WRC 571

WRC 571

WRC 571


Incorporation of Structural Reliability Methods into Fitness for Service Procedures

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P.H. Wirsching, A.E. Mansour

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The late 1990’s were the period when advances in fracture mechanics and other advanced concepts for assessment of the safety and reliability of operating equipment were beginning to be widely recognized as useful and practical tools for implementation by, among others, the process and power industries. Important, but missing at the time, were codes and standards detailing the necessary steps and methods for implementation of assessment procedures for evaluation of operating equipment. Such documents were in time to be written for adoption by organizations such as API and ASME.


The Materials Properties Council (MPC), now part of the Welding Research Council (WRC), was selected by specialists at major petroleum companies to be the focal point for the development of a comprehensive draft fitness-for-standard which, in time, would be adopted by the American Petroleum Institute (API) as API 579. Later, it would also be adopted by the American Society of Mechanical Engineers (ASME) as ASME FFS-1. Development of the document, which continues under a joint committee of those societies to this date, was the result of the leadership of David Osage, at the time of British Petroleum’s U.S.A. operation with considerable input from Dr. Ted Anderson, at the time a professor at Texas A&M University. The activities of the original MPC committee continues to this date under a Joint API and ASME. Both Anderson and Osage remain active in the development and standardization of the industry’s fitness-for-service technology.


Among the essential first steps by the MPC industry committee on fitness-for-service (FFS) was gathering procedures and data relevant to applications and material pertinent to fabrication and operation of process equipment. The following WRC Bulletin, originally distributed only to the MPC working committee as FFS-II-9, was prepared by Paul Wirsching and Alaa Mansour, consultants to MPC’s FFS Committee. It captures the early, but still valuable essentials of many aspects of FFS assessment at the time of the documents distribution to the MPC committee of industry engineers. The information is now published by WRC because of its ongoing relevance and the usefulness of the procedures and data presented. Anyone seeking an introduction to FFS standards and state-of -the art can profitably begin by studying the content of this publication.


In this WRC Bulletin Wirsching and Mansour provide details and data relevant to measurement of fracture toughness values and estimating distributions of crack lengths. Various concepts concerning applying the information on an interaction diagram are treated in detail. Concepts of elastic-plastic fracture mechanics as understood at the time are elaborated on as are the concepts of partial safety factors and reliability. In short, this WRC Bulletin provides an excellent and practical starting point for addressing many issues associated with FFS assessments and understanding how far current procedures have come as compared to where industry was before the current standards were published.



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