A New OCTG Strength Equation for Collapse Under Combined Loads
- Frans J. Klever (Shell Intl. E&P BV) | Toshitaka Tamano (Kogakuin University)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2006
- Document Type
- Journal Paper
- 164 - 179
- 2006. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 4.2 Pipelines, Flowlines and Risers, 1.7.5 Well Control, 1.14.1 Casing Design, 1.7.2 Managed Pressure Drilling, 1.10 Drilling Equipment, 1.6 Drilling Operations, 4.6 Natural Gas, 1.7.1 Underbalanced Drilling
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Wells are becoming more challenging, and casing designers are faced with increasing design pressures. Deep hydrocarbon targets lead to requirements for the casing to resist collapse under external pressure, while significant internal pressure and axial compression or tension may exist at the same time.
This paper describes the development, and its evaluation, of a new collapse-strength equation for oil-country tubular goods (OCTG). It is based on a generalization of a model previously proposed by Tamano et al. (1983). The new model is evaluated through comparisons with both finite-element analyses(FEAs) and test data. It is more accurate in dealing with combined internal pressure, external pressure, and axial load than, for example, the model currently provided in American Petroleum Inst. (API) Bull. 5C3 (1994).
The joint API/Intl. Standardization Organization (ISO) subcommittee (SC)5 Work Group (WG) 2B tasked with modernizing the Bull. 5C3 property equations has evaluated a number of collapse models available in the literature on their performance against several collapse databases. As a result, the model presented here is recommended for developing design collapse strengths in the new ISO TR 10400 standard (2006).
The design-collapse-strength equations currently used in the industry and provided in Bull. 5C3(1994) give a highly nonuniform failure probability over diameter, weight, and grade for downhole well tubulars (Adams et al. 2003; Ju et al. 1998). In addition, the Bull. 5C3 average collapse-strength equations are relatively poor predictors of true collapse and, therefore, no compelling case exists to use these equations for qualifying high-collapse pipe and other proprietary products. Designing deep wells is becoming more challenging because of requirements for the casing to resist collapse under external pressure while significant internal pressure and axial compression or tension may exist at the same time. This highlighted the need for revisiting the method to account for combined loading in collapse.
The situation may be improved if more accurate collapse-prediction formulas could be developed that adequately capture the physics of collapse failure, and more explicitly include the effect of imperfections. Because the collapse-failure mechanism is an instability phenomenon (i.e., the transition from an essentially round pipe to a pipe that starts to ovalize and flatten with the external-pressure capacity reaching a maximum can happen very quickly), it is not feasible to expect a simple equation to capture this failure mechanism very accurately. However, theoretical analyses, detailed FEA, and numerous collapse tests have provided a wealth of insight that has guided the development of approximate collapse equations that capture collapse failure to a satisfactory degree.
|File Size||1 MB||Number of Pages||14|
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