Integrity of Reduced Pressure EB Welds for Deep Water Pipes

Paper: Pipe 24 presented at 4 ^th International Pipeline Technology Conference, Ostend, May 2004.

Abstract

The Reduced Pressure Electron Beam (RPEB) process offers potential economic advantages for producing high integrity girth welds in heavy wall pipe for deep-water pipe-lay applications. However, in most commercial linepipe steels the RPEB process does not reliably produce levels of fracture toughness specified by some companies. A series of full-scale pipe tests was carried out to examine the fracture performance of RPEB welds containing deliberately introduced flaws under simulated pipelay conditions. Results from pipe bend tests and fracture mechanics are described. These were conducted at temperatures down to 0°C on API 5L X65 pipe with diameters of 608-713mm and wall thickness in the range 32-38mm. Flaws were positioned into the HAZ/fusion boundary of the RPEB girth welds. It is shown that from considerations of constraint and residual stresses, the pipe girth welds are tolerant to small welding flaws (< 4mmhigh) and maintain good fracture resistance to high strains of at least 2.6%.

Introduction

The Reduced Pressure Electron Beam (RPEB) process offers potential economic advantages for producing high integrity girth welds in heavy wall pipe for deep-water pipe lay applications. ^[1] It is a single pass welding process such that the welding time is relatively independent of pipe wall thickness. Typically, the welding time for a 24in diameter pipe with a wall thickness of 32mm is ~6 minutes. However, withdual electron beam guns, the welding time can be reduced to ~3½ minutes inclusive of the 30 seconds pump-down time to achieve a pressure of 1 millibar, suitable for welding. High integrity girth welds are produced which are generally free from harmful flaws as an inherent part of the RPEB process. The most critical factor, beam/joint alignment, is assured by means of an on-line seam tracking system which compensates, in real time (at a rate of 7 times per second), for any deviation of the beam from the joint line, whether caused by mechanical or magnetic effects. Therefore, the occurrence of a missed joint would be extremely unlikely. However, unless special steel compositions are chosen which resist grain coarsening, the high effective heat input, inherent in the welding process, means that the melted region in autogenous welds and grain coarsened HAZ can have relatively poor fracture toughness, but adequatetensile properties. Weld metal fracture toughness can be improved by alloying through the introduction of a nickel shim at the joint interface. Nevertheless, the problem of relatively poor GCHAZ fracture toughness remains. However, girth weld fracture resistance may be adequate for pipeline applications when standard commercial pipe steels are employed if it can be shown that the girth weld is tolerant to small flaws that are on the limit of reliable sizing bynon-destructive testing. This paper describes the results of some full-scale pipe bend tests that have been conducted which demonstrate RPEB girth welds can be resistant to brittle fracture even with small welding flaws present.

Materials and testing

Full-scale test tests

The paper describes the results from three full-scale pipe tests from a programme to evaluate the fracture performance of RPEB girth welds in pipe to API 5L Grade X65. Two of the pipes tested were 608mm OD x 31.8mm WT with a composition designed for sour service (pipe tests 1 and 2). The third pipe was 713mm OD x 38.0mm WT and was designed for sweet service (pipe test 3). The steel compositions are given in Table 1. The girth weld in pipe test 1 was an autogenous RPEB weld and was expected to have relatively poor fracture toughness, in both weld and HAZ, when assessed using conventional fracture mechanics tests. An all-weld metaltensile test indicated a 0.2% proof stress of 516MPa. This compared with a 0.2% proof strength of 467MPa measured in the pipe (parallel to pipe axis). A shallow artificial surface flaw, inclined at approximately 5° to the girthweld was introduced to intersect weld metal, fusion boundary and HAZ. This was produced using a high speed slitting wheel, with a blade thickness of 0.15mm. The flaw was 150mm long and 4mm deep.

Table 1 Chemical composition of pipe materials

In order to focus attention on the performance of the HAZ, the second pipe test was welded with a pre-placed shim (0.3mm thick) of high purity nickel. This was designed to produce weld metal with 2-3% nickel content and significantly higher fracture toughness than by autogenous welding. (This was confirmed by Charpy testing. The autogenous weld resulted in a 27J transition temperature of -35~C; whilst in the alloyed weld it was -90°C). Adeliberate welding flaw, representing a missed joint, was induced by temporarily disabling the seam tracking system and by using the beam deflection system to direct the beam to miss the root over a distance of 45mm.

The third pipe test was on a pipe different from the first two but the girth weld was made in the same way as the second pipe test except that the seam tracking/beam deflection system was not disabled. An artificial flaw was introduced into the fusion boundary on the inside diameter of the pipe by spark erosion (EDM). The nominal notch width was approximately 0.18mm. The flaw was 1.6mm deep and 20mm long. Ultrasonic inspection using TOFD UT successfully detected the flaw and sized it to be 1.7mm high.

