Power supply on the island of sylt completely converted to underground cabling

19.05.20 - Thanks to trenchless technology, electrical power on the North Sea island of Sylt will soon only flow underground. Schleswig-Holstein Networks AG (SH Netz) has invested an impressive €16.5 million to optimise the power supply and to dismantle more than 7 km of 60,000 volt overhead lines. For the new underground supply, a 15 km long, 110.000 V power line weighing around 800 t and was installed through the bottom of the North Sea and continued from there via a new high-voltage land cable to the transformer station on the island in Keitum. The installation of the empty pipes for the connection of the sea cable in the mudflats and the power cables on the island was a case for the Nodig professionals from the contractor Paasch, who mainly applied the HDD method for this task.

Trenchless technology protects the World Natural Heritage

Sylt is the largest German island in the North Sea and after Rügen, Usedom and Fehmarn the fourth largest German island anywhere, situated near the coast of Schleswig-Holstein and Denmark and is a very popular holiday destination, mainly due to its very mild climate all year round. Close to 18,000 inhabitants, share an area of 99 km² with an average of three quarters of a million holidaymakers per year. Half of this typical North Friesland nature reserve with forty miles of sandy beaches, high dunes and a species-rich flora and fauna is under strict protection. Protected areas include dunes and heathlands, cliffs and inland waters and of course the tidelands, as part of the national park of the same name and a UNESCO World Natural Heritage Site.

It is easy to understand why the more than two kilometre length of empty pipes could not be installed on the island using the open trenching method. So it was obvious that Paasch proposed to apply the trenchless construction method when this task was given to them in autumn 2018. The pipeline constructors from Damendorf have already successfully applied the NODIG technology several times in the sensitive coastal regions of the North and Baltic Sea, true to the company motto "We will solve that". These include such spectacular projects as the renewal of the complete power supply for the Hallig Northern Moor, the rewiring of a lighthouse on the pilot island Schleimünde, or the connection of a new water gauge on the Elbe coast in Krautsand. In the Sylt project they were also able to convince the client of their professionalism and the economic and ecological advantages of trenchless installation technologies.

After all construction and nature conservation approvals had been granted, the pipe installation was ready to begin in early summer 2019 in Keitum. Their client was the Christophers company, who would later lay the 800-ton undersea cable between the mainland and the island on behalf of the SH-Network, using a huge cable plough bearing the beautiful name "Nessie". To ensure that the World Heritage Site and breeding migratory birds are not disturbed, all work was accompanied and monitored by a natural conservation society.

Challenging routes

The path through the sensitive eco-system for a total power cable length of 2,100 m was divided into several sections and proved to be a real challenge. On its way from the transfer point of the undersea cable in the mudflats to the Keitum transformer station, it crossed forests, wetlands, salt marshes and the Hindenburg Dam, which connects Sylt with the mainland. With the exception of undercrossing the railway embankment, all bores were carried out with the HDD method.

The Hindenburg Dam is a lifeline for the island and especially important for tourism. Unlike many other North Sea islands, Sylt is not car-free and you can only reach the island from the mainland by car with the car train, which runs over the railway embankment to Westerland. The route is highly frequented. A red and a blue car train commute between the mainland and the island up to 44 and 26 times a day respectively in the high season and passenger trains also use this route every day. In order not to endanger the stability of the Hindenburg dam, the empty pipes beneath were not to be installed with the steerable drilling method, as specified by the Deutsche Bahn (German Rail). Instead, an OD 500 steel pipe was to be pressed in a straight line beneath the dam over a short distance of 60 m using the pipe pushing method. For the same reason, the old power cables, to be switched off after the new undersea cable is put into operation, will remain in the Hindenburg Dam.

The main part of the bore path was however carried out with the HDD method, using a GRUNDODRILL 18ACS. The steerable “All Condition System” with around 180 tons thrust and pulling force was developed specially for complex and alternating geologies and therefore precisely the right HDD bore rig for these bores under protected landscapes. The trickiest part of this was the accommodation of the sea cable. To do so a 150 m length PE pipe OD 355 mm had to be installed from the mudflats. . Ashore the power cable was divided into three service lines, so that three 160 mm OD pipes had to be drawn in from the transfer point. Three further empty pipes 50 mm OD for the future accommodation of fibre optic and telecommunication cables over 250 m length up to the railway embankment were installed in the same bundle. Behind the railway embankment, the bundle of 6 pipes had to be continued along to the transformer station.

HDD-Pros master all the challenges

The Paasch bore crew worked it’s way steadily but quickly along the bore path. This ran from the Ingiwai meadow on the beach, underneath the railway embankment and the district road K117 to Koog Street and from there again underneath the K117 over to a bus stop and then parallel to the railway line underneath a car park up to the transformer station Keitum. This somewhat complicated route, which required two undercrossings of the district road was necessary, because previously there was not enough space between the railway embankment and the road to accommodate the new power cables.
First of all, in May 2019, the pipe bundle 3 x 160 OD SDR 11 + 3 x 50 OD was installed over a total distance of 975 m in eight sections of 125, 100, 110, 150, 135, 95, 110 and 90 m from the transformer station, parallel to the railway tracks towards the railway embankment. Then followed the 450 m section from the railway embankment to the coast in three single bores of 150, 160 and 140 m. The GRUNDODRILL 18ACS and the experienced drilling pros mastered these tasks throughout without any problems. The difficult part was still to come at the end.
The eight-man team took on the trickiest of the HDD bores in the first days of June: the installation of the 355 mm OD empty pipe to accommodate the undersea cable in the mudflats up to the transfer point. Special care and timing was required here. The tides only allowed work at low tide and parts of the mud flats were not to be entered or loaded at all by vehicles. The 150 m length pilot bore ran smoothly, but when pulling in the pipe the floating dredger, which was supposed to hold the heavy PE pipe, sank into the soft ground and work had to be stopped for some time to make new plans. Later, a diving boat took over the dredger’s job and the 355 mm pipe was finally pulled in.

Goodbye to overhead service lines on Sylt

When some time later the pipe bundle was pressed through beneath the railway embankment in only one night, the new empty pipe network on the island was complete. Then, in October “Nessie”, the large cable plough pulled in the sea cable from the mainland right up to the island coast. This special machine, 18 metres long, over 10 metres wide and weighing 91 tons, is a rubber tracked vehicle with a vibration plough blade, which was applied to “jolt” the 15 km long and 800 ton heavy sea cable approximately 1,5 m deep into the mudflats between the mainland and Sylt. Now the power supply has been secured on the island for the long term and the more than 60-year-old overhead line can be dismantled.




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