Effect of Loading Frequency on Fatigue Performance of Risers in Sour Environment

Abstract

ExxonMobil requires experimental verification of fatigue performance of fracture-critical risers designed for sour environments. Interaction between the sour environment and cracks in welded risers affects the crack-growth rate and, thus, the fatigue performance of the risers. Therefore, when conducting tests on riser welds in a sour environment, the frequency at which cyclicloads are applied during testing is critical to properly capturing the physio-chemical reactions and diffusion processes at the crack tip. Unfortunately, the load frequencies required to properly capture these effects are much lower than those currently used in cost-effective, resonant fatigue testing in air. Depending on the material, sour environment composition, and loading regime, testing at too high a frequency can eliminate the potential deleterious effects of the environment acting on the riser. Yet, testing at too low a frequency may not be practical. In order to determine the most efficient but technically valid load frequency to be used in a fatigue qualification testing program, a novel experimental screening methodology has been devised and implemented. In this paper, the proposed methodology is discussed and the results of a pilot test program conducted with C-Mn steel in a mildly sour environment are presented. For the particular sour brine, C-Mn steel and loading regime, it was found that the loading frequency could be increased up to about 1Hz, thereby making the fatigue verification tests more practical and cost-effective than the 1/3Hz currently used.

Role of Sour Environments on the Corrosion Fatigue Growth Rate of X65 Pipe Steel

“…The film formation on carbon steel as a result of corrosion was shown in Equations (1) and (2). Therefore, it is feasible that a FeS film forms at the crack tip in the presence of sufficient HSions.…”

“…The hydrogen fluxes obtained in the two test environments were converted to subsurface hydrogen concentration using Equation (2). The concentration change as a function of time based on the flux obtained in NACE A solution is shown in Figure 4 as an example.…”

“…The presence of hydrogen sulfide (H 2 S) in the environment typically results in an increase in the FCGR, with FCGR at high ∆K values being as high as 50 times the values in air. [1][2][3][4][5][6][7] FCGR studies in a range of environments indicate that FCGR increases with increasing H 2 S concentration. 1,[4][5][6][7] The increase in the FCGR with respect to the air value is also affected by ∆K at which the tests are performed.…”

“…Another important effect of sour environments on FCGR is the cycle frequency; usually it is found that with decreasing frequency the FCGR increases and reaches a plateau. 2,[8][9] The role of hydrogen diffusion through the fracture process zone as the rate-limiting step in FCGR of steels under various conditions has been developed to account for the observations of FCGR. The decrease in FCGR with increasing frequency has been attributed to insufficient time for hydrogen diffusion through the fracture process zone at higher frequencies.…”

SAFEBUCK JIP: Critical Aspects of the Fatigue Limit State for Pipelines Designed to Laterally Buckle

“…Typically, the fatigue life (in cycles) is expected to diminish as the frequency decreases, and rates of fatigue crack growth will increase. Design codes do not address the influence of sour operating environments, but published data again suggest a detrimental influence of decreasing frequency [6][7][8][9]. At low cyclic loading frequencies, there is more time (per cycle) for such interactions, and the degree or extent of 'embrittlement' potentially increases.…”