Casing Strain Tests of 13 3/8-in., N-80 Buttress Connections
Authors: Weiner Peter D. , Wooley Gary R. , Coyne Patrick L. , Christman Stan A. 
Published: December 2007

Introduction

In typical oilfield casing design work, engineers specify the force that a casing must withstand. For example, it is generally necessary for each section of a casing string to support the weight of all the pipe that is suspended below. The force and stress are known quantities, and the resulting pipe deformation, called strain, can be calculated using the physical properties of the casing steel.

However, there are situations where the casing loading is of a displacement type, for which the deformation is specified. In this case, the stress is unknown. An example is surface casing subjected to soil subsidence. In soil subsidence, the compacting soils will tend to strain the adjacent casing, and the amount of resulting pipe strain can be essentially independent of the casing properties.

As a part of the Prudhoe Bay permafrost-thaw subsidence studies, Exxon Co., U.S.A., and Atlantic Richfield Co. conducted a testing program to determine the amount of casing strain that would cause failure. The casing tested, 13 3/8-in., N-80, 72-lb/ft buttress, is used as surface casing in many Prudhoe wells.

Testing Procedure

Three tension tests and three compression tests were made with strain gauges installed as shown in Fig. 1. The compression specimens were of relatively short length (2 1/2 ft) to prevent Euler buckling. (Actual surface casing will not column buckle if it is laterally supported by cement and the formation.) The tension specimens used welded end fixtures to provide a means of attachment to the test machine.

The casing tested was 13 3/8-in.-OD, controlled yield N-80 (normalized), 72-lb/ft buttress, selected from mill runs for Prudhoe Bay. Both mill- and field-end connections were constructed by the torque-turn method, which specifies both minimum turns and minimum torque required for a satisfactory connection. Mill ends were made at the mill with an epoxy thread-lock compound, while the field ends used a teflon thread compound.

Based on metallurgical test coupons, the API yield strength (0.5-percent strain) of this casing ranged from 83 to 92 ksi. The ultimate strengths ranged from 110 to 122 ksi and occurred at strains of 12 to 16 percent. Elongation (maximum strain) was 25 to 35 percent, indicating good ductility. The post-yield behavior was uniform, with plastic moduli of 550,000 to 850,000 psi. Wall plastic moduli of 550,000 to 850,000 psi. Wall thicknesses were uniform, and averaged 0.495 in. (API minimum/nominal is 0.450/0.512 in.2)

The samples were tested at the U. of California Field Research Facility at Richmond. Load increments of 50,000 to 200,000 lb were spaced 5 to 30 minutes apart.

Tension Test Results

Fig. 2 shows the tension axial-strain readings for the D locations, which are located on the pipe 7 in. from the buttress collar. There were three tests, and the mill-end and the field-end gauges provided six sets of data. Test 1 was terminated without failing the casing at a load of 2,250,000 lb and with pipe strains of 3.6 percent (mill end) and 3.7 percent (field end). Tests 2 and 3 experienced fractures near the last perfect thread of the mill end at loads of 2,350,000 and 2,250,000 lb, respectively. The mill-end strains were 3.6 to 4.2 percent.

Fig. 3 shows a comparison of strain data taken at various locations on the casing.

JPT

P. 1301