heavy wall casing in c110 grade
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HEAVY WALL CASING IN Cl10 GRADE
FOR SOUR SERVICE
C.P. LINNE, F. BLANCHARD, F. PUISSOCHET Vallourec Research Center
Corrosion & Metallurgical Department P.O. Box 17
59620 Aulnoye Aymeries, FRANCE
B.J. ORLANS-JOLIET R.S. HAMILTON Vallourec & Mannesmann Tubes Tubular Industries Scotland Ltd
Vallourec Mannesmann Oil & Gas France Imperial threading works OCTG Division Clydesdale heat treatment plant 23 rue de Leval Airdrie, SCOTLAND
59620 Aulnoye Aymeries, FRANCE
The recent developments of high pressure and sour wells in the North Sea area have increased the need for high strength H2S resistant carbon steels. Steel chemistry and heat treatment solutions have been available to provide products suitable for use in these environments within the constraints of classic well design since the early 90s but operators are now demanding higher strength and heavier wall products for HPHT wells.
Well completion design teams are now specifying from OCTG suppliers C 110 grade products in increasingly heavy wall and the challenge facing suppliers is to guarantee product integrity not only of these heavy wall casing but also the associated coupling stocks.
This paper was aimed at evaluating the performances of thick walled C 110 tubulars (up to 2) for sour environments. Metallurgical characteristics (microstructure, structure, microhardness), mechanical properties (hardness, tensile, toughness), Sulfide Stress Cracking resistance (smooth tensile, DCB) have been investigated throughout the wall thickness.
The C 110 proprietary grade proved to be an excellent material for use as Oil Country Tubular Goods (OCTG) in typical North Sea environments with improved assessment of H2S corrosion resistance properties according to both NACE and EFC (European Federation of Corrosion) philosophies.
The need for higher strength Sulfide Stress Cracking (SSC) resistant steels has become more apparent with the increasing energy demands and the decrease of easily obtained sweet oil and gas reserves. Oil fields now being explored in the USA and gas fields in the North Sea area require drilling to depths beyond 5500 m with bottom hole pressures and temperatures greater than 1000 bar and 2OOC, where hydrogen sulfide is often found in the crude oil and gas ; moreover the static well-head pressure is expected to be around 800 bar. Such depths and pressures represent the extreme limits for the use of C95 casing. Actually, wall thickness would be so wide and gaps so narrow that there will be a serious probability of uncontrolled casing wear during the drilling operations. Therefore well engineering departments have applied the pressure on the suppliers for the development of a Cl 10 casing grade, which could be used safely. It is important, from the economic aspect as well as that of safety, that appropriate materials are used to successfully withstand the demands made upon them [l]. The choice of material is dependent on adequate mechanical properties whilst ensuring their integrity in the service environment. As for the development of Central Graben area (North Sea), the conditions defined by 30-60 bar of CO2 (i.e. a pH below 3.5) and 30-50 mbar of H2S represent a sour service environment , much beyond the limits of any standard Pl 10. The development of deep high pressure high temperature (HPHT) wells of sour gas has always raised the problem of the incompatibility between high strength steels and a good resistance to SSC. The Cl 10 proprietary grade for sour service proposed in the early 90s  proved to be an interesting alternative assuring both a minimum threshold stress of 85% SMYS according to NACE TM0177 standard and a potential reduction of the string weight of about 25% . The new trends are to extend the application limits of Cl 10 casing in extremely high BHP deep reservoirs inducing a high burst requirement i.e. heavy wall casing (higher than 1 WT) associated with the corresponding coupling stock (as thick as 2 ). This need is also highlighted by additional items such as casing hangers and crossovers which are also thick-walled components. Hence, the intention of the present study was to assess the feasability of heavy wall casing and coupling stock in Cl 10 grade without impairing the mechanical and corrosion properties so research was conducted on the variation of both microstructure, toughness and SSC resistance as a function of the wall position in thick-walled tubulars.
