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Herrenknecht’s New Full Face Hole Opener in Theory and Practice

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By Michael Lubberger

In 2015, after several years of development, Herrenknecht AG launched its first horizontal directional drilling (HDD) rock cutting tools. Until now, in the field of horizontal drilling the company has only manufactured large premium HDD rigs of the maxi and mega class (180,000 lbf to 1,349,000 lbf thrust/pullback force). Along with those premium HDD rigs Herrenknecht AG makes surface equipment that covers a large part of the HDD setup. And now the company has developed and manufactured down the hole tools to expand its product line to cover more of the complete drilling process. The downhole tools are a key component in HDD and a decisive factor in the productivity of drilling projects.

With its entry into the HDD cutting tool market, in particular rock cutting tools, Herrenknecht has relied on its knowledge and experience from hard rock cutting with tunnel boring machines to develop its new line of HDD tooling. Being the market leader in mechanized tunneling technology with more than 3,000 cutterheads built, Herrenknecht has great expertise and experience in the field of cutting tool technology. Good and stable cutting tools are key for any kind of cutterhead in rock. These cutterheads range from 16 to 58 ft in diameter and based on these sizes there are significant differences in the cutting tools used. There is also a difference between the tools used in the horizontal and vertical boring depending on the type of machine and application. The types of ground conditions encountered in these applications range from soft ground to hard rock with the potential for high abrasive wear in both cases. The pressures encountered can range from no groundwater present where the discs run “dry,” up to 725 psi groundwater pressure, which acts directly on the seals of the disc cutters. The development and selection of the proper roller disc cutters according to the expected ground conditions is crucial to their success.

In developing the HDD cutting tools, Herrenknecht has drawn on the disc cutter developers from its Sumner, Washington, facility, who together with the R&D department at the headquarters in Germany took a close look at the HDD application. 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

When creating a borehole in rock the HDD method has a variety of risks, technical problems and cost, such as:
1) Breaking of the drill string directly in front of or behind the reamer head, which is caused by reversing bending stresses at the threaded connection. This repeated bending is caused by an offset between the axis of the drill string in the borehole and the axis of the reamer head. This offset is caused by the lack of centering during the various stages of reaming.
2) Premature failure of the roller cutters and short service life of the cutting tools due to high undefined forces on the roller cutters. These roller cutters are usually too small, overhung mounted and were originally designed for much smaller pilot holes.
3) High wear on the roller cutters and the body structure of the reamer head.
4) Drilling fluid losses through natural cracks in the formation that have been caused by excessive drilling fluid pressure.
5) Unclean boreholes that cause large deposits of drill cuttings to accumulate in the invert. This can result in jamming during installation or removal of the cutting tool for maintenance. In the worst case scenario this causes high frictional forces during pipe installation that can lead to the pipeline getting stuck.
6) Short service life of the roller cutters on the reamer head which cannot be easily replaced. As a result, a complete new reaming heads must be used if the roller cutters are worn or the cutter bearing is defective.
7) The torsion spring effect of the drill string due to low torsional stiffness. This especially happens with longer holes when the drill string twists and stores energy which is then discharged in short intense bursts. This usually results in breakage of the drill rod or a loosening of the threaded connections. Additionally, unacceptably high speeds and loads for the cutting tools can occur.
8) Squeezing or crumbling behaviour of the subsoil.
9) Keyhole-shaped borehole due to lack of centering during the various reaming stages.

With the above criteria and problems in mind, the development of Herrenknecht HDD tools was started with the goal of minimizing the risks while simultaneously improving performance and cost efficiency. With regard to the application range of the tools, 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 ¼ in. Ideally the contractor uses 6 5/8-in. drill rods, with the new larger 7 5/8-in. rod being even better. For smaller diameters, the 5 ½-in. drill rods can also be used.

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 number of reaming steps and the reamed diameter is determined by the drilling contractor. However, 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. during the first reaming pass).

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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. during 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. It is not uncommon that with reamed diameters greater than 48” that the diameter increase is only 4to 6 in.

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 broken off material is usually too big to be removed in the mud stream through the drilling suspension and stays in the borehole. It also usually falls into the area around the front centralizer and cannot be removed from there by the reamer head. As a result, material accumulates in the region at the front centralizer, which during the course of drilling is broken down and causes the 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 nearly impossible. 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. Most manufacturers recommend inspection intervals of about 80 to 120 hours for the assessment of the bearings, with the service life of the hole opener usually being under 150 hours.

With the latest HDD rig technology, the cutting tools which are 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 whole shifts to change the cutting tools. Therefor it is imperative to have the longest service life as possible for the tools in order to save a considerable amount of 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.

Herrenknecht’s new Full Face Hole Opener

Downhole illustration of Herrenknecht’s new Full Face Hole Opener

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. For some time now there have also been reamer heads fitted with polycrystalline diamond compacts (PDC). Although these tools promise low wear, they are not as universally usable as MT or TCI tools. Especially in heterogeneous formations with fault zones, the tools fitted with PDC reach their wear limits quickly as a result of clogging and blocking. On the other hand, PDC tools have the advantage of being a static tool and thus do not require bearings and seals which are used with roller cutters.

New Herrenknecht Full Face Hole Opener (FFHO)

The solution that Herrenknecht has developed has already been hinted at in the name of the product. 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. 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. Larger and heavier duty drill rods and their threaded connectors allow simpler and more reliable transmission of higher torques. Compared to conventional tools, with the new full face hole opener an increase in torque is not needed.

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. The main reason for the faster drill times is the 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 compared to conventional reamer heads. 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.

The roller cutters, which were developed and are manufactured at the Herrenknecht plant near Seattle, are almost twice as resilient as conventional roller cutters on the market. In addition, with a higher allowable contact force (bearing load) there is a potential for higher penetration rates.

