Drilling Under Pressure: HDD Beneath an Active Airport

Transcription

1 North American Society for Trenchless Technology (NASTT) NASTT s 2016 No-Dig Show Dallas, Texas March 20-24, 2016 MM-T2-01 Drilling Under Pressure: HDD Beneath an Active Airport Dylan Davidson, Staheli Trenchless Consultants, Lynwood, WA Laura Wetter, Staheli Trenchless Consultants, Lynnwood, WA James Campero, City of Camarillo, Camarillo, CA Bill Yates, Kennedy Jenks Consultants, Ventura, CA 1. Abstract The Camarillo Airport is a general aviation reliever airport with approximately 190,000 takeoffs and landings per year. As such, it is vital to keep airport disruptions to a minimum during construction projects. This led the City of Camarillo to consider trenchless options during a planned upgrade to the Camarillo Airport water system. The upgrade required the construction of approximately 1,900 feet of 16-inch water main pipeline beneath the airport without disturbance to the runway and parallel primary taxiway. The initial feasibility analysis compared horizontal directional drilling (HDD) with a combination of open-cut and auger boring for the water main construction. HDD was ultimately selected as the preferred method, leading to design challenges associated with a creative pullback layout requiring lofting above the County flood control channel just north of the airport. The active runway and taxiway also made drill navigation very challenging. A conventional wireline system could not be placed, a walkover system was considered to be too disruptive to airport operations, and rebar beneath the runway and taxiway was anticipated to cause magnetic interference. Ultimately, the drill was guided using a combination of wire coils near the entry and exit points and an AC beacon in the infield between the runway and taxiway. Continuous coordination with air traffic control was required during all phases of the installation which was successfully completed without major incident. This paper details the design and construction of the new waterline, focusing on lessons learned during drilling beneath the active airport. 2. Introduction The City of Camarillo needed to install a new water main; however, the proposed pipeline alignment crossed beneath an active airport. The Camarillo Airport is owned by Ventura County and is considered a general aviation reliever airport with approximately 190,000 takeoffs and landings per year. As such, it was vital to keep airport disruptions to a minimum during all phases of the construction process. That meant no surface construction was allowed to take place within the restricted area of the active runway and taxiway. This led the City of Camarillo to consider trenchless alternatives for upgrading the Camarillo Airport s water system. The upgrade required the construction of approximately 1,900 feet of 16-inch water main pipeline beneath the airport. Auger boring with a combination of open-cut construction was considered for the project; however, auger boring required dewatering and open-cut resulted in significant impacts to the airport. As such, horizontal directional drilling (HDD) was determined to be the preferred trenchless alternative for the project. It reduced the impacts to airport operations and did not necessitate dewatering. There were two challenging parts to the HDD design; pullback layout and drill navigation. Due to the County flood control channel just north of the airport, a creative pullback layout was required which included lofting the fused pipe above the channel during continuous pullback operations. In addition, due to the active runway and taxiway, tracking of the drill was challenging. A conventional wireline system could not be used, a walkover system was considered to be too disruptive to airport operations, and rebar beneath the runway and taxiway was anticipated to cause magnetic interference to any system. Ultimately the HDD Paper MM-T

2 was guided using a Paratrack-2 with a combination of wire coils near the entry and exit points and an AC beacon in the infield between the runway and taxiway. Ventura Directional Drilling Inc. (VDD) was awarded the contract for a bid price of approximately $550,000. VDD installed the HDD totaling approximately 1,685 feet of 16-inch HDPE pipe. In all, the project took six weeks to complete. 3. Geotechnical and Site Conditions To determine the soil and groundwater conditions at the project s site, two borings were drilled to depths greater than 70 feet. Both borings encountered approximately 10 feet of medium stiff to stiff clays underlain by interbedded layers of alluvial soils consisting of medium to very dense/stiff silts, clays, sand, and silty sands to the maximum depth explored (Earth Systems Southern California, 2012). Although the groundwater elevation could not be determined due to the drilling method performed (rotary wash method), historically high groundwater in the area was approximately 10 feet below the ground surface (Earth Systems Southern California, 2012). However, the moisture content of the samples in the upper 10 feet of clay was indicative of saturated soils; therefore, for purposes of design, the groundwater was assumed to be at the ground surface. 4. Feasibility Approach The primary focus of the design team was to ensure disruptions to the airport operations would be minimal. The more disruptions to the airport, the longer the construction would take. Therefore, a continuous open-cut option through the runway was eliminated from consideration. During the initial approach to the project, microtunneling, auger boring, pipe ramming, and HDD were evaluated to determine which trenchless methods were the most appropriate for the project. The project site is shown below in Figure 1. Figure 1: Project site showing critical crossings locations, boring locations, and total length Paper MM-T

