John F. Kiefner
Kiefner & Associates Inc.
Worthington, Ohio
Patrick H. Vieth
Battelle-Columbus Laboratories
Columbus, Ohio
An improved method for evaluating the remaining strength of corroded pipe reduces the original B31G method's inherent overconservatism.
The modified criterion was developed on Project PR 3805 sponsored by the American Gas Association's Corrosion Supervisory Committee.
One of the most important results of the PR 3-805 project was the development of a personal-computer program, RSTRENG. The program facilitates the analysis of corroded areas via the effective-area method.
The first of two articles (OGJ, Aug. 6, p. 56) outlined how the original criterion was developed and its shortcomings.
This conclusion discusses the modified criterion and shows its application in a sample calculation using RSTRENG.
NEW AREA REPRESENTATION
It was recognized that the parabolic area representation (discussed in Part 1) left a few things to be desired and that some pipeline operators might not choose to use the effective-area method because of the detailed measurements required.
Therefore, in addition to the RSTRENG option, a new area representation relying solely on maximum defect length (L) and maximum depth of pitting (d) was conceived.
The new representation is shown in Equation 10: A dL. (Equations 1-9 are located in Part 1.)
In Equation 10, A = area (in square inches) of crack or defect in the longitudinal plane through the wall thickness; d = maximum depth of the defect; and L = axial extent (in inches) of the defect.
The new A can be used in Equation 1 with the modified Folias factor (Equations 9 or 9a) to calculate predicted failure pressures when only d and L are known.
The calculation of remaining strength via RSTRENG or Equation 10 tends to give predictions that are in better agreement with actual burst-test results than those made with the parabolic representation of metal loss.
Kiefner and Vieth provide an expanded data base of 86 burst tests which includes the original 47 tests used to establish the B31G criterion.1 (References are located at the end of Part 1.)
The new data were gathered from tests conducted by individual pipeline operators on corroded specimens removed from their systems. These data were provided for the purposes of the PR 3-805 project.
A final point that must be made with regard to any type of profile representation of metal loss is that some excess conservatism will always be present when the deepest parts of the corrosion are not lined up along the axis of the pipe and when deeper portions of the pitting are separated by islands of greater remaining wall thickness.
Within the present state of technology, it is impractical to deal analytically with these variables.
ANALYSIS OF CORRODED AREAS
The improved criterion also takes advantage of the fact that the shallow corrosion does not affect the serviceability of a pipeline at its operating pressure (0.72% specified minimum yield strength; SMYS) regardless of the length of the corrosion.
Shannon proposed a "no-failure" boundary for corroded pipe which can be used to define the depth of pitting below which failure is not a concern in a pipeline operating at a given maximum stress level.?
The "no-failure" boundary was developed from Equation 1 and validated by British Gas using its own and early AGA corroded-pipe burst tests. It is in fact derived from Equation 1 by letting A/Ao equal d/t (true for a rectangular flaw), MT approach , and S be 1.15 SMYS. Hence, Equation 11 is derived: S = 1.15 SMYS (1 - d/t).
Here, S = hoop-stress level (in psi) at failure; d - as noted previously; and t wall thickness (in inches) of the pipe.
The failure stresses of all corroded pipe tested up to that time exceeded S regardless of the length of the corrosion. Kiefner and Vieth showed that the results of all 86 burst tests in AGA data base satisfy Equation 11 also.1
For pipelines operating at stress levels less than 72% SMYS, it would seem acceptable to say that S should be at least 90% SMYS. Setting S = 0.9 SMYS in Equation 11 and solving for d/t yields d/t = 0.217.
Therefore, it is reasonable to state that a pipeline containing corrosion not exceeding 20% W.T. can be safely left in service regardless of the length of the corrosion as long as the operating stress level is less than 72% SMYS and as long as the remaining wall thickness is at least 80% of that required by the design stress level.
CLOSELY SPACED AREAS
No provision exists in the original B31G criterion for considering the effects of interaction between closely spaced corrosion anomalies. As a result, users of the criterion must either choose to treat such areas as being continuous (introducing excessive conservatism) or treat them as separate defects based on an arbitrary criterion of separation distance (possibly resulting in a nonconservative assessment).
Examples of the kinds of interaction commonly encountered in corroded pipe are illustrated in Fig. 1. Interactions between flaws of these three types were studied7 under Pipeline Research Committee sponsorship about the time the original B31G criterion was being developed.
- Type 1 defects. Type 1 interactions are those in which the flaws are separated circumferentially, but their individual profiles overlap when projected to a single plane through the wall thickness forming a projected profile of length (L) as shown in Fig. 1.
The calculated remaining strength of the resulting profile tends to underestimate the remaining strength because the islands of full-thickness material between the individual flaws prevent their acting as a single flaw.
Experiments have suggested that at a separation distance of 6 x W.T., Type 1 defects can be expected to exhibit little or no interaction.7
The appropriate means of handling a Type 1 situation would be to treat the defects as a single defect if the circumferential spacing is less than 6 x W.T. and to treat the defects separately if the spacing is equal to or greater than 6 x W.T.
