PROTECTION OF PIPE JOINTS

Abstract: A method of protecting a welded joint (7) between two weight-coated metal pipelines (1, 2) comprises positioning a wraparound sheet steel mould (8) around the joint, retaining that mould around the joint using a wraparound heat shrinkable fabric sleeve (9) with high tear strength, and filling the mould (8) with a corrosion resistant mastic (10).

Description: Protection of Pipe Joints Description This invention relates to the protection of joints in elongate substrates and more particularly to the protection of welded joints in submerged, weight-coated pipelines. Submerged weight-coated pipelines are usually installed by a purpose- built offshore vessel known as a laybarge. In a typical operation, twelve metre long steel pipes are coated onshore with an anti-corrosion coating, for example, coal tar enamel, asphalt, or a fusion-bonded epoxy coating, leaving a bare section at each end of the pipe to allow subsequent welding of adjacent lengths to take place without contamination or coating damage. This bare area normally extends up to 250 mm from each end of the pipe. In the next stage, the weight coating, which is usually a mixture of concrete and iron oxide, is applied over the anti-corrosion coating to a thickness of from about 25 mm to about 150 mm, depending upon the degree of negative buoyancy required. The weight coating is also cut-back from the pipe ends to expose the bare section of pipe and about 200 mm of the anti-corrosion coating, in order to allow subsequent overlap and completion of weld area corrosion protection. On the laybarge the coated pipes are strung together, aligned, welded, and the welds x-ray inspected. An anti-corrosion layer is then applied over the welded region which may, for example be a sealant-coated tape, or a heat- shrinkable wraparound sleeve. Finally the joint area is mechanically protected, to restore the weight coating at the joint, and to protect the joint from mechanical damage during subsequent operations. Typically the laybarge is divided into a number of work stations each carrying-out a specific function at a pipe joint area. The completed pipeline leaves the rear of the laybarge and encounters a series of rollers and guides which assist, correct and control entry of the pipeline into the sea, river or lake. This collection of rollers and guides, known as the stinger, is towed along by the laybarge and is subjected to any sea movements or wave impacts, hence the pipeline supported by the stinger is itself buffeted about and impacts with the rollers and guides, particularly in rough weather. Such impacts can be severe and could result in damage to the joint area weight coating and exposed anti-corrosion coating if no mechanical protection system were to be employed. Owing to the high cost of operating a fully equipped laybarge all operation must be carried-out quickly and efficiently, with no single station taking more than 8 minutes to complete its elected task. Various mechanical protection systems for welded pipe joints have been proposed and are commercially available. The most common system involves the use of hot marine mastic, and asphalt /bitumen based material reinforced with aggregate. This marine mastic, which is normally supplied in blocks which have first to be broken up, is fed into a large heated hopper and maintained at a temperature between 180°C and 240°C, at which the marine mastic is in a molten /fluid state. The hopper is positioned at the last working station on the laybarge, directly above the pipeline joint. A single, flat metal sheet is wrapped around the joint area, overlapping the adjacent weight coating, and onto itself to form a mould. Steel straps are wrapped around the mould and tightened to hold the mould in place. The hot marine mastic from the hopper is introduced through a hole cut in the top of the sheet mould and fills up the annulus formed by the mould. The hole is then sealed by means of a metal sheet that is held in place by means of straps. One example of a hot mastic approach is described in EP-A-0079610 where a covering is formed on a joint between covered steel pipes each having a protecting concrete layer by first wrapping a heat-shrinkable sheet of low shrinkage temperature around the joint, then installing a metal tube around the joint to form a closed space around the joint, and finally pouring fused mastic into an opening in the metal tube, the mastic shrinking the shrinkable sheet and solidifying to form a covering protecting