Three pipes, processed commercially, from 3 different heats were included in the investigations : one casing length 10 314 OD x 1.05 WT (273 mm x 26.67 mm) and 2 coupling stocks 289 mm OD x 37.8 mm WT, and 3 12 mm OD x 49 mm WT. The products were manufactured via a BOS / electric arc furnace + ladle furnace + vacuum degassing + continuous casting route and seamless rolling mill. To reach the required mechanical properties, i.e. a minimum yield strength of 110.000 psi, the heat treatment was optimized according to quench and tempering steps. The combination of steel chemistry design (Chromium-Molybdenum and microalloying element additions) and external/internal quenching was designed to achieve both high hardenability and, hardening and a high tempering temperature (69OC) to give optimum mechanical properties and SSC resistance.
Elemental analysis of the steels was determined using the glow discharge spectrometer technique. As for C and S contents, the LECO t&ion technique was involved as a more accurate means of determining these. Reported chemical compositions are shown in Table 1.
Mechanical tests. The actual yield strength values (0.2% offset) were measured longitudinally by tensile tests according to ASTM ES standard. O1Omm and 05mm round bar specimens were taken respectively at midwall thickness (MW) and throughout the thickness (OD-MW-ID) as described on figure 1.
Full size (10 x 10 mm) Charpy V notched specimens taken in the longitudinal direction throughout the wall thickness were tested from -6OC up to 20C to determine the influence of the m location on the brittle/ductile transition. Tests were also performed on coupling stocks in he transverse direction at -40C as shown onto figure 2.
Finally, the microstructure homogeneity was evaluated on both as quenched and quenched & tempered products by microscope observations after Nital etching and microhardness investigations.
Tensile tests at high temneratures. The development of the Cl10 grade steel is aimed at broadening consistently the application range for HPHT wells, especially in terms of temperature, In order to evaluate the mechanical behavior of casing up to servrce limits, high temperature tensile tests were performed on round specimens. These investigations were carried out at stabilized temperatures from 1OOC up to 25OC in 50C stages.
The interest was focussed on the evolution of the 2 main parameters involved in the design of completions : Yield Strength (YS, 0.2% offset) and Ultimate Tensile Strength (UTS).
SSC testing. A corrosion test is described by four elements : a sample, a corrosive medium, a stress and an evaluation criterion. For a casing string, this means classical mechanics and stresses, and smooth tensile specimens provide the most relevant results. Two philosophies are proposed to deal with medium and stress : NACE  that has established for decades the use of a defined set of parameters (NACE solution + %SMYS as applied stress) and on the other hand the European point of view summarized hereafter . The corrosive medium must meet the two leading parameters, pH and PH2S that better replicate the environment. The stress must be the maximum stress which can be applied in service on the steel. Indeed, this stress may reach the actual YS. However, for experimental reasons? the stress applied must be limited to 90% of the actual YS. The present paper is also aimed at companng the two approaches, Finally, since cracking normally occurs either quickly or never, the exposure time is limited to one month. The acceptance criterion is no more than one failure out of three tested samples.
s.~ooth.Ten~:l!e.~~.~).testing. Sulfide stress cracking tests were conducted according to NACE TMO177-90 method A. Specimens were machined according to a schematic provided on figure 3 in parallel with the mechanical study to characterise the SSC performance relative to the WT position : OD, ID and MW.
The test environment was initially the NACE solution with pH = 2.7 obtained by acetic acid addition at the beginning of the test. Then the saturation was maintained by a pure H2S gas. The applied stress level was 85% of the Specific Minimum Yield Strength (SMYS).
Complementary SSC testing conditions were involved specifically to simulate the Elgin and Franklin (EEC) well conditions : higher pH, lower H2S partial pressure and lower salinity.
Moreover, the conditions defined by the European Federation of Corrosion (EFC)  were also listed as an alternative standard severity to reproduce both oil environment (high pH 4.5) and gas environment (low pH 3.5). The lower environmental severity is balanced by increasing the stress level up to 90% of actual yield stress.
