This article outlines the procedure to evaluate the fitness-for-service of corroded buried pipe, and explains what CC N-806 covers … and what it does not cover.
What are the Damage Mechanisms in Metallic Buried Pipe?
The inspection of a buried metallic pipe, after years of service, can reveal that the pipe has little if any damage, or on the contrary that the pipe has undergone one of four types of damage:
- Wall thinning caused by ID erosion, or by ID or OD corrosion. This wall loss can be in the form of pitting, local thin areas, or general metal loss. Wall thinning is the damage mechanism addressed in CC N-806.
- Cracking caused by fatigue, corrosion, or both. It can also happen that construction flaws are discovered that were undetected when the pipe was installed. Cracking is not addressed in CC N-806.
- Embrittlement, typically in the form of leaching. Embrittlement is not addressed in CC N-806.
- Structural damage such as dents, gouges, buckles, etc. Structural damage is not addressed in CC N-806.
What are the Failure Modes?
The failure mode of a corroded section of buried pipe depends on the type of damage it is undergoing. CC N-806 addresses the first damage mechanism, wall thinning, which is detected and measured by one of the several ultrasonic examination techniques. If the pipe has lost thickness over the years, its fitness-for-service must be evaluated against four potential failure modes:
- Membrane failure: Burst caused by internal pressure.
- Ring failure: Instability failure of the pipe cross-section by ovalization or buckling, caused by soil, surface or flood loads.
- Beam failure: Failure of the pipe by excessive longitudinal or shear stresses, these can be caused by constrained thermal cycling, ground settlement, seismic wave passage, or flood-induced flotation.
- Pinhole leak failure: This is the most common failure mode of a buried pipe, often caused by pitting, either from the soil-side (OD), or fluid-side (ID).
What is the Technical Basis of N-806?
The fitness-for-service evaluation forr each of the failure modes listed above has its technical basis, let’s review them in sequence.
Membrane failure: The resistance of a pipe to burst from internal pressure, whether corroded or not, is the same as if the pipe was above-ground. It is therefore not surprising that the assessment options in N-806 are the same as for above-ground pipes, they are:
- The “Evaluation of General Metal Loss” method of N-806 Section 6.1.1 and Appendix A, which is based on the method of API-579-1/ASME FFS-1 Part 4.
- The “Evaluation of Local Metal Loss Region” method of N-806 Section 6.1.2, which is based on the methods of earlier ASME XI Code Cases N-513 and N-597. These, in turn, are based on the design rules of ASME III Div.1, and on the corrosion assessment rules for pipelines of ASME B31G.
Ring failure: If the pipe wall is thinned over a large area, then the pipe will lose its resistance to ovalization or buckling. The equations for the prevention of ring failure come from classic empirical formulas (such as the Iowa formula for ovalization) and from ring buckling limits. For the sake of simplicity, these were added as Appendix B to CC N-806.
Beam failure: The evaluation of longitudinal stresses is addressed in Section 6.3 of CC N-806, but the CC does not provide the equations to implement this assessment. So, in this case, the engineer must supplement the Code Case by using classic equations for constrained thermal expansion-contraction, settlement, seismic wave passage, and flotation. In Appendix C to N-806, the CC does provide the formula for calculating the section modulus of a corroded pipe, which will be necessary for calculating the longitudinal stress.
Pinhole leak failure: Here, Section 4 of the Code Case takes the empirical approach, in line with API-579-1/ASME FFS-1, by imposing a lower bound corroded wall thickness limit of 0.1 inch, and 20% of the nominal wall, although this last limit can be replaced by a Section III area reinforcement rule check.
What does CC N-806 not Cover?
CC N-806 does not explicitly cover the following areas, which must therefore be addresed by the engineer:
- The corrosion rate: Revision 1 of the CC, currently in development, will be more explicit on the prediction of ID and OD corrosion rates, with input from EPRI and the NRC staff.
- The equations for the protection against beam failure, mentioned above.
- The allowance for pinhole leaks.
So, what are the Options for the Assessment of Corrosion in Metallic Buried Pipes?
Since CC N-806 has not yet been approved by the NRC in R.G. 1.147, and may not be approved until the question of corrosion rate is resolved, the engineer may follow one of several options, for example:
- Reject any corrosion beyond the fabrication tolerance. This simplistic approach is most of the time severe to the extreme, and would lead to unnecessary repairs and delays. This is contrary to good engineering practice, and to the concept of fitness-for-service and ASME Section XI.
- Apply “CC N-806 plus”, where the “plus” recognizes that the CC is not explicit on all the assessment equations, and must therefore be supplemented. The “plus” would also incorporate the proposed CC N-806 Rev.1 approach to corrosion rate.
- Develop a company procedure that addresses fitness-for-service of corroded buried pipe, from first principles and fundamental equations, using the formulas in CC N-806, traced to their original references, plus the equations missing. The procedure would stand on its own merits.
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