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 involvedthe 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, theriver channel has meandered a considerable distance from its original location.The pipeline has become exposed, necessitating the replacement of the rivercrossing. A technique for replacing the crossing that would be immune to furthermeandering of the river channel was needed. The river is an important habitatfor trout. Hence, an installation technique that minimized the environmentalimpact to the river was also required.
DEGT chose an HDD crossing of the entire width of the river’s flood plain tomeet the engineering and environmental requirements for the crossingreplacement. The length of the crossing would be 4,207.5 ft. A borehole diameterof 30 in. would be required to allow the 20-in. pipeline to be pulled into placeunder the river.
The river crossing is located in a remote part of the Canadian North in theYukon Territory. The nearest permanent road is almost 63 miles away. Access tothe location can only be gained during the winter months of January throughMarch when ice roads and ice bridges can be used to traverse the difficult andenvironmentally sensitive terrain of the region.
A geotechnical investigation of the site performed by DEGT revealed that thecrossing would be installed primarily in bedrock, consisting of shale. Thebedrock is overlain by glacial till consisting of clay, gravel and cobbles. Thethickness of the overburden varies between 132 ft on the south side of thecrossing and 19.8 ft on the north side. The investigation also revealed thathigh artesian pressures are present in a water-bearing gravel seam directlyabove the bedrock on the south side of the crossing.
Typically, artesian pressures present a significant technical challenge tothe installation of HDD crossings. Inflow of water into the borehole as it isbeing drilled causes degradation of the drill fluid and destabilizes the wallsof the borehole. This results in a risk of the borehole collapsing. There isalso a risk that the cuttings generated by the drilling operation will not beable to be flushed from the borehole by the degraded fluid.
In order to mitigate the risks posed by the groundwater, DEGT hired KamloopsAugering to drive steel casing down the bedrock during the winter 2005. Thecasing was driven from the entry point on the south side of the crossing untilit penetrated the upper layer of the bedrock. The purpose of the casing wastwofold. First, it would seal off the borehole from the surrounding groundwaterin the gravel layer. Secondly, it would prevent the borehole from collapsing inthe unconsolidated gravel and sand that are present in the water bearinglayer.
In order to minimize the length of the casing that would have to beinstalled, a relatively steep entry angle of 22 degrees was chosen. Even so, thelength of casing required was just more than 330 ft. This long length requiredthe use of multiple telescoping sections of casing. A diameter of 36 in. waschosen for the longest and innermost casing to allow it to accommodate thereaming tools that would later be used in the drilling of the borehole. Theother, shorter, sections of casing had diameters of 42 and 48 in.
As expected, during the installation of the casing, groundwater flowed to thesurface through the various sections of casing. Grout was installed between thesections of casing and in the formation around the end of the longest section ofcasing to seal off the inflow of water.
A drill profile design that passed deep through the bedrock over most of itslength was chosen to minimize the risk of a drill fluid frac-out impacting theriver. Approximately 132 ft of solid bedrock and an additional 132 ft ofoverburden would separate the HDD borehole from the river bottom for a totalvertical depth of 264.
The extraordinary depth of the crossing would meanthat special care would have to be taken to ensure that the accuracy of thedownhole survey equipment was maintained as the pilot hole was steered along itstrajectory beneath the river.
Once the casing installation was complete, Mears Canada was contracted byDEGT to fabricate and install the new HDD crossing. Mears mobilized equipmentand personnel to the crossing location early in January 2006.
Upon arrival on the worksite, the Mears’ crew was met by a large bison, amember of the diversified wildlife community in the region. The bison became theproject mascot and was seen daily throughout the project duration.
It was critical the work be completed before the ice roads and ice bridgesbecame impassable with the arrival of spring. If spring breakup occurred beforethe work was complete, then Mears’ equipment would be trapped on the worksiteand 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 onweather conditions. This uncertainty placed more pressure on the project team tocomplete 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 handlethe large fluid flow rates that would be required to drill and ream the boreholethrough the bedrock. The first spread, with a drill rig with a push/pullcapacity of 160,000 lbs, would be used to drill the pilot hole. The secondspread, 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 rigwould also be used for the pipe pullback operation. All operations would be doneon a 24-hour per day, seven-day per week basis to maximize the speed of theinstallation.
