Recently, well design engineers attempted to replace traditional API 5CT pipes with cheaper UOE steel pipes in the downhole wellbore construction. The UOE method for producing longitudinally welded large-diameter pipes creates typically weaker pipes in terms of collapse resistance. The standard API 5C3 collapse design procedure no longer meets safety requirements. A practical solution to this issue is discussed.
UOE pipe casing can be designed based on the new collapse strength envelope, which is established by modifying the API RP1111 collapse strength model. The original API RP1111 collapse strength model was intended for pipeline design, not downhole tubular design. It is comprised of the elastic collapse term and yield collapse term, with the latter being proportional to material yield strength. However, the deration effects of temperature, tension, and internal pressure are not included. For downhole wellbore tubular design, yield strength is replaced with equivalent yield strength (from API 5C3 new addendum), which is a function of temperature, axial stress, and internal pressure.
A new casing design workflow has been implemented in the computer program, and case studies were performed to verify the collapse design results. A new collapse pressure envelope was generated using the computer program integrated with a commercial tubular design tool and was compared to the traditional API 5C3 collapse pressure envelope. As expected, the new collapse strength values, calculated using the modified API RP1111 collapse model, are typically much lower than the estimated values using the API 5C3 collapse formula.
Results of the collapse safety factor and maximum allowable wear are also compared between the modified API RP1111 collapse model and the traditional API 5C3 collapse model. Typically, UOE pipe using the modified API RP1111 collapse model generates lower safety factor and maximum allowable wear values, as expected.
The API RP1111 collapse strength model has been modified to include the deration effects of downhole conditions. Implementing this model in the commercial tubular design tool enables the cost-effective design of wellbore casing strings using cheaper UOE-manufactured steel pipes.