New width for directional drilling

German ductile-pipe manufacturer Buderus Guss has achieved a European first in pipeline construction using HDD. Steffen Ertelt of Buderus Giesserei Wetzlar, Hermann Lübbers of Nacap and Pablo Ramón of Construtec Canalizaciones explain the challenge

Trenchless laying of ductile cast-iron pipes with horizontal directional drilling (HDD) has been an established feature in pipeline construction for many years. But work on a project in Alzira, a small town near Valencia in Spain, has now chalked up a European first in the trenchless installation of ductile cast-iron pipes.

For the first time, a cast-iron pipe with a nominal diameter of DN 900 has been laid by HDD.

A new pipe with nominal diameter of DN 900 had to be installed for the water supply in the region around La Ribera. Part of the pipeline had to cross the river Jucar, the railway line from Valencia to Alicante, which runs parallel to the river, and a factory building.

It was not possible to cross the railway with open installation methods, nor could a nearby bridge be used, as calculations showed that it would have been over-stressed by the weight of the pipeline filled with water. The client therefore opted for a culvert construction using HDD, with pipes of ductile cast-iron.

The tender for the drilling work and for supplying the pipes was won by Nacap and TK Eelde of the Netherlands, German pipe manufacturer Buderus Giesserei Wetzlar while the consortium was represented in Spain by Construtec Canalizaciones of Bilbao.

HDD usually consists of three phases: pilot drilling, widening and inserting the pipe.

For the pilot drilling, the drilling head is driven along a stipulated drilling profile by the drilling machine. To start with, the first section of the drilling line – consisting of drill cutter, bent sub and non-magnetic drilling rods – is pressed by the drilling machine into the ground at a previously stipulated angle of entry.

Generally, drilling mud consisting of a water-bentonite suspension is pumped through the rods to the cutter nozzles, which force it out at great pressure. The ground loosened in this way is brought back to the surface, with the drilling mud flowing back through the ring chamber.

The drilling line is directed by constantly changing the working direction of the cutter, brought about by specific rotation of the drilling rods and thus the bent stub by small amounts. A measuring probe directly in the drilling head is responsible for locating the position of the drill head.

Once the directed pilot drilling phase has reached the surface again at the target point, the cutter and measuring probe are removed and a reaming tool is fitted to the machine. This is a tool for widening the drilled channel to a larger diameter.

Depending on the prevailing ground formation, barrel reamers, fly cutters or hole openers are used for this purpose.

The reaming tool is usually pulled back in rotating mode and flushed with mud from the emergence point back to the drilling rig. This procedure is repeated until the drilling hole has reached the intended final diameter.

During the last phase of HDD, the prepared pipeline is inserted in the hole, which has been widened to the required size. To insert the pipeline, the reaming tool is set rotating and pulled back through the system, flushing mud as it does so.

A swivel joint connection with the pipeline transfers the pulling force to the pipeline, but not the torque. As a result, the pipeline follows the reamer through the hole practically without turning at all, through to the entry pit at the drilling rig.

The drilling phase was carried out with a maxi rig with a pulling force of 2500kN. A 241mm milled tooth cutter was used. The quantity of mud used can be specified at about 600 l/min.

Altogether 49 drilling rods were used in the process, corresponding to a drilling length of about 456m. The channel was produced with a drilling radius of 600m. The drilling channel was widened in three reaming phases. A 813mm fly cutter was used in the first phase, flushed on average with about 1,500l/min mud. Reaming was carried out with a torque of around 4kNm and a pulling force of 80kN without any technical problems.

The second reaming phase produced a diameter of 1.17m. The fly cutter was stabilised and centred with a 711mm barrel reamer, working with a measured torque of about 7.5kNm.

The final diameter of the drilled channel was 1.37m. The reaming tool worked with a torque of about 10kNm and was pulled through the drilled hole with a pulling force of 120kN.

Ductile cast-iron pipes with positive longitudinal force fit connecting sleeves have provided outstanding service in many HDD projects. This particular project used ductile cast-iron pipes with a nominal diameter of DN 900 and restrained socket joint BLS, meeting the cement mortar coating standard DIN 30674-2.

With the BLS restrained socket joint, the locking segments are secured by a shear protection antechamber integrally cast to the pipe sleeve, which then supports the welding bead applied in the factory to the pipe shank. This means that the locking segments made of ductile cast iron can take very high forces of up to 1,845kN for the BLS joints DN 900. The sleeve connections can be angled at up to 1.5° for a nominal diameter of DN 900, permitting a minimum curve radius of 230m.

In order to meet the increased safety requirements for Europe’s first HDD project with ductile cast-iron pipes in this dimension, the BLS locking segments were also secured with fixing clamps. These fixing clamps ensured that the locking elements were firmly connected to the pipe shank while the pipe was pulled into the drilled hole.

The outer protection of the pipe in zinc with plastic-modified cement-mortar coating has proven outstanding under external mechanical loads, as can occur particularly in trenchless installation in unforeseeable magnitudes. Thanks to the addition of fibres to the cement mortar and the PE net bandage, the cement mortar coating is extremely resistant to mechanical loads.

This outer protection for the pipe can also be used in soils of any corrosiveness. A PE shrink material was used to protect the connecting sleeves. A metal cone was also applied as further protection from any possible mechanical damage.

The pipeline was to be pulled into the hole in floating state to reduce the friction forces that account for the majority of the necessary pipe pulling forces. Before assembly of the complete pipeline section, a corresponding pipe trench was excavated for this purpose. The individual pipe sections were then assembled in the trench.

Under the technical supervision of Buderus, the pipeline was assembled and subjected to a final pressure test. The pressure test was carried out at a pressure of 21 bar and accepted without any complaints by the acceptance authority.

The pipe trench was subsequently flooded from the return pipe of the drilling mud system. A volume of around 1,600m3 had to be filled before the pipeline reached its final floating position.

Parallel to the filling work, a cleaning run was carried out with a 1.1m barrel reamer. This checked free passage through the drilled channel for the last time, and also brought any solid matter left in the drilled channel up to the surface.

After coupling the pulling fitting to the traction head, the procedure for pulling the pipe into the drilled channel began at 6.33pm on March 14, 2007. By 9pm, the pipe had reached the starting pit at the drilling rig.

The pulling force was measured at maximum 750kN, which was only about 0.4 times the weight force (1,830kN) of the pipeline. The client, supplier and the drilling company worked as a well coordinated team to ensure the success of this unusual HDD project.

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