The tests were performed with proof ring devices, Double walled glass vessels were used to control and record the temperature of the solution continuously throughout the test. The test temperature was 23C, since the room temperature is known to be the worst case for SSC. After machining, the specimens were polished with 600 grit paper and electrolytically. The environment was first purged with nitrogen and then saturated with H2S (mixture) continually bubbling after initial saturation. An oxygen trap was utilized.
Table 2 gives an extensive overview of the key parameters involved in SSC ST testing.
Crack progression in cylindrical tensile specimens occurs normal to the pipe axis, whereas in the Double Cantilever Beam (DCB) specimen, the crack propagates parallel to the pipe axis. A range of different specimen configurations ensured that the cracking behaviors in both directions have been addressed. Moreover, authors have previously established that thicker steel of the same material is more subject to cracking than a thin one considering pre-existing notches . SO, the assessment of good resistance to H2S and crack propagation is of great interest.
DCB.Qe.sting. Sulfide stress cracking tests were conducted according to NACE TMO177-90 Method D. Precracked specimens were loaded to a predetermined stress by means of a double taper wedge which provided a constant displacement during the test. The specimens were then placed in NACE solution for 14 days. As the cracks extend due to SSC the load, and hence the stress intensity factor, decreases until it reaches the KISSC value beyond which the crack will not grow. Two weeks are enough to reach the final crack length of carbon steels . Transverse orientation specimens are known to have lower toughness than specimens of longitudinal orientation , so various geometry configurations were tested as shown in figure 4.
RESULTS AND DISCUSSION
Metallurgical results : The cleanness of the steel was evaluated according to ASTM E45 method A (Table 3). These results were excellent regarding elongated inclusions which are especially detrimental for toughness and corrosion.
Hardenabilitv [lo]: As a result of balancing the composition to achieve a high YS level with improved H2S cracking resistance, a high hardenability steel has resulted. The JOMINY curve shown in figure 5 highlights the efficiency of water quenching for heavy wall casing.
After water quenching, Vickers hardness readings were performed each millimeter through the thickness. The very flat curve shown in figure 6 illustrates the full penetration of the quench. The hardness level of 5OOHV, corresponding to 49 HRC, is higher than the API criterion for 90% of martensite :
HRC 2 58 %C+27=46.7 with 0,34%wt C As shown in figure 7, the typical as-quenched structure is fully martensitic and, as a consequence, well quenched and tempered after tempering.
Since SSC resistance is a function of the amount of martensite formed on quenching, with the best resistance obtained for material that possessed 100% martensite, flat microhardness profile [l l] implies homogeneous microstructure of these heavy products up to 2WT and therefore good corrosion properties of this Cl 10 proprietary grade are expected.
According to ASTM E112: with saturated picric acid etching, the austenitic grain size was measured between 9 and 10 accordmg to the thickness and the cast. An illustration is shown on figure 8. The refinement of the structure is a key point in guaranteeing high performances in sour environment [ 121.
Mechanical results : Detailed mechanical properties are displayed in table 4 for tensile tests and on figure 9 for hardness measurements. The hardness profiles are roughly flat respectively around 285, 283 and 295HV. The 2WT coupling stock presents a slightly harder OD with a 1OHV drop at Mw.
On the Rockwell scale, the hardness homogeneity is satisfied along the thickness within 2 HRC deviation and conforms with a 30HRC maximum criterion.
Prismatic tensile results are also reported and led to an interesting comparison with 010 mm round tensile : YS are systematically greater with the latter geometry as a consequence of the skin effect. For 05 and 010 mm round tensile machined on MW the difference is slight enough to be attributed either to the standard deviation of the experiment hardness or to the small drop in hardness profile.
The 26.67 and 37.8 mm thick pipes show very consistent values on round specimen between the OD-ID locations and pipe body MW. As for the 2WT coupling stock, all the portions meet the Cl 10 grade with tensile properties in accordance with the hardness trends.
