Risk-Based Hoop Stress Factors for Pressure Design

Abstract

A risk-based pressure design approach has been developed as an alternative to the class location approach currently used in the Canadian Standard Association’s (CSA’s) Standard Z662. Similar to the current approach, the new approach uses a set of hoop stress factors to calculate the minimum wall thickness from the pressure, diameter, and specified minimum yield strength. The hoop stress factors, termed class factors, are calibrated to keep the failure probability below an allowable value for the limit states representing burst of undamaged pipe under the operating pressure and failure due to equipment impact loading. Yielding under the strength test pressure is addressed as a separate limit on the class factor. To achieve a consistent safety level for all pipelines, the allowable failure probabilities are inversely proportional to the magnitude of failure consequences, as implied by a safety class determined according to the approach described in a companion IPC paper. This paper describes the calibration process used to define the class factors and provides a comparison between the wall thicknesses resulting from the risk-based approach and those obtained from the current hoop stress factors in CSA Z662.