International Pipeline Conference — Volume 1 ASME 1996

IPC1996-1845

ASSESSMENT OF LONG CORROSION GROOVES IN LINE PIPE

Duane S. Cronin

K. Andrew Roberts

Roy J. Pick Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021

Department of Mechanical Engineering University of Waterloo Waterloo, Ontario Canada

ABSTRACT method and compared the results to reported experiments. Due to coating disbondment long corrosion grooves can develop in line pipe. For simple corrosion geometries current methods of EXPERIMENTAL DATA assessing the residual strength of the pipe are adequate but conservative, particularly if only the nominal material strength is Measured burst pressures of pipe with various corrosion geometries considered. In the case of long corrosion grooves which contain pits have been tabulated from a number of sources by Vieth and Kiefher the application of current assessment procedures can lead to a [4], Table 1 presents data from Vieth and Kiefher [4] for corrosion variation in the degree of conservatism, depending on the importance geometries that can be approximated by longitudinal grooves. These applied to the pitting. This paper reviews existing methods of results are labeled with the prefix KV and include the corrosion assessing long corrosion and describes the result of a finite element depth, length and a brief description. study of pits within long corrosion grooves. The authors have also completed tests on 3 sections of 24 inch pipe that suffered corrosion damage due to disbonded polyolefin tape. INTRODUCTION The results are shown in Table 1 and labeled with the prefix RL. For these tests, material properties were determined from tensile tests on Various pipelines, coated with polyolefin tapes, have experienced straightened circumferentially oriented coupons removed from the corrosion damage due to disbondment of the tape. Disbondment and pipe. sagging or wrinkling of the tape can lead to the trapping of water between the pipe and tape. The resulting corrosion often has a \ EFFECT OF THE CIRCUMFERENTIAL EXTENT OF longitudinal orientation due to the orientation of the wrinkles in the CORROSION tape. There have been various methods developed to evaluate the significance of this corrosion: the general approaches of ASME Most assessment procedures neglect the circumferential extent of B31G [1], CSA Z662 Clause 10.10.6 and RSTRENG [2] and the the corrosion, assuming burst to be controlled by the area of metal specific approach of Mok, Pick, Glover and Hoff [3], With the loss in the longitudinal and radial directions. To confirm this exception of RSTRENG these assessment procedures approximate assumption for long defects the finite element method was used to long corrosion as flat bottomed grooves. If the depth of the corrosion model pipe with infinite length longitudinal grooves of various varies or there is pitting, the depth of the groove is normally assumed depths and circumferential widths. The finite element model (shown to be the depth of the deepest pit. This leads to a conservative in Figure 1) was a circumferential section subject to plain strain estimation of the burst pressure of the corroded pipe. boundary conditions in the longitudinal direction. A description of RSTRENG will allow the geometry of the groove to be considered. the finite element code and failure criterion is presented later. However as will be shown, RSTRENG tends to minimize the effect Longitudinal defects in X52, 864 mm diameter, 7.1 mm wall pipe of individual pits whereas experiments show that burst normally were modeled using the finite element method. Burst pressures of originates in the deepest pit. Thus RSTRENG produces an long flat bottomed defects ranging in depth from 20% to 80% of the inconsistent factor of conservatism with different geometries. wall thickness and 23 mm to ISO mm in circumferential width were The authors have investigated the various assessment procedures, calculated. The results, shown in Table 2, indicate that the calculated considered a number of specific geometries using the finite element burst pressure decreased as the defect depth increased, but remained

Copyright © 1996 by ASME constant with varying defect widths. flat bottom groove as the maximum depth of the pits in the groove. In summary, the circumferential width of longitudinal grooves does This will be a very conservative assumption if there are regions of not have a significant influence on the burst pressure of the pipe and local deep corrosion within long shallow defects. can be neglected for flat bottomed defects. The exceptions will be Table 3 compares the B31G predicted safe pressure with the actual when the groove is very narrow and the behavior is crack like and burst pressure. Predictions vary from 20% to 68% of the actual when the groove width is small compared to pits within the groove. burst pressure (average 49%) using the B31G procedure to evaluate Based on the assumption that the circumferential dimension has no longitudinal corrosion. If this B31G prediction of burst pressure is effect on the failure pressure, the pipe can be considered to be of below the operating pressure no further assessment is required. reduced thickness equal to the corrosion ligament thickness as in However due to the conservatism of the B31G assessment procedure B31G. it is possible that some safe pipe may fail this assessment procedure. In this case other assessment procedures developed specifically for ASSESSMENT PROCEDURES longitudinal grooves may be used.

