The present invention relates to a multi-layer pipe and a method of forming a multi-layer pipe/pipeline for transport of hydrocarbons, e.g. a corrosive multi-phase well flow.

TECHNICAL FIELD

In addition to meeting the mechanical strength requirements, such laminated transport pipes shall also permit heating by means of electrical power to prevent or defrost any hydrate formation. Likewise, fabrication of the said pipes at an acceptable price shall be possible.

BACKGROUND ART

Among known multi-layer pipes can be mentioned those designated clad steel pipes, consisting of two concentric steel pipes that are metallically interconnected. A clad steel pipe can therefore not be heated by means of electrical power to prevent or defrost any hydrate formation if the outer insulation layer is damaged and the steel pipe is in contact with sea water. Clad steel pipes moreover are very expensive in fabrication, considerably more expensive than the total cost of the two part pipes separately. For transport of a corrosive multi-phase well flow, the clad steel pipes are not competitive with other pipes price-wise.

Coaxial steel pipes with thermal insulation are moreover known. Both parts of the pipe here consist of carbon steel, and also

the inner pipe is therefore sufficiently strong to resist the internal pressure. In connection with heating to prevent or defrost hydrate formation, the resistance heat developed in the outer pipe outside the insulation will be lost to the sea.

Among the prior art solutions are also the pipes and methods of forming pipes set forth in Norwegian patent application no. 150.771 and British patent letter no. 1.390.873, but also these fail to meet the above-mentioned criteria: mechanical strength, heatability, acceptable price.

DISCLOSURE OF INVENTION

One object of the present invention is to provide a multi-layer pipe/pipeline for transport of hydrocarbons e.g. in the form of a corrosive multi-phase well flow, where the transport pipeline has the necessary mechanical strength properties, and can be heated by means of electrical power to prevent or defrost hydrate formation, and which can be fabricated at an acceptable price.

Another object of the present invention is to describe an advantageous method of forming a multi-layer pipe/pipeline.

These objects are achieved by the features set forth in the subsequent patent claims.

A main pipe consists of a strong and cheap material, for example carbon steel. A thinner pipe of a non-corrodible material is installed coaxially inside the main pipe, for example an appropriate aluminium alloy, titan or stainless steel.

The inner pipe will by a subsequently stated method be glued or cast to the outer main pipe by means of a polymer material,

which may be a thermosetting plastic, for example epoxy. If desired, a heat- insulating coating may be installed outside the main pipeline, covered by a weight coating.

The main pipe and inner pipe respectively are welded together to form as long lengths as practicable. A sufficient number of plastic spacers of a thickness corresponding to the thickness of the polymer layer later installed are glued on the inside of the outer pipe (the main pipe) or on the outside of the inner pipe, preferably on the former. The inner pipe is thereafter introduced into the outer pipe (main pipe) , and the annulus between them is filled with a polymer material, which may be a thermosetting plastic, for example epoxy. When the polymer is hardened, the two pipes are securely fastened to one another.

Such a transport pipe can be fabricated at a far lower price than known pipes with similar strength properties.

Since the outer and inner pipes are electrically insulated from one another, one of the following methods of electrical heating can be used: (I) Sending a direct current through the inner pipe. The outer pipe is then free from current and voltage, and can admit electrical contact with the sea water. (II) Sending an alternate current coaxially through the inner and outer pipe. Given normal thickness of the outer pipe, the current will only go on the internal side of the same. The outside remains free from current and voltage, and can still be earthed to the sea water.

BRIEF DESCRIPTION OF DRAWINGS

An embodiment of the invention is hereinafter further explained with reference to the attached schematic drawings, whereof:

Fig. 1 is a cross-sectional view of a multi-layer pipe/pipeline for transport of a corrosive multi-phase well flow according to the invention; and

Fig. 2 is a longitudinal section on a larger scale through two multi-layer pipes/pipelines that are being welded together to form one section of a pipeline.

Fig. 3 shows one end of a multi-layer pipe/pipeline where the outer and inner pipes are welded together.

