The American Petroleum Institute (API) publication number 1149 (first published in 1993) [1] is perhaps the first accepted industry procedure for the numerical assessment of uncertainty in software-based Computational Pipeline Monitoring (CPM) leak detection systems (LDS). This publication remains valid and extremely valuable within its range of applicability. Generally speaking, it is designed for crude oil and refined products pipelines. It also focuses on (while also discussing other ancillary issues) single, straight pipelines and on the Material Balance method of CPM, particularly under steady state conditions.

A recent initiative sanctioned through American Petroleum Institute (API) and sponsored by the Pipeline Research Council International (PRCI) has developed a revision of procedure for the assessment of uncertainty in CPM techniques, in light of a number of recent technological developments and operational requirements. It is also directed at engineering uncertainty factors that prove, in practice, to have a significant effect on LDS uncertainty but that were not thoroughly addressed in the 1993 version.

The new procedure's aim to follow the standard API and ASME measurement uncertainty practice of defining a Reference Value, a Bias and a Precision for LDS, just as with any other industrial measurement system. In particular, it is possible for the reference conditions of the pipeline to be estimated using a transient pipeline system simulation model - the Reference Model – that takes all the relevant engineering uncertainty factors into account. In this respect, the procedure is similar to a formalized, statistical Leak Sensitivity Study (LSS) as is often performed as part of the requirements analysis and design of a software-based LDS.

This paper provides an overview of the procedure, with a focus on the utilization of transient pipeline simulation models as the Reference Model. The process of identifying and prioritizing the key areas of input uncertainty is highlighted. In particular, experiences in running the procedure for LVL liquids, HVL liquids and natural gas pipelines are discussed. Other areas of discussion and comment include how the new API 1149 update, technical report, can be used for a relative benefit analysis of different candidate CPM techniques for a specific pipeline; and how it might fit in with industry best practices as an API Technical Report (TR).

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