PIPELINE CONSTRUCTION

4.5.5 Pipeline Construction

4.5.5.1 Pipeline Design Codes

Most of the codes of practice are derivatives from studies conducted by the American Society of Mechanical Engineers (ASMIE) and the American Standards Association (ASA), which later changed its name to the American National Standards Institute (ANSI).

The UK Pipeline Safety Code is Part 6 of the IP Model code of Safe Practice in the Petroleum industry, which includes and takes note of the British Standard Code of Practice for Pipelines, BS CP 2010, which relates to pipeline construction in the UK.

Gas distribution lines up to a working pressure of 70 bar are adequately covered by the Institution of Gas Engineers’ series “Recommendations on Transmission and Distribution Practice. The IIP Code does not claim to be a design handbook and does not replace the need for appropriate experience and engineering judgment.

The IP Code of Practice sets forth general requirements for the safe design, construction and operation of pipelines for the conveyance of petroleum (crude oil and liquid products) and gas (natural gas and gaseous products).

It specifies considerations for pipe materials, flanges fittings and valves etc.

Submarine pipelines are designed to internationally accepted codes, such as in Norway the Det Norske Veritas “Rules for the Design, Construction and Inspection of Submarine Pipelines and Pipeline Risers”.

By definition pipelines normally start at the scraper launcher and ends at the scraper receiver or slug catcher.

It should be remembered that wherever national codes are more stringent than internationally accepted codes, the national codes must take precedence.

4.5.5.2 Grades of Steel

The pipe from which flow lines and pipelines are constructed is known in the oil industry as “1ine pipe”. As with casing and tubing, line pipe is manufactured from different grades or strengths of steel and in different wall thickness to enable economical as well as safe design. The physical properties of the various grades of steels used in the manufacture of most of the line pipe of importance to the industry are set out in API Standards.

The requirement for high pressure, large diameter, cross-country, oil and gas transmission lines developed a need for a high strength, field weldable steel. As a result, API grades X-42 through X-65 with yield strengths of 42,000 psi to 65,000 psi were developed. These higher strength steels are available for use under the requirements of the IP Code.

The higher working pressures resulting from the use of the higher strength steels enable a substantial saving in steel tonnage and can be economical in use.

Submarine pipelines are subject to external stresses not considered so far in our discussions. In addition to hydrostatic pressure due to immersion depth, the motion of the sea introduces currents and swell and possibly thermal stress. During and after laying greater consideration must be given to the weight and curvature of the pipe.

4.5.5.3 Process of Manufacture

Three different processes are used to manufacture pipe that is used for line pipe. The properties and capabilities of the pipe vary with the type of process used.

4.5.5.4 Seamless Line Pipe

Seamless pipe is generally the industries first choice for high-pressure flow lines and pipelines.

Seamless pipe is a wrought steel tube without a welded seam, manufactured by hot working steel and, if necessary, subsequently cold finished to produce the desired properties.

Generally speaking, seamless pipe is preferred by the oil industry for use in well flow lines and other high pressure lines, although welded pipe described below is similarly used for high pressure lines in larger sizes where seamless pipe is not available. Availability is limited to a maximum diameter of about 20
inches because of the process of forming seamless pipe.

4.5.5.5 Furnace Welded Line Pipe

About the only type of furnace welded pipe available today is manufactured by the continuous welding, butt-weld process.

In the butt-weld process, pipe is manufactured with one longitudinal seam formed by mechanical pressure to make the welded junction after the entire steel strip from which the tube is formed has been heated to proper welding temperature.

The cost of the CW, continuous weld, butt weld line pipe is 15 to 20% lower than Grade B seamless or electric weld line pipe.

4.5.5.6 Electric Welded Line Pipe

Electric welded pipe has one longitudinal seam formed by electric flash welding, electric resistance welding or electric induction welding without the addition of extraneous metal. There is probably more pipe manufactured by the electric weld process than any other method because of the low initial
investment for the equipment and the adaptability to different wall thicknesses. Most electric weld line pipe is not fully normalised after welding. Some is normalised in the weld zone only. Therefore, there is a heat runout zone on each side of the weld resulting in non-uniformity of hardness and grain structure.

Like furnace weld, electric weld is not recommended for use where internal corrosion is expected.

Electric weld is the same price as seamless when made from the same grade of steel with the same wall thickness.

4.5.5.7 Pipe Diameters

Steel pipes are referred to according to their nominal inside diameter up to 12 in. Pipes of above 12 diameter are usually identified by their outside diameter (OD). All classes (weights) of pipe of a given nominal size have the same OD, the extra thickness for different weights being on the inside.

4.5.5.8 Pipe End Connections

Flow lines and pipelines are normally constructed with plain-end or bevelled and pipe ready for field welding. Where occasionally flanged connections are required, for example where flanged spools or block valves are fitted, the flanges will generally be specified raised face to ANSI B16.5, or its equivalent BS 1560, with weld-neck ends.

4.5.6 Pipe Coating and Protection

4.5.6.1 Land Pipelines

Before being trenched and buried, the pipe is normally cleaned and coated with a1ayer of bitumen, fusion-bonded epoxy or other type material for external corrosion protection. Many types of coating, some proprietary, are available and the type of soil influences the choice of coating. The coating is normally wrapped with tape for physical protection of the coating during subsequent operations.

4.5.6.2 Submarine Pipelines

Subsea immersion causes the pipeline to be exposed to a corrosive environment that is normally very severe. Pipe coating must be applied under stringent conditions with good mechanical strength to withstand the subsequent laying operations. Concrete coating is frequently necessary to provide negative buoyancy. Trenching may be necessary as dictated by the authorities for coastal areas, inland swamp areas, shallow waters and shipping lanes.

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