Choosing the Safety Factor in an ASME B31G Level 1 Evaluation

Introduction

Section 1.2 of ASME B31G – 2012 (R2017) establishes how to perform the evaluation of metal loss due to external or internal corrosion in buried pipelines, aboveground pipelines, and offshore piping, and other cases. It also covers metal loss produced by grinding that is used to eliminate mechanical damage, cracks, electric arc burns, manufacturing defects, or other defects on pipeline surfaces [1]. This article studies five different models of varying configurations. It considers cylindrical, elliptical, and conical metal losses, isolated and grouped, with a depth of 0.177" in the central indication, and less or equal to 50% of the thickness for the rest, in pipelines with NPS 3", 6", and 8" diameter with respective nominal thicknesses of 0.216", 0.250", and 0.250" for the central indication, and less or equal to 50% of the thickness for the rest, in pipelines with NPS 3", 6" and 8" diameter, with respective nominal thicknesses of 0.216”, 0.250”, and 0.250” in a system with fittings rating 150.

ASME B31G defines a Level 1 assessment as a simple calculation based on individual measurements of the maximum depth and axial extent of metal loss. Level 1 assessments are also suitable for use in the prioritization of metal loss anomalies identified through inspection. Furthermore, Section 1.9 describes that there is no single factor of safety (FS) that is appropriate for all types of pipeline construction, all modes of operation, or all types of defects or anomalies; and that in no case should FS be less than 1.25. Section 1.10 states that the use of software packages and spreadsheets can greatly facilitate Level 1 assessments and are practically a necessity for conducting Level 2 and Level 3 assessments.

Another factor that intervenes in the result of an ASME B31G evaluation is the S_flow, which is related to the properties of the material. In the case of carbon steel working at temperatures below 250ºF (120ºC), it can be defined by S_flow = 1.1xSMYS; however, in our case of being conservative, we considered S_flow = 0.72xSMYS.

The finite element analysis (FEA) performed does not propose any interpretation for in-service suitability since it is not a real case, but rather an approach to establish best practices in the choice of the most appropriate FS, depending on the type of metal loss, considering whether it is isolated or grouped, in the conduct of a Level 1 evaluation, section 2.2 of ASME B31G.

Finite Element Analysis (FEA)

For each model, a meshing based on quadratic element curvature with a maximum size of 3.22mm and minimum size of 0.64mm was applied. It was validated with a model without indications and subjected to an internal pressure calculated under the ASME B31.4 construction code in API 5L Gr-B steel, joint factor E = 1, design factor F = 0.72 and a maximum temperature of 200°F, S_Y = 35000, where S = FxExS_Y = 25200; solving the pressure of  with ASME B31.4 - Section 403.2; ultimately obtaining a result of 1,460 psi for a pipeline with NPS of 8". As seen in Figure 1, the result of the von Mises hard work for the pipeline is in the order of 22,522 – 26,669 psi – values that enclose the admissible of 25,200 psi, with which we accepted the meshing. These checks were performed in the same way for 3" and 6".

Table 1 corresponds to a spreadsheet with the data and results, from which we took the safe pressure (PS) of each of the six models for the evaluation in the finite element software.