A Unified Approach to Yield, Buckling, and Leak in Well Tubulars

Abstract

Summary Yield and buckling are independent of hydrostatic pressure. However, leak in a threaded connection depends on hydrostatic pressure, and hence, leak resistance is a function of connector location in the string. It also means that von Mises stress alone is insufficient to characterize connection leak. Like pipe-body yield and buckling, a simple consistent failure theory derived from principles of mechanics is proposed for leak in threaded connections. In addition, the buckling fictitious force is reformulated as a nonfictitious expression to clearly show independence of hydrostatic pressure. Two leak constants—thread modulus and makeup leak resistance—are introduced and evaluated with simple example cases. To quantify results, a 7-in. long-thread-casing (LTC) connection is modeled with the new leak criterion, and results demonstrate that the connection can withstand differential pressures higher than the published ratings because of hydrostatic pressure. A new connection safety factor is defined, and a leak line and a leak circle are developed for graphical purposes to quickly identify critical loads for leak.