Ductility : Figure 10 highlights the ductile behaviour of the steel in the longitudinal direction downto -60C where the criterion 54J/dOC (average) is well satisfied whatever the thickness and sampling location even for the 2WT coupling stock. In relation to the previous remarks concerning the hardness profile, its MW behaves a range lower than the skins which show an excellent ductility. Typical values of 130J are achieved for the casing at -40C. Finally, the transverse results obtained on 1.5 WT coupling stock are extremely good regarding the API criterion at low temperature (36J/OC) [131. 11714
The results show that mechanical characteristics are slightly influenced by the pipe dimensions but nevertheless the Cl 10 specifications are completely satisfied from the point of view of microstrwtwe, structure homogeneity, tensile, hardness and toughness properties.
Hiah temnerature mechanical results : New questions are raised through the development of HPHT wells concerning the mechanical properties of pipes at high temperature even in a range exceeding the actual service limits, up to 25OC. Table 5 provides some interesting answers. At 2OOC, a typical limit of Central Graben area, the steel loses 12% on YS and 5% on UTS.
ST testing : As shown on Tables 6 and 7, tests were carried out in 5 various environment in a view to assess extensively the SSC resistance of the steel.
Considering MW specimen, only one failure occured among the 22 tested specimens, More precisely one out of three 2 WT coupling stock specimen did crack late after 358 hours of exposure in the pH3.5 EFC gas environment. It is noticeable that the material did pass the EEC test characterised by an initial pH of 4. Specifically, the PI&tress level combination is a key parameter. Moreover as pointed out in table 2, the end pH values are respectively 4.10-4.18 for EFC gas solution and 4.20- 4.25 for EEC solution. As shown on figure 11, it clearly appeared that without HCl adjustement during the test, it is not possible to maintain the buffer effect for 30 days. On the other hand, previous qualification trials and published results  have already revealed this acceptable limit of (maintained) pH 4.1 with 10% H2S in CO2 gas saturation (PH2S =O.l bar).
Considering OD-ID location, all 21 specimens passed the NACE test. The SSC resistance of OD and ID samples is as good as that of classical MW samples even for high yield strength values around 120 ksi. Additionally, the materials passed also the EFC oil conditions at pH 4.5 with 0.1 bar H2S.
DCB testing : Data are shown in table 8. KlSSC values are situated in an average range 36 MPadm - 46 MPadm for both geometry. The results largely surpass the 33 MPadm criterion  considered as an equivallent to the no-failure criterion on ST specimen. Moreover, the very high KlSSC (46 MPadm) obtained on the 2 WT coupling stock confirms the SSC resistance of the material [ 151 even if it exhibited slightly lower mechanical performances homogeneity.
Our previous works  established in the early 90s a combination of steel chemistry and heat treatment parameters that enabled Cl 10 grade casing to be supplied for sour service. New developments in rolling and heat treatment have led to the scope of supply to be increased. Both casing and coupling stocks can now be delivered as thick wall products (up to 2) with the same propertres garanteed :
- restricted yield strength range of 10 ksi : 1 lo-120 ksi - 90% minimum martensitic quenched structure - controlled hardness : HRC < 30 - high toughness level : CVI 2 54 J at -40C - SSC threshold 2 85% SMYS in NACE solution - KlSSC > 33MPadm in NACE solution
These performances were satisfactory and consistent on the three locations throughout the thickness : on external skin (OD), on internal skin (ID) and at mid wall thickness (MW). Moreover, the materials pass SSC tests in various EFC oil and gas conditions so extensively assessing the corrosion resistance of our C 110 proprietary grade.
Thanks to Tubular Industry Scotland Limited (TISL), Vallourec & Mannesmann Tubes and Vallourec Research Center (CEV) for their participation in this research program.