layer. Another mechanical protection system for joints which is available involves the use of polymer cement in-fill systems instead of hot marine mastic. Yet another system uses special liquids that foam-up to fill in the annular space between pipe and mould. Again special chemicals must be correctly stored and then mixed using sophisticated application equipment requiring skilled operators. With the hot marine mastic approach, once the mastic has cooled, the metal mould is no longer required. It is merely a delivery container to hold the mastic while is sets. However typically cooling takes several hours, while the offshore laybarges can complete a joint in less then 15 minutes. Therefore the metal moulds are usually left in place on the pipe. This, however, can lead to problems in the lifetime of the pipeline. The metal mould and /or the straps holding it around the pipeline may corrode after exposure to the sea water. This may cause the mould to lose contact with the pipeline, or even spring away from the pipeline. If this occurs jagged edges of the metal mould project from the pipeline. These may, for example, cause damage to fishing nets, or interfere with shipping anchors. According to our invention the problems of the prior art can be overcome by providing a sleeve positioned around the mould, and engaged tightly around the mould substantially to prevent it leaving the pipeline, or projecting therefrom. The present invention provides a method of protecting a joint between two corrodible, weight coated elongate substrates which have been bared of weight coating in the joint region, comprising (a) positioning a wraparound mould around the joint region, and (b) filling the mould with a corrosion resistant material, characterised in that a wraparound sleeve is positioned around the mould, and engaged around the mould, substantially to retain the mould within the sleeve. It is not necessary for the sleeve to protect the mould from corrosion. Indeed, the mould, if metal as is usual, can corrode away completely within the confines of the sleeve. For easy installation the sleeve is preferable a recoverable sleeve, preferably a radially heat shrinkable sleeve. Heat recoverable articles are those that recover on heating toward an original shape from which they have previously been deformed, but the term "heat-recoverable" also include those articles which adopt a reconfiguration even if not previously deformed. Traditional recoverable sleeves comprise a polymeric material such as polyethylene. Examples are described in US 2027962, 3086242 and 3597372. More recently heat recoverable fabrics have been found to be useful. Heat recoverable fabrics are described in US 3669157, EP-A-115905, EP-A-116390, EP-A-116391, EP-A-116392, EP-A-116393, EP-A-117025, EP-A-117026, EP-A- 118260, EP-A-137648, EP-A-153823, EP-A-175554 and EP-A-0202898 In a particularly preferred embodiment of the invention the covering sleeve comprises a recoverable fabric. Any of those described above are suitable. Preferably the sleeve, whether fabric or not, exhibits significant tear or split resistance. This is desirable to prevent the sharp edges of the mould on corrosion splitting open the covering sleeve, and also to prevent splitting when the newly joined pipeline is fed over the rollers on the laybarge into the sea, river or lake. Thus the split resistance is preferably exhibited at operation temperatures, and also at elevated temperature e.g.. up to 180°C, 200 or even 250°C. Temperatures of this order will be encountered if the corrosion resistant in-fill material within the mould is a marine mastic or the like, having a melting point of 180°C or higher, e.g.. 200°C or 250°C. Preferably the wraparound sleeve has a tear strength at 25°C, 180°C or 200°C or even 250°C of at least 20N preferably at least 30N especially at least 50N, more especially at least 100N when tested in on Instron tensometer employing a draw rate of 100mm /mm. Preferably the tear strength is greater than 300N/25mm, preferably greater than 400N/25mm or even as high as 500N/25mm. The material of the sleeve, also preferably consists of components that can all resist temperatures encountered in use e.g.. 180°C, 200°C or even 250°C. Where the sleeve comprises a recoverable fabric the fabric is typically used in conjunction with a polymeric matrix laminated on one or both sides of the fabric preferably have melting points above, preferably 10, 20 or 50°C above the temperatures encountered in use that are described above. An