RSTRENG

Introduction

• Taking the same basic formula ass the initial B31G criterion, but using the more accurate representation of Flow Stress, the modified Folias factor, and the effective area representation, a less conservative criterion was derived.
• However, calculating failure stress using these factors is complex and cumbersome without a computer. It is for this reason the program RSTRENG was developed.

MAOP To Be Used For RSTRENG Calculations

• In both RSTRENG (See AGA/PRCI Research Report PR 3-805, included here as Reference (7), and ASME/ANSI (Part 4, Article 4.2) the maximum allowable operating pressure that must be used in the RSTRENG calculation is the greater of either the established MAOP or the pressure calculated by Barlow’s formula including design factor. The same pressure can be calculated using the equations from CFR Part 192 or Part 195, excluding a joint factor.

MAOP = (2St / D) F

where: S = SMYS(psi)
D = dia(in)
t = wall thickness(in)
F = design factor

If Corrosion < 20% of Wall thickness

• British Gas proposed a “no-failure” boundary and validated it using their own and AGA pipe burst tests.
• If a pipeline has corrosion that does not exceed 20% of the wall thickness, the pipeline may be left in service regardless of the length of the corrosion if the operating stress level is less than or equal to 72% of SMYS and the remaining wall thickness is at least 80% of that required by the design.

Modified B31G Criterion Summary, RSTRENG

• Are accepted by US DOT/OPS, CFR Sections 192.485, 192.7, 192.933 for gas lines and 195.3, 195.452, 195.587 for hazardous liquid lines
• Both are less conservative than the Original B31G Criterion
• RSTRENG Automatically address complex corrosion geometry
• Both require fewer cutouts than the Original B31G

Continued Validation of RSTRENG

As we have discussed, the PRCI report prepared under Contract No, PR 3-805 was the basis and background for RSTRENG. The initial work was based on 86 burst tests, and subsequent to the generation of RSTRENG, an additional 82 burst tests were evaluated to continue the validation of RSTRENG. A significant number of these additional tests were performed on pipe that had the corrosion-simulating grooves machined.

This continued validation effort is presented in the PRCI report prepared under Contract No. PR 218-9304, “Continued Validation Of RSTRENG”, by Kiefner, Vieth, and Roytman. This report (Ref 8) is intended to provide qualitative validation to define applicability and limitations and quantitative validation to compare actual with predicted pipe failure pressures.

The continued validation efforts included comparing predicted test failure pressure using RSTRENG against actual failure pressures for the:

• Interaction of corroded areas
• Behavior of very long flaws
• Behavior and interaction of isolated pits
• Effects of diagonal/spiral corrosion
• Effects of patches of corrosion having significant longitudinal and circumferential extent
• Effects of large axial stresses on the behavior of corroded pipe
• The following series of slides summarize a large part of the continued validation effort.

RSTRENG/Effective Area

• Level 2 evaluation a number of pit depths and spacing measurements are required for a projected profile of metal loss. Each additional corrosion depth entered in a profile increases algorithmically the number of sub-sections that can be evaluated.
• More than one profile may be necessary for agreement on the lowest failure stress of an area of multiple patches and pits.
• Interaction rules must be used as applicable.

Related Links