Various methods have been applied in order to build steel pipelines over river and road crossings. Pipe jacking and segmental lining allow for the construction of concrete protective tunnels with subsequent insertion of the product pipeline. The horizontal directional drilling (HDD) method allows the laying of a steel pipeline but includes the construction of a pilot bore and several hole opening steps prior to pipe pull-in. Recent developments such as the Easy Pipe method comprise jacking processes where tight connection interim steel pipes are pushed towards the target shaft, coupled to a pipeline and then can be pulled back together in a second step. All these methods include a two- or multi-step pipe installation process.

The development of the Direct Pipe method, which is a combination of microtunnelling and HDD, was not only based on the creation of a one-step pipe laying method but also on the provision of an efficient alternative to existing methods, which is able to minimise the geological risks (e.g. drill-hole collapse using HDD). With Direct Pipe steel pipelines can now be jacked efficiently and fast in one operation process. The direct installation of the pipeline allows for continuous drill-hole support preventing borehole collapse. The Direct Pipe method

The Direct Pipe machine is mounted in front of the pipeline and is welded onto it. The Pipe Thruster, which is located in the launch pit, operates as thrust unit - clamping the pipeline on the outside and pushing the machine as well as the pipeline into the ground.

The tunnel face is excavated by the Direct Pipe machine similar to the pipe-jacking method, which has been established for several decades. The cutting wheel can be equipped with cutting tools adapted to the specific geological conditions. In contrast to HDD technology, larger boulders, hard rock as well as soft, unstable soils (gravel) can be crossed.

The excavated material is removed via a slurry circuit with separation plant in order to separate the spoil from the slurry liquid before feed pumps transport the liquid back to the tunnel face.

The Direct Pipe machine is controlled from the operating container. A gyro compass and a hydrostatic water levelling system are used for the horizontal and vertical machine surveying. To facilitate the controlled steering of the machine (and the connected pipeline), the Direct Pipe machine is longer than an ordinary microtunnelling machine at approximately 12 m instead of 6 m.

At the end of the Direct Pipe machine a lubrication ring is mounted in the transition area between and the product pipeline, where most of the bentonite is added in the annular gap, in order to reduce the friction between the pipeline and the ground to a minimum. The annular gap which is uses with the Direct Pipe method is 2-3 times larger than with the miroctunnelling method to improve the swimming behaviour of the pipeline in the borehole.

The Herrenknecht Pipe Thruster clamps the pipeline on the outside and pushes the pipes as well as the Direct Pipe machine strokewise (5 m each) forward with its two thrust cylinders. It can be adapted to diameters ranging between 500 mm and 1,200 mm. The clamping units can be deployed on any pipe type and coating. The two thrust cylinders are designed for a maximum pull and push force of 5,000 kN (500 tonnes) by advance rates of 5m/min.

Pilot project: Rhine crossing near Worms During the Rhine crossing near Worms, Germany in September 2007, The Direct Pipe method made its debut with the Rhine crossing near Worms, Germany in September 2007. Herrenknecht installed a pipeline with a total length of 464 m, which was designed as steel casing with an outer diameter of 1,220 mm.

The method was selected due to its economic benefits and project-specific advantages such as the minimum space requirements on the target side in Worms. It was also chosen due to the tight schedule, which required a timely realisation of the project because of the flooding danger.

After start of excavation, the new method exceeded even the most optimistic expectations. Average performance rates of 15 cm/min (maximum 25 cm/min) allowed for a fast installation of the pipeline. The average thrust forces were 70 to 80 tonnes, even though the pipe lubrication was not carried out automatically through the pipe string. Upon arrival in the target shaft, it became evident that the largest part of the thrust force had to be applied due to face pressure, which was no longer generated after the target shaft had been reached. Conclusion

Direct Pipe is an alternative method for pipeline installation, which not only has theoretical advantages over existing methods (in particular over HDD technology and especially regarding large diameters and difficult geologies) but also achieves impressive practical results. The Direct Pipe method allows for project requirements and jacking performances that are not possible with other methods. Future projects will show how far the limits can be pushed with regard to alignment length and performance rates. There is already a large number of possible upcoming projects due to the high and continuously growing worldwide demand for pipelines and undercrossing structures required for the oil and gas industry, for freshwater and sewage transportation as well as for supply and communication lines.

Direct Pipe is a registered trademark of Herrenknecht.