Oil Country Tubular Goods (OCTG) : Overview

OCTG include three types of seamless tubes, delivered in quenched and
tempered condition:

• Drillpipe – heavy seamless tubes that rotate the drill
  bit and circulate the drilling fluid. Joints of pipe 30 ft (9m) long are
  coupled together with tool joints


• Casing pipe is used to line the hole.


• Tubing – a pipe through which the oil or gas is produced
  from the wellbore. Tubing joints are generally around 30 ft [9 m] long with a
  thread connection on each end.

Oil Country Tubular Good (OTCG) Steel Grades Low Alloy Grades
Yield strength (ksi)	API Grade Code	% Alloy content						Yield Strength (0,2% proof stress) (N/mm2)	Tensile Strength min (N/mm2)
		C	Mn	Ni	Cr	Mo	Cu
40	H40	0.5	1.5					276-552	410
55	K55	0.5	1.5					379~552	655
75	C75-1	0.5	1.7	0.5	0.5	0.40	0.5	517~620	665
90	C90-1	0.35	1.9	0.9	1.2	0.75		620~724	690
95	T95-1	0.35	1.2	0.9	1.5	0.85		655~758	724
125	Q125	0.35	1	0.9	1.2	0.75		860~1035	930
140		0.3	1		1.6	1.1	0.05	965~1171	1034

Table 1: Chemical composition and strength properties of common low alloy
OCTG steels

Traditionally the grades used for for OCTG applications 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 with contaminants causing
corrosive attack have created strong demand for higher strength materials
resistant to hydrogen embrittlement and sulphide stress cracking (SCC).

Highly tempered martensite has been identified as the structure which is most
resistant to SCC at higher strength levels, and 0.75% has been found to be the
Mo concentration to obtain the optimum combination of yield strength and
resistance to SCC ⁽¹⁾.

This is reflected in the list of Mo containing low alloy API standard grades
in Table 1 above. For the 75 ksi strength level 0.4% Mo is sufficient, while
each of the the higher strength grades up to 125 ksi show the optimum Mo level
of 0.75 or 0.80 % .

For higher strength up to 140 ksi (yield strength 965-1171 MPa) dispersion
has been introduced as an additional strengthening mechanism by the addition of
niobium (columbium). The according grade at the end of the list in table 1 is a
non API specialized grade with 0.05% niobium; the molybdenum range is extended
to 1.1%.

Oil Country Tubular Good (OTCG) Steel Grades Stainless Steels
Yield strength (ksi)	API Grade Code	% Alloy content						Yield Strength (0,2% proof stress) (N/mm2)	Tensile Strength min (N/mm2)
		C	Mn	Ni	Cr	Mo	Cu
9% Chromium Stainless
75	C75-9Cr	0.15	0.6	0.5	9	1	0.25	517~620	665
13% Chromium Stainless
80	L80-13Cr	0.22	16	0.5	13		0.25	552~655	655
95/110		0.04 max	0.6	4	13	1.5
95/111		0.04 max	0.6	5	13	2.5

Table 2: Chemical composition and strength properties of OCTG stainless
steels

For service in oil and gas fields with more aggressive corrosion environments
stainless API grades are standardized with 9% Cr, 1% Mo and 13% Cr (without Mo).
(Table 2).

For high temperature environments with CO2 and H2S, the non API specialized
grades shown in table 2 with improved corrosion and SCC resistance have been
developed ⁽²⁾. The reduced C content increases Cr in solid solution,
which effectively improves the corrosion resistance. Ni and Mo secure both hot
workability and corrosion resistance. In particular the addition of Mo improves
the pitting corrosion, thereby eliminating initiation sites for SCC.

Owing to its higher Mo content the 2% Mo grade can be used in lower pH and
higher H2S environments.

Literature

(1) J.A. Straatmann, A.P. Grobner in ‘Molybdenum containing steels for Gas
and Oil Industry Applications’ Climax Molybdenum Company , 1978.

(2) Dishimaru et al. in JFE Technical Report No.2, (Mar 2004)

source: http://www.imoa.info/

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What is OCTG

What does OCTG stands for ?

OCTG stands for Oil Country Tubular Goods.

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