Considerations for Line Pipe Material Reliability
Authors: Kiefner J.F. , Morris W.G. 
Published: October 2008

Line pipe material selection can be a critical element in the safety and reliability of any pipeline, but the penalties for inadequate quality are usually higher in an offshore environment than onshore. Problems can be avoided if the focus during material selection includes consideration of strength, toughness, weldability and quality assurance. This paper addresses the factors which affect line pipe quality such as grade, seam type, transition temperature, toughness, and chemical content. Also discussed are mill inspection; hydrostatic testing; and the transportation of line pipe.

Introduction

Line pipe quality should be a major concern for any pipeline operator. First and foremost, high quality pipe enhances pipeline safety and reliability. Second, the overall cost of constructing a pipeline can turn out to be higher through an ill-considered attempt to sacrifice quality as a means of "saving" money. Line pipe material selection should he focused on strength, toughness, weldability, and coatability. A line pipe purchaser needs to be aware of the many factors which affect these properties.

Background

Thirty years ago almost all line pipe materials were essentially carbon-manganese steels made by the open hearth process, ingot casting, blooming, billeting and hot rolling or hot forming. Very little was done or could have been done to effect enhanced toughness and weldability. On the other hand obtaining adequate strength for levels up to and including X52 was not a problem. Most current line pipe materials are low-carbon, microalloyed steels usually made by either the basic oxygen process or electric-arc steelmaking, continuous casting and sophisticated rolling or forming techniques.' These processing steps alone result in cleaner steels with lower carbon contents both of which facilitate enhanced toughness and weldability. Additional steps such as hot-metal pretreatment2 between the blast hence and the steelmaking process, and ladle metallurgy processing after steelmaking can lead to very clean (low sulfur), fine-grained steels with optimum strength, toughness, and weldability. However, these outcomes are not guaranteed because problems can arise even with the newer materials which can adversely affect pipe quality. The question for pipe purchasers then becomes: How does one assure without being a metallurgical expert and without following and understanding every step of the process, that the quality of a given line pipe order will be satisfactory? The answer lies in preparing an adequate specification, selecting a manufacturer in which one can have confidence, and in providing proper monitoring to see that the specification is followed.

Developing Specifications

Since 1928 almost all line pipe has been made in accordance with API Specification 5L. This standard gives minimum requirements, and many if not most purchasers opt for requirements which exceed the minimum levels embodied in Specification 5L. Ofien the company-specific specifications rely heavily on Specification 5L and deviate from it only for issues of particular importance to the purchaser.