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DRILLING IN PROTECTED LANDSCAPES

When it comes to line installation in ecologically highly sensitive environments, it is usually a job predestined for the horizontal drilling technology. This was also the case when a new wiring for the lighthouse of Schleimünde was required. The job site is approached by ship; Schleimünde is normally impossible to reach with land-based means of transportation. The northern German peninsula situated in the Schleswig-Flensburg district separates the river Schlei from the Baltic Sea. The area covering 112 hectares is predominantly a nature reserve and bird sanctuary, it is prohibited to enter it and there are no public roads. In the summer months, the harbour of refuge at the tongue of the peninsula is popular with yachtsmen and people with sports boats alike. The 14 metres high lighthouse, built at the end of a mole in 1871, is used by ships and water sports enthusiasts for orientation.




New cables for the lighthouse

"This lighthouse needed renovation", Jürgen Wilhoeft explained; he works for the responsible water and shipping authorities Lübeck, the federal agency in charge of safety and facility of navigation and, in this case, the mole and sea marks. Within the bounds of the elaborate project, updating the technical facilities and renovating the facade of the lighthouse, the old cables providing the power and data for the light beacon also had to be replaced.
The existing cable path starts from a distribution station and takes course along the property of the "Lighthouse Foundation". This foundation, bent on the sustainable development of seas and oceans, had originally purchased the area with its buildings belonging to the federation at a public property auction in 2008. Only the distribution station and the mole remained in the possession of the federation. In view of future accessibility, the new path leaves the distribution station taking the shortest possible way and runs beneath the shore protection covered with large rocks on public grounds at the seaside, in parallel with the water's fringe, to a cable manhole at the foot of the mole. From here, one cable string leads to the lighthouse at the tip of the mole and another to a distribution manhole at the harbour.

In the planning phase, the water and shipping authorities attached greatest importance on avoiding any greater interference with the sensitive ecology in the course of the building measure. To obtain the required permits from the nature conservation authorities, the trenchless, fluid-assisted horizontal drilling technique was chosen. The closed breeding season for the birds ruled out any other time for the installation than the months of autumn/winter. "Actually, that is the time when you shouldn't be building anything here because of the weather", Jürgen Wilhoeft told us.

Difficult marginal conditions

There was a public call for tender for this installation measure and the company Paasch Rohrleitungsbau from Damendorf in Schleswig-Holstein, having submitted the most cost-effective offer, received the commission.

Given the marginal conditions, a reliable calculation of the costs for this measure posed quite a challenge for the tenderers. The transportation of the drill rig and job-site equipment through the nature sanctuary with all the required permits, the restrictions of material and machinery transport to the job-site, the specific properties of the building ground, the provision with soft water for mixing the drilling fluid and the disposal of the used suspension, all these factors were heavily laden with uncertainties and therefore hard to evaluate monetarily. In this case, Paasch had an advantage over the other competitors joining the bidding because the company had formerly drilled in Schleimünde for two other projects and was acquainted with the conditions on site quite well.
 
The job-site equipment was loaded onto two trailers and pulled through the nature sanctuary with the help of a tractor. As a means of avoiding field damage as far as possible, the path was covered with ground plates in places where the soil was too soft. For safe manoeuvring through the rough terrain, a tractor was also put before the truck for the drill rig on the way through the nature protection area to the job-site.
 
The water for mixing the drilling fluid was provided by a well with a limited yielding capacity of 1.5 cubic metres per hour. "For this reason, we stored tanks with water at the job-site to buffer the water supply in cases of shortage", Martin Paasch, managing director of Paasch Leitungsbau, told us.
 
Drilling was started in early October. A 15-ton drill rig from Tracto-Technik came into operation. The bore path leads all the way from the distribution station to the foot of the mole on a stretch of 200 metres. Four cable protection pipes made of HDPE with outer diameters of 140 mm and a wall thickness of 12.7 mm each were intended for the installation. The drilling operation demanded special aiming precision. The mole is bordered by two natural stone walls; the exit point of the bore was aimed directly at the centre between these walls. Therefore the bore was laid out approximately seven metres deep, the foundation of the mole had to be undercrossed at a relatively steep exit angle in order to reach the target pit. "And that worked really well!", Jürgen Wilhoeft exclaimed. Expansion and pipe pulling caused no further trouble, the same goes for the 60 metres long drilling operation from the mole to a distribution manhole at the harbour.

Unwelcome surprise

The bore to the lighthouse had a surprise in store for the crew, however. Backed up by the as-built drawing from the construction period, the plan was to drill the 200 metres to the lighthouse in the mole. The documents indicated that the core of the mole consisted of a sandy, drillable filling material. Instead, the company Paasch found coarse rubble which could not be penetrated with the machinery on site. "When it became clear that we were not making any progress at a depth of 1.50 metres, we tried drilling deeper. But even at 6 metres in depth, the underground did not improve", Guido Röhlich, the resident engineer of the company Paasch explained. In agreement with the customer, the decision was made to install the line in the mole with an open trench. "That made the job more expensive for us, but owing to the circumstances, we have no other choice", Jürgen Wilhoeft stated. Following the suggestion of the company Paasch, a strip as wide as the cable trench was cut out of the large-sized concrete slabs and put aside for the restoration of the surface after the protection pipes were installed.

Pulling in the cable with the power of water

For the process of pulling the cables into the protection pipes, the company Paasch, in co-operation with the manufacturer of blowing-in technology and winches Bagela, had come up with a brilliant idea. They applied the not yet widely used method of floating the cables into the protection pipe with the power of water. Compared with the conventional way of pulling-in, this variant saves time and protects cables with great pulling lengths from too much strain caused by the pulling forces and friction. "It was the first time we ever tried out this method, and it worked optimally", was Guido Röhlich's conclusion.
 
At the beginning of the new year, the job-site was basically finished. There were only a few spots here and there where inevitable damage was done to the surfaces and growth which had to be reinstated. In spite of the unforeseen problems at the mole - when taking stock of the complete installation method, the results turn out positively: the lighthouse Schleimünde with its modern technology is now fit for the coming decades and nature had hardly suffered, thanks to the discerning way a modern installation method was employed.
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