API 5CT Tubing Coupling

Tubing Coupling Selection

Tubing couplings must match both the pipe size and the connection type. An EUE coupling has a larger outside diameter than the pipe body because it must accommodate the external upset, while an NUE coupling is close to the pipe outside diameter. This dimensional difference is the reason a NUE coupling cannot be substituted for an EUE coupling even on the same nominal tubing size. In wells with tight clearances, or where a wireline or coiled tubing operation must pass the coupling, a special clearance or slim-hole coupling may be required. State the tubing size, weight per foot, connection type, grade and quantity when ordering, and specify if a special clearance coupling is needed.

Sour Service Coupling Requirements

For sour service the coupling must satisfy NACE MR0175 / ISO 15156 in addition to API 5CT. This means a hardness limit, typically 22 HRC maximum for carbon and low alloy steels, and a chemistry that limits the risk of sulfide stress cracking. L80 is the most common sour service coupling grade. It is equally important to use a thread compound containing no free zinc, copper or other reactive metals, because the decomposition products of an unsuitable compound can accelerate cracking. Where the well is both sour and high pressure, discuss the string design with the coupling manufacturer before ordering, because the coupling is frequently the governing component.