This section deals with cross-country transmission pipelines which conduct natural gas or liquid products such as crude oil.

Pipes of reasonably long lengths are produced in a tube mill. They are either seamless or electric-resistance welded, or submerged-arc welded. Laying of pipelines involves only circumferential welding in the field.

Seamless pipes are made from solid round billets of proper diameter and length. Surface defects of the billets are initially removed by scarfing. The billets are heated and pierced to make a hole in the solid billet. The so-formed pipe is passed successively through a plug-rolling
mill to elongate it and reduce the wall thickness to the desired dimension. The pipe is rounded and smoothed on the inside and outside surfaces by passing through a reeling machine. The pipe is finally sized by passing through sizing rolls, straightened, expanded, hydrostatically tested and beveled at the two ends.

Resistance-welded pipes are made from rolls of steel strip in a tubemaking machine. In this machine, the continuously fed strip is passed through forming rolls to form a straight O-shaped section, which is electric-resistance welded at the seam. The emerging pipe is tested continuously by means of a non-destructive testing device and cut to the desired length. A

coiler is used if a long length of pipe is to be supplied in coil from.

The operation of producing large diameter pipes by the submerged-arc process is best

understood by referring to the procedure followed by a firm in the U. S.A.

The firm produces mild steel pipes up to 13 m length and diameter between 500 and 900 mm and thickness between 6.3 and 12.7 mm in the following stages :

1. Shearing the edges to exact widths, bevelling the edges and pre-forming the plate by an initial bending of the edges.

2. U-ing press.

3. O-ing in a semi-cylindrical die with another top semi-cylindrical die activated by two massive hydraulic rams of 6,000 tons capacity.

4. Tack welding and tack grinding.

5. Cleaning the pipe in degreasing bath.

6. Tab is weld at each seam end to assure proper lead-in and cut-off of finish welds.

7. The pipe is welded finally by the submerged-arc process, one run on the inside and another run on the outside. For the first pass, water-cooled backing is used.

8. The finished pipe is moved on to the expander, where it is surrounded by locked

restraining dies, while water at extreme pressure is pumped in, expanding the pipe against the enclosing dies. The expander does the following functions:

(a) Pipe ends are mechanically expanded to size.

(b) Hydrostatic Pressure expands the pipe to the exact size of the mechanically locked

dies.

(c) Pipe is tested to code requirements.

(d) Hammers are dropped, while pipe is under maximum code pressure.

(e) Inspector examines welds for leaks.

Two 13 m long pipes may be welded to make 26 m lengths, again using submerged-arc welding.

Finally, there is end facing and bevelling.

The forming is at the rate of 20 m/min and output is up to 3,000 tons in eight hours.

A typical boom welder used for the internal welding of pipe by the submerged-arc process is shown in Fig. 11.8. It is fitted with a television monitor. The 375 mm diameter boom enables pipes of 450 mm and large diameters and lengths up to 10 m to be welded internally.

Pipes are also welded by the submerged-arc process, using the so-called spiral welding technique. The main advantage is that with a given width of plate or coil, a wide range of pipe diameters can be fabricated.

In this technique, the edges of plates or coils are trimmed to the required width and bevelled. They are then subjected to a modified three-roll bending arrangement supported by internal or external cage rolls, and the result is a continuous helix. The first welding pass is laid on the internal diameter of the seam and then on the external diameter, 180° away. The conventional single electrode or two electrodes in tandem may be used for the submerged-arc process. To feed the stock continuously into the machine, ends of plates or coils are welded only on the inside by the submerged-arc process prior to forming. After seam welding, the required length of pipe is cut off and the external cross-weld is completed. The maximum outside diameter of seamless pipes is 650 mm. High frequency resistance seam welding is used to produce pipes and tubes of diameters ranging from 12.5 mm to over 1,250 mm and with wall thicknesses of between a fraction of millimetre and 25 mm. Submerged-arc welding is best suited for large diameter pipes, which can be internally and externally. Penstock pipes of 10 m diameter and above have been welded by this process.

Generally, pipes for the transmission of liquid products are smaller in diameter than pipes meant for natural gas. The common diameters used for gas transmission are 600, 750 and 900 mm (24, 30 and 36 inch), though recently these have been increased to 1,400 or 1,500 mm.

Transmission pipelines are usually manufactured to the API specifications for Line Pipe. They specify, among other things, the strength levels of various steels to be used, working stress levels and longitudinal joint efficiency of pipes, and tests for the qualification of proce­dures and welders.