embodiment of the present invention is now described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a longitudinal sectional view of a joint region prior to attachment of a metal mould and in-fill mastic; and Figure 2 shows the joint region of Figure 1 similarly inside sectional view, after the mould in-fill mastic and covering sleeve according to the invention has been installed. Referring now to the drawings, Figure 1 shows two steel pipes 1 and 2, having anti corrosion coatings 3 and 4, and weight coatings 5 and 6. The anti corrosion coatings and the weight coatings are cut back to expose lengths of bare pipe which are welded at 7. The lengths of the bared region is typically of the order of 700 mm. Referring now to Figure 2, this shows the arrangement of Figure 1 after a sheet steel mould 8 has been wrapped around the bared region. The mould 8 is wrapped so that it overlaps the weight coatings 5 and 6 on either side of the bared joint region. Hot marine mastic 10 has been poured into the mould 8, and the entry hole sealed. Over the mould 8 a heat recoverable fabric sleeve 9, has been installed into close conformity with the mould 8 and also to project beyond the edges of the mould 8 on to the weight coatings 5 and 6. The sleeve 9 may have any composition as described hereinbefore, and is radially heat recoverable. As seen in Figure 2 the sleeve 9 completely encloses the sheet steel metal mould 8. However it is not necessary for sleeve 9 to prevent corrosion of the sheet steel mould, and therefore, it is acceptable for sea water or the like in use to be accessible to the mould 8. Therefore sleeve 9 does not have to be sealed or bonded in any way to the mould 8 or weight coatings 4 and 5. It merely needs to retain within its confines the mould 8, to prevent the mould loosening and springing or pulling away from the pipes in use, which could cause anchor foulage or interfere with fishing nets. The fabric also needs to be resistant to tearing so that it is not damaged when the finished joint is pulled over rollers on the laybarge when the finished joint is entered into the sea.

Claims: Claims 1. A method of protecting a joint between two corrodible, weight coated elongate substrates which have been bared of weight coating in the joint region, comprising (a) positioning a wraparound mould around the joint region, and (b) filling the mould with a corrosion resistant material, characterised by a positioning wraparound sleeve around the mould, and engaging the sleeve around the mould substantially to retain the mould within the sleeve. 2. A method according to claim 1, wherein the wraparound sleeve has a tear strength of at least 20 N preferably at least 30 N, especially at least 50 N, more especially at least 100 N when tested in an Instron tensometer employing a draw rate of 100 mm mm at 25°C. 3. A method according to claim 1 or 2, wherein the wraparound sleeve has a tear strength of at least 20 N preferably at least 30 N, especially at least 50 N, more especially at least 100 N when tested in an Instron tensometer employing a draw rate of 100 mm/mm at 180°C or at 200°C or at 250°C. 4. A method according to claim 1, 2 or 3, wherein the wraparound sleeve has a tear strength of at least 300N/25mm, preferably at least 400N/25mm, especially at least 500N/25mm at 25°C. 5. A method according to any preceding claim wherein the wraparound sleeve has a tear strength of at least 300N/25mm, preferably at least 400N/25mm, especially at least 500N/25mm at 180°C, or at 200°C or at 250°C. 6. A method according to any preceding claim, wherein the wraparound sleeve is heat-recoverable, preferably radially heat shrinkable. 7. A method according to claim 6, wherein the wraparound sleeve comprises a recoverable fabric. 8. A method according to any preceding claim wherein the mould comprises a wrapped metal sheet. 9. A method according to any preceding claim, wherein the mould is metal, and retention of the mould by the sleeve does not prevent corrosion of the mould. 10. A method according to any preceding claim where the corrosion resistant material used to fill the mould is a marine mastic. 11. A method according to claim 10, wherein the marine mastic is poured into the mould at a temperature of at least 180°C.

Inventors: DUNNING ROBERT, JOHN ROBIN, VAN BEERSEL JOZEF, DUNNING, ROBERT, JOHN, ROBIN, VAN BEERSEL, JOZEF
Applicants: RAYCHEM LTD, RAYCHEM SA NV, N.V. RAYCHEM S.A., RAYCHEM LIMITED
Application year: 1993
Main classification: F16L58
IPC codes: B29C63, F16L58