Oil Country Tubular Goods (OCTG)

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)