Strain Limit Design of 13 3/8-in., N-80 Buttress Casing
Authors: Wooley G.R. , Christman S.A. , Crose J.G. 
Published: December 2007

A mathematical model of a buttress-threaded connection was developed to compute the strain limit of casing. The model results are compared with full-scale test results and predictions are made of the casing strain when the connection fails for various conditions of temperature, pressure, and connection assembly.

Introduction
Usually, well casing is designed so that the applied axial forces are not sufficient to cause yielding. However, for limited strain problems it is possible to load certain casing types beyond the yield point without causing a failure. Two examples of limited strain problems are soil subsidence and thermal expansion and contraction. For these types of problems, a displacement load is imposed and the stress is determined by the material properties.
Permafrost soil subsidence caused by thawing induces a displacement-type load. To design for such loads, Atlantic Richfield Co. and Exxon Co., U.S.A.. have performed tests and calculations to determine the strain limit performed tests and calculations to determine the strain limit of 13 3/8-in., 72-lb/ft, N-80 buttress casing, which is used as surface casing in many Prudhoe Bay wells.
Although full-scale casing connection tests have been conducted, the experimental measurements provide information for only a few special loading conditions. To investigate the effects of changes in loads, geometry, or material properties, the ability to calculate stresses and strains in the casing connection is needed. A computer model has been developed to perform a stress analysis of the threaded connection. Test data and model calculations establish the criteria for strain limit design of a 13 3/8-in., N-80 buttress casing.

Model Description
The problem of concern is diagrammed in Fig. 1. Since the geometry and loading are axisymmetric, an axial cross-section of the pipe may be analyzed as a twodimensional problem, with circumferential effects accounted for in the calculations. Moreover, a plane of symmetry exists at the center line of the collar. Therefore, it is possible to hold the center-line position rigid and apply the axial deflection across the end of the pipe section. pipe section.The mathematical model used to perform the stress analysis of the threaded connection was developed by Prototype Development Associates. The computer code Prototype Development Associates. The computer code uses the finite-element method to calculate stresses and strains for a given load system.
Several features made the code adaptable to a threaded connection analysis. Mechanical properties may be approximated by a bilinear constitutive relation that accounts for yielding in the threads during deformation. The particular stress-strain relation selected for calculation is described in another section. Another nonlinearity is introduced by modeling the interface between mating teeth. This surface is capable of carrying a considerable compressive load, but the thread surfaces must separate without resistance under a tensile load. A special interfice element accounts for this nonlinearity and allows for resistance to shear motion, which is used to approximate frictional resistance to radial thread separation. This friction is most important near a failure condition where deformation is significant. The effects of connection assembly loads are modeled by specification of initial displacements that describe the appropriate interference fit between pipe and coupling.
For the 13 3/8-in. buttress-connection analysis, approximately 2,000 finite elements and 2,200 nodes are used.
JPT
P. 355