Stress Analysis of a Gas Pipeline Installed Using HDD and Auger Bore Techniques

As part of an important project to reinforce the natural gas transmission network, a new pipeline has been constructed to transport natural gas from a major UK LNG storage facility into the national transmission system. The project involved the installation of several sections by trenchless methods, namely auger boring for a number of road crossings and significant lengths of horizontal directional drilling (HDD) beneath railroads, canals and marshland. The installation of pipelines using trenchless techniques such as HDD continues to increase in popularity. The various methods available offer advantages over traditional open cut techniques, in particular much reduced disruption during the construction of road and rail crossings. Furthermore, increased awareness and responsibility towards the environment leads us to seek installation methods that cause the least disruption at the surface and have the least impact to the environment. It was required to assess the proposed crossing designs against acceptable stress limits set out in company specifications and against the requirements of UK design code IGE/TD/1 Edition 4 [1], which requires that ‘additional loads’ such as soil loadings, thermal loads, settlement and traffic loading are accounted for within the stress calculations. However, it does not stipulate the sources of such equations and the pipeline engineer must rely on other methods and published sources of information. This paper presents the method used to analyse those sections of the new pipeline installed by auger boring and HDD focusing on the methods and formulae used to calculate the stresses in the pipeline from all loading sources.