ABSTRACT
It is well-known that HDD uses the borehole as a discharge line to remove the cuttings out of the borehole. A slow velocity of the mud is not productive when large cuttings are transported as they tend to settle along the borehole. Remaining cuttings left in the borehole lead to large overcut dimensions to avoid pipes getting stuck during pullback. Furthermore, excessive pumping of mud can lead to frac-outs when the fluid pressure is higher than the maximum allowable borehole pressure. The complexity of the mud design increases which results in costly mud monitoring and adjustment on jobsites.
With the development of a downhole jet pump system for HDD operations, Herrenknecht has set a milestone in the pipeline industry. In conjunction with a new tooling series, the downhole jet pump system allows drilling and cutting faster at lower cost and risk. The full face hole opener has been developed for hard rock drilling with HDD, where time-consuming reaming passes are reduced to one single step.
The successful concept of the jet pump system for HDD has now been transferred to Pipe Jacking technologies. The long existing demand for increasingly long drives in simultaneously smaller diameters, has now been met with the development of the AVNS technology. The AVNS tunnelling machine (minimum ID 350 mm) with powerpack and slurry jet-pump in the machine is designed for up to 1,000 m drive length, depending on geological conditions. Furthermore, this machine concept can also be used for Direct Pipe® and opens up the lower diameter range from 20” to 30” pipelines.
This paper presents the use of the jet pump systems in HDD, Pipe Jacking and Direct Pipe operations and gives an overview of applications fields and recent case studies.
