Seamless pipe is formed by hot working steel to form a pipe without a welded seam. Several processes are available, illustrated schematically in figure 1.
The initially formed pipe may be subsequently cold-worked to obtain the required diameter and wall thickness and heat-treated to modify the mechanical properties. A solid bar of steel, termed a billet, is cut from a slab and is heated and formed by rollers around a piercer to produce a length of pipe. The Mannesmann mill is perhaps the best-known type of piercing mill. In this mill, the steel billet is driven between rotating, barrel-shaped rolls set at a slight angle to each other. The rolls rotate at about 100-150 rpm. The billet also rotates. The piercer is placed just beyond the point where the billet is squeezed by the rolls, so that as the formed billet passes through the pinch zone between the two rollers, the reducing stress tend to open the metal over the piercer.
Fig. 1 Seamless pipe-manufacturing processes
The piercing mill produces the primary tube that requires finishing to form the pipe. During finishing, the wall thickness is further reduced. In a plug-rolling mill, the pipe is driven over mandrels fitted with plugs of increasing diameters between rollers that extrude the tube to the required external diameter. Other methods use multiple conical or conventional horizontal rolls or offset rollers. Pipe is finished in a reeling process in which it is driven between slightly conical rollers, followed by passage through a sizing mill that ensures circularity.
An older process is the Pilger process. This process uses eccentric rolls to form the pipe in discrete stages. A mandrel is inserted into the partly formed pipe from the piercing mill. The assembly is driven into the open rolls and, as the rolls rotate back and forth, sequential sections of the pipe are drawn into the eccentric rolls; the outer diameter formed to the required dimension set by the roller eccentricity. This process is also used for production of corrosion-resistant alloy pipe.
After forming, the pipe may be delivered as produced or, more usual for the oil industry, as either normalized or quenched and tempered. These heat treatments homogenize the mechanical properties and further improve strength and toughness. After forming is completed, the pipe is inspected for internal laminations and pressure-tested hydraulically.
This type of pipe is generally available in diameters up to 16 in but can be obtained from a restricted number of suppliers in sizes up to a maximum of 28 in with wall thickness to 2 in. The larger diameter pipe is made by hydraulically expanding smaller diameter pipe. Seamless pipe is the preferred material of several operators for small-diameter pipelines. Its main advantages are its good track record in service and the absence of welds in the pipe sections. However, the larger-diameter, seamless pipe may be more expensive than pipe fabricated by the alternative processes. The disadvantages of seamless pipe are fairly wide variation of wall thickness, typically +15% to -12.5% and out-of-roundness-and-straigtness. The premier pipe fabrication mills can produce seamless pipe to closer tolerances.
The outer surface of the pipe may be highly distorted such that when it is grit-blasted, prior to coating, tiny slivers of steel rise up. These slivers can be a drawback when the pipe is to be coated with a thin anticorrosion coating such as fusion-bounded epoxy (FBE), and it is prudent for the pipeline engineer to check for such effects when prequalifying pipe suppliers.
Source:
- Subsea Pipeline Engineering by Andrew Clennel Palmer, Roger A. King
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