Ramgopal Thodla

Hydrogen Permeation and Corrosion Fatigue Crack Growth Rates of X65 Pipeline Steel Exposed to Acid Brines Containing Thiosulfate or Hydrogen Sulfide

Corrosion fatigue crack growth rates were obtained for X65 pipeline steel in acid brines containing thiosulfate (S2O32−) or hydrogen sulfide (H2S). Samples were exposed for 72 h at the open-circuit potential to allow bulk hydrogen charging. The corrosion fatigue crack growth rate increased with partial pressure of H2S and correlated with the steady-state flux of hydrogen permeation during corrosion in the same solutions. The rate of hydrogen absorption increased with increasing S2O32− concentration to a maximum at 10−3 M S2O32−, owing to a competition between increased surface concentration of H2S from S2O32− reduction and increased rate of iron sulfide film formation. Corrosion fatigue behavior in S2O32−-containing acidified brines is the same as in solutions with low partial pressures of H2S, in accordance with predictions from the hydrogen permeation results. This suggests that, for corrosion fatigue studies, S2O32− solutions are possible candidates for replacement of H2S gas, as long as the H2S partia...

Effect of Liquid Impurities on Corrosion of Carbon Steel in Supercritical CO2

The increasing urgency to mitigate global warming has driven many efforts to control green house gas emissions. One solution among many is carbon capture and storage. However, CO2 emitters are not necessarily in the close vicinity of potential geologic storage sites. In consequence CO2 will be transported from generation site to storage sites under high pressures. This will necessitate a network of pipelines gathering supercritical CO2 from diverse sources and transporting it through transmission lines to the storage sites. These pipelines will be under corrosion risks, particularly because of possible carryover of trace impurities produced from the different sources, such as water, chloride, NOx , SOx , and O2 . The effects of impurities on corrosion in supercritical CO2 have yet to be evaluated systematically. Corrosion of carbon steel associated with water and impurities in supercritical CO2 was studied by Electrochemical Impedance Spectroscopy in autoclaves. Five impurities were studied by introducing them in the liquid condensed phase: water, amine, HCl, HNO3 and NaOH. Results were analyzed in terms of the phase behavior and speciation.Copyright

Fatigue Crack Growth Rate and Fracture Toughness of Non-Sour Service Grade X65 and X80 in Sour Environments

Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion.There has been some work performed to understand the performance of carbon steels in dynamic applications like risers and flow lines in sour service. This work is focused on exploring a range of environmental and loading conditions that are relevant to offshore production to evaluate the non-sour service grades X65 and X80 to gain greater insight into the role of chemistry and thermo mechanical processing on the sour service fatigue and fracture behavior of line pipe steels. Two different grades of non-sour service grade seamless pipe X65 and X80 were investigated in a range of sour environments. The concentration of diffusible hydrogen in the range of environments to relate the environment assisted cracking properties to the concentration of hydrogen. Fracture toughness measurements indicated that crack initiation and tearing modulus were dependent on the concentration of diffusible hydrogen. The lowest toughness properties was obtained in pH = 5, 4.6psia H2S. FCGR exhibited a1/√f dependence in all conditions suggesting that hydrogen diffusion was likely the rate limiting step before reaching a plateau. X65 and X80 exhibited the highest FCGR in pH = 5, 4.6psia H2S. The FCGR of X80 was lower than X65 under the same environmental conditions in all cases while the plateau frequency was very similar for both X65 and X80. Paris law measurements performed at constant Kmax and decreasing ΔK at plateau frequencies exhibited elevated FCGR levels above the in-air values consistent with the frequency scan data.Copyright

Engineering Critical Assessment in Sour Service by Fracture and Fatigue Small Scale Sour Testing

Engineering critical assessments (ECA) have increasingly become a routine part of pipeline design to determine tolerable flaw sizes for weld defects. These assessments are now being applied to pipeline systems in deeper water with increased loadings arising from responses to thermal and pressure cycling. Often these are flowline systems in which fatigue damage is exacerbated by the presence of aggressive internal conditions.Deep water flowlines operating at high temperatures and pressures need to be designed to accommodate issues such as significant end expansion, walking and lateral buckling. Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion.Currently for a number of offshore sour service pipelines, Engineering Critical Assessment (ECA) for flaw acceptance criteria are performed using knockdown factors up to 50X on fatigue crack growth rates. The toughness values used for the analysis are performed assuming a CTOD of 0.1 mm. In order to develop material chemistry and heat treatment procedures for offshore pipelines, it is important to be able to understand the fracture toughness behavior as well in sour environments.In order to evaluate the safety of offshore flowlines and risers applications fatigue and fracture toughness properties of X80 parent pipe material in sour environments were determined. The data developed was used to develop an ECA framework for flowlines and riser applications.Copyright

