From the previous article in this blog, you may already know about pipeline material and grade selection. Now, in this post, I will write more about the difference and usage of high strength steel and low strength steel pipeline material.

Low carbon (carbon content less than 0.29%) steel is mild and has a relatively low tensile strength so it is used to make pipes. Medium or high carbon (carbon content greater than 0.3%) steel is strong and has a good wear resistance so they are used to make forging, automotive parts, springs, wires, etc. Carbon equivalent (CE) refers to method of measuring the maximum hardness and weldability of the steel based on chemical composition of the steel. Higher C (carbon) and other alloy elements such as Mn (manganese), Cr (chrome), Mo (molybdenum), V (vanadium), Ni (nickel), Cu (copper), etc. tend to increase the hardness (harder and stronger) but decrease the weldability (less ductile and difficult to weld).

Pipes are graded per their tensile properties. Grade X-65 means that SMYS (specified minimum yield strength) of the pipe is 65 ksi. The yield strength is defined as the tensile stress when 0.5% elongation occurs on the pipe, per API-5L . The DNV code defines the yield stress as the stress at which the total strain is 0.5%, corresponding to an elastic strain of approximately 0.2% and a plastic (or residual) strain of 0.3%

The research and development of new steels are for both sour and non-sour service. The materials being developed for subsea pipelines and risers are grades X70 and X80 for nonsour service and grades X65 and X70 with a wall thickness of up to 40 mm for sour service.

For offshore pipelines, the current trend is towards linepipe in grade X70 with a wall thickness up to 40 mm. Fulfillment of the requirements for DWTT transition temperature will be progressively difficult as the wall thickness increases. For wall thickness in excess of 30 mm, low transition temperatures can only be achieved by means of highly expensive rolling processes. Until now, there has been only limited offshore use of X70 material. The main installation contractors have completed three projects with X70 and have two planned until 1997.

For high pressure transmission land lines. Grade X70 is now widely used for high pressure transmission lines in many countries.

Meanwhile, five onshore projects have been identified in which X80 pipe has been used.A period of seven years elapsed before Ruhrgas AG in Germany began in 1992 to place an order for linepipe for the construction of the world's first ever grade X80 pipeline.

Large diameter X80 pipe can be manufactured consistently for long pipelines. The approach to the X80 projects was significantly different when the welding procedure and consumables were selected. The field welding of the X80 pipe did not present any difficulty for the Ruhrgas and Mitzihwin projects. These projects demonstrated that conventional mechanized welding using the GMAW process could produce consistent, high quality welds for onshore pipelines.

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Source:

Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007.

Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.