In most jurisdictions the B31G assessment procedure with Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021 variations is regulated as the assessment procedure. In Canada CSA Mok. Pick. Glover and Hoff [3] Z662, Clause 10.10.6 also allows an Engineering Critical Assessment Mok et al developed a model to predict the behavior of line pipe using other established procedures or analysis techniques. Therefore with long defects in various orientations (spiral corrosion). This it is assumed that the initial assessment of corroded pipe with long model was developed from burst tests on pipe with long flat bottomed corrosion grooves will be undertaken using the B31G or CSA Z662 machined defects in spiral and longitudinal orientations. Vieth and procedure. If this assessment procedure indicates an acceptable burst Kiefiier [4] have included the results for longitudinal defects in their pressure no further assessment is required. If the predicted burst database as KV97 ,KV98 , KV119 and KV120 as listed in Table 1. pressure is not acceptable it may indicate the pipe is unsafe or that the The equation developed by Mok et al to describe the strength of assessment procedure is overly conservative and an Engineering longitudinally oriented defects is shown in equation 4. Critical Assessment is more applicable. P = 1.5 P * |l - i (4) B31G Assessment B31-G was developed from a database of full size tests of corroded pipe. A semi-empirical formula was developed from this data which relates the failure pressure to the flow stress of the pipe material and where: to the size of the defect for different pipe geometries. These P = predicted burst pressure of pipe with defect equations have since been supported by additional full scale testing. “ = predicted burst pressure of plain pipe B31G models short defects as parabolas, but for long defects with P a length greater than a minimum Length given by:

, . 4 'J e t t ... with P* being approximated by: Limn = — *------(1) 0.893 P ' = SMYS — (5) D

The defects are considered to be flat bottomed. The maximum safe pressure is described by: This is similar to the B31G formula except a factor of 1.5 is used (compared to a factor of 1.1 in B31G), based on consideration of the d P' = 1.1 P * 1 (2) strain hardening behavior of a series of typical pipeline steels. t Table 3 shows the ratio of the Mok et al predictions of burst pressure to the actual pressure. The Mok et al equation predicts burst and P* is given by: pressures between 27% and 92%'(with an average of 67%) of the actual burst pressure. In summary this assessment procedure is, on average, more P ’ = SMYS — (3) accurate than B31G, but like B31G makes use of the deepest point D in the corrosion as the depth of the flat bottomed corrosion groove. This leads to a conservative prediction of the burst pressure of the pipe. If this is not acceptable a more accurate assessment procedure In this case the length is not considered and can be infinite. For this should be attempted. study lengths exceeding Lmin will be considered infinite The assumption that the flow stress is 1.1 times the SMYS leads to RSTRENG a high degree of conservatism. Flow stress approximations based on The RSTRENG assessment procedure developed by Kiefner and the actual yield stress such as the yield stress plus 69 MPa are more Vieth [2] allows the use of a more complete description of the appropriate. longitudinal geometry of the corrosion compared to B31G and Mok For defects longer than Lmin B31-G approximates the depth of the etal.. Kiefner and Vieth recognized that the main sources of conservatism confirmed by the stress distribution through the pipe wall in the pipe in B31G were the assumed value of the flow stress and the away from the defect which approached that of plain pipe. The simplification of the corrosion geometry. RSTRENG redefines the model was not allowed to contract or expand longitudinally to create flow stress as the yield stress + 69 MPa and uses the actual corrosion a condition of plane strain in this direction. This simulates a buried geometry to describe the defect. An “effective area” technique is pipe in which longitudinal expansion and contraction is restricted by contained within the RSTRENG code that makes use of the variation the soil. These boundary conditions actually model a pipe with an in the depth of the corrosion in the longitudinal direction. These infinite series of equally spaced pits in the longitudinal groove as changes require more accurate measurement of the corrosion but shown in figure 5. Varying the length, L, allowed the interaction of reduce the conservatism in the assessment procedure compared to the pits to be investigated. B31G and Mok et al. Comparing the RSTRENG predictions of the burst pressure with Failure Criteria the measured burst pressures in Table 3 shows that the predicted Burst or failure of the pipe occurs by plastic collapse and is indicated

