Use of Directional Drilling for Installation of Raw Water Supply Line in Fort Smith, NWT

USE OF DIRECTIONAL DRILLING FOR INSTALLATION OF RAW WATER SUPPLY LINE AND SLUDGE DISPOSAL LINE IN FORT SMITH, NWT K.R. Johnson UMA Engineering Ltd. Edmonton, Alberta T5S 1G3 ABSTRACT UMA Engineering Ltd. was commissioned by the Town of Forth Smith in 1990 to undertake the design of a 650 metre raw water supply line and a sludge disposal line as part of a new water treatment system for the community.

The Town of Fort Smith is located in the region of discontinuous permafrost where frost penetration may reach a depth of 3.5 metres, therefore freeze protection for the pipeline was a major concern. The options considered for freeze protection included pipeline insulation, continuous liquid circulation, pipeline draining, pipeline heating, and deep burying.

Although all of these options were feasible for the Fort Smith application, UMA concluded that a deep bury system would offer the simplest operation, and minimize the risk of freezing for the system. However, the capital cost for completing a deep bury installation to a depth of 3.5 metres by ordinary excavation was very high.

A pipeline installation method using directional drilling as a means of deep burying the pipeline was identified as a viable alternative to ordinary excavation. The directional drilling installation of the raw water line and sludge disposal line began in 1991. The project was completed in 1992, with some modifications to the procedure because of the difficulties encountered in 1991.

Forage directionnel pour l'installation d'une conduite d'alimentation en eau potable et de disposition des boues à Fort Smith dans les Territoires du Nord-Ouest.

K.R. Johnson Gestionnaire du district du Yukon UMA Engineering Ltd. 206 - 303 Jarvis St. Whitehorse, Yukon Y1A 2H3 UMA Engineering Ltd. a obtenu de la municipalité de Fort Smith en 1990, le mandat de concevoir une conduite d'alimentation en eau potable et de disposition des boues dans le cadre de l'installation d'un nouveau système de traitement d'eau potable pour la communauté.

La municipalité de Fort Smith est située dans une région de permafrost intermittent et le permafrost n'a donc pas été une préoccupation majeure. Toutefois, la pénétrarion du gel jusqu'à une profondeur de 3,5 mètres requérait une protection efficace des conduites contre le gel. Les options considérées ont été l'isolation, le drainage, et le chauffage de la conduite, de même que la circulation continue de liquide et l'enfouissement profond.

Bien que toutes ces options étaient applicables au cas de Fort Smith. L'enfouissement profond a été retenu par UMA Engineering pour sa protection efficace contre le gel et sa simplicité d'opération. Par contre, le coût élevé de l'enfouissement à une profondeur de 3,5 mètres était très élevé.

L'enfouissement de la conduite par forage directionnel a alors été identifié comme technique applicable permettant des économies appréciables par rapport à l'enfouissement conventionnel.

Le forage directionnel est une technique de forage à haute pression utilisé en exploitation pétrolière qui consiste à creuser un tunnel pilote horizontal à partir d'une foreuse installée en surface. Le tunnel est ensuite élargi et la conduite mise en place grâce à de la boue de forage servant de lubrifiant.

Le forage et l'installation des conduites d'alimentation en eau potable et de disposition des boues débuta en 1991. Des difficultés dues au sol ont requis d'interrompre les travaux pour une réévaluation et une modification de la technique de forage. Le projet a t complété en 1992, la modification de la technique de forage directionnel permettant son utilisation sur la plus grande partie du projet.

INTRODUCTION UMA Engineering Ltd. was commissioned by the Town of Forth Smith in 1990 to undertake the design of a 600 metre raw water supply line and a 600 metre sludge disposal line as part of a new water treatment system for the community. A new water treatment plant was needed to replace the aging existing facility which lies on a geotechnically unstable site.

The raw water supply pipeline will convey raw water from the Slave River from the old water treatment plant site to the new water treatment plant site. This sludge disposal pipeline will convey waste sludge from the new water treatment plant site to the old water treatment plant site.

Both pipelines will operate in conjunction with the operation of the water treatment plant.

The sludge disposal pipeline will flow into either the existing disposal line which discharges into the Slave River, or the Town's sewer system, depending upon the season of operation. During the summer months the sludge will be discharged into the existing disposal line to the Slave River. During the winter months the sludge will be discharged into the nearby sewer system to avoid the freezing problems with the disposal line to the Slave River.

BACKGROUND The Town of Fort Smith is located in the region of discontinuous permafrost in Canada's north, therefore permafrost was not a major concern in the pipeline design. However, frost penetration may reach 3.5 metres in this region, therefore freeze protection for the pipelines was a major concern.

Water mains in Fort Smith are generally buried to a depth of 3.0 metres. Although this is within the frost penetration zone, the risk of freezing to the pipe is tolerable for the water distribution system (2 repairs to the water lines between 1988 and 1991).

The minimum depth of bury for the raw water supply and sludge lines was chosen to be approximately 3.8 m, which is slightly below the expected frost penetration of 3.5 m. These lines are critical to the operation of Fort Smith's water supply system, therefore the pipeline system should be capable of continuous operation. A failure in either one of these lines would create an unacceptable disruption in water supply service to the community.

CONSTRUCTION OPTIONS The two options considered for the pipeline configuration were shallow buried pipeline and a deep buried pipeline (below frost penetration). Within these two configurations were a number of construction options, which included shallow bury, direction drilling, deep excavation, and deep auger constructed pipelines.

Shallow Bury Constructed Pipeline A shallow bury constructed pipeline would utilize High Density Polyethylene Pipe (HDPE) installed within the frost penetration zone (less than 3.5 m). This pipe would be at risk of freezing, therefore freeze protection utilizing such methods as pipe insulation, pipeline heating, pipeline draining and recirculation would have to be considered.

