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.
The fatigue performance of fracture-critical production lines, such as risers and flowlines, has been shown to significantly degrade in the presence of sour hydrocarbon production caused by water injection of reservoirs. To ensure the reliability of the fatigue design under such conditions, experimental verification of the degradation effect on fatigue life due the presence of H2S is required. To that end and over the past several years, ExxonMobil has developed new testing methodologies to evaluate the riser fatigue performance for both in-air and sour conditions. This paper reviews the general elements of the fatigue qualification process and presents new sour fatigue data aimed at assessing performance at the high-cycle fatigue (HCF) and low-cycle fatigue (LCF) regimes. These new data are relevant to that seen in steel catenary riser (SCR) and flowline thermal responses, respectively. Testing methodologies for each regime are discussed and results presented. The new data are interpreted within the context of previous data in the intermediate-cycle fatigue (ICF) to provide a more robust basis for riser design. The main finding is that the new data support a constant slope S-N curve for the practical domain of fatigue lives to which offshore lines are typically designed under sour conditions.
One alternative to developing offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG using tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a cryogenic hose, whose main body is a vacuum insulated, pipe-in-pipe hose made of corrugated stainless steel pipe (c-pipe) and flanged terminations. Given the novelty of the transfer system, ExxonMobil conducted an experimental program to understand the structural performance of the basic c-pipe under static and cyclic loading at room and cryogenic temperatures. This paper discusses overall qualification issues and presents the experimental methodology and results of structural performance tests of the full-scale c-pipe at both ambient and cryogenic temperatures. Fourteen full-scale, c-pipe static tests are reported, including tension, compression, bending, torsion, and internal pressure. In addition, 11 axial and three pressure fatigue tests are presented. One key result is that, overall, cryogenic temperature improves structural performance for the limit states tested, indicating that future qualification at room temperature would be sufficient. Moreover, the fatigue performance at both ambient and cryogenic temperatures surpassed the design curve reported in the literature for c-pipe.
Early in the life of the Genesis spar, cracking developed at the welded connections between the riser guide supports and the hull wall plate. The cracking was caused by the movements of the top-tensioned risers within the steel guide frames in the moon pool of the structure. The remedial action taken to minimize the riser movements and its effects on the hull involved the use of novel rubber bumpers, which were installed in lieu of the steel guides. The bumpers around the periphery of the moon pool were fastened to the hull wall via threaded studs that were friction welded to the hull wall plate underwater. This paper describes a testing program specifically designed to qualify the fatigue performance of the stud-plate friction welds. Results verify the use of the F2 S-N curve from British Standard 7608 with a single slope for the design of the friction-welded connections subjected to axial load. It was also found that the fatigue performance of friction welds is sensitive to the stud preload. One unique feature of the fatigue failure mode of the connection, when the load is transferred through the stud into the plate, is that cracking takes place along the semi-circular heat-affected zone (HAZ) of the bond-line between the stud and the plate, and not through the hull plate thickness. As a result, failure of a stud connection does not compromise the structural integrity of the spar hull.
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