API 5CT Casing Coupling

Why the Coupling Grade Matters

The coupling is often the weakest link in a casing string, so its grade and heat treatment should be specified deliberately rather than defaulted. API 5CT requires that the coupling be of the same grade as the pipe body unless otherwise agreed, and be subjected to the same heat treatment. In practice a higher grade coupling is sometimes specified to increase joint efficiency, or a coupling of the same grade but with controlled hardness is used for sour service. Couplings are seamless because a welded coupling would introduce a seam in the highest stress region of the connection. Each coupling is thread gauged with API 5B plug and ring gauges and marked with its heat number so it can be traced back to the pipe it was made up with.

Thread Forms and Gauging

STC and LTC both use an API round thread but differ in thread length, with LTC being longer and therefore stronger. BTC uses a buttress thread with a 3 degree load flank and a 10 degree stabbing flank, giving a much greater shear area and higher tensile efficiency. All three are inspected with API 5B gauges. Thread compound is applied before make-up, and the correct compound must be selected for the service: standard compounds for normal service, and compounds containing no free metals such as zinc or copper for sour service, since these metals can accelerate sulfide stress cracking. Make-up torque must follow the mill or API recommendation, and torque-turn monitoring is recommended for critical strings.

Casing couplings: thread form first, then grade

A coupling carries the same load as the pipe it joins and is the part of a joint most likely to be replaced. It is bought to the same API 5CT grade as the pipe and to the thread form used on the string, so an enquiry that gives the pipe size without the thread form is usually answered with a question.

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. A coupling is ordered to the same grade as the pipe it joins, and to the same end and thread form; the grade is not chosen separately.

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
• Q125

Casing coupling dimensions: what to confirm on the order

The coupling dimensions come from the casing size and thread form, not from a free choice. The order should name the casing OD, weight designation, grade, thread and drift so the correct coupling is selected from the API 5CT/5B table.

Order fields for casing coupling

• Casing size Outside diameter and weight per foot
• Thread STC, LTC or BTC
• Grade J55, K55, N80, L80, P110 or Q125
• Sour service Hardness limit and NACE MR0175 / ISO 15156 requirements when required
• Documents EN 10204 3.1 or 3.2, thread gauging record and heat traceability

Download API 5CT Casing 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.