COBBLE ROCK AND HORIZONTAL DIRECTIONAL DRILLING 1
Cobble Rock and Horizontal Directional Drilling
Michael Phillips
Southern Utah University
Abstract
A study of horizontal directional drill toolingfor a Ditch Witch directional drill was conducted pertaining to effectiveness, durability, and longevity. Three different drill bits manufactured by two different companies used to drill in cobble rock type soils were tested. The study’s hypothesis was that drill bits that are less aggressive to cut through cobble rocks, as well as withstand the harshness of drilling in cobble rock. The study concluded that while less aggressive drill bits were more durable over all, 95% of the tests resulted in each drill bit being too damaged to be considered feasible to be used again for another drilling application.
Keywords:horizontal drilling, Ditch Witch, cobble rocks
Cobble Rock and Horizontal Directional Drilling
Horizontal directional drilling (HDD) is a trenchless technology that replaces conventional forms of underground construction. It is used to install utilities or pipelines under roads, through wetland areas, under existing pipelines/utilities, or any other situation where typicalexcavation is impractical. Environmentally speaking drilling horizontally significantly reduces the damaging impacts excavation creates (Mud, Saun, & Zall, 2013). A study was conducted and found that digging in a pipeline caused 30% more damage to the existing environment (Mud et al.) If a river crossing is involved the ecosystem of that river could be compromised for up to fifteen years after excavation is complete (Mud et al.).
Horizontal directional drilling is often more efficient, safer, and cheaper than classic means of pipeline installation. However, it is not a fool proof technology. Given soft soil conditions or even solid rock conditions HDD works exceptionally well, the difficulties arise when faced with cobble rock. Because cobble rock tends to not stay in place it is difficult to drill through a single cobble rock. Different sizes of cobble stones makes controlling the drill shot almost unobtainable. Vibrations created when working in rocky soil conditions damage the drilling machine, locating devices, and the tooling. Drilling out is only half the battle.
When reaching the target exit hole the other half of the operation begins, pulling product back through the hole. If the project involves pulling back large heavy walled steel pipe it is important that the drill shot is as flat and straight as possible; as this type of product does not easily bend and tends to get stuck where the boar path has been corrected. Furthermore, it is crucial that the drill bit being used is able to endure and keep the drill shot in line. Because of the nature of cobble rock it is difficult to keep a good open hole for pulling product. It is important to understand pulling forces involved when it comes to HDD (Chehab & Moore, 2012). In a study conducted by Chehab and Moore, both engineers, all of the pulling forces pertaining HDD is mathematically solved. However, it is hard to plan for a cobble rock falling in between drill stem and product causing things to break and pulling forces to sky rocket.
Method
Materials
In this study the products of two manufactures of HDD tooling will be tested. Both manufactures will have three drill bits tested. Two of the three bits being tested from both manufactures are similar in design; the other from each manufacture completely different in design.
Procedure
To determine what drill bit is the most durable for drilling is cobble rock; each drill bit was used on the job with analysis conducted. However, each drill bit will be tested on a different job location. That being said certain ground conditions will not be consistent, cobble rock being present will be the only constant variable.
After each drill bit went through the ground an evaluation of their performance and damage was conducted. The evaluation was based on the following three questions:
- Was able to complete the job?
- How much of the drill bit was still intact and is it reusable?
- Was the drill bit controllable during the drill shot?
Results
Analysis of each drill bit when it finished drilling indicated that in each differing design each had significant damage. Both Manufacture A’s & B’s tri-cone type drill bits completed the job, but with all three of the roller cones missing. Manufacture A’s iron fist type drill bit completed a job with zero structural integrity left. Manufacture B’s iron fist type drill bit was used until no further progress was being made, when pulled back zero structural integrity left. Manufacture A’s three tooth design drill bit was used until indication of broken teeth, pulled back with zero teeth left and no option to put new teeth on. Manufacture B’s single roller cone design completed a drill shot with roller still intact and miniscule carbides left.
Drill Bit / Job Completed / Intact / ControllableManufacture A / Tri-Cone Type / Yes / No / Yes
Iron Fist Type / Yes / No / Somewhat
Three Tooth / No / No / No
Manufacture B / Tri-Cone Type / Yes / No / Yes
Iron Fist Type / No / Yes / Unknown
Single Roller Cone / Yes / Yes / Yes
Table 1 Test results of manufacture A & B drill bits
Discussion
Less aggressive drill bits proved tomake if further than aggressive type drill bits. The original hypothesis therefore is true. However the iron fist type drill bits for both manufactures are not aggressive type drill bits. One of the two completed a job, indicating that less aggressive drill bits give a better chance to complete that job but are not guaranteed. Originally hypothesized was the less aggressive tri-cone type drill bits would be able to cut the rock and still being intact after drilling. This statement is only half true; tri-cone types completed jobs with control but were severely damaged. Out of all of the drill bits tested the single roller cone type drill bit proved to be the most efficient, rugged, and longer lived.
Lastly, because of the variables in this study results are inconclusive. Further testing should be done in order to compare and contrast better. Each drill bit should be tested as close together as possible. Even if testing could be done five feet from each other, every cobble rock is diverse and could compromise the integrity of drill bit differently. Further studies needs to be conducted.
References
Chehab, A. G., & Moore, I. D. (2012). Analysis for Long-Term Response of Pipes Install Using Horizontal Directional Drilling. Journal OfGeotechnical & Geoenvironmental Engineering, 138(4), 432-440. doi:10.1061/(ASCE)GT.1943-5606.0000608
Mud, B. A., Saun, G. F., & Zall, B. M. (2013).Horizontal drilling and the environment.Journal of Horizontal Drilling, 56, Article 2561hdd. Retrieved June 18, 2014, from