No abstract

Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments. SCRs can be subject to low-stress high-cycle fatigue loading, for example from wave and tidal motion, vortex induced vibration (VIV) and operating loads, and corrosive environments (internal and external). When the production fluids are sour, higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared to performance in air, as a result of the interaction between fatigue crack growth and sulphide stress cracking. Successful design of risers is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced and rates of fatigue crack growth are increased. Historically there has been a discrepancy between experimental sour fatigue endurance data and fracture mechanics-based estimates of the corresponding stress-life (S-N) curves. This paper summarises the results of recent sour FCGR tests on C-Mn pipeline steel. Tests were performed under conditions of increasing applied stress intensity factor range (ΔK), on specimens containing shallow initial flaws and at very high stress ratios (R), to obtain data close to threshold. In many cases it is material behaviour at these low values of ΔK that dominate the fatigue life (e.g. VIV loading). The FCGR data are then compared to sour fatigue endurance data, both published and from a TWI Joint Industry Project (JIP). The observed environmental reduction factor (ERF) for endurance tests is compared to that expected from the difference in fatigue crack propagation rates, to examine whether FCGR data might provide an alternative means of predicting ERFs. This paper offers valuable insight into current best practice methods for generating sour FCGR data when qualifying girth welds for sour service, and the relationship between fatigue crack growth and fatigue endurance.

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behavior. One area where both experimental data and a validated assessment methodology are lacking is the behavior of shallow cracks. This paper describes the early results of an ongoing research program aimed at addressing the shortfall in experimental data to characterize material behavior in the shallow-crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking. There is an industry need for a better understanding of material behavior under these conditions and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared with air depending on the applied stress intensity factor range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential nonconservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

Fatigue crack growth rate of line pipe steels in sour environments typically exhibits a steady-state value at low frequencies. However, in highly inhibited sour environments, there is no evidence of a steady-state fatigue crack growth at low frequencies. This is likely a result of static crack growth rate at K max . Stable static crack growth measured under constant stress intensity factor (K) conditions in inhibited sour environments was in the range of 10 −7 mm/s to 10 −8 mm/s. The crack growth rate in inhibited sour environments is likely associated with crack tip processes associated with metal dissolution/film formation and associated hydrogen evolution. The results obtained were modeled based on a crack tip strain rate based approach, where the rate limiting step was the metal dissolution/FeS formation and the corresponding hydrogen generation reaction. CORROSIONJOURNAL.ORG100 150 200 250 300 350 400 450 500 K J 0.2 mm -Air K J maxload -Air K J 0.2 mm -Env K J maxload -Env K J (MPa√m) dK/dt (MPa√m/s) X65 parent pipe pH = 5 p H2S = 0.46 psia FIGURE 2. Effect of K-rate on the measured J 0.2 mm for API 5L X65 in sour environments on coated samples. 15 140 J 0.2 mm 5 wt% NaCl Uncoated pH 5, 1 psia H 2 S J maxload J 1 mm 120 100 80 60 40 20 0 0.01 0.1 K-Rate (N . mm -3/2 /s) J (N/mm) FIGURE 3. Effect of K-rate on the measured J 0.2 mm on API 5L X6 in sour environments on uncoated samples. 16

In 2002, TWI Ltd. carried out a questionnaire-based survey of “user experience of plant life management practices,” to gain a better understanding of the reality of plant life management and the needs of plant operators [Iravani and Speck, 2002, “Industry Survey of Risk Based Life Management Practices and Their Relationship to Fitness-for-Service Assessment,” TWI Report No. 13032/5/02]. In 2003, the European fitness-for-service network reported the results of their survey on “current application and future requirements for European fitness-for-service (FFS) technology” [Filiou et al. 2003, “Survey of Current Application and Future Requirements for European Fitness-for-Service Technology,” Technical Report No. FITNET/TR2/03, FITNET Consortium]. In 2006, the management of aging plant became a regulatory hot topic in the UK with a health and safety executive document on the subject being released [Health and Safety Executive, 2006, “Plant Ageing: Management of Equipment Containing Hazardous Fluids or Pressure,” RR509]. Considering also the recent release of the new API/ASME joint FFS standard [2007, API 579-1/ASME FFS-1, Fitness-For-Service, 1st ed., The American Petroleum Institute and the American Society of Mechanical Engineers, Washington, DC], TWI Ltd. decided 2007 was the ideal time to carry out an updated industry survey to assess how developments such as these might affect plant life management practices in different industry sectors across the world. The aims of this survey were to gain an insight into current FFS trends across several industry sectors and how these may change in the future. Information was gathered as to how different companies handle their FFS activities, both in terms of the types of flaw they assess and the complexity of the assessments they carry out. The survey also investigated how safety regulating authorities view FFS activities and whether or not they accept the results as the basis for plant integrity management decisions. Closely related to this is whether there is a need for better regulation of FFS activities, FFS training, or, indeed, whether FFS qualifications should be introduced. This paper presents the results of the online industry survey and draws pragmatic conclusions that will be of interest to all those involved with FFS activities, from inspectors to researchers and from engineers to insurers.