The average weld metal hardness close to the root was 344HV5. This corresponds to a yield stress of about 915MPa and this compares with a parent pipe 0.2% proof stress of 512MPa.

The three pipe samples were each 600mm in length with the girth weld in the middle. These were then welded into longer sections of pipe to make a test piece 6000mm long. The pipe was instrumented with strain gauges, displacement transducers and clip gauges and placed in a four-point bend rig. The inner loading span was 1220mm and the outer loading span was 5600mm. The notched area was located at the 12 o'clock position, on the maximum tension side. The firstpipe test was conducted at ambient temperature which was 13°C. The other two tests were conducted at 0°C and this was achieved by wrapping plastic tubing around the pipe through which coolant was pumped.

Fracture toughness tests

Near full pipe wall thickness, square section, single edge notch bend (SENB) specimens were extracted transverse to the girth weld. These prepared and tested to generally meet BS 7448:Parts 1 and 3 requirements. The specimens included the following types of notches:

- Shallow surface (a/W ≈ 0.2) and deep through-thickness notches (a/W = 0.5) in the autogenous weld metal (representative of pipe test 1).

- Shallow surface notches into the HAZ (close to the fusion boundary) at the weld root (a/W = 0.2) in weld representative of pipe tests 1 and 2 and a/W = 0.12 in pipe test 3, and deep (a/W = 0.5) through-thickness notches into the HAZ (pipe test 3 only).

The fracture toughness tests were conducted at 0°C and +10°C and the specimens were instrumented to enable J-integral to be derived from crack mouth opening displacement. ^[2] The results are expressed in terms of critical stress intensity factor, K _J , derived from J.

Results

Pipe tests

The results from the three full-scale pipe bend tests are summarised in Table 2. The first survived an overall strain of 2.6% before the pipe was unloaded. Local strains in the weld metal were about 0.5% lower. The test set-up and curvature achieved in the pipe is shown in Fig.1. A metallographic section taken at mid-length of the notch confirms that the tip was located in the fusion boundary and that initiation by ductile tearing had taken place, see Fig.2. Fractography and metallography indicated that approximately 0.2mm of crack growth had taken place at a CTOD (estimated from a pair of clip gauges mounted above the notch) of 0.98mm.

Table 2 Full-scale pipe bend tests

The second pipe fractured in a brittle manner at 0°C and a stress of 276MPa or 60% of the parent pipe SMYS. Post-test fractography showed a much larger and more irregular shaped flaw than expected. In fact, two fusion boundary flaws were created, see Fig.3

. The first was an irregular shaped surface flaw on the pipe inside diameter approximately 45mm long with a maximum depth of 15mm. The second was an embedded flaw, ahead of the first flaw, which was about 4.5mm high and11mm long.

The final, third pipe test survived an overall strain of 3.01% at 0°C before being unloaded. The local strain reached in the girth weld was lower, at about 2%, probably because of the significantly overmatched weld metal strength compared with the parent pipe. This may explain the relatively low CTOD achieved of 0.2mm, although the notch depth was 1.6mm.

Post-test metallography confirmed that the notch tip was located on the grain coarsened HAZ side of the weld, very close to the fusion boundary, see Fig.4. Indeed, ductile tearing (about 0.1mm) had just commenced.

Fracture toughness tests 

In girth welds representative of pipe tests 1 and 2, HAZ and weld metal fracture toughness values were similar and there was only small increase in toughness when temperature was increased from 0 to 10°C, see Fig.5. Although there is a weak affect of notch depth (a/W), higher weld metal fracture toughness can be achieved in shallow surface notched specimen compared with deeply notched through-thickness specimens.

In the HAZ of the girth weld representative of the third pipe test, there was a more discernible increase in fracture toughness with reduction in notch depth, see Fig.6. However, lower bound results are the same irrespective of notch depth

