In Theory and in Practice
In 2015, after several years of development, Herrenknecht AG launched its first horizontal directional drilling HDD rock cutting tools to complement Herrenknecht’s other HDD offerings, which include maxi and mega class HDD rigs and ancillary surface equipment. The down-the-hole tools now allow Herrenknecht to cover the complete drilling process.
In developing the HDD cutting tools, Herrenknecht has drawn on its experience in making cutterheads and cutting tools for tunnel boring machines. The resulting new tools have a lower investment cost and reduce some of the risks encountered during HDD operation, along with increased production during rock drilling.
Current State of the Art
The most common method for borehole creation in rock today is the multi-stage expansion of the pilot hole by pulling a reaming tool through the progressively expanding drill hole in several successive steps.
The current state of the art and accepted practice is that the diameter of the borehole increases in steps, with the first step being from the pilot hole and increasing the diameter 15 to 18 in. (e.g. 8.5-in. pilot hole is expanded to 24 in. in the first reaming pass). Then an additional reaming step is done to increase the diameter an additional 10 to 12 in. (e.g. 24 in. is expanded to 36 in. in the second pass). This general practice is followed as the reamed diameter gets larger with the additional expansion of the diameter getting smaller during each reaming step.
As the diameter increases it becomes increasingly difficult to keep the cutting tool centered. With every revolution, the drill rod is lying on the invert of the previously drilled hole and tries to tilt the reamer head downward.
Additionally, during the reaming steps in larger diameter holes only a small part of the roller cutters are subject to load, which causes uneven bearing load and wear of the roller cutters. As a result of uneven tool engagement larger pieces of material can break off around the previously drilled hole. This material is usually too big to be removed in the mud stream through the drilling suspension and stays in the borehole, causing wear protection to wear away.
Most hard rock reamer heads, called hole openers, are made with the roller cutter welded directly onto the hole opener body. Because the hole opener is one piece it makes changing the roller cutters difficult. This means that the service life of the entire cutting tool is dependent on the life of the roller bearing and the wear behaviour of the roller cutters.
With the latest HDD rig technology, cutting tools built into the drill rod can be removed from a borehole relatively quickly and rapidly reinstalled. However, the longer the bore hole, the longer this “tripping” of the tool out of the bore hole can take. With this “tripping” time along with the time it takes for the inspection or changing of the cutting tool, it can take up to two shifts to change the cutting tools. Therefore it is imperative to have the longest service life as possible for the tools in order to save time and money.
In heterogeneous formations with differing hardness along the advancement of the borehole, it is advisable to change the roller cutters in order to achieve more rapid drilling through the differing formations. For tools with welded, non-replaceable cutters, this means that several different reaming heads are required as the ground conditions change.
Nowadays, the state-of-the-art rock drilling tools include roller cutters that are fitted with tungsten carbide inserts (TCI cutters) for harder rock formations and milled tooth cutters (MT cutters) for softer ground conditions. In addition, some reamer heads are fitted with polycrystalline diamond compacts (PDC).
Herrenknecht Full Face Hole Opener (FFHO)
The Herrenknecht FFHO reams from the initial pilot hole to the final borehole diameter in one pass instead of multiple reaming stages, and therefore only one cutting tool is needed. The reamer heads are only built in diameters of 30 in. (DN700) and above with preference given to pilot holes with a diameter of 12.25 in. Ideally the contractor uses 6.625-in. drill rods, with the new larger 7.625-in. rod being even better. For smaller diameters, the 5.5-in. drill rods can also be used.
A major reason for the decision to develop the single pass, full face hole opener is the excellent centering of the rotating drill rod in the pilot hole. It reduces the tilting of the reaming head and the damaging bending loads on the drill rod.
By eliminating the multiple stepped reaming heads with additional reaming passes, there are significantly less drill rod and cutting tool changes. This leads to a savings of time and money during the drilling operation. Jobsite experience indicates that the actual drilling process is also faster due to improved centering, which guarantees a more continuous concentricity and even loading of the roller cutters.
Since only one reamer head is required, the initial investment is reduced. It also allows the roller cutters to be quickly and simply changed on site to match the ground conditions. This is because both TCI and MT cutters can be mounted on the same reamer body. If a change in geology is encountered, an average of 20 minutes per roller cutter can be expected for replacing them.
An advantage of the FFHO is that the reamer body does not wear and the roller cutters are the main wear part, which are easily replaceable. The reamer body structure can therefore outlive several sets of roller cutters and thus is a long-term acquisition, whereas the roller cutters will serve as a project-related investment.
One of the first contractors to try the Herrenknecht Full Face Hole Opener was North American driller Laney Directional Drilling of Houston, Texas, as part of a multi-year test program. In late 2013, Herrenknecht supplied a 48-in. Full Face Hole Opener, for testing purposes.
