The Vector Approach to Safety Factors for Tubular Design

Abstract

Summary Traditionally, the collapse safety factor (SF) is the ratio of tubular-collapse resistance to collapse load. The collapse resistance is usually calculated by use of formulas suggested by API TR 5C3 (2008) or ISO/TR 10400 (2007). If the collapse load is very small, whereas the tension stress and/or internal pressure are high, the collapse SF could be very large, which is unrealistic because the load point is close to the collapse envelope. This paper presents a vector approach for the collapse-SF calculation to overcome this problem. Moreover, the application of this vector approach can be properly extended to the calculation of International Organization for Standardization (ISO) connector SF for arbitrary load combinations. The new SF is named the vector SF. It is the multiplier that would scale the load-point vector to reach the strength envelope in stress space. It must meet the following requirement: When all the tubular loads (axial force, internal pressure, and external pressure) are scaled by this SF, the load point must fall exactly on the failure envelope. Therefore, for American Petroleum Institute (API) collapse, the new SF must simultaneously satisfy the API TR 5C3 (2008) collapse formulas and the through-load-point radial-line equation. Ridders' method (Ridders 1979) is used to solve the SF for a specific load case. The algorithm has been implemented in a casing-design computer program (Landmark 2017) and integrated with commercial software. This vector approach has been applied to both pipe body and connectors. Example cases are used to study the effect of the vector approach on the collapse-SF values. The collapse-SF value is observed to become much smaller when the effective axial stress (actual axial stress plus internal pressure) is very high, which is expected because of the much-smaller vector collapse resistance (at the cross point of the radial line with the collapse curve). When the effective axial stress is not positive, the collapse SF slightly changes. A North Sea high-pressure/high-temperature well is also presented. The results indicate that the vector collapse SF is more realistic. In particular, it is most conservative in load cases with high effective axial stresses. In summary, it is shown that the vector approach to calculate the SF is equivalent to the current method of comparing the von Mises equivalent stress with the material yield strength (YS). The vector SF has critical safety implications for load cases in which high effective axial stresses exist. Its software implementation can certainly assist well engineers with more-conservative and more-straightforward tubular design.