Pipeline installation by HDD: pull-back governs success

Soil reaction force at the head of the pipeline during the pull-back 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 up to 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 pull-back operation. The cost of damaged pipes if things go wrong, and the cost of additional measures during and after the pull-back, can be considerable. Recently, in the Netherlands, problems occurred during pull-back operations at a number of locations where relatively large diameter pipelines are being installed. The problems varied from high pulling forces to abandoned pull-back operations due to a jammed pipeline. The pipeline–soil interaction during the pull-back operation has been identified as the cause of these pull-back operations.

The current Dutch method for calculating the pull-back force on the pipe is based on the soil-pipeline interaction, developed over 10 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 pull-back 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 pull-back 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 pull-back equipment in the curved sections of an HDD trajectory. The authors describe the model they have developed for the pull-back operation, and simulations they performed to study the behaviour of a pipeline in the borehole during the pull-back 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 pull-back equipment. Ill11_A sub-bottom profiler is lifted back on board of the survey vessel Triad.tif

Nord Stream Pipeline series: considering environmental impacts

The Nord Stream Pipeline’s German landfall: the challenges ahead, by Nigel S Kirk and Dipl-Ing Björn Dobberstein.

The current issue of the Journal of Pipeline Engineering includes the first of three articles on one of the world’s biggest current pipeline projects – the Nord Stream Pipeline. The series will review various aspects of the project, with specific attention paid to the pipeline landfall in Germany. The German landfall crosses an area that requires a high degree of environmental protection. The first article describes the project and its general technical details, together with a description of the German landfall, including the environmental and authorisation issues, anticipated construction techniques, and the expected installation schedule.

The second article, which will be published during the construction period for the Nord Stream Pipeline next year, will provide a general update of the project and describe the installation design together with the actual construction activities and the challenges encountered. The final article, planned for 2011, will review the project on completion of the first pipeline and assess the positive and negative aspects of the German landfall’s construction.

As described in the last issue of Pipelines International, the Nord Stream Pipeline project consists of two 1,223 km long parallel 48 inch diameter offshore pipelines laid across the Baltic Sea, connecting the pig launchers close to the compressor station at Portovaya Bay, Russia, and the pig receivers adjacent to the Greifswald receiving terminal in Germany. At the Russian end, the pipelines cross the coastline southbound at Vyborg, northwest of St Petersburg. The route runs westward through the Gulf of Finland for approximately 440 km, and then turns southward and east of the Swedish island of Gotland. Following this the route turns southwest to skirt the Danish island of Bornholm, continuing in a south-southwest direction and eventually landfalling close to Lubmin, east of Greifswald in Germany. The project is very significant for Europe as, once fully operational, it will transport energy sufficient for 13–14 million people, and supply approximately 25 per cent of Europe’s imported gas requirements.

The pipeline’s route is highly sensitive from many viewpoints, not the least of which is its environmental aspects, and Nord Stream AG has gone to great lengths to ensure that the environmental impact of the pipeline is kept to an absolute minimum along its complete length. The pipeline profile in the German landfall area has been designed to satisfy a variety of criteria in respect of the burial depth: the cover to the pipeline varies from 1–4.5 m depending on pipeline stability, pipeline protection, coastal erosion, and local shipping authority requirements. The pipeline’s route crosses a sandbar known as the Boddenrandschwelle, an area where the water depth is relatively shallow, varying between 2.5 m and 4.5 m deep, and through which the pipe trench has to be widened over a length of about 1,100m to ensure a minimum trench width of approximately 50 m to allow access for the laybarge.

After installation of the pipelines, the dredged material contained within the offshore storage area will be re-dredged and backfilled into the trench. Selected coarse-grained material will be placed directly around the pipeline to ensure that liquefaction of the material does not occur and induce buoyancy, and cohesive soil will be placed above the coarse material, with topsoil finishing off the layered backfill. The backfilled trench will be accepted on completion of a detailed bathymetric survey, after which the offshore and pull-in sections of the landfall will be hydrostatically tested in combination with the remainder of the Nord Stream Pipeline.