Due to the increasing demand for oil and gas, coupled with the fact that oil reserves are becoming rather scarce, the petroleum industry is pushed to drill and complete deeper wells. Threaded connections are often the weakest link in this process and are therefore the subject of research and optimization. At first, this paper presents a brief overview of the design characteristics of today’s premium connections. Secondly, the failure mechanisms of Oil Country Tubular Goods (OCTG) are discussed. At last, an in-depth analysis of the API 5C3 formulae is given. Four formulas for collapse are given in API 5C3, each valid for a specific D/t range. With increasing yield strength of the steel, the difference between the yield strength collapse and the plastic collapse gets larger. Also, the elastic collapse zone gets bigger, so stronger materials with relatively large D/t ratios will collapse in the elastic zone instead of the plastic or transition zone. These four formulas can be approached by a third-order polynomial equation that is valid for all D/t ratios