Fatigue Crack Growth Rate and Fracture Toughness of API5L X65 in Sweet Environments

Corrosion fatigue and fracture toughness in sour environments of APIX65 5L have typically been studied in relatively severe environments like NACE A and NACE B solutions. There are very limited data in sweet and mildly sour environments that are of interest in various applications. This paper presents fatigue crack growth frequency scans in a range of sweet and mildly sour environments as well as on different microstructures: Parent Pipe, Heat Affected Zone (HAZ) and Weld Center Line (WCL). The fatigue crack growth rate (FCGR) increased with decreasing frequency and reached a plateau value at low frequencies. FCGR in the sweet environments that were investigated did exhibit a frequency dependence (increasing with decreasing frequency) and had plateau FCGR in the range of 10–20× the in-air values. In the mildly sour environments that were investigated, FCGR was found to be about 25 to 30× higher than the in-air values. By comparison, in NACE A environments the FCGR is typically about 50× higher than the in-air values. The FCGRs of parent pipe and HAZ were found to be similar over a range of environments, whereas the WCL FCGR data were consistently lower by about a factor of 2×. The lower FCGR of the WCL is likely due to the lower concentration of diffusible hydrogen in the weld. FCGRs as a function of ΔK (stress integrity factor range) were measured on parent pipe at the plateau frequency. The measured Paris law curves were consistent with the frequency scan data.Rising displacement fracture toughness tests were performed in a range of sweet and sour environments to determine the R-curve behavior. Tests were performed in-situ at a slow K-rate of 0.05Nmm−3/2/s over a range of environmental conditions on parent pipe. The initiation toughness and the slope of the R-curve decreased sharply in the sour environments. The initiation toughness and slopes were largely independent of the notch location as well as environmental conditions. Typical values of initiation toughness were in the range of 90–110N/mm.Copyright

Effect of Reeling on Sour Service Fatigue Crack Growth Behavior of Welded API5LX65 Line Pipe

The effect of reeling on the fatigue crack growth rate (FCGR) behavior of welded pipe was investigated both in-air as well as in sour environment. The FCGR behavior of the reeled pipe in various notch locations, such as parent pipe (PP), weld center line (WCL), and heat affected zone (HAZ), did not exhibit any effect of reeling (i.e., the properties in the strained and aged conditions were similar to the as-fabricated welds). Frequency scan FCGR tests in sour environment (pH = 5/0.0031 MPa H2S) exhibited maximum FCGR in the range of 10× to 35× higher than the in-air values at frequencies in the range of 3–1 mHz and 3× to 5× at frequencies in the range of 0.3 Hz (risers). In sour service, WCL exhibited better fatigue performance than the PP and HAZ in all conditions. Fatigue performance of PP and WCL was independent of reeling. The poorest fatigue performance was observed in unstrained HAZ. Fatigue performance of HAZ extrados (side last strained in compression) and intrados (side last strained in tension) was similar and better than unstrained HAZ. It was also found that the FCGR in sour environments was controlled by the internal hydrogen due to bulk charging from the sour environment. The overall conclusion is that reeling has no detrimental effect on sour service fatigue crack growth behavior, i.e., sour service fatigue performance of reeled pipe is the same as unreeled pipe.

Effect of Reeling on Fracture Toughness Behavior of Welded API5LX65 Line Pipe

The effect of reeling on the fracture toughness (FT) behavior of welded pipe was investigated both in-air as well as in sour environment. The FT behavior of the reeled pipe in various notch locations parent pipe (PP), heat affected zone (HAZ), weld center line (WCL) did not exhibit any effect of reeling (i.e. the properties in the strained and aged condition was similar to the as-fabricated welds). Fracture toughness in sour environments is 4× to 6× lower than the In-Air values. The fracture toughness behavior of parent pipe in sour environments was affected by reeling at strains associated with 2.12%/cycle and 2.72%/cycle. WCL fracture toughness was not affected by reeling in sour environments. The intrados HAZ exhibited the lowest fracture toughness properties. Some of the sour service fracture toughness tests on parent pipe and HAZ strained to 2.72%/cycle on the intrados samples exhibited pop-in behavior possibly due to higher YS. Parent pipe strained to a lower level of 2.12%/cycle did not exhibit any evidence of pop-in’s. Reeling appears to decrease the parent pipe toughness in the extrados and intrados in sour service both at 2.12%/cycle and 2.72%/cycle. The extrados parent pipe and intrados HAZ showed the lowest fracture toughness values in sour environment.Copyright

