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.

Tubing couplings: NUE, EUE or slim hole

Tubing couplings come in the same two thread forms as the pipe, and the outside diameter of the coupling is what decides whether the string will pass the completion equipment above it. Class T and class SM differ in wear protection, which matters in deviated wells rather than in straight ones.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the API 5CT grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example API 5CT P110 casing. L80, C90, T95 and C110 also sit in this ladder; they are the grades with a hardness limit for sour service rather than the next step up in strength. R95 is the 95 ksi strength grade, and API 5CT sets no hardness limit for it. Tubing couplings are supplied to the same grade as the tubing, and the wall is set by the coupling size rather than by the strength of the grade.

Source: specified minimum yield and tensile strength for each grade, per API 5CT / ISO 11960. The standard is the governing document.

• J55
• K55
• N80
• L80
• P110

NUE and EUE dimensions to state on the order

The coupling OD and length are fixed by the tubing size and thread form, not by the wall thickness of the tubing. Confirm the upset type and the coupling OD against the API 5CT/5B table for the order.

Order fields for tubing coupling

• Size and thread Tubing OD, weight or wall, NUE or EUE
• Grade J55, K55, N80-1, N80Q, L80 or P110
• Sour service Hardness limit and NACE MR0175 / ISO 15156 requirements when H2S is present
• Surface treatment Phosphating, coating or bare
• Documents EN 10204 3.1 or 3.2, thread gauging record and heat traceability

Download API 5CT Tubing Coupling data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.