The situation in coastal areas is becoming critical. More and more people are moving to the world s urban centers. Big cities are turning into megacities with more than 5 million i...
In the construction of utility infrastructures in coastal areas or in river regions, tunneled Outfalls, Intakes and Landfalls are effective and sustainable method. With the help of sea outfalls (including river outfalls wastewater can be transported away from the coastline or river bank and discharged at locations where diffusion, dispersion and decomposition are enhanced. The municipal wastewater may be fully treated, pre-treated or untreated.
Water intakes can be constructed, for example, to supply water to desalination or power plants. If no beach or sandy floor exists near the plant location, or if the site conditions are inadequate for infiltration, a tunneled offshore intake system is the ideal choice. The worldwide growing demand for oil and gas makes the construction of pipelines on and offshore necessary. Here are methods for trenchless installation of offshore lines pipe jacking and micro tunneling.
Installation of pipeline
A launch shaft is excavated at the land side of the pipeline. The dimensions and the design of this shaft may vary according to the specific requirements of the site with economics being a key factor. A thrust wall is constructed to provide a reaction against which to jack. The initial alignment of the pipe jack is obtained by accurately positioning guide rails within the thrust pit on which the pipes are laid.
Powerful hydraulic jacks are used to push the jacking pipes through the ground. At the same time, excavation at the tunnel face is taking place within a steerable shield. Remote controlled microtunnelling machines are operated from a control panel in a container which is located on surface next to the launch shaft. This is an advantage regarding safety regulations, because no staff has to work in the tunnel during construction. The position of the remote-controlled machine is supervised by a guidance system.
Today, the developed tunneling technique enables the realization of long distance advances, also in difficult ground conditions. The following possibilities reduce friction and jacking forces.
Automatic lubrication system
During the pipe jacking process the whole pipeline is pushed through the ground. Rising friction forces between the surrounding ground and the pipe string lead to increasing jacking forces. However, the maximum jacking force is strictly limited by the maximum permissible pipe load. To reduce friction, the pipeline should be lubricated continuously. Bentonite suspensions act as lubricants during the pipe jacking process. They are mixed in bentonite plants at the job site and are pumped into the tunnel via hoses or pipes. Through injection nozzles within the jacking pipes, the lubricant is squeezed into the annular gap between the pipeline and the surrounding ground. Thus, the jacking forces can be reduced considerably. Reduced jacking forces optimize the performance of the pipe jacking process in terms of lower pipe loads and thus longer jacking distances.
A new generation of our bentonite lubrication system enables the automatic distribution of bentonite suspensions along the alignment, while controlling and recording the injected bentonite volumes along each pipe at the same time. This permits to select and adjust the desired bentonite volume of each meter along the tunnel route, also considering changes in geology. Monitoring, control and recording of relevant data, such as bentonite volumes, pressures and friction forces, is done in the control container where the machine operator supervises the pipe jacking process.
Use of intermediate jacking stations
Another, mostly additional possibility to keep jacking forces down in difficult ground or long drive lengths, is the use of intermediate jacking stations. They are installed to reduce and distribute the jacking forces, thereby reducing the forces applied by the main hydraulic jacks in the start pit and secondly on the thrust wall. The interjacking stations are integrated in several distances in the pipeline and serve to separately advance the pipeline in sections.
Additional push force with telescopic station
To realize longer advance distances in accordance with the given maximum load, telescopic stations are a further possible option. The Telescopic station consists of 2 steel pipes, the inner and the outer part, which can be pushed into each other. These steel pipes are connected by integrated jacking cylinders. The telescopic station is used as a jacking station following the machine. Situated behind the tunneling machine, it controls the forces applied to the cutting head of the machine. Now, only the part in front of the telescopic station moves forward. When the telescopic station has reached its maximum stroke, the remaining pipeline is jacked forward with the use of the main jacking station in the launch shaft, whereby the telescopic station is pushed together again. The result is a divided jacking force. Consequently, the allowed maximum loads of the jacking pipes will not be exceeded and longer distances can be realized. Furthermore, after a longer standstill, the tunnelling machine is able to be started up again without problems, because it is not necessary to move the whole pipeline at once.
Access to cutting wheel
A deciding factor for attainable drive length is the lifetime of the cutting tools. Due to strongly worn cutting tools the tunneling performance will be decreased, and when cutting tools are worn-out, they have to be changed. tunneling machines have been designed to give the possibility to enter the excavation chamber to inspect the cutting head and to change worn cutting tools. If necessary, an airlock can be used to be independent from possible ground water in front of the tunneling machine. Consequently, longer drives can be realized
Machine Recovery
Tunneling machines to be used for Sea Outfalls are equipped with an additional recovery module, consisting of a steel can with bulkhead to close the machine and hydraulic cylinders to separate tunnel and machine. The supply of hydraulic oil for these cylinders is done by divers and connected from the outside skin of the recovery module. After complete installation of the tunnel, the seaside end of the pipeline is mostly closed with a bulkhead equipped with a valve.
Source
Peter Schm�h
Member of the Executive Board
Business Unit Utility Tunnelling
E-mail schmaeh.peter herrenknecht.de
Lutz zur Linde
Sales Manager
Business Unit Utility Tunnelling
E-mail ZurLinde.Lutz herrenknecht.de
Herrenknecht AG