Mears Canada Completes HDD of the Labiche River

Mears Canada recently completed a challenging horizontal directional drilled(HDD) crossing of the Labiche River in the Yukon Territory in Northern Canada.

The project was undertaken for Duke Energy Gas Transmission (DEGT) and involved the installation of a replacement crossing of the river for an existing 20-in.high pressure sour gas pipeline.

During the decades that have passed since the pipeline was first built, the river channel has meandered a considerable distance from its original location. The pipeline has become exposed, necessitating the replacement of the river crossing. A technique for replacing the crossing that would be immune to further meandering of the river channel was needed. The river is an important habitat for trout. Hence, an installation technique that minimized the environmental impact to the river was also required.

DEGT chose an HDD crossing of the entire width of the river’s flood plain to meet the engineering and environmental requirements for the crossing replacement. The length of the crossing would be 4,207.5 ft. A borehole diameter of 30 in. would be required to allow the 20-in. pipeline to be pulled into place under the river.

The river crossing is located in a remote part of the Canadian North in the Yukon Territory. The nearest permanent road is almost 63 miles away. Access to the location can only be gained during the winter months of January through March when ice roads and ice bridges can be used to traverse the difficult and environmentally sensitive terrain of the region.

A geotechnical investigation of the site performed by DEGT revealed that the crossing would be installed primarily in bedrock, consisting of shale. The bedrock is overlain by glacial till consisting of clay, gravel and cobbles. The thickness of the overburden varies between 132 ft on the south side of the crossing and 19.8 ft on the north side. The investigation also revealed that high artesian pressures are present in a water-bearing gravel seam directly above the bedrock on the south side of the crossing.

Typically, artesian pressures present a significant technical challenge to the installation of HDD crossings. Inflow of water into the borehole as it is being drilled causes degradation of the drill fluid and destabilizes the walls of the borehole. This results in a risk of the borehole collapsing. There is also a risk that the cuttings generated by the drilling operation will not be able to be flushed from the borehole by the degraded fluid.

In order to mitigate the risks posed by the groundwater, DEGT hired Kamloops Augering to drive steel casing down the bedrock during the winter 2005. The casing was driven from the entry point on the south side of the crossing until it penetrated the upper layer of the bedrock. The purpose of the casing was twofold. First, it would seal off the borehole from the surrounding groundwater in the gravel layer. Secondly, it would prevent the borehole from collapsing in the unconsolidated gravel and sand that are present in the water bearing layer.

In order to minimize the length of the casing that would have to be installed, a relatively steep entry angle of 22 degrees was chosen. Even so, the length of casing required was just more than 330 ft. This long length required the use of multiple telescoping sections of casing. A diameter of 36 in. was chosen for the longest and innermost casing to allow it to accommodate the reaming tools that would later be used in the drilling of the borehole. The other, shorter, sections of casing had diameters of 42 and 48 in.

As expected, during the installation of the casing, groundwater flowed to the surface through the various sections of casing. Grout was installed between the sections of casing and in the formation around the end of the longest section of casing to seal off the inflow of water.

A drill profile design that passed deep through the bedrock over most of its length was chosen to minimize the risk of a drill fluid frac-out impacting the river. Approximately 132 ft of solid bedrock and an additional 132 ft of overburden would separate the HDD borehole from the river bottom for a total vertical depth of 264.

The extraordinary depth of the crossing would mean that special care would have to be taken to ensure that the accuracy of the downhole survey equipment was maintained as the pilot hole was steered along its trajectory beneath the river.

Once the casing installation was complete, Mears Canada was contracted by DEGT to fabricate and install the new HDD crossing. Mears mobilized equipment and personnel to the crossing location early in January 2006.

Upon arrival on the worksite, the Mears’ crew was met by a large bison, a member of the diversified wildlife community in the region. The bison became the project mascot and was seen daily throughout the project duration.

It was critical the work be completed before the ice roads and ice bridges became impassable with the arrival of spring. If spring breakup occurred before the work was complete, then Mears’ equipment would be trapped on the worksite and would have to wait until the following year before it could be demobilized. Obviously, the exact timing of spring breakup is uncertain, as it depends on weather conditions. This uncertainty placed more pressure on the project team to complete the work as quickly as possible.