burst pressures are between 62% and 106% of the actual burst by one of two criteria. When no significant defect is present, such as Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021 pressures (with an average of 82%). While on average RSTRENG plain pipe, plastic collapse is indicated by a global geometric is more accurate than other techniques there remains a degree of instability of the model. This is the result of a decreasing wall scatter which leads to non conservative (106%) predictions. The thickness and increasing pipe radius which lead to an increasing error in the RSTRENG prediction is plotted as a function of stress. When this increasing stress overcomes the strain hardening of maximum defect depth in Figure 2 for the data in Table 3. Although the material, instability occurs. This instability can be predicted a large degree of scatter exists, it is evident that RSTRENG becomes numerically using Rik’s method [6]. Failure by plastic collapse when less conservative as the long defects increase in depth. Using a lower a defect is present is predicted to occur locally in the corrosion bound line drawn through the data (Figure 2) shows that RSTRENG ligament by a stress-based criteria. Collapse is predicted when the may become non-conservative at defect depths greater than 66% of von Mises stress exceeds the ultimate tensile strength of the material the wall thickness. The extrapolated degree of conservatism for through the entire ligament. This method has been confirmed with defects which are 10% of the wall thickness deep is 25%. experimental results for single pits [7].

Assessment of Machined Grooves Pit Interaction Figure 3 compares the predicted burst pressures of RSTRENG, The interaction of pits was investigated by varying the longitudinal B31G and Mok et a] to the measured burst pressures for the five length, L, of the three-dimensional model and thus the spacing pipes that had machined flat bottomed grooves (KV97, KV98, between the pits. Both the finite element results and RSTRENG KV119, KV120 and KV124). It can be seen that each method has predictions are plotted in Figure 6. For this geometry RSTRENG a relatively constant degree of conservatism with the method of Mok predicts a conservative burst pressure with an accurate prediction of et al having the least amount of conservatism. the interaction between pits (assuming the finite element results The remaining data in Table 3 is for actual corrosion defects and represent the true behavior). All RSTRENG analyses were carried suggests that the scatter in the conservatism for the RSTRENG out using a longitudinal defect 356 mm in length with 60% deep pits predictions is partially due to the variation in the depth of the equally spaced. Use of a greater length did not alter the results and corrosion and pitting. Variation between the specified and actual so an infinite length could be assumed. Both RSTRENG and the material strength will also contribute to the scatter. Therefore a more finite element results predict that the interaction of pits begins at a pit accurate analysis will require consideration of these effects. separation of 6 wall thicknesses. The most significant interaction and reduction in failure pressure occurs at less than 1 to 2 wall FINITE ELEMENT MODEL OF A PIT WITHIN A LONG thicknesses. Similar results have been reported for pits in full CORROSION GROOVE thickness pipe [7]. For all geometries B31G considers the defect to The finite element method has been shown to accurately predict the be infinite in length and 60% of the wall thickness deep, the depth of burst pressure in corroded pipe [5], Therefore a study was the pit The burst pressure predicted by B31G is 2.60 MPa which is undertaken of the behavior of pits within a long corrosion groove . very conservative.

