API 5CT K55 Oil Country Tubular Goods

According to API Spec 5CT, K55 is a Group 1 steel grade and one of the most widely used Oil Country Tubular Goods (OCTG) for shallow to medium-depth oil and gas wells, available in both casing and tubing forms. In terms of mechanical properties, K55 features a yield strength ranging from 379 MPa to 552 MPa (55 ksi to 80 ksi) and a minimum tensile strength of 655 MPa (95 ksi).

Although its chemical requirements for impurities are identical to those of grade J55-limiting both phosphorus and sulfur contents to ≤0.030%-K55 provides a higher tensile strength. K55 is typically supplied in as-rolled, normalized, or normalized-and-tempered conditions. It is a standard choice for oilfield development, striking an optimal balance between cost-effectiveness and medium-strength performance, though it is not designed for sour service environments containing hydrogen sulfide (H2​S).

 

API 5CT K55 OCTG Color Identification Table

Component	Color	Application Method	Remarks
Pipe Body	Bright Green	Two Bands	Painted within 0.6m (24") of the end
Coupling	Bright Green	Solid Color	Entire outside surface is painted
Accessories	Bright Green	Solid Color	Applies to pup joints, connectors, etc.

 

Chemical Composition Table for API 5CT K55 casing pipes (Mass Fraction, %)

According to API Spec 5CT (Group 1), the chemical requirements for K55 are relatively flexible, focusing primarily on the control of impurities (Phosphorus and Sulfur).

Element Name	Symbol	Requirement (Mass Fraction %)	Remarks
Carbon	C	Not Specified	Controlled by manufacturer to meet mechanical properties
Manganese	Mn	Not Specified	Controlled by manufacturer to meet mechanical properties
Phosphorus	P	≤0.030≤0.030	Ladle / Product Analysis maximum
Sulfur	S	≤0.030≤0.030	Ladle / Product Analysis maximum
Silicon	Si	Not Specified	-
Copper	Cu	Not Specified	-
Nickel	Ni	Not Specified	-
Chromium	Cr	Not Specified	-

 

Mechanical Properties of K55 Oil Well Drilling Tube

Property	Metric Units	US Customary Units
Yield Strength	379 ~ 552 MPa	55,000 ~ 80,000 psi
Tensile Strength	≥655≥655 MPa	≥95,000≥95,000 psi
Hardness	Not Specified	No mandatory hardness limit

 

Delivery Condition / Heat Treatment of API 5CT K55 Oilfield Casing

K55 casing and tubing are typically delivered in the following states:

As-rolled (No heat treatment, if properties are met via rolling).

Normalized (N).

Normalized and Tempered (N&T).

K55 is an economical, mid-strength OCTG grade. Its chemical management focuses on "Low Phosphorus and Low Sulfur," while other alloying elements are determined by the manufacturer to satisfy the required strength levels.

 

API 5CT K55 Casing Size Chart

Casing is used to stabilize the wellbore. K55 casing is commonly available with BTC (Buttress Thread), LTC (Long Round Thread), or STC (Short Round Thread) connections.

Outside Diameter (OD)	OD (mm)	Nominal Weight (lb/ft)	Wall Thickness (mm)	Drift Diameter (mm)
4-1/2"	114.30	9.50 / 10.50 / 11.60	5.21 / 5.69 / 6.35	101.27 / 100.31 / 98.99
5"	127.00	11.50 / 13.00 / 15.00	5.59 / 6.43 / 7.52	113.11 / 111.43 / 109.25
5-1/2"	139.70	14.00 / 15.50 / 17.00	6.22 / 6.91 / 7.72	124.56 / 123.19 / 121.57
6-5/8"	168.28	20.00 / 24.00	7.32 / 8.94	150.93 / 147.68
7"	177.80	20.00 / 23.00 / 26.00	6.91 / 8.05 / 9.19	161.27 / 158.98 / 156.70
7-5/8"	193.68	24.00 / 26.40 / 29.70	7.62 / 8.33 / 9.53	175.74 / 174.32 / 171.91
8-5/8"	219.08	24.00 / 28.00 / 32.00	6.71 / 7.72 / 8.94	203.02 / 200.99 / 198.55
9-5/8"	244.48	36.00 / 40.00	8.94 / 10.03	223.88 / 221.70
10-3/4"	273.05	40.50 / 45.50	8.89 / 10.16	252.32 / 249.79
13-3/8"	339.72	54.50 / 61.00 / 68.00	9.65 / 10.92 / 12.19	317.50 / 314.96 / 312.42
16"	406.40	65.00 / 75.00 / 84.00	9.53 / 11.13 / 12.57	384.45 / 381.25 / 378.37
20"	508.00	94.00 / 106.50 / 133.00	11.13 / 12.70 / 16.13	482.85 / 479.71 / 472.85

 

Which Casing Thread Connection Should You Choose?

Thread failure is one of the most common causes of casing leakage and field operational failures.

