Developed as an auxiliary tool for the pullback of pipe in horizontal directional drilling (HDD), the Direct Pipe method involves operating the Herrenknecht Pipe Thruster as a thrust unit, which is a similar technique to that of the jacking frame used for pipe jacking.
The Pipe Thruster holds the prefabricated and laid out pipeline and pushes it into the ground at thrusts of 5 m each. The requisite bore hole is excavated by a slurry microtunnelling machine (AVN), which is placed at the front of the pipeline. The soil excavated by the cutterhead at the tunnel face is mixed with the slurry in the excavation chamber and then pumped through the entire pipeline to a separation plant, using a slurry pump integrated inside the machine. The slurry also provides support at the tunnel face, and after treatment in the separation plant, it is conveyed back into the circuit via a feed line.
In terms of its general function, the Pipe Thruster is very similar to a conventional microtunnelling machine, however a key difference is the machine’s length. In order to ensure the requisite curve motion of the machine and subsequent pipeline in culverts, the Pipe Thruster features two to three backup pipes. Considering that all the individual backup pipe connections feature articulated tensile couplings, optimum control of the machine is ensured. Another advantage is that in an emergency, the machine and pipeline can be extracted along with the Pipe Thruster.
Just like the microtunnelling method, prior to launching, the machine is positioned at the requisite access angle on a launch rail in front of the launch seal. This protects against ingress of groundwater and soil. This is followed by the welding of the pipeline, which is mounted on rollers behind the launch pit, with the conical section of the machine at the rear.
The clamping unit of the Pipe Thruster embraces the pipeline and thrusts it into the ground along with the machine. The current maximum pipeline diameter that can be clamped is 1,524 mm. The forces to be anchored depend on the pipeline access angle and the maximum thrust or retraction force to be applied. Horizontal forces applied can be absorbed by lateral support profiles mounted in the shaft, while deep sheet pile or bore pile walls can be used for the vertical forces.
In the course of the Direct Pipe development process outlined below, the individual process components have been permanently improved and adapted to changing demands. For example, this year an innovative launch rail was used for the first time in order to set up the machine in the launch pit. It should be possible to save two to three days during setup using this hydraulic height and angle-adjustable support.
The Worms pilot project
With the aid of a Pipe Thruster (HK500PT) and an AVN1000 Direct Pipe machine, it was possible to successfully install a 464 m steel pipeline under the Rhine River in 2007. The steel pipe was intended to serve as a protective pipe for a water line and various protective cable pipes at a later stage. As the lack of space prevented the 1,219 mm diameter pipe from being installed in one piece, it was laid in sections of approximately 90 m.
The Worms project demonstrated that the friction arising from the Direct Pipe method is low, as the pipeline was pushed 80 t into the small target pit in only 13 days with no lubrication being applied along the entire length of the pipeline. This advantage has also been displayed in the form of relatively low thrust forces required within the framework of various other projects.
The first gas pipeline installations
The next phase of development undergone by the Direct Pipe method (i.e. the direct installation of product pipes) was preceded by a lab-quality load test conducted on the pipeline coating.
Confirmation was given to a Dutch gas supplier who intended to complete one of its projects using the Direct Pipe method that the Pipe Thruster would not cause any damage to the polypropylene (PP) coating.
Following successful testing, a total of five projects were completed using the Direct Pipe method in the Netherlands in 2010. The approximately 500 km long, 1,219 mm diameter North-South Route, which would transport gas throughout the Netherlands, involved crossings of between 360 and 540 m in length that bypassed obstacles such as archaeological sites, smaller rivers and a railway line. This was the first time gas pipelines had been installed using the Direct Pipe method, representing yet another milestone in the method’s history.
The most unusual of these five projects was the 540 m crossing of the very deep and busy Hartelkanaal in Rotterdam’s Europoort in the summer of 2010. What made this project so different was the requisite course depth of 30 m under the ground surface and the very steep access and exit angles of 10 and 12 degrees respectively (approximately 3–5 degrees had been the standard to date). The altitude to be overcome necessitated the application of a slurry pump within the pipeline (in addition to the one at the end of the machine). In geology comprising sand and silt, the entire pipeline was installed successively in ten sections of 54 metres each over a period of two weeks in September 2010.
Another milestone was achieved during the last two of the five Dutch projects (both over 500 m long) involving the first-time crossing of a railway embankment using the Direct Pipe method. The overburden under the railway tracks on the Zwolle-Almelo line totaled 15 m. The 1,219 mm diameter gas pipeline was thrust in one single piece and inserted into the target pit together with the machine with only 150 t of thrust force. Advance rates of up to 124 m were achieved in a shift lasting approximately ten hours.
Conclusions
All the projects completed since Worms show that the Direct Pipe method allows for fast installation speeds. This has made the process a practical alternative to HDD and microtunnelling. The improved installation reliability in difficult soil – compared to HDD – as well as the economic advantage over conventional pipe jacking translate into considerable competitiveness on the part of this method.
The method’s ability to overcome invariably new challenges indicates that the limits of the process will shift increasingly from one year to the next, making areas of application increasingly obvious – something which was difficult to assess in the early days. It will be exciting to see what other new applications are uncovered, especially for those wishing to use the Direct Pipe method for their own projects.
