API 5CT OCTG Grade Guide

An enquiry for "7 inch N80 casing" cannot be filled. The grade letter gives yield strength, and the outside diameter gives one dimension, but a casing joint is also defined by its weight per unit length, which sets the wall; by its connection, which is frequently a proprietary design rather than an API thread; by its range, which sets the length band; and in some grades by its delivery condition. This API 5CT OCTG grade guide works through what completes the designation. Ranges are underAPI 5CT casing and tubing.

What the Grade Letter Covers, and What It Does Not

Element	Where it comes from
Minimum yield strength	The grade designation
Wall thickness	The weight per foot, not the grade
Burst and collapse capability	Yield strength and wall together
Joint strength	The connection, not the pipe body
Length	The range designation
Heat treatment	The grade in some cases, the type or suffix in others
Sour suitability	The grade, where it is specified for that purpose

The most common gap is weight. Two joints of the same grade and outside diameter with different weights have different walls and therefore different pressure capability, and neither is a substitute for the other.

The Common Grades

Grade	Character	Where it is used
J55 and K55	Same minimum yield strength	Surface and intermediate strings. K55 costs more for stronger round thread joints, not for a stronger body
N80	Two production routes under one designation	Type 1 is normalised; N80Q is quenched and tempered. The specification should say which
L80	Controlled hardness for sour service	The standard sour tubing grade. ISO 15156-2 lists L80 Type 1 at 23 HRC rather than the general 22
C90 and T95	Higher strength sour grades	Where L80 strength is insufficient but sour service still governs
P110	High strength	Deep strings. The strength that makes it useful also makes it vulnerable where a sour well runs cool
Q125	Higher again	Extreme depth and pressure

J55 and K55 share a minimum yield strength, so burst and collapse of the pipe body are the same. The price difference buys joint strength in round thread connections, which is a connection property rather than a body property.

N80 is the designation most often ordered incompletely. Type 1 and Q are produced differently and behave differently, particularly where a sour or low temperature consideration exists.

Sour Service Changes the Grade Logic

In sour wells the logic inverts relative to normal strength selection. Hardness rises with strength and the sour ceiling does not move, so:

L80, C90 and T95 exist as controlled hardness grades specifically for sour service
P110 is not a sour grade, and the risk is concentrated where a well runs cool, because lower temperature increases susceptibility
Going up in strength to solve a pressure problem makes the sour qualification harder, not easier
The region approach in ISO 15156-2 decides which grades are acceptable, using partial pressure and pH together

The mechanics are in theNACE MR0175 and ISO 15156 guide.

Connections Are Usually Not API

API 5CT defines threads such as short round, long round and buttress, and a great deal of modern casing and tubing uses proprietary premium connections instead, chosen for gas tightness, torque capacity or clearance.

For a buyer this means:

The connection is a separate specification item from the pipe, and may be a licensed design
Connection and coupling material have their own requirements, which in sour service include hardness
Mixing connection types across a string is a design decision, not a supply convenience
A grade substitution can affect the connection's rating

Range, Coupling and Finish

Item	What it sets
Range 1, 2 or 3	The length band of the joint
Coupling supplied or not	Whether couplings are included, and in what material
Thread protectors	Specified, because damage in transit is common
End finish	Plain end, threaded, threaded and coupled
Drift requirement	Whether a drift test to a stated diameter is required

What a Complete OCTG Enquiry Contains

Outside diameter
Weight per unit length, which sets the wall
Grade, with the type or delivery condition where the grade has one, such as N80 Type 1 or N80Q
Connection, by designation, and whether couplings are supplied
Range
Sour service requirement, with partial pressure, pH, chloride and temperature
Any supplementary requirements: impact testing, hardness testing, HIC
Certification level, from theMTC and EN 10204 guide

OCTG Grades: Buyer Questions

Why is grade and diameter not enough to order casing?

Because weight per unit length sets the wall, the connection sets joint strength, and range sets the length band. Two joints of the same grade and diameter with different weights are not substitutes.

What is the difference between J55 and K55?

They share a minimum yield strength, so pipe body burst and collapse are the same. K55 costs more for stronger round thread joint strength, which is a connection property.

Is N80 one product?

No. N80 Type 1 is normalised and N80Q is quenched and tempered. They are produced differently and the specification should state which is required.

Why is L80 the standard sour tubing grade?

It is a controlled hardness grade intended for sour service, and ISO 15156-2 lists L80 Type 1 at 23 HRC rather than the general 22 HRC limit.

Can a higher grade solve a sour pressure problem?

No. Hardness rises with strength while the sour limit does not move, so going up in strength makes qualification harder rather than easier.

Are casing connections covered by API 5CT?

API defines short round, long round and buttress threads, but much modern casing uses proprietary premium connections, which are a separate specification item.

Applying This to an Enquiry

Send outside diameter, weight, grade with its type or condition, connection and range together. Any one of those missing turns the enquiry into a question rather than a quotation, and on sour wells the condition matters as much as the grade.

Quotations are available in USD, OMR, AED, SAR, QAR, KWD and BHD, on EXW, FOB, CFR or CIF terms, with EN 10204 3.1 certificates as standard and 3.2 on request. Lead time is confirmed at quotation.

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API 5CT OCTG Grades: Reading the Designation