assessment of long corrosion grooves in line pipe - asme … asme 1996 assessment of long corrosion...

International Pipeline Conference — Volume 1ASME 1996

ASSESSMENT OF LONG CORROSION GROOVES IN LINE PIPE

Duane S. Cronin

K. Andrew Roberts

Roy J. Pick

Department of Mechanical Engineering University of Waterloo

Waterloo, Ontario Canada

ABSTRACTDue to coating disbondment long corrosion grooves can develop in

line pipe. For simple corrosion geometries current methods of assessing the residual strength of the pipe are adequate but conservative, particularly if only the nominal material strength is considered. In the case of long corrosion grooves which contain pits the application of current assessment procedures can lead to a variation in the degree of conservatism, depending on the importance applied to the pitting. This paper reviews existing methods of assessing long corrosion and describes the result of a finite element study of pits within long corrosion grooves.

INTRODUCTIONVarious pipelines, coated with polyolefin tapes, have experienced

corrosion damage due to disbondment of the tape. Disbondment and sagging or wrinkling of the tape can lead to the trapping of water between the pipe and tape. The resulting corrosion often has a longitudinal orientation due to the orientation of the wrinkles in the tape. There have been various methods developed to evaluate the significance of this corrosion: the general approaches of ASME B31G [1], CSA Z662 Clause 10.10.6 and RSTRENG [2] and the specific approach of Mok, Pick, Glover and Hoff [3], With the exception of RSTRENG these assessment procedures approximate long corrosion as flat bottomed grooves. If the depth of the corrosion varies or there is pitting, the depth of the groove is normally assumed to be the depth of the deepest pit. This leads to a conservative estimation of the burst pressure of the corroded pipe.RSTRENG will allow the geometry of the groove to be considered.

However as will be shown, RSTRENG tends to minimize the effect of individual pits whereas experiments show that burst normally originates in the deepest pit. Thus RSTRENG produces an inconsistent factor of conservatism with different geometries.

The authors have investigated the various assessment procedures, considered a number of specific geometries using the finite element

method and compared the results to reported experiments.

EXPERIMENTAL DATAMeasured burst pressures of pipe with various corrosion geometries

have been tabulated from a number of sources by Vieth and Kiefher [4], Table 1 presents data from Vieth and Kiefher [4] for corrosion geometries that can be approximated by longitudinal grooves. These results are labeled with the prefix KV and include the corrosion depth, length and a brief description.The authors have also completed tests on 3 sections of 24 inch pipe

that suffered corrosion damage due to disbonded polyolefin tape. The results are shown in Table 1 and labeled with the prefix RL. For these tests, material properties were determined from tensile tests on straightened circumferentially oriented coupons removed from the pipe.

\EFFECT OF THE CIRCUMFERENTIAL EXTENT OF CORROSION

Most assessment procedures neglect the circumferential extent of the corrosion, assuming burst to be controlled by the area of metal loss in the longitudinal and radial directions. To confirm this assumption for long defects the finite element method was used to model pipe with infinite length longitudinal grooves of various depths and circumferential widths. The finite element model (shown in Figure 1) was a circumferential section subject to plain strain boundary conditions in the longitudinal direction. A description of the finite element code and failure criterion is presented later.Longitudinal defects in X52, 864 mm diameter, 7.1 mm wall pipe

were modeled using the finite element method. Burst pressures of long flat bottomed defects ranging in depth from 20% to 80% of the wall thickness and 23 mm to ISO mm in circumferential width were calculated. The results, shown in Table 2, indicate that the calculated burst pressure decreased as the defect depth increased, but remained

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constant with varying defect widths.In summary, the circumferential width of longitudinal grooves does

not have a significant influence on the burst pressure of the pipe and can be neglected for flat bottomed defects. The exceptions will be when the groove is very narrow and the behavior is crack like and when the groove width is small compared to pits within the groove. Based on the assumption that the circumferential dimension has no effect on the failure pressure, the pipe can be considered to be of reduced thickness equal to the corrosion ligament thickness as in B31G.

