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Technical Library > PIPES > PIPES MATERIALS > API 5CT OCTG Pipes Materials
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OCTG PIPES API 5CT


In this article, we explain the meaning of the OCTG pipes (which stands for “oil country tubular goods”), give some indications about the differences between casing and tubing pipes and their manufacturing processes, show the chemical composition of the common API 5CT material grades including their mechanical properties, and production tolerances.


OCTG PIPES

OCTG (which stands for “Oil country tubular goods”) is a family of tubular that includes drill pipes and casing/tubing pipes used in upstream oil and gas operations:

Casing Pipes API 5CTCasing pipes are used on the external side of the borehole. The casing is particularly exposed to axial tension and internal pressure by the pumped oil or gas emulsion. They are also subject to additional axial tensions by their dead weight and the external pressures coming from the surrounding rocks

 

 

Tubing pipes API 5CTTubing pipes are tubular goods through which the oil or gas is transported from the wellbore to the surface. Tubing segments are generally around 30′ long with a threaded connection on each end (standard or premium connections are available)

 

 

Drill PipesDrill pipes are heavyweight seamless tubular that rotates the drill bit and circulates the drilling fluid. Pipe segments of 30′ are coupled with tool joints. Drill pipe is subject to high torque by drilling, axial tension by their weight, and internal pressure due to the purging of the drilling fluid. Additionally, alternating bending loads due to non-vertical or deflected drilling may be superimposed on these basic loading patterns

 

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API 5CT CASING AND TUBING PIPES

CASING/TUBING PIPES GENERAL INFORMATION

The API 5CT standard covers seamless and welded casing and tubing pipes for upstream operations. Casing pipes are available in the diameter range 4 1/2 . to 20 inches, and in the following materials: H-40/J-55/K-55/N-80/L-80/C-90/T-95/P-110/Q-125.

The main types of connections for casing pipes are STC (short threads), LTC (long threads), BTC (buttress threads) and premium gas-tight connections.

Tubing pipes are manufactured in seamless and welded execution, in the size range 1.050 to 5 1/2 inches and in the following material grades: H-40 / J-55 / K-55 / N-80 / L-80 / C-90/ T-95 / P-110 / Q-125. The main types of connections for tubing pipes are NUE (non-upset), EUE (external upset) and premium. Corrosion resistance under sour service conditions is a very important OCTG characteristic, especially for casing and tubing.

OCTG pipes

The manufacturing processes of casing and tubing pipes include (all dimensional ranges are approximate):

  • Continuous mandrel-rolling process and the push bench process for sizes between 21 and 178 mm OD.
  • Plug mill rolling for sizes between 140 and 406 mm OD.
  • Cross-roll piercing and Pilger rolling for sizes between 250 and 660 mm OD.

These processes typically do not allow the thermomechanical processing customary for the strip and plate products used for welded pipes. Therefore, the high-strength seamless pipe must be produced by increasing the alloying content in combination with a suitable heat treatment such as quench & tempering.

Meeting the fundamental requirement of a fully martensitic microstructure even at large pipe wall thickness requires good hardenability. Cr and Mn are the main alloying elements used to produce good hardenability in conventional heat-treatable steel. However, the requirement for good sulfide stress cracking (SSC) resistance limits their use. Mn tends to segregate during continuous casting and can form large MnS inclusions that reduce hydrogen induced cracking (HIC) resistance. Higher levels of Cr can lead to the formation of Cr7C3 precipitates with coarse plate-shaped morphology, which acts as hydrogen collectors and cracks initiators. Alloying with Molybdenum can overcome the limitations of Mn and Cr alloying. Mo is a much stronger hardener than Mn and Cr, so it can easily recover the effect of a reduced amount of these elements.

Traditionally, OCTG grades were carbon-manganese steels (up to the 55-KSI strength level) or Mo-containing grades up to 0.4% Mo. In recent years, deep well-drilling and reservoirs containing contaminants that cause corrosive attack have created a strong demand for higher strength materials resistant to hydrogen embrittlement and SCC. Highly tempered martensite is the structure most resistant to SSC at higher strength levels, and 0.75% is the Mo concentration that produces the optimum combination of yield strength and to SSC resistance

 

CASING/TUBING PIPES MATERIAL GRADES (API 5CT)

Group Grade Type C Mn Mo Cr Ni max. Cu max. P max. S max. Si max.
min. max. min. max. min. max. min. max.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1
H40
0.03
0.03
J55
0.03
0.03
K55
0.03
0.03
N80
1
0.03
0.03
N80
Q
0.03
0.03
R95
0.45 c
1.9
0.03
0.03
0.45
2
M65
0.03
0.03
 
L80
1
0.43 a
1.9
0.25
0.35
0.03
0.03
0.45
 
L80
9Cr
0.15
0.3
0.6
0.9
1.1
8
10
0.5
0.25
0.02
0.01
1
 
L80
13Cr
0.15
0.22
0.25
1
12
14
0.5
0.25
0.02
0.01
1
 
C90
1
0.35
1.2
0.25 b
0.85
1.5
0.99
0.02
0.01
 
T95
1
0.35
1.2
0.25 d
0.85
0.4
1.5
0.99
0.02
0.01
 
C110
0.35
1.2
0.25
1
0.4
1.5
0.99
0.02
0.005
3
P110
e
0.030 e
0.030 e
4
Q125
1
0.35
1.35
0.85
1.5
0.99
0.02
0.01
a The carbon content for L80 may be increased up to 0.50 % maximum if the product is oil-quenched.
b The molybdenum content for Grade C90 Type 1 has no minimum tolerance if the wall thickness is less than 17.78 mm.
c The carbon content for R95 may be increased up to 0.55 % maximum if the product is oil-quenched.
d The molybdenum content for T95 Type 1 may be decreased to 0.15 % minimum if the wall thickness is less than 17.78 mm.
e For EW Grade P110, the phosphorus content shall be 0.020 % maximum and the sulfur content 0.010 % maximum.
NL = no limit. Elements shown shall be reported in product analysis.

