A limit states design approach for onshore pipelines has been developed as part of a three-year joint industry project. The approach covers both natural gas and low vapor pressure (LVP) liquid pipelines and addresses key design conditions, including burst, equipment impact loads and plastic deformations during hydrostatic testing. The results are described in a set of design guidelines that can be used as a basis for a limit states design standard.
The design methodology is based on two limit states categories, namely ultimate limit states representing large leaks and ruptures, and serviceability limit states representing excessive deformations. For ultimate limit states, a safety class system is used to characterize pipelines based on the anticipated severity of failure consequences as determined by pressure, diameter, product, population density and environmental sensitivity. Simple deterministic design rules are then developed to ensure that appropriate reliability levels are met for each limit state category and each safety class.
Comparisons between the wall thicknesses resulting from limit states and conventional design methods are given for a wide range of pipeline diameters, pressures, grades and location classes (for gas pipelines). The results show that the limit states design approach results in thicker walls for small diameter pipelines (especially in highly populated or environmentally sensitive areas) and lower wall thicknesses for larger diameter, higher pressure pipelines (especially in areas with low population densities). The reason for this trend is explained and the potential for its acceptance by designers and regulators is discussed.