ASSESSMENT PROCEDURESIn most jurisdictions the B31G assessment procedure with

variations is regulated as the assessment procedure. In Canada CSA Z662, Clause 10.10.6 also allows an Engineering Critical Assessment using other established procedures or analysis techniques. Therefore it is assumed that the initial assessment of corroded pipe with long corrosion grooves will be undertaken using the B31G or CSA Z662 procedure. If this assessment procedure indicates an acceptable burst pressure no further assessment is required. If the predicted burst pressure is not acceptable it may indicate the pipe is unsafe or that the assessment procedure is overly conservative and an Engineering Critical Assessment is more applicable.

B31G AssessmentB31-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 to the size of the defect for different pipe geometries. These equations have since been supported by additional full scale testing.

B31G models short defects as parabolas, but for long defects with a length greater than a minimum Length given by:

, . 4 'J e t t ...Limn = — *------- (1)0.893

flat bottom groove as the maximum depth of the pits in the groove. This will be a very conservative assumption if there are regions of local deep corrosion within long shallow defects.Table 3 compares the B31G predicted safe pressure with the actual burst pressure. Predictions vary from 20% to 68% of the actual burst pressure (average 49%) using the B31G procedure to evaluate longitudinal corrosion. If this B31G prediction of burst pressure is below the operating pressure no further assessment is required. However due to the conservatism of the B31G assessment procedure it is possible that some safe pipe may fail this assessment procedure. In this case other assessment procedures developed specifically for longitudinal grooves may be used.

Mok. Pick. Glover and Hoff [3]Mok et al developed a model to predict the behavior of line pipe

with long defects in various orientations (spiral corrosion). This model was developed from burst tests on pipe with long flat bottomed machined defects in spiral and longitudinal orientations. Vieth and Kiefiier [4] have included the results for longitudinal defects in their database as KV97 ,KV98 , KV119 and KV120 as listed in Table 1. The equation developed by Mok et al to describe the strength of longitudinally oriented defects is shown in equation 4.

P = 1.5 P * | l - i (4)

where:P = predicted burst pressure of pipe with defect P “ = predicted burst pressure of plain pipe

with P* being approximated by:

P ' = SMYS — D (5)

The defects are considered to be flat bottomed. The maximum safe pressure is described by:

P' = 1.1 P * 1 dt (2)

and P* is given by:

P ’ = SMYS — D (3)

In this case the length is not considered and can be infinite. For this study lengths exceeding Lmin will be considered infinite The assumption that the flow stress is 1.1 times the SMYS leads to

a high degree of conservatism. Flow stress approximations based on the actual yield stress such as the yield stress plus 69 MPa are more appropriate.For defects longer than Lmin B31-G approximates the depth of the

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 strain hardening behavior of a series of typical pipeline steels.

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 pressures between 27% and 92%'(with an average of 67%) of the actual burst pressure.

In summary this assessment procedure is, on average, more accurate than B31G, but like B31G makes use of the deepest point 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 should be attempted.

RSTRENGThe RSTRENG assessment procedure developed by Kiefner and

Vieth [2] allows the use of a more complete description of the longitudinal geometry of the corrosion compared to B31G and Mok etal..


Kiefner and Vieth recognized that the main sources of conservatism in B31G were the assumed value of the flow stress and the simplification of the corrosion geometry. RSTRENG redefines the flow stress as the yield stress + 69 MPa and uses the actual corrosion geometry to describe the defect. An “effective area” technique is contained within the RSTRENG code that makes use of the variation in the depth of the corrosion in the longitudinal direction. These changes require more accurate measurement of the corrosion but reduce the conservatism in the assessment procedure compared to B31G and Mok et al.

Comparing the RSTRENG predictions of the burst pressure with the measured burst pressures in Table 3 shows that the predicted burst pressures are between 62% and 106% of the actual burst pressures (with an average of 82%). While on average RSTRENG is more accurate than other techniques there remains a degree of scatter which leads to non conservative (106%) predictions. The error in the RSTRENG prediction is plotted as a function of maximum defect depth in Figure 2 for the data in Table 3. Although a large degree of scatter exists, it is evident that RSTRENG becomes less conservative as the long defects increase in depth. Using a lower bound line drawn through the data (Figure 2) shows that RSTRENG may become non-conservative at defect depths greater than 66% of the wall thickness. The extrapolated degree of conservatism for defects which are 10% of the wall thickness deep is 25%.

