Abstract: This paper discusses the principle of triaxial stress design of casing strings, deduces the strength formula of triaxial stress, and analyzes the overall external loads applied on the casing string.The modified equal safety coefficient method is put forward on the basis of equal scatter coeffi-
cient method. The new method is more reasonable than equal safety coefficient method when dealing with external loads, and it improves the design procedure. Based on the proposed method and in combination with the practice of oilfields, the software for strength design of casing string is developed and has been widely used in oilfields.

Key words: Casing string design, Casing strength, 3D model, Casing pressure, Casingstress, Computer program