M.B. Kermani, D. Harrop, R.D. Mac Cuish, J.R. Vera Sulfide stress cracking of downhole Tubular, Corrosion 91, Houston, paper 272, (1991)
M.B. Kermani, D. Harrop, J.L. Crolet, M.L.R. Truchon Experimental limits of sour service for tubular steels, Corrosion 91, Houston, paper 21, (1991)
NACE Standard MRO175-97, NACE International, (1997)
B.J. Orlans, F.A. Pellicani, G.C.Guntz, J.J. Ser-vier Development of Cl10 grade for sour service I, Corrosion 93, New Orleans, paper 147, (1993)
NACE TM01 77-90 standard (1990)
J.L. Crolet, Materials selection policy for II% media, Corrosion 94, Baltimore, paper 66 (1994)
EFC report n16, Guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production, The Institute of Materials (1995)
J.Brison Greer Effects of metal thickness and temperature on casing and tubing design for deep, sour wells, Journal of Petroleum Technology, April (1973), p.499-510
John P. Frick Variations in environmental cracking resistance of thick-walled low alloy steel tubulars, Corrosion 88, St Louis, paper 53, (1988)
G.M. Waid, R.T. Ault The development of a new high strength steel with improved hydrogen sulfide cracking resistance for sour oil and gas well applications , Corrosion 79, Atlanta, paper 180 (1979)
M.Watkins Microstructure - The critical variable controlling the SSC resistance of low alloy steels, Corrosion 95, Orlando, paper 50, (1995)
H. Asahi, M. Ueno, Effect of austenite grain size of low alloy martensitic steel on SSC resistance, Corrosron 90, Houston, paper 66 (1990)
APISCT, fith edition (1995)
J.L. Crolet, J. Jelinek, S. DAgata, M. Bonis, M.F. Louge Selection of a Cl 10 casing grade for mildly sour service, EUROCORR 94, Boumemouth UK, (1994)
D. L. Sponseller Interlaboratory testing of seven alloys for SSC resistance by the DCB (TM0177-90D) method, Corrosion 91, Houston, paper 3, (1991)
TABLE 1 : CHEMICAL COMPOSITION OF THE DIFFERENT HEATS (1O-3 wt %)
Dimensions Heat C Mn Si P S Cr MO Ni Nb Al V
273 x 26.7 Tl 339 460 324 12 1,9 931 759 28 35 29 49 312 x 48.6 Cl 318 502 303 7 1 988 850 89 38 28 46 289 x 37.8 C2 338 422 296 8 1 947 853 79 38 32 45
TABLE 2 : SSC TESTS CONDITIONS
EFC oil 1 EFC gas EFC oil 2 EEC NACE
Applied stress 9O%YS 9O%YS 9O%YS 9O%YS 85%SMYS Gas 1 OO%H2S 10% H2SKO2 10% H2s/co2 10% H2s/co2 lOO%H2S
NaCl (g/l) 50 50 50 1 50 Acetate(g/l) 4 4 4 10.464 Acetique(g/l) 5
HU yes yes yes yes Start pH 4.5 3.5 4.5 4 2.7 End pH 4.5 4.10-4.18 4.5 4.20-4.25 3.50-3.60
TABLE 3 : ASTM E45A INCLUSION RATING
We A B C D Code Fine Thick Fine Thick Fine Thick Fine Thick
Tl 0.5 - - - 2 - Cl 0.5 - 1 - 1 - 1 - c2 1.5 - 1 - 0.5 1.5
TABLE 4 : MECHANICAL PROPERTIES
