Mechanically lined pipe

Abstract

A reelable mechanically lined pipe (MLP) (30) having at least a liner (32) and an outer pipe (34), the outer pipe having an outer diameter, DH, with the MLP formed from a plurality of pipe joints having conjoining girth welds (36), wherein the ends of each pipe joint terminate with clad overlay welds (40) having a length in the range L.sub.min=100 mm and L.sub.max=4D.sub.H, and wherein the liner thickness, t, is equal to or less than a value calculated by formula I as defined.

Claims

1. A reelable mechanically lined pipe (MLP) formed from a plurality of pipe joints having conjoining girth welds, and comprising at least a liner and an outer pipe, the outer pipe having an outer diameter, D.sub.H, and wherein the liner thickness, t, is less than or equal to a value calculated by formula I:
t=a.sub.00(D.sup.0.75).sup.0g.sup.0+a.sub.01(D.sup.0.75).sup.0g.sup.1+a.sub.10(D.sup.0.75).sup.1g.sup.0+a.sub.11(D.sup.0.75).sup.1g.sup.1+a.sub.20(D.sup.0.75).sup.2g.sup.0+a.sub.21(D.sup.0.75).sup.2g.sup.1+0.16 where: t is in mm: a.sub.00, a.sub.01, are constants defined by Table 1: TABLE-US-00002 Ij a.sub.ij 00 2.39846 01 0.239488 10 5.48161 11 2.35153 20 6.50598 21 1.37840 is the maximum reeling strain, g is the radial insertion gap in mm, and D=D.sub.H2t.sub.H is the liner outer diameter in mm, wherein D.sub.H is defined above and t.sub.H is the wall thickness of the outer pipe of the MLP in mm; wherein the ends of each pipe joint terminate with overlay welds of a length in the range of L.sub.min=100 mm and L.sub.max=4D.sub.H, and wherein the MLP is able to be reeled onto a reel having a radius R and $\mathrm{.Math.}=\frac{\frac{D_H}{2}}{\frac{D_H}{2}+R}.$

2. A reelable MLP as claimed in claim 1, wherein the outer pipe comprises a carbon steel outer pipe.

3. A reelable MLP as claimed in claim 1, wherein the liner comprises a Corrosion Resistant Alloy (CRA) liner.

4. A reelable MLP as claimed in claim 3, wherein the liner is alloy 316L, 825, 625, 904L, 22% Cr duplex or 25% Cr super duplex.

5. A reelable MLP as claimed in claim 1, spooled on a reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP during spooling.

6. A reelable MLP as claimed in claim 1, wherein at least two pipe joints of the MLP have one of different outer pipe thicknesses, different outer diameters, and different outer pipe thicknesses or outer diameters.

7. A method of manufacturing a reelable mechanically lined pipe (MLP) as defined in claim 1, comprising at least the steps of: (a) providing a plurality of pipe joints having a liner and an outer pipe as defined in claim 1; (b) terminating the ends of each pipe joint with clad overlay welds whose length is in the range L.sub.min=100 mm and L.sub.max=4D.sub.H; (c) girth welding the pipe joints together to provide pipe sections; and (d) girth welding the pipe sections together to provide the MLP.

8. A method of reeling onto a reel a reelable mechanically lined pipe (MLP) as defined in claim 1 comprising at least the steps of: (a) providing a reelable mechanically lined pipe (MLP) as defined in claim 1; and (b) reeling the MLP onto the reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which;

(2) FIG. 1 is a schematic cross-sectional view of an MLP;

(3) FIG. 2 is a diagrammatic view of a method of spooling an MLP onto a reel;

(4) FIG. 3 is an illustration of the effect of a high mismatch during reeling in a prior art MLP; and

(5) FIG. 4 is a cross-sectional longitudinal view through a portion of a reelable MLP according to one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(6) Referring to the drawings, FIG. 1 shows a schematic cross-sectional view of parts of a mechanically lined pipe (MLP) 2. The MLP 2 generally comprises a number of layers (including coating), only two of which are shown in FIG. 1 for clarity, comprising an outer layer 4 which can be a carbon steel pipe, and an inner layer or liner 6 being formed from a corrosion resistant alloy (CRA), such as alloy 316L. The relative dimensions shown in FIG. 1 are not to scale, and are provided for clarity of representation.

