Recently,well design engineers attempted to replace traditional API 5CT pipes with cheaper UCE steelpipes in the downhole wellbore construction.The UCE method for producing longitudinally welded large-diameter pipes creates typically weaker pipes in terms of collapse resistance.The standard API 5C3 collapsedesign procedure no longer meets safety requirements.A practical solution to this issue is discussed.UCE pipe casing can be designed based on the new collapse strength envelope,which is establishedby modifying the API RP1111 collapse strength model.The original API RP1111 collapse strength modelwas intended for pipeline design,not downhole tubular design.It is comprised of the elastic collapse termand yield collapse term,with the latter being proportional to material yield strength.However,the derationeffects of temperature,tension,and internal pressure are not included.For downhole wellbore tubulardesign,yield strength is replaced with equivalent yield strength(from API 5C3 new addendum),which isa function of temperature,axial stress,and internal pressure.A new casing design workflow has been implemented in the computer program,and case studies wereperformed to verify the collapse design results.A new collapse pressure envelope was generated using thecomputer program integrated with a commercial tubular design tool and was compared to the traditionalAPI 5C3 collapse pressure envelope.As expected,the new collapse strength values,calculated using themodified API RP1111 collapse model,are typically much lower than the estimated values using the API5C3 collapse formula.Results of the collapse safety factor and maximum allowable wear are also compared between themodified API RP1111 collapse model and the traditional API 5C3 collapse model.Typically,UCE pipeusing the modified API RP1111 collapse model generates lower safety factor and maximum allowable wearvalues,as expected.The API RP1111 collapse strength model has been modified to include the deration effects of downholeconditions.Implementing this model in the commercial tubular design tool enables the cost-effective designof wellbore casing strings using cheaper UCE-manufactured steel pipes.
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