Justification of a Novel Pipe in Tunnel Approach to a River Crossing

As part of a major pipeline construction project, Laing O’Rourke had a requirement to install a section of pipeline beneath a river estuary. Due to a number of reasons it was not possible to negotiate the crossing using conventional techniques such as horizontal directional drilling (HDD) and an alternate method had to be sought. A feasibility study was undertaken and it was decided that a pipe-in-tunnel approach was the most viable. Due to constraints at the points of entry and exit it was necessary to cut two vertical shafts, one on each river bank. Linepipe sections were to be welded together, in the entry shaft, and pulled through the 2.44m diameter tunnel on plastic rollers, which were later to become the permanent supports during operation. On completion of the installation within the tunnel, two vertical sections of pipeline were to be installed in the shafts for connection to the main pipeline system. Due to the length of the crossing it was decided that back filling the void between the pipe and the tunnel wall with a suitable grout was not viable. A particular consideration was the likely occurrence of voids which would reduce the effectiveness of the CP system. For this reason, following installation, the tunnel was to be sealed with a concrete plug and flooded with water, and the shafts are to be backfilled with soil. This unique design arrangement presented a number of challenges and hence a requirement for the use of more complex modelling techniques than would normally be required. Models of the pipeline in various stages of installation were produced using the finite element software ABAQUS, with a variety of element types. Sets of rollers and their contact with the linepipe were also modelled. Soil loading, pressure, weight, buoyancy and temperature were applied to simulate a range of construction, commissioning and operational conditions. These were analysed, and the results were assessed for compliance with appropriate standards. Based on the results of the study it was possible to show that with a number of modifications to the original proposed design configuration, the crossing would be fit-for-purpose.