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 has been developed.
The method is based on the original B31G method but reduces the method's inherent overconservatism.
The modified criterion permits metal-loss anomalies of greater size to remain in service at the current maximum operating pressure. And, for anomalies which exceed the newly recommended allowable size, it will require less pressure reduction for maintenance of an adequate margin of safety.
The modified criterion was developed on Project PR 3-805 sponsored by the AGA's Corrosion Supervisory Committee.
This first of two articles outlines how the original criterion was developed and its shortcomings. The second discusses the modified criterion and shows its application in a sample calculation.
DEVELOPMENT
In the late 1960s and early 1970s, a criterion was developed through research sponsored by Texas Eastern Transmission Corp. and the Pipeline Research Committee of AGA to evaluate the serviceability of corroded pipe.
This criterion has been embodied in both the B31.4 and B31.8 pipeline-design codes and is described in detail in "ANSI/ASME B31G-1984 Manual for Determining the Remaining Strength of Corroded Pipelines."
The criterion, commonly referred to as the "B31G criterion" can be used by a pipeline operator to assess corroded pipe for rehabilitation purposes.
The remaining pressure-carrying capacity of a pipe segment is calculated on the basis of the amount and distribution of metal lost to corrosion and the yield strength of the material.
If the calculated remaining pressure-carrying capacity exceeds the maximum allowable operating pressure (MAOP) of the pipeline by a sufficient margin of safety, the corroded segment can remain in service. If not, it must be repaired or replaced.
Applying this criterion, pipeline operators have saved millions of dollars by not removing corroded pipe which is still fit for service in spite of having sustained some loss of metal.
From its inception, the B31G criterion was intended to embody a large factor of safety to protect pipelines from failure. Experience has shown, however, that the amount of conservatism embodied in the criterion is excessive, resulting in the removal or repair of more pipe than is necessary to maintain adequate integrity.
Therefore, it is desirable to have a modified criterion which will still preserve adequate pipeline integrity but result in less removal of pipe.
The B31G criterion is based upon a semiempirical fracture-mechanics relationship conceived by Maxey.2 3 It was based upon a "Dugdale" plastic-zone-size model, a "Folias" analysis of an axial crack in a pressurized cylinder,4 and an empirically established flaw-depth-to-pipe-thickness relationship.
The extensive data base of flawed-pipe burst tests presented in Kiefner3 demonstrates its usefulness and validity for axial flaws in line pipe.
Subsequently, Kiefner and Duffy conducted a series of burst tests of corroded pipes which demonstrated the applicability of the NG-18 surface-flaw equation to predicting the remaining strengths of such pipes.5
The applicability of this approach to the analysis of corroded pipe was further substantiated in a program of research conducted by British Gas.6
From the work of Kiefner and Duffy,5 the B31G criterion was derived. It is based upon Equation 1 (box).
Equation 1 is used to calculate the failure stress level of a pressurized pipe containing a longitudinally oriented crack or defect. It is also used to predict the remaining strength of corroded pipe where the parameters of the metal loss are handled as shown in Fig. 1.
The overall axial length of the corrosion is taken as L even if the corrosion is an array of pitting not necessarily lined up along an axial line. The projection of the pitted profile onto the axially oriented plane through the wall thickness as shown in Fig. 1 yields the area (A) to be used in Equation 1.
The maximum depth of a corroded area (d; Fig. 1) does not appear in Equation 1 but is used in the analysis of corroded pipe as will be shown.
ASSUMPTIONS, SAFETY FACTOR
In the adaptation of Equation 1 to predicting the remaining strength of corroded pipe, the following assumptions were made.
First, the Folias factor (M) was represented by Equation 2. Second, the flow stress of the material (,S) was taken as 1.1 specified minimum yield strength (SMYS) of the material.
To simplify the evaluation of corroded pipe, the area of metal loss (A) was represented by a parabola as shown in Fig. 2. This permits one to calculate A on the basis of two simple parameters of the metal loss, its overall length (L) and its maximum depth (d). The resulting area (A) is equal to (2/3) Ld. Ao is Lt.
The format of Equation 1 as used in the B31G criterion is shown in Equation 3.
Equation 3 predicts the hoop-stress level which will cause the failure of a corroded pipe with diameter (D), wall thickness (t), and SMYS, where the metal loss has an axial length (L) and a maximum depth of d.
