The yield to tensile ratio of a line pipe material is a measure of the margin of safety against failure by plastic collapse, and indicates the ability of a pipe to experience plastic deformation before failure. Should the yield strength (σ γ) be equal to the ultimate tensile strength (UTS), any plastic deformation of the pipe could result in rupture. However, with a difference between σ γ and UTS, the ability for the steel to exhibit strain hardening provides some protection for the pipe against fracture, for example, during laying.
For example, both the American Petroleum Institute specification for Line Pipe [1] and the Det Norske Veritas Submarine Pipeline Standard [2] require a maximum σ γ/UTS of 0.93 for C-Mn steel pipes of yield strength levels up to 555MPa (80ksi). In both standards, σ γ is defined as the stress at a total strain of 0.5%.
The American Petroleum Institute specification for Line Pipe specifies a maximum σ γ/UTS of 0.99 for X120 grade linepipe steel (827MPa/ 120 ksi yield strength) while the Det Norske Veritas Submarine Pipeline standard specifies a maximum σ γ /σ γ /UTS of 0.92 for duplex and martensitic 13Cr stainless steel linepipe.
There may further be a requirement for pipes with σ γ /UTS ratios somewhat below typical values of around 0.9, particularly when the steel pipe could be subjected to significant strains during handling, for example if the pipe was to be reeled or likely to be subject to land slip or earthquake in service. In this case, it is required that the pipe is able to withstand appreciable plastic straining.
Although fracture (brittle or ductile) of pipelines is not directly related to the σ γ /UTS ratio, it should also be considered, particularly in older lines. Specification of adequate toughness is usually ensured by use of the Charpy test, although other tests (such as the drop-weight tear test and CTOD test) are sometimes also required.
References
1. Dual badged standard (Table 7): American Petroleum Institute (API) Specification 5L 'Specification for Line Pipe', Forty-Fourth Edition, October 2007, ISO 3183:2007 (Modified), Petroleum and natural gas industries - Steel pipe for pipeline transportation systems.
2. Det Norske Veritas, Offshore Standard OS-F101 'Submarine Pipeline Systems', January 2000, p 51.