Project 1 (Independence, Kansas)
The first project was the Verdigris River Crossing, which was part of the Flanagan South Pipeline located near Independence, Kansas. The project involved a 36-in. steel product pipe (DN900) to be laid under the Verdigris River using the HDD method. Enbridge, the pipeline stakeholder, commissioned Laney to drill the hole with a diameter of 48 in. over a distance of 1,757 ft (536 m).
The pipeline passed through a geology of alternating, relatively thin horizontal shale and limestone layers. The different rock strengths of each layer varied greatly and ranged from very soft to medium hard with values of up to 15,600 psi (108 MPa) axial compressive strength. The covering layer of about 23 ft (7 m) consisted of soft soil, which then transitions into shale and later into the harder limestone. The geological data was obtained from two vertical boreholes drilled down 131 ft (40 m) on either side of the river.
The horizontal borehole was done with a 670,000 lbf (3,000 kN) drilling rig developed and built by Laney. Crews used 6.625-in. drill rods with 5.5-in. FH DS threaded connections. The pilot hole was drilled with a 12.25-in. TCI Tricone drill bit in combination with a drill motor. The alignment depth was about 98 ft (30 m) and the calculated bend radius was set at 3,600 ft (1,100 m). The actual drilling was not problematic in this formation, but it was much more difficult to maintain the proper line and grade since the drilling angle was shallow relative to the layering of the limestone and shale.
After the pilot hole was drilled, reaming to 48 in. (1,219 mm) by a 48-in. Herrenknecht Full Face Hole Opener (FFHO) was done in one step. The hole opener was equipped with a set of TCI roller cutters for very hard formations along with a 47-in. centering device, which was attached behind the hole opener to increase the concentricity of the reamer head.
During boring in the upper layers of the shale it was found that under pressure the shale behaves in a similar way as clay mixed with water. An increased torque due to the increased friction within the reamer head was clearly apparent. At this point the drilling company experimented with the use of special polymers (clay busters) to eliminate clogging in the reamer head, which improved the drilling times in some sections.
The alignment then ran horizontally, mostly through limestone. An increase in drilling fluid viscosity was clearly visible on the screens of the mud system in the form of an increased discharge. Two inspection runs were made on the tool, during which it was found that material had accumulated behind the reamer body, but it could be worked into the mud stream again without major torque. It was thus possible to make an inspection run and a cleaning pass at the same time.
Pipe installation went without complications and was performed with low pull forces under 675,000 lbf (300 kN), indicating a clean and straight borehole. After project completion the roller cutters, which had been in use for 197 operating hours, were sent to Seattle for inspection. The body structure of the roller cutters and the tungsten carbide inserts showed little wear so, the roller cutters were fitted with new seals and bearings to be on the safe side. The reamer body structure also showed relatively little wear, and only in the area of the replaceable front centralizer was hardfacing rewelded for the next bore.
Project 2 (Cleveland, Oklahoma)
The second project, the Arkansas River Crossing, was another river undercrossing for the South Flanagan Pipeline described above. The 3,015-ft (920-m) long and 48-in. diameter borehole under the Arkansas River is near the small town of Cleveland, Oklahoma.
The prevailing geology on this bore consisted of mainly sandstone, combined with layers of shale and limestone. The rock layers were moderately broken with an RQD index of 50-60. This means there was a moderate risk of an unstable borehole because rocks from the borehole wall may fall into the invert. The uniaxial rock strength was determined to have a maximum strength of 8,000 psi (55 MPa), which is not considered especially hard. The presence of shale layers increased the risk of clogging the reamer head with material.
One special feature of this borehole is that the reception side is about 130 ft (40 m) higher than the launch side. The equipment used on this bore was the same as in the first project. The pilot hole was drilled with a 12.25-in. pilot hole drill from the lower side to the higher side.
After the pilot hole was completed the HDD drilling rig was set up on the higher side and the 48-in. FFHO connected to the drill string on the lower side. With the lower compressive strengths of the sandstone and limestone the decision was taken to use a set of MT cutters for reaming.
As with the first project, in the less sticky sections it could be seen that relatively low torque was achieved even with a maximum contact force of about 45,000 lbf (200 kN) per roller cutter (270,000 lbf [1,200 kN] for the whole reaming head). By altering the configuration of the 20 different nozzles on the reaming head, acceptable times were also achieved in the sticky sections.
The pipe installation itself again went smoothly, with a pull force of less than 11,200 lbf (500 kN). Despite the abrasive section of sandstone, the wear on the body structure of the reamer head was only moderate. After the second project, however, the front and rear centering units needed to be replaced. The MT cutter bodies were completely worn out and could no longer be used for another project. The river undercrossing has shown that the MT roller cutters work well and that the reamer head runs very concentrically downhole.
Michael Lubberger is Senior Product Manager for Pipelines at Herrenknecht AG, Schwanau, Germany.