Finite Element Model Effect of Pit Depth For the finite element prediction of burst pressure, a 3-dimensional The effect of pit depth was investigated using a groove depth of elastic-plastic, large displacement finite element analysis was 40% of the wall thickness and a pit separation exceeding 6 wall undertaken. The finite element model, shown in Figure 4, considered thicknesses. The pit depth was varied from 45% to 80% of the wall a 864 mm diameter, X52 line pipe with a wall thickness of 7.1 mm. thickness. Figure 7 shows the burst pressure calculated by the finite A longitudinal groove depth of 40% of the wall thickness and a pit element method and predicted by RSTRENG and B31G as a depth of 60% of the wall thickness was considered as representing a function of pit depth. The effect of pit radius is also considered. The relatively common pattern of corrosion. The pit radius was 5 mm burst pressures predicted by the Finite Element Method are slightly and the pit was spherical in shape. The groove had a total higher for the smaller radius pits and the difference between the small circumferential width of 51 mm and was flat bottomed. To reduce and large radius pits increases with pit depth. Both FEM results computing time, the pipe model was reduced in size by considering converge to the predicted burst pressure for plain pipe with a 40% symmetry and applying appropriate boundary conditions. The pipe deep groove of 6.29 MPa. The RSTRENG analysis is conservative was allowed to expand only in a radial direction at the longitudinal in all cases but the degree of conservatism decreases as the pits faces of the model. The accuracy of this simplification was become deeper. The RSTRENG prediction appears to be independent of pit depth and pit radius for single pits. In effect, defect depth, length, pipe dimensions and grade of steel. Defects RSTRENG does not account for the single pit in the long groove. which meet this criteria are safe while defects which are calculated to The projected area of the small single pit is outweighed by the area of be unsafe by B31G can be assessed with a less conservative method. the groove and as a result RSTRENG predicts a similar burst pressure The assessment method developed by Mok for long corrosion is the for all pit dimensions. least conservative of all methods when considering flat bottomed As can be seen in Figure 7, RSTRENG does not provide sufficient grooves. However, this method is similar to B31G, although slightly consideration of the importance of individual pits. This can be less conservative, when applied to irregular bottomed longitudinal explained by the RSTRENG analysis method which uses a series of corrosion grooves since it does not account for pitting or variations in data points describing the actual projected area of the corrosion groove depth. profile and iterates through various combinations of the data points RSTRENG predicts failure pressures which are 60% to 106% of to find the lowest predicted failure pressure. For long defects the the actual burst pressure when applied to the database. The lowest predicted failure pressure corresponds to the projected area of prediction which is 6% above the actual failure pressure is for a the entire corrosion profile. Thus when a pit is present in a long defect which is 79% of the wall thickness deep. In general, the Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021 groove, its projected area is very small in comparison to the area of degree of conservatism associated with RSTRENG predictions the groove and as a result doesn’t affect the RSTRENG prediction decreases as the defect depth increases. Using a reduced wall significantly. ' RSTRENG analysis for long defects captures the corrosion geometry When analyzing long flat bottomed defects with RSTRENG, one better but results in increased conservatism. This provides no benefit can consider the full thickness pipe with the long corrosion defect, or over a standard RSTRENG analysis. consider a plain pipe of wall thickness equal to the thickness of the Three-dimensional finite element analyses of single pits 10 mm ligament in the flat bottomed groove. Both of these methods will long located within longitudinal grooves indicate that the pits begin yield the same predicted burst pressure. This suggests that long to interact at a separation of 6 wall thicknesses and this interaction corrosion defects can be modeled as a pipe with a reduced wall becomes significant when the separation is below 1 to 2 wall thickness which is the assumption used by Mok et al and B31G. thicknesses. RSTRENG can predict this trend for evenly spaced pits Based on this assumption, a burst pressure prediction of a long of similar dimensions, but the predicted burst pressures are corrosion defect with single pit should be equal to that of a pipe of conservative. The RSTRENG technique does not adequately reduced wall with a single pit of reduced depth. Considering the account for single pits within long grooves and becomes less idealized geometry shown in Figure 8, RSTRENG will neglect the conservative as the pit increases in depth. effect of the pit (L,) in the long defect (L2) compared to the reduced wall analysis shown in below. For various dimensions, Table 4 REFERENCES shows that the reduced wall analysis produces a more conservative 1. ASME, 1991, “Manual for Determining the Remaining prediction of the burst pressure since it will account for the smaller Strength of Corroded Pipelines”, American Society of Mechanical defect within the larger defect. Engineers, New York. Rgure 9 shows the finite element analysis and RSTRENG data for the single pit in the long groove from Figure 7. Also shown is an 2. Kiefner, J. F. and Vieth, P. H., 1989, “A Modified RSTRENG reduced wall analysis. Although the reduced wall Criterion for Evaluating the Remaining Strength of Corroded Pipe”, analysis slightly accounts for the pit depth in the groove it produces Final Report on Project PR 3-805, Battelle Memorial Institute, a more conservative estimate of the failure pressure. Thus a reduced Columbus. wall RSTRENG analysis provides little benefit when analyzing long corrosion grooves with pits or rough bottoms. 3. Mok, D. H. B„ Pick, R. J„ Glover, A. J., and Hoff, R., Table 3 shows the results of the reduced wall analysis (RWA) 1991, “Bursting of Line Pipe with Long External Corrosion”, Int. J. applied to the long corrosion data. Here the reduced wall thickness Pres. Ves & Piping, 46, pp. 195-215. was assumed to be the maximum remaining ligament within the corrosion. With this geometry RSTRENG predicted burst pressures 4. Vieth, P. H. and Kiefner, J. F., 1994, “Database of between 59% and 96% (with an average of 78%) of the actual burst Corroded Pipe Tests”, Final Report on Contract No. PR 218-9206, pressure. Kiefner and Associates, Inc., Worthington, Ohio. Based on the limited number of cases assessed with the finite element method, RSTRENG is found to be conservative in its 5. Chouchaoui, B. A., and Pick, R. J., 1994, “A Three Level prediction of failure pressures. Pits interact significantly if there is Assessment of the Residual Strength of Corroded Line Pipe”, ASME less than 1 to 2 wall thicknesses of full thickness ligament between OMAE, Vol. V, Pipeline Technology, pp. 9-18. them. RSTRENG can account for regularly spaced pits and their interaction due to the large projected area of the pits compared to the 6. Hibbitt, Karlsson and Sorenson, Inc., Abaqus Theory projected area of the groove. However the degree of conservatism is Manual, Providence, Rhode Island. reduced when predicting the failure pressure of grooves with single or non-interacting pits, particularly when the pits are deep. 7. Chouchaoui, B. A., and Pick, R. J., 1993, “Interaction of Closely Spaced Corrosion Pits in Line Pipe”, ASME OMAE, Vol. V, CONCLUSIONS Pipeline Technology, pp. 203-214. For the data base of measured burst pressures B31G provides predictions of burst pressure which are 20% to 68% of the actual ACKNOWLEDGMENTS burst pressures. This technique is simple to apply, requiring the The authors wish to acknowledge the financial and technical support provided by British Gas Investments, Interprovincial Pipelines and Nova Gas Transmission Ltd. Samples of corroded pipe were provided by Mobil Oil and Rainbow Pipe Lines. Preliminary work was supported by an operating grant from the Natural Science and Engineering Research Council of Canada.

