A lifeline for Helgoland's dune village
The challenge of constructing a dual direction pipeline to simultaneously carry drinking water and sewage on a North Sea island has been met thanks to modern pipe materials, as Egeplast's Ulrich Winkler explains.
Using a common protective pipe, which was laid between the rocky island and the offshore dunes under the sea bed without horizontal directional drilling, drinking water and sewage flow in opposite directions. The two fluids are kept apart by separate SLA pressure pipes, manufactured by Egeplast GmbH & Co.
The unusual bundling of the piping also includes electricity and telecommunications cables, and an empty pipe.
The project was in response to plans to build a 72-house holiday village in the sand dunes in the sea 1km off Helgoland.
In order to be able to provide the necessary infrastructure for the village in such an extreme location, it was necessary to connect the dune area to a reverse osmosis plant on the lower parts of Helgoland which produces fresh water. The sewer was planned to go in the opposite direction to be processed at the Helgoland sewage treatment plant.
Power supply and telecommunications cables also had to make the jump over the narrow stretch of sea between the islands. The strong current ruled out the laying of an open pipeline on the seabed. The planners therefore designed a 1250m culvert to be laid by hydraulic rotary boring at a depth of 25m under the seabed.
In order to avoid carrying out this expensive and technically complicated operation several times for each of the various pipelines respectively, the planners decided to bundle the incompatible lines into a common steel protective 388mm-diameter, 8mm-thick pipe.
From a safety point of view, the most difficult aspect of the scheme was the question of how to provide long-term separation of the drinking water and the sewage, and also separation from the electricity power cables and telephone lines.
The answer was provided by modern SLA pipe technology from Egeplast. The concept is based on a development of PE-HP pipe to make a highly secure, three-layer pipe.
A core pipe made of PE-HP, PE 80 or PE 100 is diagonally wrapped around with overlapping pore-free aluminium foil that is stuck to it to act as a barrier layer and thus ensures reliable sealing of the pipe against diffusion over its expected lifetime of at least 100 years.
A 2-3mm thick layer of wear and abrasion resistant material is extruded onto this structure to act as a sheath. As a result, the SLA pipe has been certified for trenchless pipelaying methods as well as for installations in polluted and contaminated ground.
Responsibility for laying the pipeline under the bed of the North Sea fell to Oldenburger LMR-Drilling GmbH. The contractor put two hydraulic rotary boring units into tandem operation.
A 100t rig on the main island provided the necessary torque to the boring rods, while a 40t unit in the dunes assisted in providing the required pulling force.
The drilled tunnel was measured in three dimensions using a control sensor that reacted to the magnetic field of the earth with respect to the local magnetic fields. This measuring unit was installed 30m behind the hydraulic rotary motor and gave off a signal with the measured value every 5m during the 12 days that were required for the pilot drilling. The drilling tools had to be changed several times due to the lack of homogeneity in the ground.
The actual tunnel hole, with a diameter of 560mm, was produced in the opposite direction to the pilot boring. The steel protective pipe that had been welded up from three lines was pulled through within 12 hours.
Although SLA pipes can be joined up into one length, the water, sewage and cable protective pipes for Helgoland were extruded seamlessly in lengths of 1250m each at the Egeplast main plant in Emsdetten and supplied to the island wound onto steel drums with a total weight of up to 11t.
While a PE-100 (PN 16, 110x10mm) SLA pipe was to carry the drinking water, an SLA pipe with a PE-HP core (PN 16 125x17.3mm) was to handle the sewage.
The total of six pipes that had to be drawn through were held together in an even bundle by means of plastic skids and were pulled through the steel protective pipe from Helgoland to the dunes in the course of two working days.
The open pipes at the towing head where flooded with water as they were pulled in to ensure that they could be installed with minimal damage by minimising the pulling and friction forces.
The project was completed on schedule after just five months of work. Its successful completion was the conclusion of an unconventional procedure that was completed with minimum impact on the ecology and infrastructure at the location, and has provided a lifeline with long-term functionality for life in the dunes of Helgoland.
It is also clear that the Helgoland project would not have been possible in the form that it was done, with significant technical and economic aspects, without modern SLA pipes with their high reserves of safety.