Research On The Calculation Method Of PDF Of Casing Strength

In oil drilling and later oil extraction process casing plays an important role, poor choice will lead to various forms of casing failure, therefore accurately calculating the casing strength is very important. At present, calculating the casing strength mainly use the API method. The main problems of the standard is too conservative and can not reflect the different manufacturers, different quality and intensity between casing. It also brought many problems in casing design and appliaction. Therefore, the establishment of a casing products to reflect the strength parameters of statistical properties of the casing strength calculation is necessary.In this paper, a summary of the calculation method taking into account manufacturing defects in the pipe of domestic and abroad on the basis of comparison is given to calculate casing against external collapse, internal pressure and tensile strength of the uniaxial and triaxial strength. Selected the formula to calculate the casing's brust strength and tensile strength of the uniaxial and triaxial, finite element software were used to verify the proposed or selected casing strength calculation formula comprehensive and systematic, the results show that, in most cases theoretical values were slightly less than the calculated value of finite element analysis, and with the changes of the casing parameters the two different methods have the same trend. On this basis, given parameters and obtaining method for calculating the probability distribution of casing strength.These parameters established method for determining the probability distribution function, the determination method of use obtained intensity parameters of the probability distribution function.Obtained parameters were used to calculate the probability distribution function of the casing strength by the first order reliability method. On the basis of theory, prepared the computer programs to calculate probability distribution of casing strength, the example completed by the program shows that, API casing strength is conservative,by increasing the mill test can significantly reduce the casing strength variance, greatly increased the minimum strength of casing. To some extent provides a solution for designing some oil and gas well that can not find meeting the requirements's casing.