In order to meet the tight schedule, Mears elected to use two drill spreads, each with complete drill fluid pumping and recycling equipment that could handle the large fluid flow rates that would be required to drill and ream the borehole through the bedrock. The first spread, with a drill rig with a push/pull capacity of 160,000 lbs, would be used to drill the pilot hole. The second spread, with a rig with a push/pull capacity of 660,000 lbs, would be used, together with the first spread, to perform the reaming operation. The larger rig would also be used for the pipe pullback operation. All operations would be done on a 24-hour per day, seven-day per week basis to maximize the speed of the installation.

Typical daytime temperatures throughout the project duration were -13 F, with nighttime lows below -22 F. This required special precautions to prevent freezing of the numerous lines, pumps and other equipment that make up the large drill fluid handling systems used. Large steam generating boilers were used on both drill spreads and plumbed into the fluid systems to prevent freezing.

In order to minimize the amount of drilling fluid that would have to be disposed at the end of the project, Mears planned to maximize the amount of fluid recycling. The drill fluid would be recycled thousands of times throughout the pilot hole drilling and reaming operations. Centrifuges were incorporated into the drill fluid systems on both rigs to prevent the build up of the concentration of very fine rock particles in the fluid.

The geotechnical investigation showed that the shale is reactive. This meant that the rock cuttings would absorb water from the drill fluid and swell. Mears used pre-approved, environmentally sensitive drilling fluid additives that would encapsulate the cutting particles to prevent them from coming in contact with the drilling fluid. However, the effectiveness of these additives is limited and some swelling of the cuttings did occur. The result was that the cuttings tended to stick together and form large lumps of clay-like material that were difficult to flush from the borehole during the reaming operation. During reaming, the hole opening tool had to be withdrawn from the borehole frequently to swab the clumps of cuttings from the borehole.

In order to guarantee accuracy of the downhole surveys taken during the drilling of the pilot hole, Mears utilized a Paratrack surface coil over the entire length of the crossing. The thick ice covering the river provided a convenient surface for supporting the heavy gauge electrical cable of the surface coil.

To further mitigate the risk of a drilling fluid frac-out, the annular pressure was carefully monitored during the pilot hole drilling. This was done by means of a pressure transducer incorporated in the downhole survey probe. Annular pressure data was transmitted to the surface along the same wireline used to transmit the survey data. The pressure readings where compared with predicted pressures and previously determined allowable pressures to allow the driller to determine when measures to clean the annular space of cuttings were required.

The combination of the deep vertical depth of the crossing, the use of downhole pressure monitoring and careful management of the drilling fluid properties allowed Mears to drill and ream the entire borehole without losing any drilling fluid whatsoever. In addition, Mears’ recycling program successfully eliminated the need for disposing of drilling fluid that had become unusable. These were significant accomplishments on a crossing of this magnitude.

Finally, after nearly eight weeks of drilling and reaming, the borehole was ready for the pipeline pullback. While the borehole was being drilled, Mears’ subcontractor, Desa Pipelines, had fabricated and pre-tested the pipeline section in a single continuous length. Several cranes and other pieces of equipment were used to lift the pipeline section into the arc needed to have the pipe enter the steel surface casing. The steep entry angle of 22 degrees of the surface casing required lifting the pipe 39.6 ft into the air in order to prevent it from buckling as it passed through an arc into the casing.

The pullback operation went without a hitch and took less than 24 hours to complete. The maximum pull load recorded was a little more than 150,000 lbs, considerably less than expected. This was evidence of how smooth a trajectory the borehole followed as it passed beneath the river. It was also evidence of how good a job Mears’ crew had done of cleaning the cuttings from the borehole.

When the pullback was complete, an inspection of the pipe coating and a gauging plate run revealed that the pipe was in perfect condition.

Once the HDD installation was complete, a second pressure test proved its integrity. The new crossing was then tied into the existing pipeline. The tie-in work required a shutdown of pipeline operations of less than 24 hours.

The pipe making up the original crossing was then removed from the river and its banks. The drilling fluid was disposed of on the worksite by mixing with native soil and burying it in the containment pits.

Mears completed the project within the time period dictated by the ice roads and bridges and did so within budget, making this difficult project a resounding success.

DEGT project manager Mike Wallace commented, “DEGT acknowledges the efforts of Mears, its subcontractors and all other supporting contractors who worked with great effort to complete the project safely and within the constrained timeframe.”

Marc Bruderer is an engineer with the Mears Group, which is headquartered in Rosebush, Mich.