05 010 prismatic Code Dimensions Location YS UTS ratio YS UTS ratio YS UTS ratio
(mm) (ksi) (ksi) (%) (ksi) (ksi) (%) (ksi) (ksi) (%)
OD 121 134 90 Tl 273 x 26.67 MW 121 134 90 117 132 89
ID 118 133 89
OD 118 133 89 Cl 312x48.65 MW 110 128 86 111 128 87 111 129 86
ID 116 131 89
OD 115 127 91 c2 289x37.8 MW 113 127 89 115 127 90 113 130 87
ID 114 128 89
TABLE 5 : EVOLUTION AT HIGH TEMPERATURES OF YIELD STRENGTH (YS) & ULTIMATE TENSILE STRENGTH (UTS)
Stress at 20C At temperature (C)
W) 20 100 150 200 250
YS= 117 1.00 0.95 0.91 0.88 0.83 T2 273 x 26.67
u-l-s= 130 1.00 0.96 0.95 0.94 0.95
TABLE 6 : SMOOTH TENSILE SSC RESULTS AT MID WALL THICKNESS
Code NACE EEC EFC oil 1 EFC gas Gas lOO%H2S lO%H2S/CO2 lOO%H2S lO%H2S/CO2
Dimensions Start pH 2.7 4 4.5 3.5
(mm) Applied stress 85%SMYS 9O%YS 9O%YS 9O%YS
Tl 212 NF 212 lw 273 x 26.67
Cl 313 NF 3i3 NF 3:3 NF 213 NF 312 x 48.65 (1 failure at 356h)
c2 313 lw 313 NJ! 289 x 37.8
2/3 NF means 2 unbroken specimens & 1 valid crack for 3 tested specimens within 720h
TABLE 7 : SMOOTH TENSILE SSC RESULTS AT OD AND ID SKINS
Code NACE EFC oil 2
Gas lOO%H2S lO%H2S/C02
Dimensions Start pH 2.7 4.5
(mm) Applied stress 85%SMYS 9O%YS OD ID OD ID
Tl 313 NF 212 NF 313NF l/lNF 273 x 26.67
Cl 212 NF 212NF l/l NF 3/3 NF 3 12 x 48.65
c2 313NF l/l NF 289 x 37.8
TABLE 8 : KlSSC (MPadm) DERIVED FROM DCB TESTS IN NACE SOLUTION
Code Dimensions Location Test piece1 Test piece2 Average
Tl 273 x 26.67 transverse 33.9 39.9 36.9
OD long 39.4 32.4 35.9 Cl 312 x 48.65 MW long 47.2 45.8 46.5
ID long 37.5 37.5
FIGURE 1 : LOCATION OF ROUND TENSILE SPECIMENS THOUGHOUT WALL THICKNESS
FIGURE 2 : LOCATION OF CHARPY SPECIMENS THOUGHOUT WALL THICKNESS (1) LONG. (2) TRANS.
FIGURE 3 : LOCATION OF NACE SMOOTH TENSILE SPECIMENS THOUGHOUT WALL THICKNESS
FIGURE 4 : LOCATION OF DCB SPECIMENS THOUGHOUT WALL THICKNESS
273 x 26,67mm
FIGURE 5 : JOIMINY CURVE FOR Cl10 GRADE STEEL
40 1 E 30
10 ~ ~
0 10 20
FIGURE 6 : HARDNESS THROUGH THE WALL THICKNESS OF AS QUENCHEDPRODUCT
0 10 20 30 40
Distance from OD (mm)
FIGURE 7 : MICROSTRUCTURE (M500) (after Nital etching) AS-QUENCHED QUENCHED & TEMPERED
FIGURE 8 : PRIOR AUSTENITE GRAIN BOUNDARIES (M500) ASTM SIZE X (after picric acid etching)
FIGURE 9 : HARDNESS THROUGH THE WT OF Q&T PIPES Tl (273x26.67 mm) Cl (312x48.65 mm) C2 (289x37.8 mm)
0 5 IO 15 20 25 30
Distance from OD (mm)
35 40 45 50
FIGURE 10 : CV NOTCH RESULTS OF LONGITUDINAL (L) AND TRANSVERSE (T) IMPACT TESTS
T X OD C2(T) j
X ID C2(T)
1 + MW C2(T)
! I i-mini 54J 1
-40 -20 0 20
FIGURE 11 : CURVE pH vs time FOR VARIOUS SSC TEST ENVIRONMENTS
0 100 200 300 400 500 600 700 600
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