Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Full Ring SSC Evaluation of X65 UOE Sour Linepipe Steels in Low H 2S Content Sour
Environment
Junji Shimamura 1, Daichi Izumi1, Nobuyuki Ishikawa 2 and Joe Kondo 212

Steel Research Laboratory, JFE Steel Corporation, Fukuyama, Japan
Plate Business Planning Dept., JFE Steel Corporation, Tokyo, JapanABSTRACT

carbon contamination, dual phase microstructure and heat transfer
variation in a recent paper (Fairchild, 2019; Newbury, 2019). Preciseand strict material design should be needed in a production of Grade
X65 UOE sour linepipe steels for preventing SSC.
In the previous paper, 4PB SSC tests revealed that SSC occurred when
the inner surface hardness exceeds the limit dependin g on H 2S partialpressure (Shimamura, 2019; Shimamura, 2020; Shimamura, 2021 ).
Also, it was found that the high applied stress over plastic strain region
activated the anode reaction in 4 point bend (4PB) SSC test under 0.15
bar H 2S partial pressure condition. The results indicated that the anodic
dissolution enhanced by applied stress was dominant factor forcorrosion groove formation (Izumi, 2020).

Sulfide stress cracking (SSC) resistant property is required for thelinepipe produced by Thermo-Mechanical Controlled Processing(TMCP) which balances strength and toughness of materials and giving
an economical benefit. In designing sour linepipe materials, lowering
surface hardness is one of the most important points to prevent SSC. In
the previous paper, it was found that the high applied stress over plasticstrain region activated the anode reaction in 4 point bend (4PB) SSC
test under 0.15 bar H 2S partial pressure condition. The results indicatedthat the anodic dissolution enhanced by applied stress was dominantfactor for corrosion groove formation.
In this study, full ring SSC tests were conducted under 0.15 bar H 2S
partial pressure condition with two different applied stress (70% and
90% AYS). The objectives are to clarify the effect of applied stress on
surface corrosion groove formation behavior and to compare the resultwith previous 4PB SSC test results. As a result, the pipe samples tested
with 70% and 90% AYS showed no SSC after 720hr immersion.
Higher applied stress sample had higher localized corrosion pit depth.
The result in full ring SSC tests corresponded with that of 4PB SSC
tests. In the case of low surface hardness sample, higher applied stressgave higher localized corrosion pit depth. However, SSC crack
formation was not seen. This result seems to indicate that hydrogenamount was not reached the threshold of hydrogen embrittlement typecrack initiation in above case.

In this study, full ring SSC tests were conducted under 0.15 bar
H2S partial pressure condition with two different applied stress(70% and 90% AYS). The objectives are to clarify the effect ofapplied stress on surface corrosion groove formation behavior
and to compare the result with previous 4PB SSC test results.
SURFACEMICROSTRUCTURE
AND
HARDNESSCONTROL FOR SSC RESISTANT PROPERTY
SSC behavior in TMCP sour gas linepipe has been focused recentlydue to ‘hard spot’ shown at the pipe inner surface. In order to evaluateappropriate hardness of a hard spot in the cross section, Vickers
hardness measurement with the load of 100g and 500g at the location
0.25mm from the surface is proposed (Anderson, 2020).
Inner surface area of pipe corresponds to plate top surface area. Fig. 1
shows the schematic illustration of cooling curve at plate surface andmiddle thickness position. In general, in TMCP process using
accelerated cooling device, cooling rate of plate surface is higher thanthat of plate mid-thick position. Therefore, surface cooling rate should
be controlled carefully to ob tain lower surface hardness. Fig. 2summarizes the material design concept for achieving lower surface
hardness in inner surface of X65 UOE sour pipes. When surface
cooling rate is over 200 oC/sec, surface microstructure is mainly hardlath bainite (LB). When surface cooling rate is around 100 oC/sec,
surface microstructure is composed from soft granular baintite (GB)and LB. When surface cooling rate is less than 50 oC/sec, surface

KEY WORDS: linepipe; sour gas; sulfide stress cracking; Thermomechanical controlled processing; Bainite; hardness; full ring SSC test.

INTRODUCTION
Application of sour linepipes has been expanded toward severe sour
environment regions including higher H 2S partial pressure conditions
over 1bar. In 2013, actual sour gas pipeline failure occurred due to SSC
under higher H 2S partial pressure condition over 1bar (Newbury et al.,2018). One of the possible root cause of SSC was assumed b yformation of hard spots in the steel. Fairchild et al investigated andsummarized the three hard zone formation mechanisms including

3058