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.
Ratcheting fatigue loading arises from the superposition of elastic cyclic loads and monotonically increasing mean strains well into the plastic domain, resulting in simultaneous tearing and fatigue of initial welding flaws. The ratcheting loads may be due to thermal gradients set by startups and shutdowns or by soil uplifts and settlements. Under these conditions, fatigue and fracture phenomena could interact, accelerating the extension of initial fabrication flaws above that predicted to occur by either mechanism acting alone. Evaluation of ratcheting fatigue behavior will impact the weld inspection criteria that ensure pipeline integrity. A previous paper [Gioielli, et al ’08] described a model that evaluates 1) tearing based on applied elastic-plastic driving force (J) versus tearing resistance obtained from standard J-R curve tests and 2) tearing-fatigue based on an extension of Paris law re-expressed in terms of an effective ΔJ instead of ΔK enabling it to be extrapolated to the very high growth rates encountered in the elasto-plastic regime. The model was successfully calibrated to small-scale tests. This paper extends the model calibration to large-scale welded pipe tests subjected to cyclic tensile loads while internally pressurized. To that end, 1) new J solutions were developed for pressurized pipes under load-controlled conditions, and 2) comparisons were made of predicted flaw extensions to those obtained experimentally from full-scale tests. The model predictions using average tensile properties and SENT-based tearing resistance of flaw extensions compared favorably to those measured in the large-scale tests, but additional tests are needed before the model can be used in design.
Hydrocarbon-carrying lines can be subjected to cyclic loads superimposed on monotonically increasing mean strains well into the plastic domain, resulting in tearing and tearing fatigue of initial welding flaws. This combined demand is referred to here as ratcheting fatigue. Examples of these loads are frost-heave in pipelines and thermal cycling of flowlines. This paper presents the experimental verification of a fracture mechanics model of monotonic and cyclic crack extension under ratcheting fatigue loads and its calibration to small-scale tests. The model is an extension of one currently used to predict tearing and tear-fatigue due to reeling. Crack driving forces (J-solutions) under load- and displacement-control conditions were derived and used with the model to predict test results. A total of 24 single-edge notched bend (SEN-B) specimens, taken from a welded riser, were tested for crack extension under combined monotonic and cyclic loads. Comparisons of predicted to measured fatigue crack-growth rates, and alternatively cyclic J-R curves, provide quantitative and qualitative validation of the model. However, calibration to large–scale tests are needed before the model can be used for design. ExxonMobil has already completed the first set of large-scale pipe tests under ratcheting fatigue loading, including internal pressure.
With the advent of the development of deepwater projects, ExxonMobil developed and successfully implemented a fatigue design and verification protocol for fracture-critical components, such as risers and tendons, to ensure design performance and reliability. This protocol has now become an industry practice. This paper discusses the analytical, fabrication, and testing aspects of the design process. The linkage among actual weld performance, welding procedures and inspection reliability is addressed. From the design implementation standpoint, reliability of the fabrication inspection is the key issue. Practical methodologies were developed to conduct and interpret the fatigue tests. In particular, specimen design, instrumentation, testing protocols, and postmortem examination are discussed. Data generated by testing 56 full-scale risers of various sizes and welded by different procedures are also presented. These data, including tests past 100 million cycles, show that (1) actual riser fatigue performance can be substantially better than that recommended by codes, (2) failures can occur in the long-life regime, and (3) fatigue performance varies with riser size and thickness. However, as a matter of practice, analyses, fabrication and testing are required for particular designs.
Three flowlines are used to transport the gas produced from the Exxon Zincsubsea production template (in MC355) to the Alabaster platform (in MC397). Each flowline installation was initiated from Alabaster with a J-tube pull andlaid down into target areas at Zinc. Diverless subsea connections at Zinc werecompleted using the second end lateral deflection method and a diverlessROV-operated collection system. Description of Flowline System The Zinc subsea production system is a gas development in 1460 feet ofwater, located in the Mississippi Canyon area of the Gulf of Mexico. Fullwellstream production will be delivered to the Alabaster platform located 6miles to the West in 468 feet of water through a dual 8-inch and single 4-inchflowline system. The 4-inch flowline is designed for a maximum working pressure of 5000 psito transport gas from the high pressure wells. The 8-inch flowlines aredesigned for a maximum working pressure of 3705 psi to transport gas from thelow pressure wells. Table 1 summarizes the characteristics of the threeflowlines. The maximum working pressures were selected to withstand wellshut-in pressure. The flowlines and associated piping are designed to allow pigging operationsfrom the Alabaster platform through a removable Pigging Valve Assembly (PVA) onthe manifold piping of the subsea template. The PVA is controlled fromAlabaster through the control system at Zinc and can be configured for variouspigging operations. The dual 8-inch lines allow round trip pigging operationsto be carried out from Alabaster. A parking block in the PVA also allows an8-inch pig to be parked and returned through the same 8-inch line. Pigging ofthe 4-inch line can be performed by pumping the pigs from Alabaster and dumpingthem into the 8-inch loop at Zinc or by using a parking block on the PVA whichallows the pig to be returned to Alabaster in the 4-inch line.
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