Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion. In the presence of hydrogen sulfide (H2S), these acidizing treatments could cause environmentally assisted fatigue and fracture (i.e. increased fatigue crack growth rates and reduced fracture toughness). A test program is underway to evaluate and quantify the effect of sour acidizing treatments on the fatigue and fracture behavior of welded C-Mn line pipe steels. This paper describes the preliminary findings from fatigue crack growth rate (FCGR) and fracture toughness (FT) tests on as-welded (i.e. unstrained) pipe. All tests were conducted at room temperature (RT) using compact tension (CT) specimens notched in the parent pipe (PP). Frequency scan FCGR tests were performed in the following sour acid conditions: simulated production environment (PE), spent acid without inhibitor and spent acid with residual corrosion inhibitor. The PE consisted of a simulated brine with pH = 4.5 and partial pressure of H2S (pH2S) = 0.21psia. FCGRs in the sour PE were of the order of 20 times faster than in air. The pH2S was the same for the tests in spent acid environments, but the pH was lower (approximately 3.5). As would be expected, the FCGRs were much higher in the low pH environment. The highest FCGRs were observed in the inhibited sour spent acid environment and were up to 100 times faster than in air. Sour FT tests were also conducted in the PE and in spent acid with and without inhibitor. In all cases, the measured FT values were significantly lower than in air. The test in PE exhibited higher FT than in the sour acidizing environment. The lowest FT values were observed in spent acid with inhibitor. Future work will investigate the effect of reeling on the fatigue and FT performance of pipe girth welds in sour acidizing environments.
DNV-OS-F101 Appendix A provides procedures for the assessment of circumferential flaws located in subsea pipe girth welds using fracture mechanics methods, commonly referred to as engineering critical assessment (ECA). The purpose of the ECA approach is to provide critical flaw dimensions for given material properties and loading conditions in a conservative way. The results of the assessment are used to derive weld flaw acceptance (or weld repair) criteria to be used during pipeline installation. An ECA will typically consider flaws under installation and operation loading conditions, including fracture and fatigue crack growth (FCG) calculations. Internal and external surface-breaking flaws are assessed, along with embedded flaws. DNV-OS-F101 provides guidance on the appropriate FCG law to be used for the assessment of each flaw type under operational loading. For internal surface flaws exposed to sour production fluids (i.e. containing H2S) FCG rates (FCGRs) are known to be significantly higher than in air and, in the absence of relevant published data, project-specific testing is commonly performed to quantify fatigue performance. The recommendation for the assessment of embedded flaws is to use an air curve, as long as it can be substantiated that the fatigue performance is not reduced due to the environment. It has been demonstrated that the FCG behavior of C-Mn pipeline steels exposed to sour environments is dominated by bulk hydrogen charging effects, i.e. hydrogen charging by absorption from the exposed surfaces rather than the corrosion process at the crack tip. Therefore, it is expected that an embedded (or external) flaw in a sour pipeline will be located in steel containing absorbed hydrogen. This paper describes the results of an investigation aimed at understanding and quantifying the FCG behavior of embedded flaws in sour pipelines. For the purposes of this work, an embedded flaw refers to a crack propagating in hydrogen charged material but whose crack tip is not directly exposed to the sour environment. Hydrogen diffusion modelling and simulation studies were performed to predict the through wall hydrogen concentration in standard fracture mechanics specimens based on sour environmental conditions. Two novel test methods were developed to accurately measure FCGRs in hydrogen charged steel, one for single edge notched bend (SENB) specimens and one for compact tension (CT) specimens. FCGR tests were carried out using both methods. The FCGRs measured in hydrogen charged API 5L grade X65 pipeline steel were significantly higher than in air. In some cases, the observed FCGRs in hydrogen charged steel were higher than for specimens fully immersed in the sour environment. This is believed to be due to reduced environmental crack closure/blunting effects; the steel is charged with hydrogen, but there is no active corrosion process occurring inside the crack. The results of the present study indicate that the use of an air FCG curve for embedded (or external) flaws located in hydrogen charged steel may be non-conservative. Further work is required to establish the relationship between FCGR and hydrogen concentration in steel and to evaluate the implications for pipeline ECA calculations.
Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion, but could also lead to environmentally assisted fatigue and fracture. A study was performed to evaluate the effect of acidizing treatments on the fatigue behavior of welded C-Mn line pipe steels. This paper describes the results of fatigue crack growth rate (FCGR) tests on as-welded (i.e. unstrained) pipe. FCGR tests were conducted at room temperature (RT) in three different acid conditions: fresh acid with corrosion inhibitor, spent acid with corrosion inhibitor and spent acid without corrosion inhibitor. Frequency scan FCGR tests were performed on compact tension (CT) specimens notched in the parent pipe (PP), heat affected zone (HAZ) and weld centerline (WCL). The FCGRs in all three environments were higher than in air and exhibited a frequency dependence. Tests in fresh acid with inhibitor exhibited plateau FCGR values around 20–30 times higher than in air. Tests in spent acid with inhibitor exhibited a strong frequency dependence with plateau FCGR values approximately 80–100 times higher than in air. In spent acid without inhibitor, the plateau FCGR was around 20 times higher than in air, however at the lowest frequencies the FCGR decreased, most likely due to crack closure/blunting effects. This behavior is consistent with the higher corrosion rate in this uninhibited environment. The role of corrosion products in causing crack closure/blunting was further evidenced in tests performed at elevated temperature (165°F / 74°C), where the FCGR at 1Hz was significantly higher than at RT. The plateau FCGR in fresh acid and spent acid with inhibitor was approximately 40–50 times higher than in air, but the FCGR decreased at lower frequency. This is similarly believed to be due to the higher corrosion rates at elevated temperature causing crack closure/blunting. The FCGR in spent acid without inhibitor at 165°F (74°C) was high initially at 1Hz but then decreased sharply, which is consistent with the highest corrosion rates expected at elevated temperature and in the absence of corrosion inhibitor. Paris curve FCGR tests were subsequently conducted at 0.1Hz. Tests were performed in the worst case combinations of microstructure/environment/temperature identified from the frequency scan tests.
Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion, but could also lead to environmentally assisted fatigue and fracture. A study was performed to evaluate the effect of cyclic plastic strains associated with reeling installation on the subsequent fatigue crack growth rate (FCGR) behavior of welded C-Mn line pipe steel in acidizing environments. The influence of the pH of the acidizing environment on the FCGR performance was also investigated as part of this study. This paper compares the results of FCGR tests on as-welded (i.e. unstrained) pipe with those from strained and aged welds, as well as quantifying the effect of the pH of the acidizing treatments. Strained and aged welds were obtained by subjecting the as-welded pipe to 4 cycles of full-scale reeling simulation, with each cycle corresponding to 1% strain. Small-scale compact tension (CT) specimens were then extracted from the strained welds and aged at 250°C for one hour to simulate strain aging. FCGR tests were performed in spent acid with corrosion inhibitor on specimens notched in the parent pipe (PP), heat affected zone (HAZ) and weld centerline (WCL) in both the as-welded and strained and aged condition. The majority of the tests were conducted at room temperature (RT) along with a select few tests at elevated temperature (165°F / 74°C). Overall, the results of frequency scan tests indicated that reeling did not have a significant effect on the FCGR behavior of welded C-Mn line pipe steel in spent acid with inhibitor, regardless of which microstructure was sampled. Frequency scan FCGR tests were also performed on strained and aged samples extracted from the intrados side of the strained welds and notched in the PP, HAZ and WCL to investigate the influence of pH on FCGR behavior. Tests were performed in spent acid with inhibitor at RT, with the pH ranging from 3.7 to 6. The observed FCGRs were higher than in air and all microstructures exhibited a frequency dependence (i.e. the FCGR increased with decreasing frequency). At pH = 3.7, the maximum FCGR was approximately 30 times higher than in air and at pH = 5 the FCGR increased to approximately 80 times higher than in air. However, a further increase in pH to 6 produced a decrease in FCGR. The increase in the maximum FCGR is believed to be due to the decrease in corrosion rate with increasing pH leading to reduced crack closure/blunting. However, as the pH increased to around 6, the corrosion rate decreased substantially, which is likely due to a substantial decrease in the concentration of hydrogen being generated, resulting in a lower FCGR. Paris curve FCGR tests were subsequently conducted on strained and aged samples at 0.1Hz.
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