowing agent is applied to the surface of the steel pipe in a uniform layer thickness by a thermal spraying process, e.g. plasma or flame spraying, preferably high-velocity flame spraying. Such a process for coating any desired substrates with aluminum foam has been disclosed by DE 195 01 659 Cl. Used in this case is a metal powder having very fine particles of 5-18 pm. grain size, to which up to 5% by weight of blowing agent is added. The blowing agent preferably has an average particle size of 50-1SO Pm and therefore has a larger grain than the metal powder. This ensures that the blowing agent has still not decomposed during the very short time of the spraying onto the surface of the steel pipe but is enclosed in the solidified matrix of the briefly liquefied aluminum particles. Liquefaction and thus foaming of the aluminum layer are in turn effected by subsequent heating of the coating. The heating may be effected, for example, in a furnace, but may also be applied by a heating mold surrounding the pipe or also by the flame of a welding torch or another heating-gas flow.

A wide variety of substances may be used as blowing agent. It is merely necessary that they decompose at the melting temperature of the aluminum. In a manner kno per se, in addition to the titanium hydride already mentioned, other metal hydrides, carbonates, sulfates, sulfides, oxides, nitrides or azides are also suitable. Correspondingly decomposable organic or metalorganic compounds and salts of organic acids may also be suitable.

The subsequent heating for foaming the aluminum coating is expediently effected to a temperature of at most up to about 1000C above the liquidus temperature. The temperature is preferably limited to a maximum temperature of 500C above the liquidus temperature.

A higher temperature and a longer application time of the heat tend to produce a greater degree of foaming. In this way, the formation of the foamed insulating layer can be influenced in its structure.

In a preferred embodiment of the invention, provision is made for a foam structure to be produced which is designed to be variable in the radial direction, i.e. is provided with pores of different size. This may be done, for example, by a plurality of layers being applied one after the other and foamed individually. The layer having the larger pores and thus the higher insulation value is preferably arranged close to the pipe surface, whereas foam layers having a smaller pore size and thus a poorer insulation value but markedly greater mechanical stability are arranged on the outside. In this case, the average pore size increases toward pipe surface. The outermost layer in this case may consist of a solid aluminum skin, which can function as a protective layer and tightly close any possible open pores in the surface region of the jacket.

The pipes according to the invention, with regard to the mechanical stability of the jacket, are expediently designed for a laying depth of at least 2000 m, preferably at least 3000 m. Since this type of heat insulation performs a substantial independent supporting function with regard to the acting water pressure, this results in a considerable advantage for the dimensional stability of the deep-sea pipe (greater safety against collapsing), and this advantage may be reflected i n -a' possible reduction in the requisite wall thickness for the actual steel pipe. Due to the encasement with aluminum foam, the mixture of the steel pipe is increased only to a relatively small degree. As already mentioned above, instead of pure aluminum, aluminum alloys may also be used for the production of line pipes according to the invention. These aluminum alloys are generally even to be preferred, since they not only have lower thermal conductivity, that is to say a higher insulation value, but also offer better strength properties. Suitable materials are, for example, the alloys AlMg5 and AlCuNi.

Claims

Patent claims:

1 A line pipe made of steel, having a wall thickness designed for deepsea laying and having an outer thermally insulating jacket which is made of aluminum foam (2) and whose outer surface has no open pores.

2. The line pipe as claimed in claim 1, characterized in that the jacket made of aluminum foam (2) is surrounded by a mechanical protective layer (3), in particular a protective layer made of a thermoplastic.

3. The line pipe as claimed in either of claims 1 and 2, characterized in that the thickness of the jacket made of aluminum foam (2) is at least 3 mm, in particular at least 20 mm, and preferably does not exceed 60 mm.

4. The line pipe as claimed in one of claims 1 to 3, characterized in that the average pore size is within a range of about 0.03-4 mm, preferably within a range of 0.3-2 mm.

S. The line pipe as claimed in one of claims 1 to 4, characterized in that the density of the aluminum foam (2) is within a range of 10-60% of the density of the corresponding solid aluminum material.

6. The line pipe as claimed in one of claims 1 to 5, characterized in that the average pore size is variable, preferably increases, in the direction of the surface of the steel pipe (1).

7. The line pipe as claimed 'in one of claims 2 to 6, characterized in that the mechanical protective layer (3) is formed from polyethylene or polypropylene.,

8. The line pipe as claimed in one of claims 2 to 7, characterized in that the thickness of the mechanical protective layer (3) is at least 2 mm and is preferably within a range of S80 mm.

9. The line pipe as claimed in one of claims 1 to 8, characterized in that the wall thickness is at least 8 mm, in particular at least 12 mm, and is preferably within a range of IS50 mm.

10. The line pipe as claimed in one of claims 1 to 9, characterized in that the diameter is at least 100 mm and is preferably within a range of about 200-600 mm.

11. A method of producing a thermally insulated line pipe made of steel, having a wall thickness designed for deep-sea laying, as claimed in claim 1, by cleaning the surface of the steel pipe and subsequently jacketing the cleaned surface of the steel pipe with a heat insulation layer made of foamed aluminum, any possible pores of the jacket which open outward being closed.

12. The method as claimed in claim 11, characterized in that the aluminum foam jacket is finally encased with a plastic, preferably a thermoplastic.

13. The method as claimed in either of claims 11 and 12, characterized in that the thermal insulation is applied to the surface of the steel pipe by direct extrusion of foamed aluminum.

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14. The method as claimed in either of claims 11 and 12, characterized in that the thermal insulation is applied by jacketing with prefabricated, extruded aluminum half-shells containing a blo wing agent which has not yet decomposed and by subsequent foaming by heating the half-shells above the liquidus temperature of the aluminum while the blowing agent decomposes.

15. The method as claimed in either of claims 11 and 12, characterized in that the thermal insulation is applied by applying a layer of aluminum and blowing agent which has not yet decomposed with a thermal spraying process and by subsequent foaming by heating the sprayed-on layer above the liquidus temperature of the aluminum while the blowing agent decomposes.

16. The method as claimed in either of claims 14 and 15, characterized in that the heating is effected in a heatable mold, in a furnace or by means of hot gases, preferably by a welding torch flame.

io New patent cla:Lm I 1 A line pipe made of steel, having a wall thickness designed for deep- sea laying and having an outer thermally insulating jacket which is made of aluminum foam (2) and is applied directly to the steel pipe and whose outer surface has no open pores.