AFAA were contracted to investigate the fitness for purpose of a novel design in which a 1.8km section of a RAMSAR protected river estuary needed to be crossed. Due to a number of reasons it was not possible to negotiate the crossing using conventional techniques such as horizontal directional drilling and an alternate method had to be sought.
Following a feasibility study, it was decided that a pipe-in-tunnel approach was the most viable. A 2.44m diameter smoothbore bolted ring concrete tunnel was to be constructed underneath the river, including the associated flood plains and railway crossings. Due to constraints at the points of entry and exit it was necessary to cut two vertical shafts, one on each river bank, which would later be filled with soil to support the pipework.
The 600mm OD linepipe sections were to be welded together in the entry shaft and pulled through the tunnel, on pre-installed plastic-roller supports, by a winching mechanism. The roller supports would become the permanent pipe supports during pipeline operation. The tunnel pipeline ends would be connected to the conventionally laid pipeline sections leading to the AGIs. The pipeline was designed to be piggable using standard on-line inspection tools.
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.
Inside the tunnel, supports are spaced 15m apart, with typically one quad roller support followed by 3 dual roller supports.
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 and the pre-processing software Patran, with a variety of element types. Sets of rollers and pipe sections they contact were modeled using 3D solid elements. Pipe soil interaction elements were used to apply the vertical, axial, and lateral soil restraints in nominal cover, and within the vertical shafts. Pipe and elbow beam elements were used for the majority of the linepipe sections, with shell models used for the elbows of greatest concern.
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. The pipeline was assessed for gross section yield, fatigue, buckling and ovalisation failure modes to IGE/TD/1.
Shell Bend at Base of Shaft and Initial Set of Supports