Abstract
The collapse resistance of oil country tubular goods has been a major concern for drilling engineers; i.e., the strong demand for high collapse resistant pipes for deep wells in 1970's, the question about collapse resistance under steam injection wells in the 1980's and, in these days, the collapse resistance of worn pipes and bent pipes in dog-leg wells. These collapse resistances vary with existence of axial load, reduction of material strength with temperature, or reduction of wall thickness at worn portion. However, the standard collapse test results of commercial pipes are always referenced as the base for estimating collapse resistances in various conditions. Thus the understanding of collapse properties of commercial pipes is very important.
This paper deals with collapse resistance of commercial pipes under the uniaxial test condition. For this purpose, the influences of roundness error, eccentricity and residual stress on collapse resistance are discussed which are followed by the statistical analysis of collapse resistance in the light of those analyses.
As a result, the following things are clarified. Firstly, rotary straightening develops residual stress in the pipe which degrades collapse resistance in the plastic collapse region. Secondly, FEM analysis is conducted which predicts reduction of collapse resistance due to roundness error and eccentricity. As an example, both 0.5% roundness error and 10% eccentricity reduce collapse resistance by 5%. Thirdly, less than 10% eccentricity and 0.3% roundness error seem to have no influence on collapse resistance for commercial pipes in practice. Finally, three experimental collapse resistance equations are obtained which are referred to regions of elastic collapse, plastic collapse and yield collapse. The commercially available pipe is characterized by significantly lowered collapse resistance in plastic regions affected by the circumferential residual stress.
