Improved Specifications Needed for Line Pipe Steels
Authors: Squire J.W. 
Published: December 2007

This paper discussed some of the technical and economic factors involved in the use of line pipe. Some of the technical factors, such as notch-toughness and weldability, should be considered for inclusion in the API line pipe specifications. There are sufficient areas of common technical interest to the operators of both liquid and gas lines to make it worthwhile for both groups to continue to work together for improvement in the present pipe specifications.Economic factors affect the liquid-line design differently from gas-line design. In turn, these differences affect the technical improvements which can be made economically.

History

A brief review of the major technical developments in the past reveals that the first butt-welded pipe was made in 1832 and that commercial production of seamless tubes was inaugurated in 1890. Electric-resistance-welded pipe was first produced in 1928, electric-fusion or electric-welded pipe was introduced in 1927 by A. 0. Smith and the submerged arc-weld process for large-diameter pipe was developed by Consolidated Western Steel Div. of United States Steel in 1946.

Growth

Concurrent with these technical developments, there has been a tremendous growth in the use of line pipe. About 860,000 miles of crude-oil, products and natural-gas pipelines exist in the United States today. During the past 10 years, the miles of gas transmission and distribution lines have almost doubled. Fig. 1 illustrates this rapid growth of the industry.

Specifications

Most line pipe for gas or oil lines today is bought to meet API Standard 5LX "Specification for High Strength Line Pipe". This specification, which was first published in 1948, was an outgrowth of the API Standard 5L "Specifications for Line Pipe", first issued in 1928. The 5LX specification was written to meet the demand for higher strength steel to be used in larger-diameter pipe.Pipe made to API 5L and 5LX specifications must meet certain minimum standards for chemical and physical properties. The steel may be made by Bessemer, open-hearth, basic-oxygen, or electric-furnace processes and may be rimmed, capped, semi-killed, or killed. Both 5L and 5LX specifications place maximum limits on carbon, manganese, phosphorus and sulfur, depending on the process of manufacture, the grade being produced and whether the pipe is seamless or welded. Cold-expansion in API 5L is delegated to the option of the manufacturer and does not affect specified maximum chemistry. In API 5LX, the maximum chemistry specified varies with grade and whether or not cold expansion is used. Limits are shown in Tables 1 and 2.

Fractures

It is of prime importance to avoid a failure of any type in an operating pipeline. It is of further importance to confine any such failure to a local area.

Types of Fractures

Two fundamental types of fractures are generally considered by the pipe-liner-shear and cleavage. A shear fracture exhibits a high degree of ductility and requires high energy for continued propagation. As shown in Fig. 2, the appearance of a shear fracture is dull gray and somewhat silky.Fig. 3 shows a cleavage fracture. It is not very ductile, requires comparatively little energy to become self-propagating, and is bright arid crystalline in appearance. It is characterized by chevron marks pointing toward the origin of the fracture.Fig. 4 shows cross sections of both the cleavage and shear fractures. The difference in ductility can be observed by comparing the "necking-down" of the shear-fracture specimen with that of the cleavage-fracture specimen. There is an infinite variety of combinations of shear and cleavage fractures in a fractured surface.

JPT

P. 370^