In recent years, there has been development of several significant offshore pipeline projects for the transmission of oil and gas from harsh, deep water environments. The production of gas transmission pipelines for offshore application demands heavy thickness, high strength, lower temperature toughness, good weldability and sour service crack resistance.
This paper reports on the experience with the production of linepipe up to 31.8mm in wall thickness grade X70 intended for offshore applications in China. The plate was produced at Shougang Steel’s 4.3 m heavy wide plate mill in Qinhuangdao China (Shouqin). Alloy/processing design along with production parameters are introduced. The achieved mechanical properties in both plate and pipe are reported. The pipe tensile test results in transverse and longitudinal directions achieved yield strength and ultimate tensile strength of 571MPa and 668MPa respectively, which are higher than plate yield and ultimate strengths of 510MPa and 658MPa respectively. The increase of strength during pipe making is due to the creation of a fine uniform acicular ferrite microstructure from the alloy/processing design. Even with this X70 strength level and heavy wall thickness the low temperature fracture toughness achieved was excellent. Transverse charpy impact testing resulted in absorbed average energy of 480 J @ – 20 °C with 100% shear. Drop weight tear testing (DWTT) achieved an average of 94% shear at −15 °C.
Evaluation of the heavy gauge X70 sour service performance was performed according to specifications of NACE TM 0284 and DNV-OS-F101 utilizing HIC and SSCC testing in low pH (∼3) solution A. The low carbon, low sulfur, clean steel and homogeneous fine acicular ferrite microstructure had extremely high resistance to HIC and SSCC failure in the testing. All of these results demonstrated that the heavy wall X70 production at Shougang Steel’s 4.3 m wide heavy plate mill (Shouqin) can meet the technical specifications of “The South China Sea Deepwater Gas Development” and DNV-OS-F101.