API 5CT K55 STC Casing

API 5CT K55 STC (Short Thread Coupled) casing is one of the most widely used tubular products in the oil and gas industry, classified as a Group 1 steel grade under the API Spec 5CT standard. K55 is renowned for its high cost-effectiveness, tensile strength superior to that of J55, and its suitability for production operations in medium-to-shallow depth oil and gas wells.
API 5CT: The American Petroleum Institute's standard governing casing and tubing.
K55 Steel Grade: Possesses the same yield strength as J55 (55,000 psi) but features a higher tensile strength (95,000 psi); this attribute gives it an advantage over J55 in deep wells or complex geological formations subject to significant tensile loads.
STC (Short Thread Coupled): A connection type utilizing short round threads. It offers excellent sealing performance and ease of handling, making it suitable for environments characterized by moderate depths and pressures.

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API 5CT K55 STC Casing pipe Chemical Composition

API 5CT K55 STC OCTG Mechanical Property

Size Chart and Tolerance for API 5CT K55 STC Oil well Casing

API 5CT Casing Tolerances‌

‌OD Tolerance‌:

D < 4-1/2": ‌±0.79 mm‌

D ≥ 4-1/2": ‌+1.00% / -0.50%‌

‌Wall Thickness‌: ‌-12.5%‌ (minimum allowable under tolerance)

‌Weight‌: ‌+6.5% / -3.5%‌ per single joint

‌Straightness‌: Total length curvature shall not exceed ‌0.2%‌ of the total length

Manufacturing & Heat Treatment

Manufacturing Types
Seamless (SML): Produced via piercing and rolling; contains no welds.
Electric Resistance Welded (ERW): Produced via high-frequency welding. For Grade K55, the ERW weld seam must undergo heat treatment.
Heat Treatment Methods
As-Rolled: Grade K55 is permitted to be delivered directly in the hot-rolled condition (without additional heat treatment).
Normalizing: Heated above the critical temperature and cooled in air to refine the grain structure.
Stress Relieving: Localized heat treatment applied specifically to the ERW weld seam.

Non-Destructive Testing & Physicochemical Analysis (Testing & Inspection)

Non-Destructive Testing (NDT)

API 5CT mandates that casing undergo the following inspections to ensure the pipe body is free of defects:
Hydrostatic Test: 100% pressure testing to ensure no leakage occurs under the specified pressure.
Ultrasonic Testing (UT): Detects internal defects within the pipe body (applicable to PSL2 level).
Magnetic Particle Inspection (MPI): Checks for minute cracks at the pipe ends and on the surface.
Electromagnetic Testing (ET): Serves as an auxiliary method for detecting longitudinal defects.

Charpy Impact Test

Requirement: K55 is classified as a basic grade within API 5CT Group 1.
Stipulation: Unless the buyer explicitly requests the application of PSL2 or PSL3 standards in the contract, the standard K55 grade (PSL1) does not mandatorily require a Charpy Impact Test. If PSL2 standards are applied, specific absorbed energy requirements (determined by wall thickness) must be met at 0°C.

Reference Inspection Items

Visual Inspection: Visual examination of the internal and external surfaces of the pipe.
Dimension Check: Measurement of outside diameter, wall thickness, and length.
Drift Test: Verification using a drift gauge to ensure that downhole tools can pass through smoothly.
Thread Inspection: Examination of STC threads to verify thread profile, lead, taper, and standoff.

Application for API 5CT K55 STC

K55 is considered the "economical" choice in oilfield operations; its specific applications are as follows:

• Shallow to Medium-Depth Wells: Commonly used for surface casing or intermediate casing strings at depths of approximately 2,000 to 3,000 meters.
• Low-Pressure Formations: Suitable for oil and gas fields characterized by low formation pressure gradients and an absence of severe tectonic activity.
• Non-Sour Environments: K55 lacks resistance to H2S corrosion (it is not sulfur-resistant); therefore, it is utilized exclusively in non-sour oil and gas wells.
• Onshore Operations: Due to its low cost and reliable strength, it is the most widely used steel grade in conventional onshore oil and gas production.
• Coalbed Methane and Water Wells: It is also extensively applied in the development of coalbed methane fields and in deep water wells.

GNEE API 5CT K55 STC OCTG Pipe factory equipments

GNEE API 5CT K55 STC Casing test equipments

GNEE API 5CT K55 STC Casing Certificate

FAQ

Is K55 carbon steel?

 Yes, K55 is carbon steel.
K55 steel pipe is classified as carbon structural steel and is primarily used for oil well drilling casing and tubing. Its material composition complies with the API 5CT standard; it possesses excellent strength and machinability, is typically manufactured as seamless pipe, and is suitable for downhole environments involving moderate loads.

What is stc in pipes?

STC stands for "Short Thread Connector"-specifically, a short round-thread connection-and represents a type of coupling used in API casing, commonly employed in the drilling and transport operations of oil and gas wells.
It features a short round-thread design with a 60° thread profile angle and a taper of 1:16. Sealing is achieved through contact between the thread flanks, making it suitable for shallow wells or low-to-medium pressure applications; furthermore, it is easy to manufacture and offers lower production costs.

What is the difference between STC and LTC threads?

The primary difference between STC (Short Round Thread) and LTC (Long Round Thread) lies in their thread length and connection strength. STC threads are shorter and are suitable for shallow wells or low-stress environments, offering a connection strength of approximately 60%–80% of the pipe body's strength; LTC threads are longer, providing superior connection strength and better sealing performance, making them suitable for medium-to-deep well applications involving moderate axial loads.

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