The advantages of a shallow bury constructed pipeline include the ease of installation, and maintenance because of the shallow depth of bury. The disadvantages of this system include the need for freeze protection, the associated risks of freezing, and the daily operation requirements.

Directional Drilling Constructed Pipeline A directional drilling constructed pipeline option would utilize HDPE pipe installed at a depth of at least 3.5 metres. The pipe depth is necessitated by the need to install the pipe below the frost penetration depth, and below the existing water and sewer service connections.

Directional drilling utilizes high pressure oilfield drilling technology to drill a horizontal pilot hole from a drilling rig at the ground surface. The service pipes are then pulled back through the pilot hole utilizing a reamer to enlarge the hole and drill mud as a lubricant (See Figure 1). The system can be controlled to maintain the pilot hole in a predetermined horizontal and vertical alignment.

The advantage of this system is the installation below the frost line, the minimal disturbance to roads and utilities, and the relative low cost for a deep installation. The disadvantage of this system is the potential difficulty for maintenance of the installed pipeline. However, the use of HDPE pipe minimizes the risks of pipe failure because of the robust nature of this pipe material.

Deep Excavation Constructed Pipeline A deep excavation constructed pipeline option would utilize HDPE pipe installed at a depth of at least 3.5 metres by open cut of a trench. The advantages of this system include the familiarity of this type of installation with the local contractors, and the installation below the frost line. The disadvantages of this system include the construction cost, the difficulty of construction, and the potential maintenance problems for the pipeline and road surface.

The difficulty of construction would include the need for caging to reduce pavement restoration, and maximize construction safety. The potential for a serious disturbance of the existing services is another possibility resulting in a disruption in service and additional costs to repair the lines.

The settlement under the backfilled road area would create future problems, and costs for road repairs.

Deep Auger Constructed Pipeline A deep auger constructed pipeline would utilize horizontal augering to install a carrier pipe in which the HDPE service pipes would be installed. A deep auger constructed pipeline option is similar to direction drilling construction, with similar advantages and disadvantages.

Comparison of Options The initial analysis of the options produced a recommendation for a shallow bury constructed pipeline. However, after further investigation UMA concluded that a system of shallow and insulated lines would add some degree of unreliability to the system, as well as an operational burden. A deep bury system would offer the simplest operation, and minimize the risk of freezing for the system.

A deep auger constructed pipeline, and a deep excavation constructed pipeline have very high capital costs (See Table 1), therefore these options were eliminated for the construction in Fort Smith. As previously mentioned, the deep excavated pipeline also poses risks to the integrity of the existing water and sewer services and the paved roadways.

The directional drilling constructed pipeline offered advantages over the shallow buried line from an operation and maintenance perspective because freeze protection is not required on such a system. The risk of pipeline freezing in a shallow buried line may be greatly reduced by utilizing freeze protection methods such as pipe insulation, and pipe draining. However,

northern experience has proven that pipeline freezing may occur in spite of the best possible freeze protection.

Although a shallow bury constructed pipeline offers the least capital cost expenditure (See Table 1), the associated operating costs detailed in Table 2 suggest that directional drilling is an equitable alternative.

PIPELINE INSTALLATION The pipelines were aligned for installation within the existing road right-of-ways in Fort Smith (See Figure 2). The pipelines were offset 3 metres from the property line. Directional drilling was undertaken over distances of at least 100 metres, therefore the installation was completed with approximately 6 access trenches.

The directional drilling installation of the 200 mm in diameter raw water line and a 150 mm in diameter sludge disposal line began in 1991. Difficulties were encountered because of the unexpected water absorption capabilities of the soil, which caused the service pipe to bind during the pull back operation. The 1991 construction was stopped because of the impending cold weather, and to allow evaluation of the soil and modifications to the directional drilling methodology.

A particle size analysis of the soil revealed that the in situ material was a fine to very fine grained sand. The soil was also quite dry, and therefore absorbed the water injected during the drilling process. No water was left to lubricate the service pipe during the pullback operation, therefore a great deal of friction was generated. This friction could not be overcome by the drill rig and resulted in breaking the pullback connection to the service pipe.

The project was continued in 1992, and the modifications to the system allowed directional drilling to be utilized successfully for the majority of the project. The contractor chose in one instance to utilize pipe augering as an alternative to directional drilling because of space limitations.

The construction modifications to the directional drilling methodology included: 

changing the consistency of the drill mud.



increasing the volume of water and mud injected during the process.



increasing the pilot hole size.



injecting drilling mud from both ends of the operation during the service pipe pullback.

CONCLUSIONS AND RECOMMENDATIONS

Directional drilling was a cost effective approach for the installation of the raw water line and sludge disposal line in Fort Smith in order to provide a reliable means of frost protection for the pipelines. Although some difficulties were encountered at the beginning of the project, modifications to the methodology allowed the work to be successfully completed.

Table 1.

Summary of Construction Estimated Cost for Various Construction Options (1991 dollars).

Construction Estimated Cost Potential Low Potential High Option of Construction Variation Variation Shallow Bury $290,000 $270,000 $330,000 Direction Drilling $355,000 $320,000 $380,000 Deep Excavation $490,000 $450,000 $550,000 Deep Auger $530,000 $480,000 $580,000

Table 2.

Summary of Estimated Capital and Operating Costs for Shallow Bury and Directional Drilling Back Constructed Pipelines (1991 dollars).

Shallow Bury Directional Drilling Capital Cost $290,000 $355,000 Operating Cost/Year $1,750 $0 (91 hr/hr @ $19/hr) 30 Year Present Value $30,000 $0 (4 % Discount Rate) TOTAL COST $320,000 $355,000 Freezing Risk Exists None (Supply Interruption)