The paper provides a review of the state of knowledge regarding the impact of soil response in the touchdown point region on Steel Catenary Riser (SCR) fatigue. For almost 20 years the impact of soil-pipe interaction on SCR fatigue has received considerable attention within the offshore geotechnical community. Over this course of time field measurements and a variety of experimental and analytical studies have been performed to determine the soil response necessary to characterize the soil-pipe interaction under long term loading conditions appropriate for fatigue. Little of this work has been integrated into existing codes and standards. This paper will summarize much of the new work to provide better insights on how to address the SCR fatigue problems and to serve as reference for future code modifications.
When risers are designed it is common for corrosion to be accounted for by including a corrosion allowance in the wall thickness [3]. However, when designing risers which are subject to fatigue loading from various sources, simply allowing extra thickness in the wall is inadequate to protect against the accelerated fatigue crack growth driven by corrosion. This paper illustrates a methodology for assessing the fitness for service of a steel catenary riser with various levels of pitting corrosion. The methodology uses FEA tools, as well as classical fracture mechanics, to predict the rates of crack growth and arrive at predictions of life. Once corrosion begins and pits form, the structure may experience an increase in crack growth rate, caused by the influence of the chemistry of the produced fluid on the steel and by the stress effects of the pit geometry. Further complications arise if extreme storms cause riser stresses to exceed yield, which then requires the use of strain based methodology. The results of the illustrative study demonstrate that riser designs should account for the potential of accelerated crack growth where there is a potential for pitting corrosion, and that by only adding a corrosion allowance to account for loss of burst capacity, an inadequate design can easily result.
Hybrid risers represent an excellent way to isolate the riser from most of the host vessel motions and thereby limit riser fatigue. A common arrangement features the riser supported by a buoyancy can via a tether chain. The tether chain is a cheap simple way to make the connection while providing flexibility for installation. However, in service the tether is under very high tension, and the chain is not really flexible in the face of small amplitude fatigue loads. The friction effectively “welds” the chain together. Moment and torque input to the system by first order vessel motions and vortex induced vibrations are carried through the chain and induce fatigue loading in the links. Analysis of the chain can be problematic because the determination of the detailed stress in the chain requires a refined FEA model with contact element between the links. From the global sense the analysis may require running hundreds of sea-state realizations in the time domain and the vortex induced vibration (VIV) assessment of thousands of current profiles. In this paper an efficient numerical method is described to rigorously determine fatigue damage at locations throughout the chain.
Inspection of deepwater risers for flaws or pits using ILI tools can be challenging. Some lines are designed as “non-piggable”, and it is not unusual for an inspection to be incomplete because of physical constraints. As with any measurement, there will be a degree of error. While deterministic conclusions cannot be reached based on such incomplete data sets, probabilistic methods can be used effectively to make judgments about fitness for service. Commonly, different sections along a riser or flowline experience different fatigue spectra and extreme loads. Applying the loads from the sections with the highest loading to all flaws/pits can be too conservative. It is useful to employ statistical methods to assess the probability that a large defect occurs in a region with critical loads. These methods are especially useful when ILI data are incomplete or when estimates of damage must be made based on lines in similar corrosion environments. Properties and parameters other than inspection findings have an element of uncertainty. Fracture toughness, yield stress, and fatigue crack growth rates will be known in terms of mean and standard deviation. Soil properties may be known in terms of upper and lower bound. Likewise, there will be a range of uncertainty about service history and chemical environment. In such cases where fitness for service is based on the interaction of multiple random variables, Monte Carlo methods are appropriate for determining if the probability of failure is sufficiently low to tolerate. In the case of deepwater risers and flowlines where failure could result in loss of containment of hydrocarbons, permissible failure rates are on the order of 1E−5 to 1E−6 per year. This paper examines a riser and a flowline case study. For each case, a fitness for service analysis is conducted using a Monte Carlo simulation to evaluate the probability of failure based on incomplete ILI data and statistical characterization of other pertinent parameters. The results are compared against the conclusions of deterministic analysis.
The development of HPHT oilfield equipment has typically resulted in the construction of heavy-walled designs, where the increase in rated working pressure is accommodated by an increase in sectional thickness. This manner of design, however, is limited by practical difficulties which arise in the areas of manufacturing, handling/lifting, and uniformity of through-thickness material properties. Designs of more efficient size and weight may be developed by relaxing assumed design factors and hydrotest pressures, but this requires more rigorous analysis, validation, and QA measures. In particular, designers must address the fatigue susceptibility of HPHT equipment which, even in purely static conditions, may fail under cycles of shut-in pressure alone. These failures typically originate from stress risers such as cross-bores, seat pockets, or transitions in bore diameter, which exhibit complex stress states under the action of internal pressure. A fracture mechanics (FM) based analysis of such features has presented a longstanding challenge to designers and analysts as general solutions for their KI and σref are not presently available. It is therefore the objective of this paper to provide a useful methodology for conducting FM-based analysis of arbitrary geometry using the KI and σref solutions provided in API 579-1/ASME FFS-1. The method is presented in the form of a case study which describes the FM-based fatigue analysis of a seat pocket radius within a valve body. Here, the mode I behavior of a hypothetical surface-breaking, semi-elliptical flaw located at the seat pocket radius is evaluated by means of 3D finite element analysis. This method generally comprises two parts. The first involves the development of a 3D finite element model similar to what would be used in a conventional durability analysis. From this model, stresses are extracted along an anticipated fracture plane and used in conjunction with a weight function method to derive KI and σref from solutions provided in API 579-1/ASME FFS-1. These solutions are then used to compute the number of cycles to unstable fracture. The second part involves the direct incorporation of cracks into the finite element model. The approach benefits from a submodeling technique which reduces computational expense and allows the method to be used on complex structures. The numerical model is used in conjunction with conventional linear-elastic fracture mechanics assumptions to derive KI solutions for the geometry of interest. These KI results are used to confirm the conservatism of the code-based solutions and, thereby, the conservatism of the previous FM analysis. The method described in this paper allows designers to rapidly develop and execute FM-based fatigue analyses of arbitrary geometric features in timeframes similar to those associated with traditional S-N analysis.
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