The Radial Approach to Safety Factors for Tubular Design

Abstract

Abstract Traditionally, collapse safety factor (SF) is the ratio of tubular collapse resistance to collapse load. The collapse resistance is usually calculated using formulas suggested in API 5C3 or ISO 10400. If the collapse load is very small while 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 radial approach for the collapse SF calculation to overcome this problem. Moreover, the application of this radial approach can be properly extended to the calculation of the burst SF and the connection SF for arbitrary load combinations. The new SF is named the "radial safety factor." It must meet the following requirement: when all the tubular loads (such as axial force, internal pressure, and external pressure) are scaled by this SF, the load point must fall exactly on the failure envelope. Therefore, for collapse, the new SF must simultaneously satisfy (1) API 5C3 collapse formulas, and (2) the through-load-point radial line equation. The Ridder's method is used to solve the SF for a specific load case. The algorithm has been implemented in a computer program and integrated with commercial software. This radial approach has been applied to both pipe body and connectors. For burst, it is shown that the radial approach to calculate the SF is equivalent to the current method of comparing the von Mises equivalent stress to the material yield strength. An example case is used to study the effect of the radial 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 radial collapse resistance (at the cross point of radial line with 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 radial collapse SF is more realistic. In particular, it is most conservative in load cases with high effective axial stresses. The radial collapse SF has critical safety implications for load cases in which high effective axial stresses exist. Its software implementation can certainly assist well engineers with safer and more straightforward tubular design.