Abstract

During the process of conducting an Engineering Criticality Assessment to determine weld flaw acceptance criteria, the fatigue crack growth rate (FCGR) must be modelled to accurately predict fatigue cracking behavior. Standard curves exists in industry codes for C-Mn steels in-air and in seawater environments exposed to cathodic protection; however, in the presence of produced fluids, the FCGR can be accelerated. This paper presents the results of a FCGR testing program to evaluate the effects of CO2 on FCGR in multiple compositions of produced fluids. Parent pipe, weld center line, and heat-affected zone metallurgies were evaluated by both frequency scanning and Paris curve tests. Baseline tests were performed at 150°F, pH of 5.4, and 1 atm pCO2. Sensitivity tests were also performed to evaluate the effect of test sequencing (decreasing ΔK vs. increasing ΔK), pH, temperature, pCO2, and presence of a corrosion inhibitor. Frequency and ΔK ranges corresponding to Steel Catenary Riser (SCR) and lateral buckling flowline applications were also addressed. Results indicate that for SCR applications (frequency ~ 0.2Hz; ΔK < 200 Nmm-3/2), the effect of CO2 on FCGR is minimal. However, for flowline applications (frequency ~ 0.001Hz; ΔK > 800 Nmm-3/2), FCGR can be accelerated by as much as 15x, depending on fluid composition.

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