Herrenknecht has also created a new standard with regard to the bearing service life of its roller cutters. The first inspection interval is 200 operating hours, which is about half as often as other products. As a general rule the roller cutters can be completely overhauled as long as the wear of the tungsten carbide inserts and the cutter body does not exceed the maximum allowable wear limits. The inspection and overhaul of the roller cutters is offered by Herrenknecht, but can be carried out by the customer with the help of training and special tools.

The structure of the reamer heads, is produced at Herrenknecht AG’s main plant in Schwanau, Germany. The entire manufacturing know-how and welding technology goes into the production of high-tech reaming heads. The basic idea 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. Unlike conventional tools, special consideration was given to optimizing the bore hole material transport by the design of reamer heads. This optimization helps minimize wear of the roller cutters and the body structure.
One of the first contractors to try the Herrenknecht Full Face Hole Opener was North American driller Laney Directional Drilling as part of a multi-year test program. In late 2013, Herrenknecht AG supplied a 48-in. Full Face Hole Opener, for testing purposes.

Project 1—Independence, Kansas

The first project was the Verdigris River Crossing located near the city of Independence, Kansas which is part of the Flanagan South Pipeline. This pipeline runs parallel to the existing Spearhead Pipeline from Pontiac, Illinois to Cushing, Oklahoma. The purpose of the pipeline is to transport petroleum.

At Independence the 36-in. steel product pipe (DN900) was to be laid under the Verdigris River using the HDD method. Enbridge commissioned drilling contractor Laney Directional Drilling from Houston, Texas to drill the hole with a diameter of 48-in. over a distance of 1,757 ft.

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 axial compressive strength. The covering layer of about 23 ft 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 on either side of the river.

The horizontal borehole was done with a 670,000 lbf drilling rig developed and built by Laney themselves. The jobsite was equipped with a mud trailer on both sides of the river, making it possible to process and pump the drilling fluid from either side. The separation and high pressure pump capacity was 1,057 gpm, and 6 5/8-in. drill rods with 5 ½-in. FH DS threaded connections made from S-135 material were used. When new, these drill rods allow maximum make-up torque of about 60,000 ft-lbs, which was not exceeded during operation.

The pilot hole was drilled with a 12 ¼-in. TCI Tricone drill bit in combination with a drill motor. The alignment depth was about 98 ft and the calculated bend radius was set at 3,600 ft. 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. Specifically, the transition zones from soft ground to harder rock were critical in maintaining the proper alignment of the borehole. If an impermissible angle change was measured in such a transition zone, a delay in drilling occurred as the material had to be “ground away” in order to get back to a normal value. After the pilot hole was drilled, reaming to 48 in. by a 48-in. Herrenknecht Full Face Hole Opener (FFHO) was done in only 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 increased the concentricity of the reamer head.

In this test it was learned how relatively non-aggressive TCI cutters behaved in very soft formations. This is because TCI cutters are typically used in formations that are harder then 11,600-14,500 psi and MT cutters are used in softer formations. The maximum rock strength that were expected to be encountered during this bore was 15,660 psi.

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. In these zones the drilling time per rod (about 32 ft R2 drill rods) was between 4:10 and 5:40 hours. 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. In this section the fastest rod times were achieved with a drilling time per rod being between 2.5 and 3 hours. The pumped mud volume was about 925 gpm, with the viscosity of the drilling fluid being mostly below 60 MV. An increase in the 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, indicating a clean and straight borehole. When only the drilling times in rock are considered, an average drilling time per rod of 3 hours was achieved. With the average drilling time for the entire drilling distance being 3.5 hours per drill rod. If one takes into account that the reaming head was fitted with roller cutters for hard rock, although most of the bore was more in soft formations with less than 7,250 psi compressive strength, these times can be considered a total success. 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 borehole. By adding additional hardfacing, the complete replacement of the front centralizer was delayed.

Map of Enbridge’s Flanagan South Pipeline

Map of Enbridge’s Flanagan South Pipeline, on which Herrenknecht’s new Full Face Hole Opener was used in projects in Kansas and Oklahoma.

Project 2—Cleveland, Oklahoma

The second project, the Arkansas River Crossing, is another river undercrossing for the South Flanagan Pipeline. The 3,015 ft 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, 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 higher than the launch side. This means that for over a distance of about 755 ft the borehole is dry. The horizontal layering of the different formations meant that the long horizontal part of the borehole could be run through the more stable sandstone as compared to the first project. With better stability, the sandstone seemed the best layer to drill in. However, it was feared that the abrasiveness of the sandstone due to its high quartz content could cause high wear on the roller cutters and reamer body.

The equipment used on this bore was the same as in the first project. The pilot hole was drilled with a 12 ¼-in. pilot hole drill from the lower side to the higher side. The rod times for the pilot hole seldom went over 45 minutes.

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. Generally, in appropriate geology, higher penetration rates can be achieved with MT cutters as opposed to TCI cutters. This is because of the tooth protrusion with the MT cutters is at least 3 times more than tungsten carbide insert protrusion of the TCI cutters. As a result, the MT cutters theoretically allow more than 2.5 times higher penetration per revolution.

During the drilling process the rod times for reaming through the sticky shale formations were always kept under 4:40 hours, with the average drilling time in the sticky sections being 2.5 hours. The fastest rod time of 1:02 was achieved in the longer sandstone formations, with the average rod time for the entire drilling distance with the FFHO being 1:35. 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 per roller cutter (270,000 lbf 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. 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. It has also been found that the nozzles in the reamer head have an important role to play in reducing the clogging of the reamer head.

Michael Lubberger is senior product manager, pipeline, for Herrenknecht AG.

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