TABLE 1: SUMMARY OF BURST TEST DATA

Test Pipe Material Properties Defect Dimensions Burst Descrip. OD t SMYS Sy Suts dm/t dmax L Press. Defect Description (mm) (mm) (MPa) (MPa) (MPa) (-) (mm) (mm) (MPa)

kv43 610 9.3 241.5 371.9 NA 0.75 6.99 381.0 10.2 rough bottom, single deep pit (25 mm long) Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021 kv44 610 9.2 241.5 358.8 NA 0.70 6.45 330.2 8.7 rough bottom, deepest pit 152 mm long kv48 610 9.5 241.5 371.2 474.0 0.79 7.49 406.4 5.1 relatively smooth bottom kv51 508 7.7 358.8 380.2 521.6 0.69 5.33 266.7 8.1 relatively fiat bottom kv63 508 7.0 241.5 279.5 442.3 0.47 3.30 304.8 12.0 rough bottom, deepest pit 102 mm long kv66 508 6.8 241.5 277.4 420.9 0.54 3.66 393.7 10.4 general corrosion with 2 deep pits kv68 762 9.4 358.8 409.9 NA 0.35 3.30 914.4 12.7 relatively smoot bottom kv71 762 9.7 358.8 429.2 NA 0.38 3.68 508.0 13.1 general corrosion with 1 deep pit kv72 762 9.6 358.8 387.8 NA 0.35 3.30 508.0 12.3 relatively smooth bottom kv73 762 9.6 3588 439.5 NA 0.29 2.79 838.2 13.2 relatively smooth bottom kv80 762 9.3 358.8 404.3 521.0 0.63 5.82 406.4 6.8 relatively smooth bottom kv81 762 9.5 358.8 474.5 580.3 0.65 622 685.8 6.8 relatively smooth bottom kv84 914 8.4 448.5 506.6 NA 0.66 5.54 406.4 5.3 relatively smooth bottom kv92 610 8.1 358.8 396.8 528.5 0.28 2.29 482.6 13.0 relatively smooth bottom kv97 508 6.6 414.0 444.1 598.9 0.39 2.62 381.0 11.3 machined groove kv98 508 6.7 414.0 427.8 601.0 0.40 2.66 1016.0 11.6 machined groove kv119 508 6.4 414.0 430.2 672.5 0.54 3.46 899.2 8.0 machined groove kv120 508 6.4 414.0 430.2 672.5 0.34 2 18 899.2 11.8 machined groove kv124 508 6.4 414.0 435.4 672.5 0.50 3.18 1000.8 8.4 machined groove RL0405 610 6.4 358.8 421.7 630.4 0.51 3.21 899.2 9.5 rough irregular bottom RL07 610 6.4 358.8 421.7 630.4 0.54 3.43 1422.4 7.9 rough irregular bottom RL08 610 6.4 358.8 421.7 630.4 0.39 2.48 1371.6 9.8 rough irregular bottom, single pit 203 mm long