Assessment of Machined GroovesFigure 3 compares the predicted burst pressures of RSTRENG,

B31G and Mok et a] to the measured burst pressures for the five pipes that had machined flat bottomed grooves (KV97, KV98, KV119, KV120 and KV124). It can be seen that each method has a relatively constant degree of conservatism with the method of Mok et al having the least amount of conservatism.

The remaining data in Table 3 is for actual corrosion defects and suggests that the scatter in the conservatism for the RSTRENG predictions is partially due to the variation in the depth of the corrosion and pitting. Variation between the specified and actual material strength will also contribute to the scatter. Therefore a more accurate analysis will require consideration of these effects.

FINITE ELEMENT MODEL OF A PIT WITHIN A LONG CORROSION GROOVEThe finite element method has been shown to accurately predict the

burst pressure in corroded pipe [5], Therefore a study was undertaken of the behavior of pits within a long corrosion groove .

Finite Element ModelFor the finite element prediction of burst pressure, a 3-dimensional elastic-plastic, large displacement finite element analysis was undertaken. The finite element model, shown in Figure 4, considered a 864 mm diameter, X52 line pipe with a wall thickness of 7.1 mm. A longitudinal groove depth of 40% of the wall thickness and a pit depth of 60% of the wall thickness was considered as representing a relatively common pattern of corrosion. The pit radius was 5 mm and the pit was spherical in shape. The groove had a total circumferential width of 51 mm and was flat bottomed. To reduce computing time, the pipe model was reduced in size by considering symmetry and applying appropriate boundary conditions. The pipe was allowed to expand only in a radial direction at the longitudinal faces of the model. The accuracy of this simplification was

confirmed by the stress distribution through the pipe wall in the pipe away from the defect which approached that of plain pipe. The model was not allowed to contract or expand longitudinally to create a condition of plane strain in this direction. This simulates a buried pipe in which longitudinal expansion and contraction is restricted by the soil. These boundary conditions actually model a pipe with an infinite series of equally spaced pits in the longitudinal groove as shown in figure 5. Varying the length, L, allowed the interaction of the pits to be investigated.

Failure CriteriaBurst or failure of the pipe occurs by plastic collapse and is indicated

by one of two criteria. When no significant defect is present, such as plain pipe, plastic collapse is indicated by a global geometric instability of the model. This is the result of a decreasing wall thickness and increasing pipe radius which lead to an increasing stress. When this increasing stress overcomes the strain hardening of the material, instability occurs. This instability can be predicted numerically using Rik’s method [6]. Failure by plastic collapse when a defect is present is predicted to occur locally in the corrosion ligament by a stress-based criteria. Collapse is predicted when the von Mises stress exceeds the ultimate tensile strength of the material through the entire ligament. This method has been confirmed with experimental results for single pits [7].

Pit InteractionThe interaction of pits was investigated by varying the longitudinal

length, L, of the three-dimensional model and thus the spacing between the pits. Both the finite element results and RSTRENG predictions are plotted in Figure 6. For this geometry RSTRENG predicts a conservative burst pressure with an accurate prediction of the interaction between pits (assuming the finite element results represent the true behavior). All RSTRENG analyses were carried out using a longitudinal defect 356 mm in length with 60% deep pits equally spaced. Use of a greater length did not alter the results and so an infinite length could be assumed. Both RSTRENG and the finite element results predict that the interaction of pits begins at a pit separation of 6 wall thicknesses. The most significant interaction and reduction in failure pressure occurs at less than 1 to 2 wall thicknesses. Similar results have been reported for pits in full thickness pipe [7]. For all geometries B31G considers the defect to be infinite in length and 60% of the wall thickness deep, the depth of the pit The burst pressure predicted by B31G is 2.60 MPa which is very conservative.

