Robert Sutherby

Transgranular Stress Corrosion Cracking of X-60 Pipeline Steel in Simulated Ground Water

Abstract An investigation was carried out on API X-60 steel specimens with multiple edge cracks to determine transgranular stress corrosion crack (TGSCC) growth rates in simulated ground water (NS-...

Cyclic Crack Growth Rates of X-60 Pipeline Steel in a Neutral Dilute Solution

Abstract An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel specimens exposed to neutral (pH 7) dilute simulated ground water (designated NS4) while being cyclically loaded in cantilever bending. Testing was carried out over short-term (generally < 40 days) and long-term periods (∼ 1 y). Loading and environmental conditions in the long-term tests were similar to those for buried natural gas pipelines, with the R ratio (minimum/maximum load) varying from 0.82 to 0.98 and frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditions for specimens in the short-term tests were more severe, with R ratios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,000 cycles/day. Post-mortem metallographic examination by optical (OM) and scanning electron microscopy (SEM) was used to evaluate crack growth. Average CGR from 4.50 × 10−8 mm/cycle to 1.25 × 10−4 mm/cycle (equivalent to 1.4 × 10−9 mm/s to 7 × 10−7 mm...

Corrosion Fatigue and Near-Neutral pH Stress Corrosion Cracking of Pipeline Steel and the Effect of Hydrogen Sulfide

Abstract Crack advance has been studied in an X-70 pipeline steel, using the compliance technique. The electrolyte used in the study was a very dilute brine bubbled with 10% carbon dioxide (CO2). C...

Corrosion Consequences of Secondary Oxidation of Microbial Corrosion

Abstract Microbial corrosion scenarios based on the action of sulfate-reducing bacteria (SRB) under disbonded tape coatings on the outside of gas pipelines have been described previously. Secondary oxidation of these sites leads to formation of unique corrosion products. These have been associated with serious corrosion damage in the field. Laboratory studies were performed under simulated field conditions on an iron sulfide-steel (FeSx-Fe) galvanic corrosion cell sustained by anaerobic microbial activity. The effects of transitions from anaerobic to aerobic conditions on corrosion rates were documented. Results were correlated with field observations, and potential mechanisms were discussed.

Environmental Crack Growth under Variable Amplitude Loading of Pipeline Steel

Abstract An investigation was carried out on API X60 steel specimens with surface cracks to study environmentally assisted crack growth rates in simulated groundwater (NS-4 solution). The environmental conditions were similar to those for buried natural gas pipelines in service, and loading was aggressive. The maximum stress intensity factor at the tip of the crack on the surface ranged from 20.6 MPa√m to 31.4 MPa√m. A periodic underload was applied to pre-cracked cantilever bending specimens. Tests were carried out for a period of 40 days at a frequency of either 1 cycle or 40 cycles per day. Crack growth rates were obtained for two stress ratios (R = minimum/maximum load) of 0 or 0.5 under both constant and variable amplitude loading conditions. Depending on loading conditions, the crack growth varied from 0 μm to 2,640 μm. However, the majority of crack growth measured was <100 μm. The experimental results showed that there was an acceleration in growth following an underload. The growth was higher whe...

Characterizing Pressure Fluctuations on Buried Pipelines in Terms Relevant to Stress Corrosion Cracking

Pressure records for high pressure hydrocarbon pipelines that have previously shown to have occurrences of trans-granular stress corrosion cracking (tSCC) were supplied by ten Canadian Energy Pipeline Association (CEPA) member companies. These pressure records were analyzed by performing six methods of pressure cycle counting described in the ASTM standard E1049-85. A comparison was made of the magnitude and frequency of these counted pressure cycles at compressor/pump discharge and suction locations as well as intermediate locations along the pipeline. An attempt to relate the derived pressure cycles to tSCC growth was made by applying previously published “superposition” type cumulative damage models in calculating tSCC growth rates. Information about the relative importance of fatigue growth in the total tSCC lifecycle as well as the ability to distinguish tSCC susceptible areas on a pipeline based on differences in pressure cycles was gained. Characteristic pressure cycle spectra were developed from the supplied pressure data which describes the different modes of pipeline operation including liquid or gas hydrocarbon transportation as well as mainline or lateral operation which may be of use in further research efforts in this area.Copyright