TABLE 2: EFFECT OF WIDTH ON CALCULATED BURST PRESSURE

Defect Burst Pressures for Pipe: Depth Defect Widths of: OD = 864 mm d/t 25 mm 50 mm 152 mm t = 7.1 mm (MPa) (MPa) (MPa) Material: X52 0.2 8.3 8.3 8.3 SMYS = 358.8 MPa 0.4 6.3 6.3 6.3 Sy = 406.6 MPa 0.6 4.2 4.2 4.3 Suts = 546.1 MPa 0.8 2.1 2.1 2.1 TABLE 3: SUMMARY OF BURST PRESSURE PREDICTIONS

Test Pipe Material Properties Defect Dimensions Act. Burst Predicted Burst Pressure Desc OD t SMYS Sy Suts dm/t dmax L Press. / Actual Burst Pressure (mm) (mm) (MPa) (MPa) (MPa) (-) (mm) (mm) (MPa) B31-G Mok RSTR RWA kv43 610 9.3 241.2 371.9 NA 0.75 6.99 381.0 10.2 0.20 0.27 0.91 0.85 kv44 610 9.2 241.2 358.8 NA 0.70 6.45 330.2 8.7 0.28 0.38 0.96 0.96 kv48 610 9.5 241.2 371.2 474.0 0.79 7.49 406.4 5.1 0.35 0.47 1.06 0.89 kv51 508 7.7 358.8 379.6 520.9 0.69 5.33 266.7 8.1 0.46 0.63 0.91 0.91 kv63 508 7.0 241.2 279.0 441.6 0.47 3.30 304.8 12.0 0.32 0.43 0.62 0.59 kv66 508 6.8 241.2 277.0 420.3 0.54 3.66 393.7 10.4 0.31 0.43 0.70 0.67 Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021 kv68 762 9.4 358.3 409.3 NA 0.35 3.30 914.4 12.7 0.50 0.68 0.73 0.70 kv71 762 9.7 358.3 428.6 NA 0.38 3.68 508.0 13.1 0.48 0.65 0.83 0.81 kv72 762 9.6 358.3 387.2 NA 0.35 3.30 508.0 12.3 0.53 0.72 0.76 0.75 kv73 762 9.6 358.3 438.9 NA 0.29 2.79 838.2 13.2 0.53 0.73 0.79 0.76 kv80 762 9.3 358.3 403.8 520.2 0.63 5.82 406.4 6.8 0.53 0.72 0.91 0.74 kv81 762 9.5 358.3 473.8 579.4 0.65 6.22 685.8 6.8 0.50 0.68 1.00 0.91 kv84 914 8.4 447.9 505.8 NA 0.66 5.54 406.4 5.3 0.57 0.78 0.95 0.95 kv92 610 8.1 358.3 396.2 527.8 0.28 2.29 482.6 13.0 0.58 0.79 0.83 0.83 kv97 508 6.6 413.4 443.4 598.9 0.39 2.62 381.0 11.3 0.64 0.87 0.78 0.78 kv98 508 6.7 413.4 427.2 601.0 0.40 2.66 1016.0 11.6 0.62 0.84 0.67 0.67 kv119 508 6.4 413.4 429.6 672.5 0.54 3.46 899.2 8.0 0.66 0.90 0.75 0.75 kv120 508 6.4 413.4 429.6 672.5 0.34 2.18 899.2 11.8 0.64 0.88 0.72 0.72 kv124 508 6.4 413.4 434.8 672.5 0.50 3.18 1000.8 8.4 0.68 0.92 0.78 0.78 RL0405 610 6.4 358.8 421.7 630.4 0.51 3.21 899.2 9.5 0.43 0.59 0.77 0.71 RL07 610 6.4 358.8 421.7 630.4 0.54 3.43 1422.4 7.9 0.48 0.65 0.85 0.78 RL08 610 6.4 358.8 421.7 630.4 0.39 2.48 1371.6 9.8 0.51 0.70 0.83 0.71

