ASME B31G Equations

The most commonly used equation to assess the remaining strength of a pipeline with metal loss is the Modified ASME B31G. This is dependent on several variables such as diameter, wall thickness and length and depth of the anomaly. For reference the equation is as follows.

\[\sigma_f=(\sigma_y+10ksi)\frac{1-0.85\frac{d}{t}}{1-0.85\frac{d}{t}\frac{1}{M}}\]

The factor \(M\) is known as the folias factor and accounts for the bulging effects of an anomaly as it reaches the limits of its capacity to resist the pressure.

\[M=\sqrt{1+0.6275\left(\frac{L}{Dt}\right)^2-0.003375\left(\frac{L}{Dt}\right)^4}\qquad for\space \frac{L^2}{Dt}\leq50\] \[And\]

\[M=0.032\frac{L^2}{Dt}+3.3 \qquad for\space \frac{L^2}{Dt}>50\]

Example

In this example we will look at how the changes in the anomaly can change remediation decisions. An ILI run can identify several thousand anomalies and it is the job of the engineer to discern which ones should be remediated. It is not only important to look at the current state of the anomaly but also to consider the growth of the anomaly over time.

In this example an anomaly is reported at 35% of wall thickness and 5 inches long. Looking at the current state this anomaly would of even survived a hydrotest to 100% of SMYS if you look at Year 0 on the plot. But if the corrosion rate is factored in over time a relatively benign anomaly can become significant over time. In this example the corrosion rate was assumed to be 10 mils per year (mpy) and the length is growing at 0.20 inches per year.

It can be seen from Figure 1 that by the end of one year, that same anomaly would not of survived a hydrotest to 100% of SMYS. In 6 years the anomaly would be projected to meet the criteria for an immediate condition since the failure pressure is less than the 1.1 times the MAOP and by the time of reassessment in 7 years the anomaly would nearly fail at MAOP.

Figure 1

Conclusion

In your fitness for service analysis it is important to consider the likely corrosion rate and the amount of time until reassessment in making your decisions. Since the anomaly is going to change over time it is necessary to consider how fast it is changing and what the effect is on the predicted failure pressure.