Effect of Reeling on Fatigue Crack Growth Behavior of Welded API5LX65 Line Pipe

The effect of reeling on the fatigue crack growth rate (FCGR) behavior of welded pipe was investigated both in-air as well as in sour environment. The FCGR behavior of the reeled pipe in various notch locations parent pipe (PP), weld center line (WCL), heat affected zone (HAZ) did not exhibit any effect of reeling (i.e. the properties in the strained and aged condition was similar to the as-fabricated welds). Frequency scan FCGR tests in sour environment (pH = 5/0.46psia H2S) exhibited maximum KDF’s in the range of 10× to 35× at frequencies in the range of 3 mHz to 1 mHz and 3× to 5× at frequencies in the range of 0.3Hz (Risers). In sour service WCL exhibited better fatigue performance than the parent pipe and HAZ in all conditions. Fatigue performance of parent pipe and WCL was independent of reeling. The poorest fatigue performance was observed in unstrained HAZ. Fatigue performance of HAZ extrados and intrados were similar and better than unstrained HAZ. It was also found that the FCGR in sour environments was controlled by the internal hydrogen due to bulk charging from the sour environment. The overall conclusion is that, reeling has no detrimental effect on sour service fatigue crack growth behavior i.e. sour service fatigue performance of reeled pipe is the same as unreeled pipe.Copyright

Effect of Reeling on Sulfide Stress Corrosion Cracking of Welded API5LX65 Line Pipe

The effect of reeling on sulfide stress cracking (SSC) resistance of welded line pipe was investigated in two different environments, a modified NACE B environment and a fitness for service environment (pH = 5/pH2S = 0.46psia). Micro hardness maps were performed to characterize the welds both in the as fabricated condition as well as in the strained and aged condition. The hardness values in all of the conditions, was less than 250VHN (in compliance with NACE requirements). Triplicate specimens were tested in the as fabricated, strained and aged intrados and extrados in both the environments. SSC resistance in a severely sour environment (pH = 3.5/1psia H2S) was affected by reeling with cracking observed in both the intrados and extrados samples. No cracking was observed in the as-fabricated welds. However, in a moderately sour environment (pH = 5/0.46psia H2S) reeling did not have a detrimental effect on the SSC performance. No evidence of cracking on the as-fabricated, intrados, and extrados welds. In moderate sour service reeling doesn’t appear to have a detrimental effect on the SSC behavior.Copyright

Fatigue Crack Growth Rate of Reeled Pipe in Sour Environments

Fatigue Crack Growth Rate (FCGR) of reeled pipe (strained & aged) in sour environments was investigated. FCGR frequency scans on different microstructures, i.e. heat affected zone (HAZ), and weld center line (WCL), revealed that, FCGR in corrosive environments increased with decreasing frequency and reached a plateau value at low frequencies of 10mHz to 3mHz. At these ‘plateau frequencies’, FCGR in the moderately sour environment that was investigated were found to be about 10–18× or 30× higher than the in-air values for the WCL and HAZ, respectively. There was no effect of the reeling cycles on the FCGR of the WCL or HAZ specimens. The FCGRs of the WCL were consistently lower than that of the HAZ by about a factor of 2–3× under various conditions. The reason for the lower FCGR of the WCL is not well understood. It is possible that it may be due to the higher yield strength (YS) of the overmatched welds, differing hydrogen concentration and/or diffusion coefficient or possibly due to the differences in the microstructure between the HAZ and WCL.Paris law curves, FCGRs as a function of ΔK (stress intensity factor range), were measured on the HAZ, and WCL (both intrados) at the plateau frequency (10mHz), representative of flowline cyclic loading. They were also measured at a higher frequency of 0.33Hz, representative of Steel Catenary Risers (SCR) cyclic loading associated with wave motion. Comparisons of measured Paris law curves in corrosive environments to those in air were consistent with the results of the frequency scans. There was no effect of number of cycles of reeling on the Paris law curves in the sour environment tested for WCL and HAZ specimens at both the plateau frequency and 0.33Hz.The results of the test program suggest FCGR of WCL and HAZ in the sour environment tested are not affected by number of cycles (up to 5) of straining on the intrados side for the strain level (1.93% per cycle) used in this study.Copyright