how a pipline is built...

Definition

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow - liquids and gases (fluids), slurries, powders, masses of small solids. It can also be used for structural applications,  hollow pipes are far stiffer per unit weight than solid members.

Depending on the standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness.  The term "pipe"  implies a level of rigidity and permanence. 

The  Manufacture

There are three processes for metallic pipe manufacture. Centrifugal casting of hot alloyed metal is one of the most prominent process.  Ductile iron pipes are generally manufactured in such a fashion. Seamless pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. As the manufacturing process does not include any welding, seamless pipes are perceived to be stronger and more reliable. Historically seamless pipe was regarded as withstanding pressure better than other types, and was often more easily available than welded pipe.
Advances in materials, process control and non-destructive testing allow correctly specified welded pipe to replace seamless in many applications. Welded, and Electric Fusion Welded pipe is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated to make the seam less visible. Welded pipe often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities
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There are a number of processes that may be used to produce welded pipes. Each of these processes leads to coalescence or merging of steel components into pipes. Electric current is passed through the surfaces that have to be welded together; as the components being welded together resist the electric current, heat is generated which forms the weld. Pools of molten metal are formed where the two surfaces are being connected as strong electric current is passed through the metal; these pools of molten metal form the weld that binds the two connected components.
Welded pipes are manufactured from the longitudinal welding of steel. The welding process for pipes is continuous as opposed to welding of distinct sections at intervals.

For large-diameter pipe (25 centimeters or greater) there are two technologies that can be used to manufacture steel pipes of sizes larger than the steel pipes that can be produced by seamless and welded processes. The two types of pipes produced through these technologies are Longitudinal Submerged Arc Welded and Spiral Submerged Arc Welded pipes. Both processes consist of  bending and welding wide steel plates and most commonly used in oil and gas industry applications. Due to their high cost, these pipes are not used in lower value non-energy applications such as water pipelines.

Materials

Pipe is made in many materials including ceramic, fiberglass, many metals, concrete and plastic. Typically metallic piping is made of steel or iron, such as unfinished, black steel, carbon steel, stainless steel or galvanized steel, brass, and ductile iron. Iron based piping is subject to corrosion in highly oxygenated water stream.

Aluminum pipe may be utilized where iron is incompatible with the service fluid or where weight is a concern; aluminum is also used for heat transfer tubing such as in refrigerant systems. Copper tubing is popular for domestic water plumbing systems; copper may be used where heat transfer is desirable. Inconel, chrome moly and titanium steel alloys are used in high temperature and pressure piping in process and power facilities.

Pipe supports

Pipes are usually either supported from below or hung from above, using devices called pipe supports. Supports may be as simple as a pipe "shoe" which is akin to a half of an I-beam welded to the bottom of the pipe; they may be "hung" using a clevis, or with trapeze type of devices called pipe hangers. Pipe supports of any kind may incorporate springs, dampers, or combinations of these devices to compensate for thermal expansion, or to provide vibration isolation, shock control, or reduced vibration excitation of the pipe due to earthquake motion.

Pipe hanger assembles are usually attached with pipe clamps. Possible exposure to high temperatures and heavy loads should be included when specifying which clamps are needed

Joining

Underground pipe typically uses a "push-on" gasket style of pipe that compresses a gasket into a space formed between the two adjoining pieces. Push-on joints are available on most types of pipe. A pipe joint lubricant must be used in the assembly of the pipe. Under buried conditions, gasket-joint pipes allow for lateral movement due to soil shifting as well as expansion/contraction due to temperature differentials.

Large above ground pipe typically uses a flanged joint, which is generally available in ductile iron pipe and some others. It is a gasket style where the flanges of the adjoining pipes are bolted together, compressing the gasket into a space between the pipe.