Technical Field The invention relates to a method of producing steel pipes having at least two outer layers.

Background Art It is known to produce metal pipes such as steel pipes for burial in the ground, each pipe being coated with a layer of material in form of a mixture of bitumen and polypropylene. These pipes are not always satisfactorily protected, the pipes being provided with insufficiently hard surfaces, ie offer no protective shield against mechanical forces and/or exposure to chemicals.

Brief Description of the Invention The object of this invention is to provide a method of the above type and by which the steel pipe is made particularly physical impact-resistant, eg in connection with the handling of the steel pipe during transport and burial in the ground and further resistant to external forces after burial. It is further the object to render the pipe more resistant to most chemicals than hitherto known.

The method according to the invention is characterised in that a) the outside of a steel pipe is coated, eg. by extrusion, with a first layer of a material chosen from among the following materials: bitumen, SBS-modified bitumen, APP-modified bitumen, polyalphaolefin-modified bitumen, bitumen enamel and tar enamel, at least one fibre-reinforcement felt, eg of fibre glass, being applied at the same time, and that subsequently thereto

b) a layer of polypropylene is coated directly on top of the fibre-reinforcement felt so as to form a shield around the pipe.

As a result an effective impact resistance is provided on the outside of the steel pipe, whereby the pipe is able to withstand the mechanical forces associated with transport and burying of the pipe. When buried the pipe is also less susceptible to external forces, eg. physical impacts from stones, rocks, soil stresses and digging tools, which inadvertently come into to contact with the pipe as well as exposure to chemicals such as oil, salts and the like present in the ground.

According to the invention the first layer may be coated at a temperature of between 180-245°C, preferably between 190-220°C, and in particular of 200°C, at which temperature the said layer may be subjected to a controlled cooling process and inspection before the polypropylene layer is applied. The effective life of the steel pipe is thus prolonged.

Furthermore according to the invention the first layer may be applied in a thickness of 2-6 mm, preferably 2.5-5 mm, and most preferably about 3 mm. This has proved to provide the steel pipe with an excellent corrosion proofing.

Moreover according to the invention the layer of polypropylene may be applied in a thickness of 0.25-3.0 mm, preferably 0.5-2.0 mm and most preferably 1. 0-1.5 mm, which has proved particularly advantageous in that the polypropylene shield obtains a suitable thickness.

According to the invention at the application of the first layer the polypropylene may have a temperature of between 180 and 220°C, preferably between 190-210°C, eg 200°C, whereby the polypropylene is evenly distributed on the steel pipe.

Moreover according to the invention after application the polypropylene may be

subjected to a controlled cooling process, preferably water cooling resulting in a reduced production time of each coated pipe.

Moreover according to the invention the polypropylene may be extruded by means of a flat nozzle to form a polypropylene band, the pipe slowly rotating while the band is applied onto the fibre-reinforcement felt on the first layer, the feed rate of the band at the pipe being 5-60 m/min., preferably 10-50 m/min., and most preferably 25-30 m/min, whereby a particularly easy manufacture of the pipe is obtained.

Furthermore according to the invention the polypropylene band may be applied by providing an overlap between a band winding and the succeeding band winding, said overlap having at least half the width of the band so as to obtain a smooth surface and to strengthen the shield.

According to the invention the polypropylene maybe extruded by means of an annular nozzle, the pipe preferably being advanced at a rate of 0.5-10 m/min, eg. 3-6 mlmin.

This embodiment of the method has proved particularly advantageous at pipes of a diameter less than about 500 mm According to the invention the first layer may be applied directly onto the steel pipe after cleaning thereof for instance by sandblasting, blasting with metal grains or by means of a wire brush, a primer layer, however, optionally also being applied. As a result, the first layer adheres strongly to the steel pipe, which in turn is highly corrosion-proof.

Finally according to the invention the controlled cooling of the first layer may be performed by applying water thereon for at least 0.5 minutes, whereby the production time of the finished pipe is reduced.

Brief Description of the Drawings

The invention is explained in detail below with reference to the drawings, in which Fig. 1 is a diagrammatic view of the steps of the method according to the invention, Fig. 2 is a diagrammatic and inclined side view of a first machine for carrying out the method according to the invention, Fig. 3 is a diagrammatic and inclined side view of a second machine for carrying out the method according to the invention, Fig. 4 is a diagrammatic and inclined side view of a third machine for carrying out the method according to the invention, Fig. 5 is a longitudinal sectional view through a pipe produced by means of the method according to the invention.