Better means of handling those spaced at less than 6 x W.T. await further experimental or analytical work to develop a well-validated relationship.
- Type 2 defects. Type 2 defects as shown in Fig. 1 are those which lie on the same axial plane but are separated by a length of full wall-thickness pipe. Experiments involving Type 2 defects were also presented in Kiefner.7
In fact, the data suggest that significant interaction occurs only when the spacing is 1 in. or less. This result has led some operators to conclude that corrosion anomalies do not interact at all unless they are continuous, As a compromise until more data are available, it is suggested that corrosion anomalies separated along the axis of the pipe by more than 1 in. be considered to act independently.
When separated by less than 1 in., they can be analyzed by means of RSTRENG as though the area were continuous.
- Type 3 defects. Experience suggests that Type 3 defects can be analyzed safely by one of two methods.
Method 1: Short, deep defect in a reduced-thickness pipe. For this method, the short, deep defect is considered to exist in a pipe the thickness of which is equal to the net thickness beneath the long defect. The length is that of the short defect.
Because the pipe is assumed to have the net thickness below the long shallow defect, the hoop stress must be calculated from the Barlow formula using the net thickness.
Method 2: Effective area. In this case, successive trial calculations are used to find the minimum failure pressure based on the effective area and length via RSTRENG.
The one available test result for this type of flaw was obtained on a 36-in. diameter, 0.400-in. W.T. pipe material with a flow-stress level of 73,100 psi. The defect in the specimen was 13 in. in length overall with a centrally located shorter flaw of 6.5 in. in length.
The depths, respectively, were 0.120 in. and 0.300 in. Its failure pressure level was 749 psig. The failure pressure predicted by the effective-area method was 735 psig.
Thus, the failure pressure of the test flaw was adequately predicted by the effective-area method but was overestimated by the exact-area method.
The failure pressure predicted by Method 1 falls considerably below that predicted by the effective-area (RSTRENG) method indicating that it is more conservative.
USING RSTRENG
To illustrate RSTRENG's use, let us consider a sample problem involving a 30-in. OD x 0.375-in. W.T. X52 pipe with a 20-in. corroded area.
The maximum depth of corrosion is 0.140 in., and the pipeline operator has made detailed depth measurements at 1-in. intervals along the axis of the pipe deviating occasionally to pick up the locally deepest portions of the affected region.
The pipeline operates at maximum allowable operating pressure (P) of 936 psig.
First, let us see what the original B31G criterion yields.
Because d is 0.140, d/t = 0.373. According to Equation 6, the B value is then 1.025. The acceptable length of corrosion then is L 1.12 B /Dt = 1.12 (1.025) /30 x 0.375 = 3.85 in.
The length is too long, and we must calculate a safe operating pressure. When we calculate Q = 0.893L//Dt, we find that Q is 5.32. This forces us to use Equation 8 as follows: P' = 1.1 P [1 - d/t] = 1.1 (936)[1 - 0.373] = 645 psig.
Thus, under the existing B31G criterion, the operator must reduce the operating pressure to 645 psig or remove or repair the pipe.
Using RSTRENG yields a more sophisticated analysis. (The RSTRENG disk is supplied with the report on Project PR 3-805 from the AGA.1)
The IBM-compatible software program can be run from any 5 1/4-in. disk drive or can be loaded onto a hard-disk drive system.
At the appropriate prompt,
the user should enter RSTRENG to start the program. During the first few screens, RSTRENG requests information in order to store the results of any analyses conducted.
Next, the program requests the user to determine whether the corrosion profile measurements will be supplied as "pit depth measurements" or "remaining wall thickness measurements."
For this example, pit-depth measurements will be entered.
RSTRENG will now prompt the user for all of the information necessary to conduct the analysis. This information includes any identification, pipe diameter and thickness, yield strength and SMYS of the pipe, and length of the corrosion.
This analysis method requires that the measurements be evenly spaced along the axial length of the pipe so the user is also prompted to enter the spacing increment.
All of the data entered by the user will be displayed on the screen. If any errors are made when entering the data, the user can use the EDIT DATA option to make corrections.
Once the data have been entered, the user is ready to ANALYZE DATA. RSTRENG will prompt for the file name containing the data characterizing a given corrosion profile. The analysis will then be conducted and within a few seconds the results will appear on the screen as presented in Fig. 2.
Fig. 2 presents the file name, appropriate pipe data, profile of the corroded wall thickness, minimum predicted failure stress, and minimum predicted failure pressure. Note that this is a failure pressure, not a safe operating pressure.
The minimum predicted failure pressure is 1,239 psig for the example, and the effective length of the pitting is 14 in. in the center of the 20-in. region as indicated by the arrows and the dashed line. Multiplying this pressure level by 0.72 yields the safe operating pressure of 892 psig.
This pressure level is much higher than the 645 psig obtained via the existing B31G analysis.
This example illustrates the most valuable feature of the effective-area method and the greatest weakness of the existing B31G criterion.
The new method permits the calculation of the realistic failure pressure of long corroded areas, whereas the existing criterion restricts the analysis to relatively short corroded areas falling back on the overconservative net-thickness concept when the corrosion is extensive in length.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
