*TWI, Cambridge, CB1 6AL.
**Sassol Synfuels, Private Bag X1000, Secunda, 2302.
Paper presented at Failures 2006, 7 th International Symposium on Risk, Economy and Safety, Failure Minimisation and Analysis. 13-17 March 2006. Villa Via Hotel, Gordon's Bay, South Africa.
The possibility of up rating and changing the service of existing pipelines can prove to be both financially attractive and time saving when involved in large project developments. However the ability to validate and prove that a pipeline is safe for operation involves a variety of mechanical testing, metallography, fracture mechanics, statistical analysis and good engineering judgement. The work undertaken will either validate or restrict a pipeline's future. The paper details some of the test work that was conducted, and the findings that necessitated some difficult engineering decision-making to determine the integrity of an ageing pipeline.
Pipelines are most commonly used to transport some form of product; liquid, gas or both between a set distance and follow a simple or complex route. It is also true that pipelines are exposed to some of the worst environmental conditions, as well as the possibility of damage through military and terrorist activities. Most pipelines, however, tend to operate in less hostile conditions. The financial outlay for a pipeline can consume a very large part of a project budget if being built from new. However, once a pipeline is installed, the operation and maintenance costs are relatively low and one can expect the pipeline to have an indefinite. In this event, a pipeline will have to have been designed adequately to satisfy the structural integrity requirements of its operational life.
It goes without saying that over the expected life of a pipeline, due to changes in operating philosophies and expansions of production plants, instances will occur where the duty (pressure, temperature and product) of the line will change from original design. In most cases, re-rating of existing lines makes the most business sense, however, this requires that various criteria be evaluated to ensure the integrity of the line.
During a recent major oil and gas development in South Africa an opportunity arose for such an investigation. A 30 year old 140km, 14in OD x 5.0mm WT, electrically resistance welded (ERW) pipeline was considered for potential up rating. The pipeline was originally designed for oxygen service at 60barg was operated for approximately 16 years at a pressure not exceeding 39barg. In 1998 the service was changed to ethylene and operation continued at 39barg. The operator wished to upgrade the system to operate at 50barg and change the contents to natural gas. Significantly, the re-rating also required that the minimum design metal temperature (MDMT) be lowered to minus 10°C. Unfortunately, very limited certification for the pipeline existed as most documents had previously been destroyed in a fire. If a new pipeline was built today, the relevant design code would be ASME B31.8  .
Various avenues of investigation were required to establish the fitness for service of the ERW pipeline.
Six 3m sections marked as A1 to A3 and B1 to B3, were selected from a recent inspection carried out on the pipeline. All six sections contained an ERW seam weld; sections A2 and B2 also contained a single girth weld. As identified by the operator, sections A1 to A3 showed corrosion in the body and sections B1 to B3 corrosion at the ERW seam. Sections B2 and B3 also had an external coating. It was decided that 6 full scale burst tests would be conducted on the2.5m lengths, while the remaining 0.5m ring would provide the material required for all small scale mechanical tests. A summary of the small-scale tests is shown in Table 1.
Table 1 - Summary of tests carried out on each section
|Tensile (base metal, ERW, girth weld & girth weld HAZ)
|Charpy V-notch (base metal & transition)
|Charpy V-notch (ERW)
|Charpy V-notch (girth weld & HAZ)
|Fracture toughness (ERW, ERW HAZ, girth weld, & girth weld HAZ)
* The drop weight tear test (DWTT), specified in API RP 5LR or ASTM E436, was developed in the early 1960s at the Battelle Memorial Institute, USA, to overcome some limitations of the 'Pellini' drop-weight test. Drop weight tear testing is a material characterisation test aimed at avoiding brittle fracture and ensuring crack arrest in pipelines (seamless or welded).
The six sections were inspected visually for internal and external defects and features of interest were marked accordingly. The sections did have visible defects due to the operational history and these defects had depths in the region of 5-10% of the nominal pipe wall thickness, and 5-40mm in length; often orientated at an angle to the pipe longitudinal axis.
All six sections were strain gauged at mid-length at 3, 6, 9 and 12 o'clock positions around the circumference, using uniaxial resistance gauges. For features such as notable internal or external surface damage, an extra set of gauges was attached adjacent to the feature but only on the external surface. Fig.1 shows internal features noted prior to testing but examined after testing. Fig.2 shows a case of additional strain gauging at the site of external damage.
The sections were initially pressurised with water up to 60barg and at ambient temperature (in the range of 10 to 17°C). They were held at that pressure for about three hours and finally pressurised to failure. During the tests, temperature, pressure and strain measurements were continually recorded.
The lowest failure pressure was recorded in section A3; 116barg (2.3 times higher than future operating pressure of 50barg), whereas the burst pressures for the other sections was in the range of 134.3 (2.69 times future operating pressure) to 145.1barg (2.9 times future operating pressure). Post-test visual inspection revealed that the material around the area of rupture was significantly thinner (about 1.14mm in section B3) than the rest of the section. It was not initially clear whether this locally thinner material was caused by a result of plastic instability of the base metal or the area had been present before the test. However, examination of the metallographic section revealed that the flow lines are interrupted by the lower surface, rather than parallel to it. This indicates that the thinning was caused by material removal (erosion or corrosion) prior to the test rather than plastic deformation during the test.
Except for sections A1 and B2, all failures were associated with a pre-existing defect. In sections A1 and B2, the failure occurred by ductile initiation and propagation parallel to the ERW seam weld, within the parent material. Only in section B3 did failure occur from the HAZ. Fig.3 shows the failed section B2, while Figures 4 and 5 show micrographs of section B3.