STC (Short Thread Coupling)

‌Optimal Application Scenarios‌:

Shallow wells

Low-load requirements

Cost-sensitive projects

 

LTC (Long Thread Coupling)

‌Optimal Application Scenarios‌:

Medium-deep wells

Working conditions requiring improved tensile performance

 

BTC (Buttress Thread Coupling)

‌Optimal Application Scenarios‌:

Deep wells

High axial load requirements

More demanding drilling conditions

For most export projects of GNEE, BTC is the preferred choice of many customers, thanks to its excellent bearing capacity.

 

API 5CT K55 Tubing Size Chart

Tubing is used to transport oil or gas to the surface. K55 tubing typically uses NUE (Non-Upset End) or EUE (External Upset End) connections.

Outside Diameter (OD)	OD (mm)	Nominal Weight (lb/ft)	Wall Thickness (mm)	Drift Diameter (mm)
1.050"	26.67	1.14 / 1.20	2.87 / 3.18	19.33 / 18.72
1.315"	33.40	1.70 / 1.72	3.38 / 3.56	25.12 / 24.77
1.660"	42.16	2.30 / 2.33	3.56 / 3.68	33.45 / 33.22
1.900"	48.26	2.75 / 2.76	3.68 / 3.91	39.35 / 38.89
2-3/8"	60.33	4.60 / 4.70	4.83	48.36 / 48.36
2-7/8"	73.03	6.40 / 6.50	5.51	59.61 / 59.61
3-1/2"	88.90	9.20 / 9.30	6.45	72.82 / 72.82
4"	101.60	9.50	5.74	87.71
4-1/2"	114.30	12.60 / 12.75	6.88	97.97 / 97.97

 

Which tubing Thread Connection Should You Choose?

When selecting a tubing thread connection for API 5CT K55 (or other OCTG tubing), the choice depends on well depth, pressure, load type, and operational requirements.

EUE (External Upset End) Thread

Standard tubing connection where the tubing body is upset externally at the thread to allow proper engagement with the coupling.
Best For: Most conventional oil, gas, or water wells.
Advantages:
Widely available and standardized
Good tensile strength
Compatible with most casing and tubing sizes
Limitations:
Not designed for extremely high axial loads

 

NUE (Non-Upset End) Thread

Tubing body has uniform OD; thread is machined directly without upsetting the end.
Best For: Shallow wells or low-pressure service.
Advantages:
Lower cost
Simple machining
Limitations:
Lower tensile capacity than EUE
Not recommended for deep or high-load wells

 

K55 Casing vs K55 Tubing

Many buyers use these terms interchangeably, but they perform different functions.

Item	K55 Casing	K55 Tubing
Primary Function	Support wellbore	Transport production fluids
Installation Position	Outer string	Inside casing
Design Load	Collapse and external pressure	Internal pressure
Wall Thickness	Generally heavier	Generally lighter
Common Connections	BTC, LTC, STC	EUE, NUE

 

K55 vs J55: What Is the Difference?

One of the most common questions from overseas buyers is whether K55 and J55 are the same.

Property	J55	K55
Yield Strength	55–80 ksi	55–80 ksi
Tensile Strength	75 ksi min	95 ksi min
API Standard	API 5CT	API 5CT
Typical Application	Conventional wells	Conventional wells
Price	Slightly lower	Slightly higher

 

Inspection Requirements for API 5CT K55 Petroleum Casing Pipe

Before shipment, reputable manufacturers typically perform the following inspections.

Dimensional Inspection

OD measurement

Wall thickness measurement

Length verification

Thread inspection

Mechanical Testing

Yield strength test

Tensile strength test

Hardness test

Non-Destructive Testing (NDT)

Ultrasonic Testing (UT)

Electromagnetic Inspection (EMI)

Thread inspection

Hydrostatic Testing

Hydrostatic testing verifies pressure integrity and leak resistance before shipment.

 

API 5CT K55 BTC Casing inspection

 

API 5CT K55 OCTG Certificate

 

FAQ

Is K55 Suitable for Deep Wells?

K55 is commonly used in shallow to medium-depth wells.

For high-pressure or deep wells, operators often select:

N80

L80

P110

The final grade selection should be based on collapse, burst, and tensile calculations rather than price alone.

 

Can K55 Be Used in H₂S Service?

Standard K55 is generally not recommended for severe sour-service environments.

For wells containing hydrogen sulfide (H₂S), buyers should evaluate grades that comply with sour-service requirements.

 

What is API 5CT K55 casing used for?

K55 casing supports the wellbore and isolates geological formations during drilling and production.

 

What is API 5CT K55 tubing used for?

K55 tubing transports oil, gas, water, or production fluids from the reservoir to the surface.

 

Is K55 stronger than J55?

K55 and J55 have the same yield strength range, but K55 has a higher minimum tensile strength requirement under API 5CT.

 

Can K55 replace N80?

No. N80 offers higher strength and is designed for more demanding well conditions.

 

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