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Abstract

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.

Introduction

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.

P. 359

axial load, axial stress, axial stress equivalent grade, collapse equation, collapse mode, collapse rating, drill string component, Drilling Equipment, drilling tool, equation, internal pressure, pi sz, pipe, pressure rating, strength, thick wall pipe, Triaxial Collapse Design Consideration, Upstream Oil & Gas

Unsolicited. This document was submitted to SPE (or its predecessor organization) for consideration for publication in one of its technical journals. While not published, this paper has been included in the eLibrary with the permission of and transfer of copyright from the author.

**Summary**

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.

NOMENCLATURE

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)

SPE Disciplines:

Fatigue testing under simulated service conditions has advantages over constant amplitude tests. For example the variability in both amplitude and frequency content of service loading can he reproduced by carrying nut tests under simulated service conditions. This means that the complex interactions between environment and loading which govern fatigue crack growth mechanisms in offshore structures are taken into account. Existing fracture mechanics models and fatigue crack growth prediction methods generally rely on using the overall equivalent stress range with a suitable crack growth law for fatigue crack growth prediction under variable amplitude loading. For S-N type analysis this method is by far the hest when dealing with variable amplitude sequences. However. for fracture mechanics (PM) crack growth prediction employed after an in-service inspection schedule. the use of the overall sequence equivalent stress range will not allow for sequence effects to the accounted for. These effects can he significant under service loading conditions as crack growth is largely dependent on the stress intensity fader range-which is a function of stress range and crack size. It is possible that the use of the overall sequence equivalent stress concept in a fracture mechanics analysis procedure may not he robust enough to handle the high degree of variability observed in service as crack growth acceleration and retardation can not account for. A different and mo]''e realistic fracture mechanics hazed approach is required. This paper presents a new fracture mechanics based model for predicting fatigue crack growth in offshore structures. The model relies on the use of measurable sea state properties to determine crack growth associated with each sea state over its duration.

Fatigue is the main source of structural degradation of structures in the North Sea and has been the focus of many major research programmers.

crack growth, equivalent stress, equivalent stress concept, fatigue crack growth, History, hydraulic fracturing, loading condition, Offshore Structure, prediction, probability, sea state, sea state equivalent stress, sea state equivalent stress concept, sequence, service condition, state equivalent stress, stress range, variation

Country:

Triaxial static test and constant-amplitude fatigue test were carried out on plain concrete cubes to investigate the multi-axial compressive fatigue strength of concrete in this paper. Based on the concrete triaxial compressive constitutive behavior, the inflexion of confining loading evolution was chosen to be the fatigue damage criterion during the test. According to Fardis − Chen criterion model and the concept of equivalent fatigue life and equivalent stress level, a unified S − N curve for multi-axial compressive cyclic loading condition was proposed. The fatigue strength factors obtained by the curve can provide information for the fatigue design of concrete structures.

Many reinforced and prestressed concrete platforms, offshore engineering structures are subjected to cyclic loadings generated by winds and waves. Fatigue failure is one of the most primary failure modes. In practice, the upper stress level at N = 2 ×10

compressive cyclic loading, compressive static loading, criterion, cyclic loading, equation, equivalent fatigue life, equivalent stress level, evolution, fardis chen model, fatigue life, fatigue loading, Fatigue Strength, fatigue test, loading, plain concrete, strength, strength factor, stress level

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)

**Abstract**

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.

application, Artificial Intelligence, collapse analysis, collapse behaviour, deflection, dynamic collapse analysis, dynamic collapse behaviour, Equivalent stress distribution, Fujikubo, hydraulic test rig system, lateral deflection, plate panel, shell element, shipbuilding, specimen, strain rate, strength, thrust, ultimate strength, velocity value

SPE Disciplines:

Technology: Information Technology > Artificial Intelligence > Representation & Reasoning > Mathematical & Statistical Methods (0.34)

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