Summary
Yield and buckling are independent of hydrostatic pressure. On the other hand, 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 based on principles of mechanics is proposed for leak in threaded connections. Also, the buckling fictitious force is reformulated as a non-fictitious expression to clearly show independence of hydrostatic pressure. Two leak constants, thread modulus and make-up leak resistance, are introduced and evaluated with simple example cases. To quantify results, a 7″ 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.
