Effect of Stress Concentration Factors due to Corrosion on Production String Design
- Kai Sun (U. of Louisiana at Lafayette) | G. Robello Samuel (Landmark Graphics Corporation) | Boyun Guo (U. of Louisiana at Lafayette)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 2005
- Document Type
- Journal Paper
- 334 - 339
- 2005. Society of Petroleum Engineers
- 4.6 Natural Gas, 1.7.5 Well Control, 2 Well Completion, 4.2.3 Materials and Corrosion, 5.8.3 Coal Seam Gas, 1.6 Drilling Operations, 2.2.2 Perforating, 4.2.4 Risers
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- 435 since 2007
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With the increase in operating costs and the need to withstand the cyclical swing of the oil prices, there is a growing demand for cost-effective production operations. Challenges associated with extreme depth, pressures, and temperatures at which corrosion is a problem can translate to additional problems caused by tubing burst, collapse, and tension. Tubular goods subjected to corrosion suffer strength deterioration. This becomes detrimental when coupled with cyclic loading. Failure of tubings could lead to disastrous consequences and the loss of the well, and if tubing strings are not designed in consideration of corrosion effects, it could also result in problems that will require well-control operations. Stress concentration caused by corrosion cavity plays an important role in tubing design under corrosive environment. The objective of this study was to develop a new criterion for designing corrosion-resistant tubular strings in deepwater and ultradeep high-temperature/high-pressure wells.
This paper presents results of our theoretical investigations of the corrosion effects on tubing-strength degradation. A new method and a revised criterion have been proposed to predict the threshold pressure for degraded tubing strength. Analytical formulae have been developed for calculating stresses around semispherical cavities, shallow surface cavities, and deep spherical cavities in the body of tubular strings. The effects of stress concentration on tubing strength are analyzed with these formulae. Solutions are also presented in the form of plots that can be easily used by the production engineers. An application example is presented in this paper.
Corrosion pits act as stress risers and decrease the pressure integrity of tubing, resulting in tubing failure. There are many references that sought to quantify the design of tubing subjected to different loads during production operations. Comparatively little research has assessed the effect and integrity of the tubing strength on the basis of the corrosion pits' geometry shapes and dimensions. Thus, it is highly desirable to predict the extent of stress concentration caused by corrosion-induced pits and cavities for both the designing and evaluating processes.
Schmitt et al.1 experimentally studied the localized corrosion caused by erosion and pitting corrosion. The induction period and the effects of the precut grooves on the localized corrosion were studied. They analyzed the cause and effect of the pitting and corrosion during sweet-gas production and presented methods to inhibit the attack. Pitting corrosion studies indicate that pitting corrosion is a localized form of corrosion by which holes are produced in the structure wall.2-11 Pitting causes localized attacks on the tubing and is one of the most destructive forms of corrosion. The loss of weight because of pits is much lower, thereby making it difficult to detect the intensity of pitting corrosion. The initiation period of pitting is long, and, once initiated, the rate of pitting increases at a much higher rate. The loss of weight caused by pits is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Small scratches, defects, and impurities in the steel pipe wall can initiate the pitting process. Mechanism analysis has shown that, because pits can be either hemispherical or cup-shaped apart from the localized loss of thickness, corrosion pits on the tubing wall can cause severe local-stress concentrations if the tubing is subjected to loads. The most damaging load for tubing is the burst load. Burst loads to the well tubing originated from the column of production fluid that holds a very high pressure and acts on the inside wall of the tubular structure. Even though the tubing is initially designed with proper safety factors, the change in the loading condition during the life of the well may lead to bursting of the tubing because of degradation of the tubing strength caused by corrosion. If the tubing strings are not properly designed, it may result in a tubing burst and, thereby, blowout and loss of the well.12
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