Experimental Study on the Effect of Axial Tension Load on the Collapse Strength of Oilwell Casing

This paper discusses a newly developed collapse testing machine that permits investigation of practical performances of oilwell casings. Although a theoretical performances of oilwell casings. Although a theoretical analysis has shown that "axial tension stress has no effect on collapse pressure in the elastic case," this theory is not applied to the design of casing string because of lack of useful experimental data or authorized recommendation. To investigate the effect of axial tension load, full-size commercial casings have been tested under combined loading of axial tension load and external pressure. From the experimental results, the theory mentioned was proved in the case of so-called high-collapse casing, which has been used widely in recent years. Also shown is the applicable d/h range, which is wider than API's elastic collapse range. If the results of this experiment were applied to the design of a casing program, an economical and safe one could be obtained. program, an economical and safe one could be obtained. Introduction Recently, improved drilling techniques have permitted deeper and deeper oil and gas wells. As well depth increases, steel pipes for well casings receive greater external pressure and axial tension load because of the weight of the casing string. High-collapse casing, which has higher collapse strength per unit weight, has become easily available. To select and to design casing for such wells properly and economically, estimating collapse strength of the casing under axial tension load is very important. Much research and many experiments concerning collapse problems on casing, drillpipe, and tubing has been conducted by 1939. A theoretical analysis showed that axial tension stress lowers the collapse pressure in the case of plastic collapse and that axial tension stress has no effect on collapse pressure in the elastic case. Although collapse tests under axial tension load simulating oilwell casing in service were conducted on 2-in.-OD tubings, the theory for the effect of axial tension stress in the elastic collapse had not been proved sufficiently. There are few published experimental proved sufficiently. There are few published experimental data on collapse strength under axial tension load. In 1968, API summarized the collapse data and showed the formulas for collapse pressure and for collapse pressure under axial tension stress in the case of plastic collapse. The purpose of our study is to show how the collapse strength of commercial casings with large OD's behaves under the axial tension load, especially in the case of elastic collapse. To test the large-size casings, a multipurpose collapse testing machine that can simulate the service condition of oilwell casing has been developed. Statement of the Problem The collapse strength of casings under combined external pressure and axial tension load may be calculated from pressure and axial tension load may be calculated from Ref. 6's Formula 1.1.5.1: ....................(1) SPEJ p. 609