At present a large horizontal directional drilling and trenching project is taking place that extends over three states and encompasses over thirty plus miles of new and replacement natural gas infrastructure. The project will provide gas service to thousands of residents and also ensures continued work for hundreds over the next year or so. The following describes one of the many directional drilling jobs required on this project along with some issues they encountered. The bore hole is predominantly in rock and required specialized tooling to complete it within the required time frame.
The proposed supply line that would feed into the multiple distribution lines route would take it through a pristine part of Seneca, Missouri’s landscape that was owned by a local rancher. The supply line would have to cross a county road that could not be disrupted by normal open cut methods. Where open cut methods could be used, the contractors were utilizing a Vermeer 755 heavy trencher with rock teeth. On completion of the site investigation it was determined that the majority of the bore hole consisted of solid flint rock which is a hard, sedimentary cryptocrystalline form of the mineral quartz. The proposed bore path cover a distance of over 400 feet, crossing the road and proceeding up-hill to a pre-determined exit point. To carry out the job with conventional directional mud drilling tooling would be extremely difficult. The contractor decided to complete the job with Vermeer D24x40 horizontal directional drilling equipment using a Mincon HDD50 hard rock pneumatic hammer drilling system.
The system consists of a hammer that utilizes a piston that strikes a specially designed drill bit to fracture the rock. The hammer cycles at high frequency (approximately 2000 times per minute) to ensure smooth drilling and to maintain steer ability. Steering is achieved through a slant faced bit coupled with a two degree bent sub. Locating the system is made possible with industry standard locating sonde beacons mounted in a shock resistant sonde housing attached to the bent sub.
The hammer requires a mixture of compressed air, rock drill oil and a small amount of water to operate. This delivered to the Directional drill using the Mincon Support Station.
With minimal engineering issues encountered after the site survey gave this particular part of the project was considered a relatively low difficulty level. Traditional open cut methods were not achievable due to the road crossing, ground cover at the approach of the hill, and layered rock formations. The absence of additional utilities such as sewer, electric, and communications also provided a relatively low site difficulty level.
The design of this crossing, just outside of the town of Seneca Missouri, called for a bore in the area of 427 feet and just over 5 feet deep taking into account parameters such as bend radius and depth of cover, country road and progression up the hill. The lack of existing utilities made the layout and design of this part of the project relatively straight forward.
Using the Mincon HDD50 Hard Rock System for the horizontal directional drilling along with the Vermeer D24x40 proved to be the right fit. Setup of the system was carried out reasonably quickly. The Doosan XHP1170 compressor was coupled to the support station with the supplied safety whip checks and high pressure hose, as was the drill machine and the water supply. The sonde beacon was installed in the sonde housing and checked to ensure it was clocked correctly.
The pilot bore was completed in just less than eight hours with 90% of the drilling being in dark colored solid flint rock formations with the remainder in gravel and black soils. The HDD50 achieved a pilot hole dimension of 5.5”, after completion of the pilot hole the HDD50 was removed and a Specialty Drilling Tools 10” four cone hole opener was installed on the end of the drill pipe.
Utilizing twin MX240 Vermeer mix systems with 1000 gallon fluid tanks, bentonite slurry with a marsh funnel test at around 65 to 70 was mixed. The contractor also added additional additives of soda ash and suspension agents to assist with the heavier rock material and gravel that had been seen at the exit pit. This however did not provide the same high efficient results experienced in the pilot bore. Utilizing the roller cone reaming method on this same rock would require an additional 8 days and thousands of gallons of fluid to complete the pullback and installation of the 6” product pipe.
Several issues crept into the back reaming process and caused numerous problems. The initial identification of gravel at the exit pit was assumed to be only minimal in depth and consistency. This however proved to be incorrect and in-fact was up to six feet in depth and over 30 feet in length along the exiting point of the bore path. With the steeper angle of the bore path as it was directed to the exiting point the gravel made an attempt to fall back into the bore path. While the reamer had no problem moving the material out of the bore path and the thicker fluid mixtures could remove it from the area, the addition of the steeper angles continued to allow the gravel to fall back into the void made by the reamer.
To overcome this the contractor had to push the reamer back out of the hole trying to remove as much of the gravel material as possible. The lower mud flows of the smaller directional drill made this process slower and more time consuming. An additional process of removing the gravel in the bores path with an excavator did make the completion of the bore possible.
The total time for the pilot bore process was less than 8 hours and required less than 500 gal of a water and polymer mix. Fuel consumption on the compressor and additional fuel used by the D24x40 were the some of the minimal cost associated with the pilot bore process. The back reaming process however proved to be a different story. In the eight days it took to back ream and pull in the product pipe over 35,000 gallons of water, associated bentonite, and additional additives were used. Additional man power to operate the excavator and maintenance personal to hard surface the additional reamers and equipment used on the job-site was also required.
The use of air operated directional drilling equipment verses conventional methods proved to be a considerable saving on this portion of the project. As contractors become more familiar with these methods they will continue to gain acceptance and utilization in the underground markets and offer the same cost benefits.