This invention is concerned with coating of pipes of the type used in subterranean or submerged pipelines for the recovery of oil, gas, slurries or the like pipeable materials from a subterranean well . In particular it is concerned with pipes in which a steel pipe section is coated with an anti corrosion coating and then a concrete aggregate anti-buoyancy coating (hereinafter referred to as a concrete coating) . Such pipes normally include a mechanical shear transfer device in the form of a wire winding or caging applied around the anti-corrosion coating prior to the application of the concrete coating. This opposes the tendency of the concrete coating to slip or jump off the anti-corrosion coated pipe during laying operations which can impose considerable bending stresses upon the pipe, leading to loss of coating integrity.

Pipe coating methods addressing the problems of effectively applying a concrete coating to a pipe treated with an anti-corrosion coating are described (without detailed discussion on curing of concrete) , for example, in earlier patent publications including US-A-3 955 600, GB-A- 1 504 051, GB-A-1 504 052, GB-B-2 088 992, GB-B-2 101 499, EP-B-0 380 474 and WO 94/26 426. The teachings of these documents concerning application of concrete coatings to pipes are hereby incorporated by reference.

In a typical coating method uniform lengths of steel pipe are accumulated for coating and each in turn is lifted e.g. by overhead crane onto a bogie for individual coating treatment during which the length of pipe is caused to rotate about its longitudinal axis and passed longitudinally past a concrete coating station wherein concrete/aggregate is caused to impinge upon the rotating pipe thereby building up a thickness of--concrete on the pipe to form a coating. Such a concrete coating can also be applied by other methods. The term "concrete" is used herein for brevity and convenience but will be understood by those versed in this art as referring to any weight coating consisting of a high density material together with a cementitious binder delivered as a plastic mass for coating purposes, and which

after coating forms a hardened coating. Preferred weight or anti buoyancy coatings may be applied to thickness of from about 40 mm to 150 mm or more and have a typical density of from 2165 to 3215 kg/m ³ by choosing appropriate mixtures of cement, sand and iron ore.

Once the pipes are coated, adequate curing of the concrete is essential to achieve the required strength and quality. The aim of curing is to promote the proper hydration of the cement. This is accomplished by preventing moisture loss and, when necessary, by providing a controlled temperature environment. Moisture is a necessary ingredient in the curing process as hydration encompasses a multitude of chemical reactions between the water and the cement. These reactions produce highly complex calcium silicate and aluminate compounds, amongst others, which bind the aggregate in a hardened matrix.

The moisture content of the concrete is critical as hydration will cease if there is insufficient water to continue the reactions. Therefore the concrete coating must not be allowed to dry out prematurely. The chemical reactions involved are overall exothermic so that heat is generated as the cement sets and gains strength. Therefore ambient temperature also affects the setting time and the rate of gain of strength. Hence setting time and strength gain may be accelerated by artificial heating. This can be achieved by placing the green, i.e. partially cured, concrete coated pipes in a saturated steam (wet steam) environment. This has the advantages of reducing setting time, reducing shrinkage and the tensile forces it causes within the concrete, and arresting moisture loss.

Traditional curing methods may take up to 5 days from application of the wet concrete to achieve fully hardened coatings but modern accelerated curing techniques are known whereby adequate strength in certain concrete structures may be achievable in less than 18 hours from application.

As in many fields this industry would welcome more efficient ways of achieving optimum concrete curing in a commercially viable way. Accordingly it is an object of the present invention to provide an improved method and

apparatus for coating pipes with concrete whereby curing is achievable in a safe, effective and highly practical manner which is capable of complete automation.

According to the present invention there is provided apparatus for the curing of concrete coated pipes comprising a plurality of mobile pipe carriers, an insulated enclosure in which there is provided a controlled wet atmosphere maintained at an elevated temperature, said enclosure being sized to accommodate a multiplicity of pipes together with pipe carriers, and means for sequentially advancing said pipe carriers in continual succession through said insulated enclosure.

Preferably each carrier supports a plurality of pipes in a non-contiguous manner and arranged transversely to the length of the carrier. A suitable number would be 3 or 4 pipes supported in parallel and lying across the carrier.

Preferably said carriers are rail guided vehicles and said means for sequentially advancing said pipe carriers comprises a circuit of rail tracks providing in parallel a first transport path for carriers loaded with pipes having a coating to be cured in the enclosure and a second transport path for return of empty carriers from which pipes have been removed following curing of the coating in said enclosure. The second transport path may also pass through the enclosure so that the carriers remain close to the operating temperature of the curing enclosure. It will be understood that passing a succession of carriers outside the curing enclosure for a period of time may at some locations result in a substantial chilling of the carriers with respect to the operational temperature of the curing apparatus. Such chilled carriers upon return to the curing enclosure may introduce undesirable heat sinks and condensation points thus unduly disturbing the predetermined temperature profile of the curing enclosure requiring more energy input at the entry zone in said enclosure by way of compensation.