Sound engineering judgment requires that corrosion should not be allowed to reach a size (L and d) so large that the predicted failure stress level is at or below the maximum operating stress level. Therefore, a factor of safety must be applied to Equation 3.
The basis for the factor of safety consists of the reasonable requirement that the failure stress level (Sf) not be less than 100% of SMYS. In that manner, the acceptance or rejection of corroded areas by the criterion would embody the same factor of safety as a hydrostatic test of the pipeline to 100% of SMYS.
For those pipelines in which the maximum operating stress level does not exceed 72% of SMYS, the factor of safety embodied in the B31G criterion (not considering any other built-in conservatism of which there is some) is 100/72 = 1.39.
CRITERION FORMAT
A given corroded region in a pipeline is evaluated on the basis of its maximum length (L) and maximum depth (d) via a transformation and combination of Equations 2 and 3 as shown in Equation 4.
The corroded area (L) is acceptable if L is less than or equal to the value given by Equation 4.
As a means of evaluating a given anomaly, the B31G document provides the simplified relationship and a curve for determining B (Equation 5).
Pits with depths greater than 0.8 wall thickness (W.T.) are not permitted because of the chances that very deep pits would develop leaks even though the criterion predicts that they will not cause ruptures.
Because the parabolic representation becomes less and less an accurate representation of the actual area of metal loss as the length increases, the use of B values greater than 4.0 is not permitted. A value of B equal to 4.0 corresponds to a d/t of 0.175.
Anomalies with depths in terms of d/t greater than 0.125 but less than 0.175 are not allowed to have lengths exceeding 4.48 Dt. Anomalies of depths in terms of d/t less than or equal to 0.125 may be of unlimited length because such cases would be expected to have the same remaining strength as a pipe which just meets the minimum wall-thickness requirement.
The B31G criterion provides that if L exceeds the allowable length, an acceptable reduced operating pressure may be calculated by keeping the factor of safety equal to 1.39. The reduced operating pressure is defined in Equation 7.
This equation is embodied in a set of curves in the B31G document. It permits the calculation of reduced pressure levels in the event that the length of the corroded area is found to be unacceptable via Equation 6.
Because of the previously mentioned inadequacy of the parabolic approximation of a long corroded area, Equation 7 is used only for Q values less than or equal to 4.0.
(In the B31G document, the M value is shown as (1 + Q2)1/2 because one must calculate Q = 0.893L/Dt to use the curves.)
When Q is greater than 4.0, the 2/3 factor on Ld is converted to 1.0 (a rectangular rather than a parabolic representation) and M is assumed to approach infinity.
The result is that P' is calculated as shown in Equation 8 for such cases except that P' must be less than or equal to P.
ENHANCEMENTS, EXTENSIONS OF B31G
As a result of excessive conservatism embodied in the original B31G criterion, too much serviceable pipe is being removed during rehabilitation. For the sake of optimizing safety, it is better to concentrate rehabilitation efforts on the portions of the systems which need repair as soon as possible rather than stretch out rehabilitation efforts to meet a criterion known to contain an unnecessarily large margin of safety.
To obtain the desired improvement in the criterion, the sources of excess conservatism and the serious limitations of the original approach were reconsidered.
The sources of excess conservatism in the original B31G criterion are the following:
- The expression for flow stress
- The approximation used for the Folias factor
- The parabolic representation of the metal loss (as used within the B31G limitations), primarily the limitation when applied to long areas of corrosion
- The inability to consider the strengthening effect of islands of full thickness or near full-thickness pipe at the ends of or between arrays of corrosion pits.
Even when the original B31G criterion was developed, it was known that 1.1 SMYS substantially underestimates the flow stress of a line-pipe material.
Maxey clearly demonstrated that yield strength plus 10,000 psi closely approximates the flow stress.2 Even if one takes yield strength to be SMYS, this alternative definition of flowstress would exceed the 1.1 SMYS value for all available grades of line pipe.
For the modified criterion, then, the value of flow stress will be taken as SMYS plus 10,000 psi.
The two-term approximation for M, the Folias factor, used in the original B31 G criterion has been presented here in Equation 2.
A more exact and less conservative approximation of MT is shown in Equation 9, for values of (L2/Dt) 50, and Equation 9a, for values of (L2/Dt) 50.