Effect of Pit DepthThe effect of pit depth was investigated using a groove depth of

40% of the wall thickness and a pit separation exceeding 6 wall thicknesses. The pit depth was varied from 45% to 80% of the wall thickness. Figure 7 shows the burst pressure calculated by the finite element method and predicted by RSTRENG and B31G as a function of pit depth. The effect of pit radius is also considered. The burst pressures predicted by the Finite Element Method are slightly higher for the smaller radius pits and the difference between the small and large radius pits increases with pit depth. Both FEM results converge to the predicted burst pressure for plain pipe with a 40% deep groove of 6.29 MPa. The RSTRENG analysis is conservative in all cases but the degree of conservatism decreases as the pits become deeper. The RSTRENG prediction appears to be


independent of pit depth and pit radius for single pits. In effect, RSTRENG does not account for the single pit in the long groove. The projected area of the small single pit is outweighed by the area of the groove and as a result RSTRENG predicts a similar burst pressure for all pit dimensions.As can be seen in Figure 7, RSTRENG does not provide sufficient

consideration of the importance of individual pits. This can be explained by the RSTRENG analysis method which uses a series of data points describing the actual projected area of the corrosion profile and iterates through various combinations of the data points to find the lowest predicted failure pressure. For long defects the lowest predicted failure pressure corresponds to the projected area of the entire corrosion profile. Thus when a pit is present in a long groove, its projected area is very small in comparison to the area of the groove and as a result doesn’t affect the RSTRENG prediction significantly. '

When analyzing long flat bottomed defects with RSTRENG, one can consider the full thickness pipe with the long corrosion defect, or consider a plain pipe of wall thickness equal to the thickness of the ligament in the flat bottomed groove. Both of these methods will yield the same predicted burst pressure. This suggests that long corrosion defects can be modeled as a pipe with a reduced wall thickness which is the assumption used by Mok et al and B31G. Based on this assumption, a burst pressure prediction of a long corrosion defect with single pit should be equal to that of a pipe of reduced wall with a single pit of reduced depth. Considering the idealized geometry shown in Figure 8, RSTRENG will neglect the effect of the pit (L,) in the long defect (L2) compared to the reduced wall analysis shown in below. For various dimensions, Table 4 shows that the reduced wall analysis produces a more conservative prediction of the burst pressure since it will account for the smaller defect within the larger defect.

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 RSTRENG reduced wall analysis. Although the reduced wall analysis slightly accounts for the pit depth in the groove it produces a more conservative estimate of the failure pressure. Thus a reduced wall RSTRENG analysis provides little benefit when analyzing long corrosion grooves with pits or rough bottoms.

Table 3 shows the results of the reduced wall analysis (RWA) applied to the long corrosion data. Here the reduced wall thickness was assumed to be the maximum remaining ligament within the corrosion. With this geometry RSTRENG predicted burst pressures between 59% and 96% (with an average of 78%) of the actual burst pressure.

Based on the limited number of cases assessed with the finite element method, RSTRENG is found to be conservative in its prediction of failure pressures. Pits interact significantly if there is less than 1 to 2 wall thicknesses of full thickness ligament between them. RSTRENG can account for regularly spaced pits and their interaction due to the large projected area of the pits compared to the projected area of the groove. However the degree of conservatism is reduced when predicting the failure pressure of grooves with single or non-interacting pits, particularly when the pits are deep.

CONCLUSIONSFor the data base of measured burst pressures B31G provides

predictions of burst pressure which are 20% to 68% of the actual burst pressures. This technique is simple to apply, requiring the

defect depth, length, pipe dimensions and grade of steel. Defects which meet this criteria are safe while defects which are calculated to be unsafe by B31G can be assessed with a less conservative method. The assessment method developed by Mok for long corrosion is the least conservative of all methods when considering flat bottomed grooves. However, this method is similar to B31G, although slightly less conservative, when applied to irregular bottomed longitudinal corrosion grooves since it does not account for pitting or variations in groove depth.

RSTRENG predicts failure pressures which are 60% to 106% of the actual burst pressure when applied to the database. The prediction which is 6% above the actual failure pressure is for a defect which is 79% of the wall thickness deep. In general, the degree of conservatism associated with RSTRENG predictions decreases as the defect depth increases. Using a reduced wall RSTRENG analysis for long defects captures the corrosion geometry better but results in increased conservatism. This provides no benefit over a standard RSTRENG analysis.

Three-dimensional finite element analyses of single pits 10 mm long located within longitudinal grooves indicate that the pits begin to interact at a separation of 6 wall thicknesses and this interaction becomes significant when the separation is below 1 to 2 wall thicknesses. RSTRENG can predict this trend for evenly spaced pits of similar dimensions, but the predicted burst pressures are conservative. The RSTRENG technique does not adequately account for single pits within long grooves and becomes less conservative as the pit increases in depth.

