Fracture Control Testing and Acceptance Criteria for Line Pipe
Authors: Hansen D.A. 
Published: December 2007

A model specification for imposing mill test and acceptance criteria for fracture control is discussed. The specification applies to line pipe with wall thickness up to 1.5 in. (38 mm) intended for use in cross-country pipelines at temperatures between 32 and 250 deg. F (0 and 121 deg. C). Methods for establishing minimum acceptance levels (MAL) for Charpy V-notch specimens and drop-weight tear test specimens are described.Fracture controls are established to ensure crack arrest; methods for establishing acceptance criteria are based on equations modified from relationships found in Ref. 1.API grades B through X-70 are covered. Fracture control criteria are established separately for four service classes.

Introduction

Fracture control for pipelines has been of major interest to the industry for more than 25 years. This interest was prompted by the occurrence of pipeline fractures in which uncontrolled crack propagation, in lengths up to 3,000 ft (914 m), was observed.Initially, interest centered on the development of methods to prevent brittle fractures. More recently, interest has shifted to the prevention of uncontrolled ductile fractures. In both cases, the objective of studies funded by the industry and pipe vendors was to find out why such fractures occurred and to develop design and metallurgical methods of prevention.Study and development have been worldwide. While the causes of uncontrolled fractures now are understood reasonably well, controversies still exist about the best methods to be adopted for cost-effective fracture control. Nevertheless, results are sufficiently detailed to allow us to begin developing engineering standards and specifications to impose fracture controls on pipelines. These standards and specifications must take into account relevant design and metallurgical factors. A model specification was developed incorporating these factors; the specification is provided in the Appendices.

Background

Relevant literature is extensive and is not subjected to review in this paper. It is sufficient for the purpose of this paper to state that an extensive literature review effort was made and that, where possible, a conservative approach was used in those areas still subject to controversy. Key literature is cited in Refs. 1 through 3.The principles behind pipeline fracture control are straightforward - one must either incorporate crack stoppers in the pipelines or have line pipe toughness adequate to stop a fast-running crack. For a long pipeline, crack stoppers are impractical and, further, complicate construction. The preferred method is to specify adequate toughness; in most cases this can be accomplished by design and metallurgical controls at little or no extra cost.Both brittle and ductile fractures must be provided for. Brittle fracture can be eliminated by requiring a low brittle/ductile transition temperature. This sometimes is done by specifying Charpy impact energies, but a more reliable method is the use of a drop-weight tear test. The latter method is used in the proposed model specification.Much of the more recent industry work addresses the problem of specifying ductile fracture arrest in line pipe using Charpy impact toughness.

JPT

P. 527＾