CEPA Study on Characterization of Pipeline Pressure Fluctuations in Terms Relevant to Stress Corrosion Cracking

Neutral-pH stress corrosion cracking (SCC) is a topic of significant interest to pipeline operators in the Canadian Energy Pipeline Association (CEPA). In recent years there has been a shift in laboratory SCC research toward investigating cracking phenomena under more realistic environmental and physical loading conditions. To support this move, CEPA has been active in collecting pressure-time data from its varied membership of liquid and gas pipeline companies. Histories of pressure fluctuations in operating systems have been characterized in terms of loading cycles, R-values (the ratio of minimum to maximum stress) and strain rates. These parameters are relevant to the design of laboratory experiments for the investigation of SCC. An overview of the data is presented for both liquid and gas pipelines in various service applications.Copyright

Field Assessment of FBE-Coated Pipelines and the Implications for Stress Corrosion Cracking

One proposed method for preventing the initiation of SCC on pipelines is through the use of high-performance coatings in conjunction with effective cathodic protection. High-performance coatings include fusion bonded epoxy (FBE), urethanes, liquid epoxies, extruded polyethylene, and multi-layer coatings. This paper reports the results of a CEPA-sponsored project to determine the long-term performance of FBE coatings on underground pipelines using in situ field measurements. The barrier properties of FBE coatings were measured after several years field service using Electrochemical Impedance Spectroscopy (EIS). Measurements were made on four FBE-coated pipelines that had been exposed for periods of between 5 and 20 years. The coatings studied included three different formulations from two different manufacturers. A variety of site, soil and CP conditions were also examined. The combination of EIS measurements and analyses of trapped water samples demonstrated that the FBE coatings continued to perform well after having been exposed for up to 20 years. Although some blistering and disbondment was observed in some cases as a result of poor application and storage procedures, in all cases the pipe surface was still protected either by the coating itself or by the joint action of the CP system and the CP-compatible FBE coating. The results of this study provide strong evidence that FBE-coated pipelines should not be susceptible to the initiation of SCC, due to (i) the barrier properties of the coating, (ii) the CP-compatible nature of the coating, and (iii) good surface preparation techniques that involve the removal of millscale and the introduction of compressive surface residual stresses.Copyright

Long Seam Welds in Gas and Liquids Pipelines and Near-Neutral pH Stress Corrosion Cracking and Corrosion Fatigue

Near-neutral pH SCC and corrosion fatigue at the toe of the longitudinal weld has been responsible for several pipeline failures in Canada. In this paper examples of such failures and other in-service toe cracks are discussed. The results of an investigation into several factors affecting near-neutral pH SCC at the toe of the longitudinal weld are discussed and summarized. Four different pipes were evaluated including both double submerged arc welded (DSAW) pipe and electrical resistance welded (ERW) pipe. The properties considered were 1) residual stresses near the toe of the weld as determined by hole drilling and slitting, 2) stress-raising characteristics of the weld shape as determined by elastic finite element analysis and 3) electrochemical properties near the toe of the weld as determined by the scanning reference electrode technique. For one line pipe we also evaluated the room-temperature creep characteristics near the weld with and without cyclic loading. The implications of these results for near-neutral pH SCC and corrosion fatigue are discussed. It is concluded that high stresses originating from the stress-multiplying effects of the weld crown contributed significantly to the failures.Copyright

Stress Intensification and Crack Growth in the Presence of Dents on Pipelines

Historically, ruptures in near-neutral pH SCC environments on gas transmission pipelines have been associated with some kind of stress raiser, generally the long-seam weld, corrosion, or dents/gouges. The current work provides an assessment of the effect of dents on environmental cracking. A dent assessment model was combined with a recently developed crack growth rate model to simulate crack growth in high-stressed areas caused by the presence of a dent. The work suggests that unrestrained dents (indenter removed) provide higher stresses and result in faster crack growth than restrained dents (indenter held in place). A dent in a lower ductility, higher strength X-65 steel further promoted the formation of high stresses and crack growth. Small crack growth and crack growth on gas lines, when governed by a strain rate based crack growth mechanism, may be less sensitive to dent parameters than crack growth governed by corrosion fatigue. It should be noted that neither the dent assessment model nor the crack growth model have been fully validated. Further work using elastic-plastic Finite Element Analysis is needed to determine more accurately the extent of stress intensification of a crack growing in or near a dent.Copyright