Pipeline installation by HDD: pullback governs success

Soil reaction force at the head of the pipeline during the pullback operation of horizontal directional drilling, by J P Pruiksma, H J Brink, H M G Kruse, and J Spiekhout.

Horizontal directional drilling (HDD) for installing pipelines under obstacles such as river or canal crossings, and road and railway embankments, is widely used for pipes 20 inches in diameter and above. The method is particularly suited to soils such as clays, through which it is easy to drill. Considerable lengths of pipe can be installed in this way: the process involves drilling an oversized pilot hole along the planned trajectory, filling it with drilling ‘mud’ such as Bentonite, and then ‘pulling back’ the actual pipeline through the drilled hole.

There are few standards that actually provide guidance for this operation, the most critical aspect of which is the pullback operation. The cost of damaged pipes if things go wrong, and the cost of additional measures during and after the pullback, can be considerable. Recently, in the Netherlands, problems occurred during pullback operations at a number of locations where relatively large diameter pipelines are being installed. The problems varied from high pulling forces to abandoned pullback operations due to a jammed pipeline. The pipeline-soil interaction during the pullback operation has been identified as the cause of these issues.

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The current Dutch method for calculating the pullback force on the pipe is based on the soil-pipeline interaction, developed over ten years ago. The method considers the distribution of the normal forces between the pipeline and the wall of the pre-reamed borehole. For general design purposes, this is a quick and relatively simple method for the calculation of the distribution of normal forces between the pipeline and the borehole wall, and gives a reasonable estimate of the maximum pullback force. The reason for pulling problems, however, cannot be explained with this method, and recent research has shown that the behaviour of the head of the pipeline is of major importance in the pullback operation.

The joint paper from the Netherlands-based National Institute Geo-Engineering Unit in Delft and NV Nederlandse Gasunie reports on research undertaken into the behaviour of the head of the pipeline at its connection with the pullback equipment in the curved sections of an HDD trajectory. The authors describe the model they have developed for the pullback operation, and simulations they performed to study the behaviour of a pipeline in the borehole during the pullback operation of an HDD project. The model describes the complex set of interactions between the pipeline, the drilling pipe, the drilling fluid, and the soil in the borehole.

From the simulations and analytical solutions, the authors show that the soil-reaction forces are much higher when the head of the pipeline is located in the bend compared to when the head of the pipeline has passed through the bend. Depending on the ground conditions and the bending radius, these high soil-reaction stresses in the curved section may cause damage to the pipeline coating, and may lead to penetration of the borehole wall, which in turn leads to high pulling forces and may lead to a stuck pipeline or to damaged pullback equipment.