Cross weld metallographic sections were prepared using standard techniques. These were etched in 2% nital and examined using an optical microscope, taking micrographs as appropriate to document the bond line, heat affected zone(HAZ) and parent material microstructure. The ERW sections were then re-polished, etched in Saspa-Nansa at 65°C and examined in a similar manner. A macrograph of section A1 etched in Saspa Nansa is shown in Fig 6.
With the exception of B2 and B3, all sections exhibited a similar manufacturing defect, i.e. offset skelp edges (sharp notch-like features) on the internal surface of the sections. None of these defects were found at the weld seam. This type of defect is known to occur in ERW welds and may have been caused by misalignment of the plates during welding. With the exception of A3, the depth of the manufacturing defect was always between 5-10% of wall thickness.
Most samples also exhibited significant grain size differences between the parent material, the HAZ and the bond line, the latter often exhibited coarser, non-equiaxed grain (with exception of section A2). This indicates that the inline seam normalising was not carried out correctly during production, and did not achieve full microstructural refinement. This caused poorer material properties except for section A2.
With the exception of sections A1 and B2, all failures were associated with some type of surface or embedded defects. In three cases (A2, A3, B3) local reduction of thickness appeared to have been caused by material removal, the cause of which is unknown. Fig.7 shows the failure location of section A2. In section B1, a large inclusion was found that had penetrated deeply into the plate. The defect created was not perpendicular to the surfaces, which probably explains why its impact was not significant, in spite of the fact that the thickness of the material present at the point where the defect was deepest was less than a quarter of the nominal wall thickness.
Charpy Impact and CTOD Results
Table 2 - Summary of Charpy Impact Tests
|Lowest observed Charpy  result
|ERW bond line
Table 3 - Summary of CTOD tests
|Lowest observed CTOD  result
|ERW bond line
The differences observed in the Charpy results were down to the grain size in the ERW weld and the HAZ; the latter had a finer microstructure. This also repeated for the CTOD tests as low toughness results for ERW bond line specimens were due to the presence of coarse-grain microstructures. Finer microstructure for the HAZ subsequently yielded higher CTOD results than in the bond line.
Fitness for Service Assessments
Fatigue and fracture engineering critical assessments (ECAs) were carried out to the procedure specified in BS 7910: 1999  . The ECAs considered pipes with an axial surface-breaking flaw at the ERW weld seam and corresponding HAZ. It was assumed that the joints were initially proof tested (on commissioning of the pipeline in 1982) at a pressure of66barg.
The level 2B (material-specific) failure assessment diagram (FAD) was employed. The lowest experimentally determined values of fracture toughness for the ERW weld and HAZ at -10°C; CTOD =0.007mm and CTOD = 0.018mm,respectively, were used in the ECAs.
From the tests conducted on the 6 sections, the lowest stress-strain curve and tensile properties (0.2% proof strength of 372.4 and tensile strength of 499MPa) for the parent material at -10°C was used in the ECA.
A 2.5-month record of cyclic pressure variation in the pipeline during normal operation was extended over a two-year period, to estimate typical, future operation of the pipeline before shutdown. The full cyclic data was simplified to a single spectrum, using the rain flow technique; the pressure drop associated with shutdown and pressure is associated with start-up periods were also added. The number of times the pipeline was pressure tested at a pressure range of 66barg, was also considered in the fatigue spectrum. The resulting 2-year block was repeated to simulate the 22 years of pipeline in service-life. This simplified spectrum was applied to the wall thickness case of 4.23mm.
As the product will be transported in a dry condition, the appropriate fatigue crack growth equation for steel in air, as recommended in BS 7910: 1999, was implemented.
The worst case ECA is shown in Fig.8.
Discussion of the results
Upon review of the work conducted it appeared that the results would support the up-rating of the line. The six burst tests demonstrated that leakage or rupture typically initiated in the parent material, at regions of external or internal damage, suggesting that the seam weld had sufficient integrity, however the poor toughness results could not be ignored. The poor toughness results also impacted the ECA tolerable flaw size calculations resulting in some doubtas to whether they would be picked up during inspection. The ECA procedure includes a high degree of conservatism and some of the assumptions made could be relaxed if more appropriate data were available. If the operator were able to prove that the minimum temperature would be above -10°C perhaps not below 0°C, improvements in toughness could be achieved and possibly improve the ECA results. It was also apparent that the inline seam normalising process carried out during production was not as effective as it should have been resulting in a less than satisfactory microstructural refinement. This contributed to poorer material properties.
Based on the results of the study the following conclusions were drawn:
- The review of the data illustrated that the area having lowest fracture toughness at -10°C was the ERW bond line.
- The ECA showed the tolerable flaw sizes to be very small, potentially below the limit of detectability of current inspection techniques.
- The lowest rupture occurred at 2.3 times the proposed future operating pressure.
- Appropriate fracture toughness and tensile properties for the parent material, ERW and HAZ could be determined at the minimum design temperature for the pipeline (nominally 0°C), and selected parts of the ECA could be repeated.
- Consideration could be given to alternative methods of demonstrating integrity, such as the application of an in-service pressure test.
The authors would like to express their sincere thanks to Phil Robinson and Afshin Motarjemi for their contribution during this work.
- ASME B31.8-2003: 'Gas transmission and distribution piping systems ASME Code for pressure vessels and piping'. ASME, New York, NY.
- TWI frequently asked questions: 'What is a drop weight tear test?'
- BS EN 10045-1:1990: 'Charpy impact test on metallic materials: Part 1. Test method (V and U-notches)', British Standards Institution, London 1990.
- BS 7448:Part 2:1997: 'Fracture mechanics toughness tests - method for determination of K Ic , critical CTOD and J values of welds in metallic materials'. British Standards Institution, 1997.
- BS 7910:1999 'Guide on methods for assessing the acceptability of flaws in metallic structures'. Incorporating amendment No.1, British standards Institution, 2000.