REFERENCES1. ASME, 1991, “Manual for Determining the Remaining Strength of Corroded Pipelines”, American Society of Mechanical Engineers, New York.

2. Kiefner, J. F. and Vieth, P. H., 1989, “A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe”, Final Report on Project PR 3-805, Battelle Memorial Institute, Columbus.

3. Mok, D. H. B„ Pick, R. J„ Glover, A. J., and Hoff, R., 1991, “Bursting of Line Pipe with Long External Corrosion”, Int. J. Pres. Ves & Piping, 46, pp. 195-215.

4. Vieth, P. H. and Kiefner, J. F., 1994, “Database of Corroded Pipe Tests”, Final Report on Contract No. PR 218-9206, Kiefner and Associates, Inc., Worthington, Ohio.

5. Chouchaoui, B. A., and Pick, R. J., 1994, “A Three Level Assessment of the Residual Strength of Corroded Line Pipe”, ASME OMAE, Vol. V, Pipeline Technology, pp. 9-18.

6. Hibbitt, Karlsson and Sorenson, Inc., Abaqus Theory Manual, Providence, Rhode Island.

7. Chouchaoui, B. A., and Pick, R. J., 1993, “Interaction of Closely Spaced Corrosion Pits in Line Pipe”, ASME OMAE, Vol. V, Pipeline Technology, pp. 203-214.

ACKNOWLEDGMENTSThe 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

TestDescrip.

Pipe Material Properties Defect Dimensions BurstPress.(MPa)

Defect DescriptionOD(mm)

t(mm)

SMYS(MPa)

Sy(MPa)

Suts(MPa)

dm/t(-)

dmax(mm)

L(mm)

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)kv44 610 9.2 241.5 358.8 NA 0.70 6.45 330.2 8.7 rough bottom, deepest pit 152 mm longkv48 610 9.5 241.5 371.2 474.0 0.79 7.49 406.4 5.1 relatively smooth bottomkv51 508 7.7 358.8 380.2 521.6 0.69 5.33 266.7 8.1 relatively fiat bottomkv63 508 7.0 241.5 279.5 442.3 0.47 3.30 304.8 12.0 rough bottom, deepest pit 102 mm longkv66 508 6.8 241.5 277.4 420.9 0.54 3.66 393.7 10.4 general corrosion with 2 deep pitskv68 762 9.4 358.8 409.9 NA 0.35 3.30 914.4 12.7 relatively smoot bottomkv71 762 9.7 358.8 429.2 NA 0.38 3.68 508.0 13.1 general corrosion with 1 deep pitkv72 762 9.6 358.8 387.8 NA 0.35 3.30 508.0 12.3 relatively smooth bottomkv73 762 9.6 3588 439.5 NA 0.29 2.79 838.2 13.2 relatively smooth bottomkv80 762 9.3 358.8 404.3 521.0 0.63 5.82 406.4 6.8 relatively smooth bottomkv81 762 9.5 358.8 474.5 580.3 0.65 622 685.8 6.8 relatively smooth bottomkv84 914 8.4 448.5 506.6 NA 0.66 5.54 406.4 5.3 relatively smooth bottomkv92 610 8.1 358.8 396.8 528.5 0.28 2.29 482.6 13.0 relatively smooth bottomkv97 508 6.6 414.0 444.1 598.9 0.39 2.62 381.0 11.3 machined groovekv98 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 groovekv120 508 6.4 414.0 430.2 672.5 0.34 2 18 899.2 11.8 machined groovekv124 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 bottomRL07 610 6.4 358.8 421.7 630.4 0.54 3.43 1422.4 7.9 rough irregular bottomRL08 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: X520.2 8.3 8.3 8.3 SMYS = 358.8 MPa0.4 6.3 6.3 6.3 Sy = 406.6 MPa0.6 4.2 4.2 4.3 Suts = 546.1 MPa0.8 2.1 2.1 2.1


TABLE 3: SUMMARY OF BURST PRESSURE PREDICTIONS

TestDesc

Pipe Material Properties Defect Dimensions Act. Burst Press.(MPa)

Predicted Burst Pressure / Actual Burst PressureOD t

(mm) (mm)SMYS(MPa)

Sy(MPa)