TABLE 4: RSTRENG ANALYSIS OF LONG FLAT CORROSION WITH A SINGLE DEEP PIT Case Pipe Description Defect Description Burst Pressure No. OD s y t d1 d2 L2 RSTRENG Red. Wall Difference (mm) (MPa) (mm) (mm) (mm) (mm) (MPa) (MPa)

1 508 358.8 6.4 2.5 3.8 50.8 6.4 5.5 16% 2 508 358.8 6.4 3.8 5.1 50.8 4.3 3.2 34% 3 864 414.0 7.6 2.5 3.8 50.8 5.9 5.4 10% 4 864 414.0 6.4 2.5 3.8 50.8 4.4 3.9 14% 5 508 552.0 6.4 2.5 3.8 50.8 9.5 8.2 16% 6 610 358.8 5.1 2.5 3.8 50.8 3.6 - 2.7 32% 7 864 414.0 7.6 3.8 5.1 50.8 5.2 3.9 32% 8 508 552.0 7.6 3 8 5.1 50.8 9.8 8.2 19% Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40207/401/2506643/401_1.pdf by guest on 25 September 2021

GROOVE DEPTH {% WALL THICKNESS) FIGURE 3. BURST PRESSURE FOR MACHINED GROOVES AS A FUNCTION OF GROOVE DEPTH.

FIGURE 1. FINITE ELEMENT MODEL FOR PIPE WITH INFINITELY LONG LONGITUDINAL GROOVE.

FIGURE 4. FINITE ELEMENT MESH FOR A PIT WITHIN A LONG LONGITUDINAL GROOVE

FIGURE 2 ERROR IN RSTRENG AS A FUNCTION OF DEFECT DEPTH

FIGURE 5. SYMMETRY CONDITIONS DESCRIBED A SERIES OF prrs IN A LONG LONGITUDINAL GROOVE. FAILURE PRESSURE (MPa) 0.0 2.0 2.0 t.O 3.0 4.0 6.0 6.0 7.0 1 2 30 20 10 0 FULLTHICKNESSMATERIAL LOCATEDTHEPITS) BETWEEN PIT SEPARATIONNUMBERPIT THEWALL(REFERS TO OF THICKNESSESOF FIGURE 8. INTERACTION OF PITS IN LONG GROOVES - 40% - GROOVES LONG IN PITS OF INTERACTION 8. FIGURE DEEP GROOVE. 40% DEEP PIT. 5 mm PIT RADIUS PIT mm 5 PIT. DEEP 40% GROOVE. DEEP

FIGURE 8 SINGLE PIT IN LONG CORROSION AND IN REDUCED IN AND CORROSION LONG IN PIT SINGLE 8 FIGURE PIPE WITH LONGANDSINGLE CORROSION PIT PIPE (LONGITUDINAL WITH SECTION) FIGURE 9. BURST DATA FOR LONG GROOVES WITH SINGLE WITH GROOVES LONG FOR DATA BURST 9. FIGURE REDUCED WALLEQUIVALENT (LONGITUDINAL SECTION) WALL PIPE. WALL PITS: 40% DEEP GROOVE. 10 mm PIT RADIUS. PIT mm 10 GROOVE. DEEP 40% PITS: I - dl

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