Tracto-Technik — WMZ bores a pipe for the sea
Posted: 1 September 2004
Lake Constance with it’s 580 km² is the largest and with 250 m the deepest sea in Germany. Furthermore the sea is a huge drinking water reservoir, which supplies wide areas of Southern Germany. The University of Konstanz uses the sea water for the cooling of their climate and research units.
The sea water is taken from a grey cast iron pipe over a 400 m length at a depth of 45 m and supplied to a retrieval shaft. The neighbouring pumping factory sucks water out of the retrieval shaft and conveys it via a 500 m length, 250 mm grey cast iron pipe into a high container in Allmannsdorf. From there the university is supplied via a 900 length, 250 mm grey cast iron pipe with the 5°C cool water, which after usage in a natural manner, can flow back into the Lake Constance, cooling down slowly with about 16°C.
Due to the extension building to the university’s library there is a higher requirement for cooling and water. The capacity of the existing cooling water supply system is 60 l/s and unsufficient to cover the requirements all year round. Therefore, additional fresh water had to be fed from the network.
In addition the 90 year old land pipeline between the pumping factory Egg, the high container at Allmannsorf and the University Konstanz, as well as the 400 m long sea pipeline were in a poor condition, which called for a renewal of the pipes.
The government building surveyor’s office Konstanz awarded the engineering company UNGER from Freiburg with the task for planning and building measures for this project.
The land pipeline was already replaced in the Winter of 2002/2003 with a 400 mm grey cast iron pipe.
In Winter 2003/2004 the renewal of the sea pipe was on the agenda, which was to be installed over a 100 m length from the retrieval shaft right into the Lake Constance using the horizontal drilling method. This way the new pipeline is protected against any mechanical influences and frost and low water. Furthermore, the use of the drilling fluid technique was an environmental aspect, as well as of optical interest.
The new retrieval shaft, with a base approx. 7 m below the terrain top surface and a diameter of 4,20 m, is surrounded by an overcut sheet pile wall and will later be applied with two drain pumps in a dry state.
After the public tendering of the project the special civil engineering company Strobel GmbH from Pfullendorf won the award. The drilling company WMZ from Lauingen, as sub-contractor, was to carry out the drilling work and Klumpp GmbH from Offenburg the pipe installation tasks.
The new HDPE sea pipe with an outer diameter of 500 mm and a wall thickness of 45,4 mm had a total length of 320 m and should be laid over a 210 m length at an approx. depth of 40 m on the sea floor. The topgraphical and geological conditions within the bore path required a bore depth of 20 m. The exploration of the ground showed soft sandstone in the top area and clay and marl in the deeper layers.
Together with the engineer’s office UNGER, the drilling company WMZ and the pipe installation company Klumpp GmbH a detailed installation plan was developed. The bore itself showed no unsolvable problems. The challenge was seen in the controlling of the buoyancy, lowering and installation of the pipes.
The following measures were agreed upon:
- For the installation a multi-layer pipe was used, which consisted of a core pipe and a sheathed pipe, which protects the pipe against any mechanical damages during the installation. The 12 m pipes are to be connected together by butt-welding.
- The part of the welded pipe string, which is to be lowered onto the sea bed, had to be ballasted on the embankment. The engineering office UNGER calculated 50 concrete blocks for this with a descendancy of 400 kg each, placed at distances of 4 m each on the pipe.
- A steered flooding of the pipe was required for the lowering process. Lowering had to begin at the land side, so that the air could be guided out. In order to do this a filling line had to be pulled into the whole pipe length. Filling was carried out from the sea side of the pipe with a pump.
- To hold the pipe string above water in a stable position and under water, swimming at the installation height of about 20 m, 6 pontoons at distances of 55 m each were necessary. The pontoons were equipped with winches, with which the pipe string was lowered to a depth of approximately 20 m (Exit point of the bore) and held there, to make sure that the pipe pulling.in process was carried out as trouble-free and easy as possible.
- The maximal pulling capacity of the Grundodrill noring unit of 20 t had to be taken into account. With a raw weight of 15 t and an additional 20 t concrete weight a max. pulling force of 10 t was expected underwater.
- A diver team was ready for action to carry out any retrieval or assembly work under water.
- The city works Konstanz had a ferry with a cable dredger waiting, if required.
After producing the retrieval shaft the pilot bore could begin. The Grundodrill bore unit (Manufacturer: Tracto-Technik, Lennestadt) was positioned approx. 30 m away from the retrieval shaft, to be able to reach the installation depth at the height of the retrieval shaft. After 70 m the bore head suddenly met a difficult to penetrate sandy stone layer, which required the appliance of a rock drilling bore-hole motor. The pilot bore took 2 days to complete.
Several expander bores followed, in diameters of 10, 14, 20 and 25 inches. After 6 working days they were finished. After the final expanding bore the 640mm backreamer was pushed far out to sea with the drill rods through the bore hole, to keep the strain of the drill rods to a minimum for the retrieval process. The divers were required once again, who fastened a tension chain to the backreamer. The cable dredger on the ferry pulled the backreamer to the water surface, where it was finally attached to the pipe after several dramatic attempts.
In the meantime the pipe string was aligned at sea and ready for pipe installation. The browsing wind, however, caused problems for a short period. The pipe string drifted away on many occasions and always had to be pulled backed to the intended installation line.
The tension amounted with all the participants. An engineer was in position on all of the pontoons. The pipe string was flooded. Simultaneously the lowering procedure at all pontoons began. The planners had got their sums right. The pipe disappeared slowly below the water surface, until only the pontoons were visible. You could feel the ease with everybody. But the last hurdle – the pipe installation itself – was still ahead. During the pulling-in process a sudden resistance was clearly detected in the bore hole. The pulling force was already at 12 t. The WMZ team guessed that a rock could have got jammed in front of the pipe. With a great amount of sensitivity and relevant support from the drilling fluid the resistance slackened off after 2 further drill rod lengths. The pulling force dropped drastically to 7 – 8 t. There were no further problems afterwards. The section time for each drill rod was 3 minutes. The arrival of the pipe string in the retrieval shaft signalled great applause and appreciation from all the attending on-lookers. After the installation process the corresponding works were carried out. The retrieval line was finally lowered to the bottom of the sea and the pontoons were disassembled.
Due to the application of the HDD technology the ecology of the flat water zone and the immediately neighbouring litoral garden of the university in Konstanz had hardly been influenced and therefore met with great approval with the council, the fishing supervisor, the university biologists and the sea research institute in Langenargen.
For more information see www.tracto-technik.de
Posted by Richard Price,
Editor Pipeline Magazine
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