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.
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:
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:
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:
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.
CC N-806 does not explicitly cover the following areas, which must therefore be addresed by the engineer:
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:
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George Antaki, Fellow ASME, has over 40 years of experience in nuclear power plants and process facilities, in the areas of design, safety analysis, startup, operation support, inspection, fitness for services and integrity analysis, retrofits and repairs. George has held engineering and management positions at Westinghouse and Washington Group International, where he has performed work at power and process plants, and consulted for the Department of Energy (DOE), the Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI).
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