Preferably also there are provided chambers at either end of said insulated enclosure to allow the entry and exit of said pipe carriers while maintaining the elevated temperature therein thereby mitigating potential heat losses

and temperature gradients or fluctuations in the curing enclosure.

Preferably the elevated temperature is maintained by the presence of saturated steam within the insulated enclosure. Whereas the mobile pipe carriers may be self- propelled remote-controlled vehicles having means for adjustment to provide adjustable support length and diameter to accommodate differing lengths and diameters of pipe, it is more economical to provide independent drive means arranged at either end of the enclosure to shunt undriven vehicles in succession as a train through the curing enclosure.

The most preferred arrangement provides for parallel tracks of paired rails and transfer sections, outwith the entry/exit chambers and the insulated enclosure, enabling cross-over between said tracks, a first track having a section passing through said entry/exit chambers and said insulated enclosure to allow a carrier to present at least one pipe for curing in said insulated enclosure before passing over a transfer section to allow that carrier to be returned for reloading on a second track.

Preferably the transfer section comprises a sliding section of track which is displaceable laterally at right angles to the track length or normal direction of travel to bring that section into correspondence with a second track parallel to the first track. The second track may either pass through, at least partially though or outwith the insulated enclosure and entry/exit chambers.

Preferably also pipe loading and offloading devices are provided adjacent to the entry and exit chambers to facilitate the loading and unloading of the carriers, together with support means to allow the concrete coated pipes to be stored prior to and after the elevated temperature curing operation. An embodiment of the present invention will now be described with reference to the following drawings in which :

Figure 1 shows an overhead view of the layout of a curing plant in accordance with the present invention;

Figure 2 shows a plan view of a pipe carrier; Figure 3 shows a section B-B through the pipe carrier shown in Fig. 2;

Figure 4 shows a side view of the pipe carrier shown in Fig. 2; and

Figure 5 shows an end view of the pipe carrier shown in Fig. 2.

Pipes are coated with an anti-buoyancy concrete coating typically of from about 40 mm - 150 mm thick and having a density of 2645 kg/m ³ based on Ilmenite. The concrete coating density may be varied from 2165 to 3125 kg/πr by choosing appropriate mixtures of cement, sand and iron ore. The coating is cured in accordance with the invention in a curing plant 5 illustrated generally in Fig. 1 wherein the layout of the curing plant is shown. The plant 5 comprises a pre-curing area 10 dedicated to the marshalling of freshly coated pipes and their subsequent loading onto pipe carriers 15, a curing hall 20, and a post-curing area 25 dedicated to the removal of cured pipes from the carriers 15 and their storage. Freshly coated (green) pipes (not shown) are delivered to the marshalling area 10 from the coating plant (also not shown) . Upon arrival the pipes are allowed time to cure at the ambient temperature and this initial curing may take up to three hours. The pipes are then loaded, three at a time, onto the carriers 15 for accelerated curing in accordance with the invention. The residence time within the curing hall is variable but 4 to 6 hours is considered adequate depending upon coating thickness.

The carriers 15 are supported on a pair of tracks, 30 and 30", each comprising two rails. The tracks run from the marshalling area 10, through the curing hall 20 and terminate in the post-curing area 25. The first track 30 is used to convey loaded pipe carriers 15 from the marshalling area 10, through the curing hall 20, to the post-curing area. The second track 30' is then used to transport the empty carriers 15 back to the marshalling area. Transfer sections, 35 and 35 ¹ , are provided to allow the carriers 15 to be swapped between tracks 30 and 30' .

The curing hall 20 is at least partially filled with saturated steam delivered at atmospheric pressure or slightly above in order to maintain conditions conducive to concrete curing. The walls, 40 and 40', and the roof of the hall 20 are insulated to prevent heat loss and hence control operating costs. The steam filled section of the hall may be in the region of one hundred metres long and the laden pipe carriers 15 may take up to six hours to traverse the steam filled section. Once the laden pipe carriers 15 have completed their journey through the curing hall 20 the pipes are removed in the post-curing area 25. The cured pipes are then allowed to cool naturally back to the ambient temperature.

Referring now to Figures 2 through 5 there is shown a pipe carrier 15. The carrier 15 is composed of a rigid frame 45 manufactured from lengths of steel channel . Encapsulated within the frame 45 are four wheels 50 which allow the carrier 15 to move along rails (not shown) provided within the curing plant. The frame 45 is surmounted by six concave bearing surfaces 55 which serve to support concrete coated pipes (not shown) while they are conveyed through the curing plant. The bearing surfaces may be interchanged to accommodate pipes of varying diameters . This apparatus is intended to be fully automated and may be operated with confidence with minimal supervision from a central control room. Details of the automatic control systems are not essential to the understanding of the accelerated curing method and so are not described herein. The embodiment of the invention described above is given by way of illustrative example only, and the specific detail thereof given to facilitate understanding of the actual practice of the invention is not intended to limit the scope of this invention in any way.