The alternate form of MT is needed for very long anomalies because the negative term of the three-term series starts to dominate and the three-term approximation is no longer valid beyond the stated limit. It is derived by extrapolation of the MT vs. L2/Dt relationship by means of a straight-line tangent to the curve of Equation 9 at L2/Dt = 50.
The exact area of metal loss as portrayed in Fig. 1 is difficult to represent in terms of simple geometric shapes definable by maximum length and depth. Two shapes which were considered in the development of the original B31G criterion were the rectangle (A Ld) and the parabola (A 2/3 Ld).
On the basis of the 47 burst tests of corroded pipe presented in Kiefner and Duffy,5 it was easily shown that
the parabolic method was preferable. Predictions of remaining strength using the rectangular method were too conservative, but those made using the parabolic method as described in the form of Equation 3 consistently underestimated the actual failure-stress levels as shown in Table 4 of Kiefner and Duffy.5 The ratios of actual-to-predicted failure stress levels range from 1.07 to 3.07. It is apparent that even with the parabolic method, many of the predictions greatly underestimated the strengths of the pipes.
In reality, the parabolic method has significant limitations. Obviously, if the corroded area were very long, the effect of the metal loss would be underestimated, and the remaining strength would be overestimated. The fact that the method of Equation 3 underestimated the strengths in all 47 cases is probably the result of circumstances such as the pits not being lined up axially and the deepest areas being separated by islands of greater remaining wall thickness.
To prevent misuse of the criterion in cases where long, deeply corroded areas might actually have lower strengths than the criterion would predict, the method was limited as described earlier to defects where the B values obtained from Equation 6 are less than or equal to 4.0. This forces all long areas to be considered essentially on the basis of 1 - d/t.
EFFECTIVE AREA
More realistic representations of the metal loss are available if more detailed measurements of the pit-depth profile are made. For example, a contour map of pit depths can be made as shown in Fig. 3.
Then, the "exact" profile can be made by plotting points along the "river bottom" path of the contour map. The profile corresponding to the dashed (river bottom) path in Fig. 3 is shown in Fig. 4.
Experience has shown that the most accurate method to predict the remaining strength involves calculations based upon various subsections of the total area of metal loss.
For example, one could calculate 16 different predicted failure pressures based upon the profile shown in Fig. 4. Each calculation involves the length (Li) where i varies from 1 to 16. The area of each individual flaw is calculated as the sum of the areas of the trapezoids made up by the discrete depth points within Li.
The procedure usually, though not always, results in a minimum predicted failure stress.
This method, referred to as the "effective area" method, is based upon the effective area and effective length of the defect.
ACKNOWLEDGMENT
The authors are grateful for the support provided by the AGA's Corrosion Supervisory Committee. The report resulting from this project may be obtained from AGA along with the personal computer software RSTRENG.
REFERENCES
- Kiefner, J. F., and Vieth, P. H., "A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe," Project PR 3-805 Pipeline Search Committee, American Gas Association, Dec. 22, 1989.
- Maxey, W. A., Kiefner, J. F., Eiber, R. J., and Duffy, A. R., "Ductile Fracture Initiation, Propagation, and Arrest in Cylindrical Vessels," Fracture Toughness, Proceedings of the 1971 National Symposium on Fracture Mechanics, Part II, ASTM STP 514, American Society for Testing and Materials, pp. 70-81, 1972.
- Kiefner, J. F., Maxey, W. A., Eiber, R. J., and Duffy, A. R., "Failure Stress Levels of Flaws in Pressurized Cylinders," Progress in Flaw Growth and Fracture Toughness Testing, ASTM STP 536, American Society for Testing and Materials, pp. 461-481, 1973.
- Folias, E. S., "The Stresses in a Cylindrical Shell Containing an Axial Crack," ARL 64-174, Aerospace Research Laboratories, October 1964.
- Kiefner, J. F., and Duffy, A. R., Summary of Research to Determine the Strength of Corroded Areas in Line Pipe, presented at a Public Hearing at the U.S. Department of Transportation, July 20, 1971.
- Shannon, R. W. E., "The Failure Behavior of Line Pipe Defects," International Journal of Pressure Vessels and Piping, (2), 1974, Applied Science Publishers Ltd., U.K., 1974.
- Kiefner, J. F., "Fracture Initiation," 4th Symposium on Line Pipe Research, American Gas Association (catalogue no. L30075), Nov. 18, 1969.
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