Fig. 4 shows one end of a multi-layer pipe/pipeline where the outer and inner pipes are mechanically connected and electrically separated by means of a flanged connection.

MODES FOR CARRYING OUT THE INVENTION

The transport pipe according to the invention consists of a main pipe (outer pipe) 1 of a strong and cheap material, for example carbon steel. An inner pipe with a smaller external diameter than that of the outer pipe, is designated by reference numeral 2. The inner pipe 2 consists of a non-corrodible material, for example an appropriate aluminium alloy, titan or stainless steel. The outer and the inner pipes, 1, 2 are coaxially installed in relation to each other and are bound together by means of an intermediate layer 3, preferably a polymer material, that may be a thermosetting plastic, for example epoxy.

If desired, a heat-insulating coating 4 may be applied outside the outer pipe 1, covered by a weight coating 5.

The fabrication is accomplished by the outer pipe 1 and the inner pipe 2 respectively being welded together to as long pipe lengths as practicable. A sufficient number of plastic spacers

6 of a thickness corresponding to that of the polymer layer to be applied should be glued on the inside of the outer pipe 1 or the outside of the inner pipe 2, preferably on the former.

The inner pipe 2 is subsequently introduced into outer pipe 1, and the annulus between them is filled, preferably with a polymer material 3 that is injected. When the polymer 3 is hardened, the two pipes 1, 2 are securely fastened to one another.

The heat insulation 4 and weight coating 5, if any, are applied to the outside in the ordinary way.

The polymer material, e.g. epoxy, will bind the outer and inner pipes together and absorb most of the shear forces produced by varying expansion in the pipes.

Calculations of the shear forces in the polymer layer between the outer and inner pipe, when the inner pipe is exposed to heat expansion, show that the shear forces are concentrated to a few centimetres near each end of the pipeline. The size of the maximum shear stress in the ends will, in relevant cases, be close to the maximum of what e.g. epoxy can tolerate. Along the rest of the pipeline, the shear stresses will be insignificant. In a preferred embodiment, the pipe ends should therefore be mechanically interlocked. When heating the pipes by means of alternate current, the outer and inner pipes must be electrically connected in one end, and the mechanical connection can be most conveniently effected by the pipes being welded together.

In the other end, the outer and inner pipes must be electrically insulated from each other and connected to a power generator. Mechanical locking of the pipe ends can then be effected bv means of some kind of flange coupling with electrical insulation. Also the end to be electrically

short-circuited can be advantageously locked with the same type of electrically insulated flange coupling. The outer and inner pipes can then be separated or electrically short-circuited as desired by means of a contactor/switch located outside the pipe.

When the switch is open, the pipeline can be used as an electrical power cable for transmission of electrical energy for the operation e.g. of seabed control installations and/or machinery, or a platform offshore. For heating of the pipes, the switch is short-circuited.

When two coaxial pipes are mechanically interlocked in the ends but not glued together, and the inner pipe is unevenly heated, i.e. is much warmer in one end than the other, the two pipes will shift in relation to each other. In practical cases, this shifting may represent several metres near the middle of the pipeline. Calculations show that when the pipes are glued together by a polymer layer, such shifting is avoided, at the same time as the shear stress in the polymer layer will be insignificant. The intermediate layer therefore does not have to be very strong, and materials weaker than epoxy may alternatively be used.

When such multi-phase pipe lengths are fabricated, they must be joined together to even greater pipe lengths, which can be towed offshore and laid on the seabed in the form of pipelines.

This joining of multi-phase pipes can be advantageously performed the following way, referring to fig. 2, which shows the three inner layers enlarged in relation to fig. 1.

In each end of the multi-layer pipe, a ring 7 of a heat-resisting, for example ceramic, material is introduced into the annulus, which is moreover filled with polymer 3. Two pipe ends are thereafter put butt-in-butt and welded together.