(7) In the conventional manufacture of an MLP, an inner layer is usually provided into an outer layer and then expanded to provide an interference contact stress between the two layers. However, bending of a pipe formed from two such layers, such as spooling on or off the reel, and especially any buckling of the pipe, is assumed to create wrinkles which may not be removable after spooling off, and which are therefore considered to be sufficiently detrimental to the laid pipeline that buckling and the forming of any wrinkles should be avoided at all costs.

(8) Thus, for such laying methods, a high interference contact stress is desired between the inner and outer layers to prevent wrinkling of the inner liner. The interference contact stress can be lost during the coating procedure and will certainly be lost during plastic deformation which occurs during bending. However, if interference stress is lost during the 1.sup.st or 2.sup.nd strain event then wrinkles will usually appear in the 3.sup.rd or 4.sup.th strain event. It can also be difficult to control the degree of intended interference contract stress during manufacturing of such pipes, and a high degree of variation of interference stress occurs in practice.

(9) Buckling during reeling should also be avoided, preferably without having to increase the thickness of either the outer layer 4 or the inner layer or liner 6.

(10) FIG. 2 shows a diagrammatic reel 10 having a smallest bending radius R, and a mechanically lined pipe (MLP) 12 having an outer pipe diameter D.sub.H. The MLP 12 is formed in long lengths of pipe sections (or pipe stalks) joined together to form a single pipeline. Stalks are normally 1 km long, but can be longer or smaller as required. FIG. 2 shows spooling of the MLP 12 onto the reel 10.

(11) By way of example only, the reel 10 could have a bending radius R of 8.23 m, and the MLP 12 could have a diameter D.sub.H of 12.75 inches (323.9 mm) and a total wall thickness of 18.9 mm.

(12) FIG. 3 shows a portion of the MLP 12 comprising the ends of first and second pipe joints 14, 16 conjoined by a girth weld 18. Whilst it is preferred and intended that each pipe joint is exactly the same, variations therein, as discussed above, can lead to some pipe joints being stronger than other pipe joints. For example, the first pipe joint 14 is a stronger pipe joint 14 than the second or weaker pipe joint 16. As the portion of the MLP 12 shown in FIG. 3 is bent onto the reel 10 of radius R of FIG. 2, ovality and strain localise in the weaker joint 16 near the girth weld 18 because the bending moment capacity of the weaker joint 16 is significantly lower than that of the stronger joint 14. This eventually leads to buckling 20 within the weaker pipe joint 16, and wrinkling of the liner 6 therewithin.

(13) FIG. 4 shows a reelable MLP 30 according to one embodiment of the present invention. The reelable MLP comprises a liner 32 and an outer pipe 34. The outer pipe 34 has an outer diameter D.sub.H. The reelable MLP 30 is formed of a plurality of pipe sections. FIG. 4 shows three full or partial pipe joints labelled A, B and C, of a given pipe section. The pipe joints are conjoined using girth welds 36 known in the art.

(14) Conventionally, the pipe on either side of the girth weld 36 has internally welded clad overlay welds added thereto, typically 50 mm long, followed by the liner 32. This length of clad overlay weld is currently sufficient to enable inspection and any girth weld repair, if needed, and is sufficient to stop water ingress between the liners 32 and the outer pipes 34. Hitherto, the clad overlay welds have served no other significant purpose, such that they have conventionally been considered as only requiring sufficient length for these purposes. In FIG. 4, such a conventional clad overlay weld length is labelled L.sub.1.

(15) However, the present invention has determined that the clad overlay welds could accommodate the localised strain that is caused by any mismatch between adjacent pipe joints. Conventional solutions to accommodate high levels of mismatches during reeling without significant liner wrinkling only have been to increase the thickness of the carbon steel pipe and/or the thickness of the liner, either of which cause increased expense. The present invention has found that increasing the length of the clad overlay welds allows accommodating high strains due to mismatches without the need to increase the thickness of the outer pipe or the liner.

(16) Thus, as shown in FIG. 4, the present invention involves increasing the length of the overlay welds from L.sub.1 (as a standard weld length, e.g. 50 mm) to L.sub.2. L.sub.2 is between L.sub.2min=100 mm and L.sub.2max=4D.sub.H where D.sub.H is the outer diameter of host pipe. Clearly, in FIG. 4, L.sub.2>L.sub.1.

(17) L.sub.2 depends on the sizes and grades of the host pipe and liner and the magnitude of mismatch, and will generally be calculated on a pipe-by-pipe basis.

(18) Various modifications and variations to the described embodiment of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with a specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.