Best Modes for Carrying Out the Invention As shown in Fig. 1, the method according to the invention comprises two steps: step (a) and step (b). Step (a) may for instance be carried out using the machine shown in Fig. 2, said machine comprising a feed and carrier member 15 with which a steel pipe 4 may rotate while being advanced, and an extruder head 16 such as a flat nozzle. In step (a) the outside of the steel pipe 3 is coated, eg by means of the extruder head 16, with a first layer 4 of an anti-corrosive material chosen from among the following materials: bitumen, SBS-modified bitumen, APP-modified bitumen, polyalphaolefin- modified bitumen, bitumen enamel and tar enamel, at least one fibre-reinforcement felt 2, eg of fibre glass or polyester or a combination thereof, being applied at the same time. Step (b) may for instance be carried out using the machine shown in Fig.

3, said machine comprising a carrier means (not shown) and an extruder head 19, eg in form of a flat nozzle. In step (b) a layer 6 of polypropylene is applied directly onto

the fibre-reinforcement felt 2 by extrusion by means of the nozzle 19 to provide a shield around the pipe 3 to provide the pipe with excellent impact resistance.

The application of the first layer 4 may be performed at a temperature of between 180-245°C, preferably between 190-220°C, and in particular of 200°C and the said layer may be subjected to a controlled cooling process and inspection prior to the application of the polypropylene layer 6. The controlled cooling reduces the production time of the finished pipe, and the said inspection ensures against undesirable defects in the first layer 4.

The first layer 4 may be applied in a thickness of 2-6 mm, preferably 2.5-5 mm, and most preferably about 3 mm, while the polypropylene layer 6 may be applied in a thickness of 0.25-3.0 mm, preferably 0.5-2.0 mm and most preferably 1.0-1.5 mm Prior to the coating of the first layer 4 the polypropylene may be heated to a temperature of between 180 and 220°C, preferably between 190-210°G, eg 200°C.

This temperature is obtained by means of heating elements (not shown) provided in the machine used.

After application the polypropylene 6 may be subjected to a controlled cooling process, eg. water cooling, as shown by means of a cooling member provided with water nozzles 12.

As noted above, the polypropylene may be extruded by means of a flat nozzle 19 to form a polypropylene band during slow rotation of the pipe. The band covers the fibre-reinforcement felt 2 on the first layer 4. The feed rate of the polypropylene band at the pipe may range from 5-60 m/min., preferably 10-50 m/min, and most preferably 25-30 mlmin. It is also possible to extrude the polypropylene by means of an annular nozzle 20 on a separate unit as shown in Fig. 4, where the polypropylene is placed around the steel pipe 5 as a jacket. In this case the steel pipe 5 is advanced at a rate of 0.5-10 m/min, eg. 3-6 m/min.

The first layer 4 may be applied directly onto the steel pipe 5 after cleaning of the latter for instance by sandblasting, blasting with metal grains or by means of a wire brush. However it is also possible to apply a layer of a primer (not shown) onto the steel pipe.

It is further possible to perform the controlled cooling of the first layer by applying water thereon for at least 0.5 minutes. The application is carried out by means of a cooling section 13 (confer Fig. 2), in which water nozzles 14 sprays water down on the fibre-reinforcement felt and the first layer 4.

Insofar as the extruder head 16 is concerned it should be noted that this may be shaped as a pouring means (not shown) for the material of the first layer 4.

As shown in Fig. 3, the polypropylene band may be applied by providing an overlap of at least half the width of the band 6 between each band winding 6a and the succeeding band winding 6b.

When the finished pipe exits the right-hand end of the cooling section 11, the polypropylene layer has shrunk firmly onto the other layers on the pipe 3 and now forms a hard (but also ductile) shield such that the finished pipe is resistant to physical impacts to which it may be subjected during transport and subsequent burying in the ground.

Fig. 5 clearly illustrates a finished pipe manufactured by means of the method. The steel pipe is indicated at the reference numeral 3; outside thereof the first layer 4 is shown and the fibre-reinforcement felt 2 is in turn shown outside thereof. On the outside of this layer the polypropylene jacket 6 is arranged as a type of shrunk sheath forming a shield.

It is advantageous to use the method according to the invention on steel pipes with a

diameter of 10-160 cm.

The abbreviation SBS denotes styrene-butadine-styrene. The abbreviation APP denotes ataxic polypropylene. An example of a polyalphaolefin is a PAO.

The invention may be modified in many ways without thereby deviating from the scope of the invention, as it appears from the attached claims. Thus, the said primer may for instance be a synthetic primer, such as a primer of the type containing rubber compounds dissolved in xylene.

It should be noted insofar as step (a) of the method is concerned that the application of the first layer 4 may be performed by other processes than extrusion, eg. pouring and painting. As to the fibre-reinforcement felt this may be impregnated with bitumen prior to being applied.