Suts(MPa)

dm/t(-)

dmax(mm)

L(mm) 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 0.85kv44 610 9.2 241.2 358.8 NA 0.70 6.45 330.2 8.7 0.28 0.38 0.96 0.96kv48 610 9.5 241.2 371.2 474.0 0.79 7.49 406.4 5.1 0.35 0.47 1.06 0.89kv51 508 7.7 358.8 379.6 520.9 0.69 5.33 266.7 8.1 0.46 0.63 0.91 0.91kv63 508 7.0 241.2 279.0 441.6 0.47 3.30 304.8 12.0 0.32 0.43 0.62 0.59kv66 508 6.8 241.2 277.0 420.3 0.54 3.66 393.7 10.4 0.31 0.43 0.70 0.67kv68 762 9.4 358.3 409.3 NA 0.35 3.30 914.4 12.7 0.50 0.68 0.73 0.70kv71 762 9.7 358.3 428.6 NA 0.38 3.68 508.0 13.1 0.48 0.65 0.83 0.81kv72 762 9.6 358.3 387.2 NA 0.35 3.30 508.0 12.3 0.53 0.72 0.76 0.75kv73 762 9.6 358.3 438.9 NA 0.29 2.79 838.2 13.2 0.53 0.73 0.79 0.76kv80 762 9.3 358.3 403.8 520.2 0.63 5.82 406.4 6.8 0.53 0.72 0.91 0.74kv81 762 9.5 358.3 473.8 579.4 0.65 6.22 685.8 6.8 0.50 0.68 1.00 0.91kv84 914 8.4 447.9 505.8 NA 0.66 5.54 406.4 5.3 0.57 0.78 0.95 0.95kv92 610 8.1 358.3 396.2 527.8 0.28 2.29 482.6 13.0 0.58 0.79 0.83 0.83kv97 508 6.6 413.4 443.4 598.9 0.39 2.62 381.0 11.3 0.64 0.87 0.78 0.78kv98 508 6.7 413.4 427.2 601.0 0.40 2.66 1016.0 11.6 0.62 0.84 0.67 0.67kv119 508 6.4 413.4 429.6 672.5 0.54 3.46 899.2 8.0 0.66 0.90 0.75 0.75kv120 508 6.4 413.4 429.6 672.5 0.34 2.18 899.2 11.8 0.64 0.88 0.72 0.72kv124 508 6.4 413.4 434.8 672.5 0.50 3.18 1000.8 8.4 0.68 0.92 0.78 0.78RL0405 610 6.4 358.8 421.7 630.4 0.51 3.21 899.2 9.5 0.43 0.59 0.77 0.71RL07 610 6.4 358.8 421.7 630.4 0.54 3.43 1422.4 7.9 0.48 0.65 0.85 0.78RL08 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

CaseNo.

Pipe Description Defect Description Burst PressureOD

(mm)s y

(MPa)t

(mm)d1

(mm)d2

(mm)L2

(mm)RSTRENG

(MPa)Red. Wall

(MPa)Difference

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%


G RO O VE 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 2 ERROR IN RSTRENG AS A FUNCTION OF DEFECT DEPTH

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

FIGURE 5. SYMMETRY CONDITIONS DESCRIBED A SERIES OF prrs IN A LONG LONGITUDINAL GROOVE.


FAIL

URE

PRES

SUR

E (M

Pa)

7.0PIPE WITH LONG CORROSION AND SINGLE PIT (LONGITUDINAL SECTION)

6.0

6.0

4.0

3.0

2.0

t.O

0.00 10 20 30

PIT SEPARATION (REFERS TO THE NUMBER OF WALL THICKNESSES OF FULL THICKNESS MATERIAL LOCATED BETWEEN THE PITS)

REDUCED WALL EQUIVALENT (LONGITUDINAL SECTION)

I - dl

FIGURE 8 SINGLE PIT IN LONG CORROSION AND IN REDUCED WALL PIPE.

FIGURE 8. INTERACTION OF PITS IN LONG GROOVES - 40% DEEP GROOVE. 40% DEEP PIT. 5 mm PIT RADIUS

FIGURE 9. BURST DATA FOR LONG GROOVES WITH SINGLE PITS: 40% DEEP GROOVE. 10 mm PIT RADIUS.


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