Tubing is specified by three numbers - outside diameter, mass per unit length and grade - and the third is the only place where the service environment enters the order. Grade is not a quality tier; it is a yield strength, and choosing it means deciding how much load the string will carry and what the produced fluid will do to it. Getting it wrong is expensive in both directions: too low and the string fails, too high and the completion pays for strength and corrosion resistance it does not need.
Grade Numbers Are Yield Strength in Thousand psi
In API 5CT the number in each grade designation is the minimum yield strength in thousand psi, which makes the ladder easy to read:
J55 and K55: 55,000 psi minimum yield, about 379 MPa, with 75,000 psi minimum tensile
N80: 80,000 psi yield, 552 MPa, with 100,000 psi minimum tensile
P110: 110,000 psi yield, 758 MPa, with 125,000 psi minimum tensile
The ladder is not a progression of quality. It is a progression of strength, with specific service conditions attached to each step, and the top of the ladder is not the right answer for most wells.
Where Carbon Grades Stop and Alloy Grades Start
Carbon and carbon-manganese grades cover routine completions. Moving up is driven by load first and by environment second. A shallow, low-pressure well has no reason to carry the cost of an alloy grade, while a deep well with a heavy fluid column needs the tensile margin. The decision point is usually the combination of depth and fluid gradient rather than depth alone, because the string carries its own weight plus the pressure loads applied during stimulation, gas lift or shut-in. N80 in the quenched and tempered condition is the common middle choice; P110 is specified where the completion is deep enough that wall thickness would otherwise become impractical and the connection would become the limiting component.
API 5CT Tubing for Sour and Corrosive Service
Corrosive service changes the problem. L80 is supplied in several types, including a 13 percent chromium variant for carbon dioxide and mildly sour service and a 9 percent chromium variant, and it is hardness controlled to resist sulphide stress cracking, with a maximum of 23 HRC. That hardness limit is the specification worth checking on a sour order, because strength and hardness move together and a grade chosen only for yield can fail the cracking requirement. Where hydrogen sulphide is present, grade selection and hardness verification have to be read together rather than independently, and the certificate is the only place the two are reconciled.
End Finish, Threads and Connection Choice
Tubing is supplied with non-upset ends and a coupling, with external upset ends where higher joint strength is needed, or as an integral joint where the connection is machined directly into the wall. Threads and couplings are gauged to API Specification 5B, so thread form, pitch diameter and stand-off are checked with gauges rather than assessed visually. Premium connections with metal-to-metal seals are available for gas-tight and high-pressure completions and are qualified by the connection manufacturer's own test protocol, which means the connection is a separate qualification exercise from the pipe body and has to be ordered as one.
API 5CT Tubing Length, Tolerance and Inspection
Lengths are ordered in three ranges: R1 from 6.10 to 7.32 meters, R2 from 8.53 to 9.75 meters and R3 from 11.58 to 12.80 meters, with R2 the most widely stocked because it suits most rig configurations and transport limits. Dimensional tolerance matters as much as grade. Wall thickness carries a minus 12.5 percent tolerance, which directly reduces the burst and collapse margin of the lightest weight that can be ordered, and outside diameter tolerance changes with size. Hydrostatic testing is performed on every length unless the order says otherwise, full-body non-destructive inspection is available on order, and drift testing confirms the internal diameter that the rest of the completion has to pass through.
Grade is a service decision, not a quality decision. API 5CT tubing specified on yield strength against load, on the 23 HRC limit against hydrogen sulphide, and on wall tolerance against burst margin, gives a completion that matches the well rather than over- or under-specifying it.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
