Abstract: In view of the facts that the manufacturing defects were not considered in current casing strength design standards, and that the safety factors of casing was hard to identify, a risk assessment method of the casing with defects based on the target reliability index was proposed. Firstly, calculation methods of the strength and external load of casing considering manufacturing defects were established, and the distribution types and parameters of strength and load were calculated by Monte Carlo(MC) method on the basis of the randomness of factors influenching strength and load. Secondly, by determining the target reliability index and solving the Rackwitz-Fiessler(R-F) calculus points using inverse iteration, the partial coefficients of casing strength and load were determined. Finally, the minimum strength value required for casing was determined according to the partial coefficients and casing load standard value, and the casing was selected or judged based on the safety and reliability requirements. An oilfield was taken as an example to verify the proposed method. The results show that the risk assessment method established in this paper can meet the practical project needs of casing design and selection for deep and ultra-deep wells, and can help to solve the difficulty of selecting safety factors in the traditional design method.

Key words: reliability, deep well, ultra-deep well, casing strength, partial coefficient, risk assessment