Discussion

Two of the three full-scale pipe tests survived strains greater than 2.6% at temperatures down to 0°C without fracture, provided that notch depth was small (1.6 to 4mm deep). Ductile behaviour, in terms of the strains achieved and initiation of ductile tearing, was demonstrated. This contrasts with the behaviour of the deeply notched SENB fracture toughness specimens which failed by cleavage at minimum K _J values of 61.3MPam ^0.5 in the HAZ and 110MPam ^0.5 in the weld metal (representative of the autogenous girth weld in pipe test 1). Clearly, the results from deeply notched fracture toughness specimens are unrepresentative of pipe with shallow notches. Twomajor factors are considered to contribute to this behaviour, these are: differences in constraint between fracture toughness test specimen and pipe, and residual stresses. The effect of constraint (or stress triaxiality at the cracktip) is indicated by the fracture toughness tests on the shallowest notched bend tests (a/W = 0.11) which show a significant increase in toughness compared with deeply notched specimens (see Fig.6). However, one of the six fracture toughness specimens fractured at a K _J value similar to the deeply notched specimens. The reason for this may be that the constraint levels in the specimen is higher than in the pipe. This is expected since the applied through-thickness stress distribution in the pipe is essentially tensile, whilst the fracture toughness test is representative of a bending stress. It is tempting to speculate on what the results would have been had fracture toughness been determined using shallow single edge notch tension (SENT) specimens. This specimen design is recommended by Wastberg, Pisarski and Nyhus ^[3] when evaluating flaw tolerance for offshore pipeline installation. The low constraint associated with SENT specimen design would avoid cleavage fracture.

The second factor is the beneficial role played by welding residual stresses in the full-scale pipe tests. Numerical analysis was used to predict the transverse residual stresses through the thickness of the RPEB girth weld. For the pipe sizes, strength grade and welding conditions employed, compressive residual stresses were predicted at both the pipe internal and external surface, as shown in Fig.7. These are balanced by tensile residual stresses within the central two thirds of pipe wall thickness.

The brittle behaviour of the second full-scale pipe test is consistent with the above hypotheses. The pipe contained a deep notch/crack almost half way through the pipe wall, so fracture toughness determined using deeply notched specimens would be appropriate. Furthermore, most of the crack and especially the deepest part of the crack tip region was located in a tensile residual stress field. Fracture mechanics analyses conducted on the second pipe test, based on BS 7910:1999, Level 2A assessment procedures, ^[4] show that for the expected range of HAZ fracture toughness (61 to 86MPam ^0.5 ), the predicted applied fracture stress is in the range 112 to 215MPa. This compares with 276MPa established from the pipe test itself.

Although strength over matching between the weld metal and parent pipe might be considered beneficial for shallow HAZ cracks in the girth weld, published limit load solutions, such as those by Schwalbe et al, ^[5] do not appear to support this. However, a limitation of such solutions, is that they are applicable to interface cracks which do not specifically consider the strength of the HAZ. So if there is a benefit, it is difficult to quantify.

Concluding remarks

The full-scale pipe bend tests and fracture mechanics tests have shown that contrary to the expectations from conventional, deeply notched bend specimens (SENB, a/W = 0.5), girth welds made by RPEB process in Grade X65 pipe can behave in a ductile manner at temperatures down to 0°C when shallow surface flaws (<4mm deep) are present. The reasons for this are attributed to the beneficial effects of low constraint in the pipe (which is loaded predominantly in tension) containing shallow circumferential flaws, compared with standard (SENB) fracture toughness tests, and compressive transverse residual stresses near to the surface. These benefits are lost if the girth welds contain deeper flaws because of the increase in constraint and presence tensile residual stresses within the central two thirds of pipe wall thickness. However, the on-line seam tracking system, which is an integral part of the RPEB process, should avoid the creation of large welding flaws. If small flaws are produced, greater than 1.6mm high these should be readily detected by non-destructive testing such as TOFD inspection. However, such flaws (provided they are less than 4mm high) do not present a particular risk of fracture for strains expected during pipelay.

Acknowledgements

This paper is based on a joint industry project conducted by TWI and funded by NKKK, Saipem SpA, BP Ltd and Marathon Oil Corp. The authors thank them for their support. We also wish to thank Mr N Bagshaw for conducting the finiteelement analyses.

References

1. A Belloni and Punshon C S: 'Reduced pressure electron beam welding for offshore pipelines'. IIW Document IV/680/97.
2. Pisarski H G and Wignall C M: 'Fracture toughness estimation for pipeline girth welds'. International Pipeline Conference, Calgary, Alberta, Canada, 2002, ASME (Paper IPC 02-27094).
3. Wastberg S, Pisarski H G and Nyhus B: 'Guidelines for engineering critical assessments for pipeline installation methods introducing cyclic plastic strains'. To be published OMAE 2004, ASME, Vancouver, Canada, June 2004.
4. BS 7910:1999 (incorporating Amendment No.1): 'Guide on methods for assessing the acceptability of flaws in metallic structures'. BSI, London.
5. Schwalbe K-H et al: EFAM-ETM-MM 96 - The ETM method for assessing the significance of crack-like defects in joints with mechanical heterogeneity (strength mismatch) GKSS 97/E/9, GKSS, Geesthacht, 1997.