Typical daytime temperatures throughout the project duration were -13 F, withnighttime lows below -22 F. This required special precautions to preventfreezing of the numerous lines, pumps and other equipment that make up the largedrill fluid handling systems used. Large steam generating boilers were used onboth drill spreads and plumbed into the fluid systems to prevent freezing.
In order to minimize the amount of drilling fluid that would have to bedisposed at the end of the project, Mears planned to maximize the amount offluid recycling. The drill fluid would be recycled thousands of times throughoutthe pilot hole drilling and reaming operations. Centrifuges were incorporatedinto the drill fluid systems on both rigs to prevent the build up of theconcentration of very fine rock particles in the fluid.
The geotechnical investigation showed that the shale is reactive. This meantthat the rock cuttings would absorb water from the drill fluid and swell. Mearsused pre-approved, environmentally sensitive drilling fluid additives that wouldencapsulate the cutting particles to prevent them from coming in contact withthe drilling fluid. However, the effectiveness of these additives is limited andsome swelling of the cuttings did occur. The result was that the cuttings tendedto stick together and form large lumps of clay-like material that were difficultto flush from the borehole during the reaming operation. During reaming, thehole opening tool had to be withdrawn from the borehole frequently to swab theclumps of cuttings from the borehole.
In order to guarantee accuracy of the downhole surveys taken during thedrilling of the pilot hole, Mears utilized a Paratrack surface coil over theentire length of the crossing. The thick ice covering the river provided aconvenient surface for supporting the heavy gauge electrical cable of thesurface coil.
To further mitigate the risk of a drilling fluid frac-out, the annularpressure was carefully monitored during the pilot hole drilling. This was doneby means of a pressure transducer incorporated in the downhole survey probe.Annular pressure data was transmitted to the surface along the same wirelineused to transmit the survey data. The pressure readings where compared withpredicted pressures and previously determined allowable pressures to allow thedriller to determine when measures to clean the annular space of cuttings wererequired.
The combination of the deep vertical depth of the crossing, the use ofdownhole pressure monitoring and careful management of the drilling fluidproperties allowed Mears to drill and ream the entire borehole without losingany drilling fluid whatsoever. In addition, Mears’ recycling programsuccessfully eliminated the need for disposing of drilling fluid that had becomeunusable. These were significant accomplishments on a crossing of thismagnitude.
Finally, after nearly eight weeks of drilling and reaming, the borehole wasready for the pipeline pullback. While the borehole was being drilled, Mears’subcontractor, Desa Pipelines, had fabricated and pre-tested the pipelinesection in a single continuous length. Several cranes and other pieces ofequipment were used to lift the pipeline section into the arc needed to have thepipe enter the steel surface casing. The steep entry angle of 22 degrees of thesurface casing required lifting the pipe 39.6 ft into the air in order toprevent 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 tocomplete. The maximum pull load recorded was a little more than 150,000 lbs,considerably less than expected. This was evidence of how smooth a trajectorythe borehole followed as it passed beneath the river. It was also evidence ofhow 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 agauging plate run revealed that the pipe was in perfect condition.
Once the HDD installation was complete, a second pressure test proved itsintegrity. The new crossing was then tied into the existing pipeline. The tie-inwork required a shutdown of pipeline operations of less than 24 hours.
The pipe making up the original crossing was then removed from the river andits banks. The drilling fluid was disposed of on the worksite by mixing withnative soil and burying it in the containment pits.
Mears completed the project within the time period dictated by the ice roadsand bridges and did so within budget, making this difficult project a resoundingsuccess.
DEGT project manager Mike Wallace commented, “DEGT acknowledges theefforts of Mears, its subcontractors and all other supporting contractors whoworked with great effort to complete the project safely and within theconstrained timeframe.”
Marc Bruderer is an engineer with the Mears Group, which is headquarteredin Rosebush, Mich.