The steel grades of common oilfield API-5CT casing pipe include J55, K55, N80-1, N80Q, and P110. Casing pipe is mostly used for oil well drilling. In accordance with API-5CT and ISO11960 standard, pipes can be classified as STC (short thread), LTC (long thread) and BTC (buttress thread) accompanied with their couplings.

Oilfield casing pipes are positioned into the wellbore and cemented in place to secure both subsurface formations and the wellbore from collapsing, also to enable drilling mud to circulate and extraction to take place. Casing pipes are normally manufactured using outside dimension sizes from 114.3 mm to 508.0 mm. The strict quality requirement for steel is due to the harsh working conditions of the casing. The steel product should be produced and checked in accordance with special standards or specifications. ISO 11960 and API Spec 5CT have specified the steel product standards of the casing.

L80 CASING

L80 belongs to steel grades group of the corrosion-resistant casing. L80 API oilfield casing pipe includes L80-1, L80-9Cr, and L80-13Cr. L80-1 is used in the H2S environment, while L80-9Cr and L80-13Cr are used in the CO2 environment. In the corrosive environment in which carbon dioxide predominates, Super 13Cr has a higher corrosion resistance than L80-13Cr. With a relatively high price, the product applies to more complicated geological conditions and runs down deeper into the well. In the exploitation of oil and gas, the use of L80 is less frequent than J55, N80, and other materials.

J55 CASING

The J55 API-5CT casing pipe is a comparatively low steel grade in oil drilling. It is widely applied to shallow oil and gas extraction.Because of its low cost among other grades of steel, it enjoys wider applications,  can be generally used in shallow wells, geothermal wells, and water wells.

N80 CASING

API-5CT N80 oilfield casing pipe contains N80-1 and N80-Q types. Those two materials are absolutely consistent regarding the chemical composition and mechanical attributes, they are just variations in the heat treatment. During the heat treatment, N80-1 steel is treated by normalizing and tempering, while N80Q steel is treated by quenching and tempering. Therefore, the collapsing strength and internal pressure strength of N80Q are higher than that of N80-1. N80-1 or N80Q should be clearly shown by the designer when N80 casing is selected.As its mechanical properties are higher than J55 and K55 types, it may very well be utilized to certain a little more challenging formations. With deeper downhole depth than J55 and K55 types, it can be applied for the exploitation of middle-level oil and gas. Due to the high capability of the N80 material, those API-5CT casing pipes are broadly applied in natural gas as well as coal bed methane extraction, and geothermal wells.

P110 CASING

The P110 API-5CT steel casing pipe is the high grade amongst other grades of steel.  The white band is marked on the finished casing to point out the materials. During petroleum drilling and producing, this pipe is generally used in a variety of complicated territory due to its higher tensile strength and higher yield.It goes down the deepest into the well among other types. However, the price is comparatively high, as well as the quantity of usage is comparatively few. P110 casing pipe is mainly used in a particular surrounding and when the drilling depth extends to a particular level.

 

CASING/TUBING PIPES MECHANICAL PROPERTIES

Group Grade Type Total elongation under load % Yield strength MPa Tensile strength min. MPa Hardness a max. Specified wall thickness mm Allowable hardness variation b HRC
min. max .   HRC HBW
1 2 3 4 5 6 7 8 9 10 11
1
H40
0.5
276
552
414
J55
0.5
379
552
517
K55
0.5
379
552
655
N80
1
0.5
552
758
689
N80
Q
0.5
552
758
689
R95
0.5
655
758
724
2
M65
0.5
448
586
586
22
235
L80
1
0.5
552
655
655
23
241
L80
9Cr
0.5
552
655
655
23
241
L80
13Cr
0.5
552
655
655
23
241
C90
1
0.5
621
724
689
25.4
255
≤ 12.70 12.71 to 19.04 19.05 to 25.39 ≥ 25.40
3.0 4.0 5.0 6.0
T95
1
0.5
655
758
724
25.4
255
≤ 12.70 12.71 to 19.04 19.05 to 25.39 ≥ 25.40
3.0 4.0 5.0 6.0
C110
0.7
758
828
793
30
286
≤ 12.70 12.71 to 19.04 19.05 to 25.39. ≥ 25.40
3.0 4.0 5.0 6.0
3
P110
0.6
758
965
862
4
Q125
1
0.65
862
1034
931
b
≤ 12.70 12.71 to 19.04 ≥ 19.05
3.0 4.0 5.0
a In case of dispute, laboratory Rockwell C hardness testing shall be used as the referee method.
b No hardness limits are specified, but the maximum variation is restricted as a manufacturing control in accordance with 7.8 and 7.9 of API Spec. 5CT.

 

CASING/TUBING PIPES TOLERANCES

Execution Outer Diameter Wall Thickness
Cold rolled Tube sizes(mm) Tolerances(mm)
<114.3 ±0.79 -12.5%
≥114.3 -0.5%,+1%

 

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