The outer pipe 1 of the two adjoining multi-layer pipes is welded together from the outside, as indicated by 8, while the inner pipe 2 of the two abutting multi-layer pipes is welded together from the inside, as indicated by 9, by means of an arc welding machine. The ceramic ring 7 prevents the polymer material from being destroyed by the heat from the welding, and the two part pipes 1, 2 from getting in metallic contact with one another. If desired, the diameter of the pipe ends 1 to be welded together may be increased to give more room for the ceramic ring 7.

A single short-circuited end can be accomplished as shown on a longitudinal section in fig. 3 by means of welding. A ring 11 of the same material as pipe 2 can be explosion welded to a larger ring 10 of carbon steel, which can then be machined e.g. to a flange-form for further connection. The composite end section 10 and 11 can then be welded together to pipes 1 and 2 by means of welds 8 and 9 as for a pipe joint described above.

An example of an electrically insulated pipe end is shown in fig. 4 in a longitudinal section. A flange-formed ring 12 of the same material as the pipe 2 is located inside an end ring 13 of carbon steel. A flanged ring 15 of carbon steel is subsequently connected with the end ring 13 by the weld 18. The rings 12 and 13 feature holes permitting a metal rod to be screwed to the ring 12 through ring 13 without being in contact with the ring. The rings 13 and 12 can then be connected with pipes 1 and 2 respectively by means of the welds 8 and 9, as for a pipe joint described above.

In the annulus between the rings 12, 13 and 15, and between the ring 13 and the metal rod 14, a polymer 16 can subsequently be injected, e.g. epoxy. If desired, the flange 15 can be covered with an electrically insulating composite material 17, which has a higher tensile strength than polymer 14.

The outer pipe 1 of carbon steel alone can be presupposed to absorb the pressure forces of the well flow. The thin inner pipe 2 of non-corrodible material (aluminium alloy, titan or stainless steel) will be forced against the polymer layer 3 5 which transfers the pressure forces to the outer pipe 1. With an internal pressure of e„g. 100 bars, the polymer layer 3 will be exposed to a surface pressure of somewhat below 100 bars and will be slightly compressed. Since the polymer layer is thin, the absolute compression limit will be low, since the inner LCF pipe 2 is insignificantly strained. An estimate shows that with a 6 mm thick polymer layer 3, a thin inner pipe 2 will be strained far below the yield point.

A coaxial pipeline, as described above, will electrically

15 behave like a coaxial transmission line. The difference in practise is that the pipe line has a relatively high ohmic resistance and capacitance. The ohmic resistance is here advantageously exploited for electrical heating. In very long pipelines, the high capacitance will cause part of the

20 alternating current to go through the insulation layers between the outer and inner pipeline as a displacement current and not reach the end of the pipeline. In addition, there will be a reflection of the alternate current wave from the short-circuited end, and resonance effects may be produced.

25 This means that the current intensity, and thus also the heating effect, will not be constant along the pipeline. For some pipe lengths, the current intensity may be highest at the short-circuited end and have a minimum near the middle. For very long pipelines, little current will reach the end.

30

These undesired effects are strongest when the capacitance is high, i.e. when the polymer layer is thin. Calculations show that when the polymer layer is at least 6 mm thick, a 150-km pipeline can be heated without any part of the pipeline getting

35 more than twice as much electrical effect as another. An average insulation thickness of 6 mm or more will also be

desirable on account of dimension tolerances in the outer and inner pipes. An epoxy layer of 6 mm has an electrical breakdown voltage of approx. 150 kV, which is approx. 10 times higher than relevant voltages during heating.

The typical current intensity of alternate current heating will be approx. 500 A per pipe circumference metre for an inner pipe of stainless steel, which will give a voltage drop of approx. 60 V/km.

A multi-layer pipe/pipeline according to the invention for transport of oil and gas in the form of corrosive multi-phase well flow has outstanding electrical properties with regard to heating to prevent or defrost hydrate formation, and can be fabricated at a far lower price than known multi-layer pipes/pipelines, e.g. clad steel pipes, with similar mechanical strength properties.