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Determination of the differential collapse pressure rating of pipe without evaluating the effect of the internal pressure is always non conservative. The errors are highest (5 to 10 percent or greater) for thick wall pipe (API yield collapse mode) at high axial load and high internal pressure. The errors are negligible for thin wall tubulars or other applications where the axial load or internal pressure is low. The derivation of the triaxial pressure is low. The derivation of the triaxial analysis for thick wall pipe is provided. In addition, the equation is expressed on a single figure allowing the user to quickly determine the impact of ignoring the internal pressure for an application using thick wall tubulars. Adjustments are also provided for other than thick wall pipe to facilitate quick evaluation.
The API collapse pressure ratings are based on theoretical considerations and empirical data. Equations are provided by API to account for the effect of axial loading. However, there is no provision in the API equations to account for the provision in the API equations to account for the effect of internal pressure on the collapse rating.
SUMMARY AND CONCLUSIONS
This analysis shows that operating at a differential pressure at or approaching the API collapse rating (i.e., intended collapse design factor near 1.0) may result in actually exceeding the true collapse rating of the tubular which may result in failure. Errors are highest (5 to 10 percent or greater) for thick wall pipe at high percent or greater) for thick wall pipe at high axial load and high internal pressure. For the purpose of this discussion, thick wall pipe has a purpose of this discussion, thick wall pipe has a D/t ratio less than 12+ where the API yield collapse equations are applicable. Slightly lower errors result for thin wall tubulars whose API collapse failure mode is plastic or transition collapse. The error due to ignoring the internal pressure (as is normally done) is negligible for pressure (as is normally done) is negligible for thin wall tubulars or other applications when the axial load or internal pressure is low.
The triaxial collapse equation is somewhat long; however, the maximum impact of ignoring the internal pressure may be seen easily since all cases can be demonstrated on a single figure. Use of these procedures results in less derating due to the internal pressure and are believed to resolve the problems addressed by P. D. Pattillo in reference 2.
Tubulars that are designed based on differential pressure are not affected if the internal pressure pressure are not affected if the internal pressure is actually zero. Tubulars that have been designed in the past based on loading that is stastical in origin need not be designed using these more precise equations since the design criteria has worked adequately. In these cases, the design criteria is sufficiently conservative to prevent failure due to ignoring the influence of the internal pressure on the true collapse rating and since the loads are unlikely to be imposed. For those cases where the external pressure loads will be imposed and the design pressure loads will be imposed and the design approaches a 1.0 design factor in collapse, the collapse rating should be determined considering internal pressure, temperature, and (for yield collapse) minimum wall thickness.
This article describes the procedures and results of a project performed at Southwest Research Institute (SwRI) for the purpose of assessing the collapse resistance of mill selected High Collapse Grade 95 casing. One-hundred-and-eight pieces of casing were tested representing the most popular sizes and weights currently in use. Dimensional properties, coupon material properties and residual stresses were measured and correlated with failure values.
The results show that the industry is capable of producing HC-95 casing with superior collapse performance properties. Theoretical land empirical formulas are given for calculating collapse pressures.
D/t = Average D/t ratio of specimen
E = Young's modulus (psi)
EC = Eccentricity (%)
Es = Secant Modulus (psi)
Et = Tangent Modulus (psi)
OV = Ovality (%)
Pcr = Calculated Collapse pressure (psi)
Peo = Collapse pressure of a perfect tube due to elastic instability (psi)
Song, Yupu (The State Key Laboratory of Coastal and Offshore Engineering, Dalian university of technology) | Cao, Wei (The State Key Laboratory of Coastal and Offshore Engineering, Dalian university of technology)
Lindemann, Thomas (University of Rostock) | Backhaus, Eldor (University of Rostock) | Ulbertus, Albert (University of Rostock) | Oksina, Anna (University of Rostock) | Kaeding, Patrick (University of Rostock)
In this paper, the dynamic collapse behaviour of structural components used for shipbuilding applications is investigated. To assume an appropriate material model uniaxial tensile tests are performed for different steel specimens. Interpolation functions are validated against the test results. Dynamic collapse analyses are performed for thinwalled structures in bending by using the Finite Element Method. The numerical results are validated against experimental data. For different plate panels under inplane thrust the dynamic collapse behaviour is determined numerically. An approach to extend the Idealized Structural Unit Method for dynamic collapse analyses of large structural units is presented.