3 Microtunneling was not considered a practical option for the project due to the long drive length and the costs of microtunneling; particularly the up front costs associated with mobilization. To minimize the risk of microtunneling, the 1,900 feet of new pipeline would either need to be split up into two separate drives of approximately 200 feet and 500 feet (see Figure 1) and combined with open-cut construction, or intermediate jacking stations would need to be used to increase the jacking capacity along the length of the pipeline. However, both of these options were deemed impractical as there were other feasible trenchless methods available at a much lower cost. Pipe ramming was determined to not be feasible. Similar to microtunneling, the 1,900 feet of new pipeline could be split up into two shorter pipe ram drives and combined with open-cut; however, this was considered particularly risky. In general, pipe rams are typically limited to drives less than 300 feet with a drive on the order of 500 feet considered exceptional. This project would require a 500-foot drive length which the design team determined would present an unnecessary amount of risk to the project and eliminated pipe ramming from consideration. Auger boring was determined to be a feasible option. Similar to microtunneling and pipe ramming, auger boring would involve splitting the 1,900 feet of pipeline into two shorter drives of 200 feet and 500 feet and combined with open-cut construction; thus yielding a viable alternative. Auger boring was a preferred alternative over microtunneling due to the lower costs of construction and mobilization. On the other hand, there were several drawbacks to auger boring. These drawbacks included several shafts needed for construction, substantial dewatering and a two-pass installation were required; all leading to an increased cost for auger boring. Although auger boring was the most practical pipe jacking approach for the project, it was not the chosen trenchless method. HDD was considered the least costly and most practical option for crossing the Camarillo Airport, as the technology excels where the other technologies had shortcomings. For example, HDD can be drilled to lengths upwards of 3,000 feet or more at the project s pipe diameter of 16-inches, eliminating the need for shafts. Additionally, this method reduces the disruptions to airport operations since there is significantly less work being performed near the taxiway and runway, particularly at the surface. Because auger boring and HDD were the most feasible trenchless methods, they were compared as shown below in Table 1. Based on the analysis, HDD was the most practical option for installation of the pipeline. Table 1: Comparison Table for Trenchless Method Selection. Green: most favorable; Yellow: neutral; Red: least favorable. Paper MM-T

4 5. HDD Design Once HDD was chosen as the preferred trenchless method, measures were taken to ensure drilling operations would not affect airport operations; risks were identified and work areas were established. Critical risks that needed to be mitigated included: soil settlement, heave of the runway, and hydrofracture. Soil settlement resulting from HDD operations is primarily caused by collapse of the annular space between the borehole and the product pipe. The risk of soil settlement increases in unstable soils, or where depth of cover is low. Fortunately for this project, the majority of the pipeline was anticipated to be within medium to very dense/stiff silts, clays, and sands, resulting in a stable borehole which greatly reduced the likelihood of settlement occurring. However, even if catastrophic borehole failure were to occur, a depth of cover of 40 to 50 feet was designed to protect the airports critical locations, the runway and taxiway, against settlement. Surface heave resulting from HDD can occur if excess drilling fluid is pumped after circulation is lost, if reaming is performed without sufficient depth of cover, or if reaming is performed too quickly. It was critical that surface heave did not occur at the airport s runway and taxiway. To mitigate these issues, the bore was designed to have significant depth of cover at the critical crossing locations, the entry pit was to be constantly monitored to ensure the drilling fluid was always in circulation, and the reaming rates were to be carefully monitored. Hydrofracture was also considered. Hydrofracture occurs when drilling fluid pressure exceeds the strength and confining stress of the soils surrounding the borehole. The excess pressure fractures the soil around the bore, allowing drilling fluid to escape. Although drilling fluid is non-toxic and is typically 97 to 99 percent water, it is still important to reduce the risk of hydrofracture. Drilling fluid is extremely slick and if a plane or car were to drive across it, there is a high potential risk of losing all steering control of the plane or car. To reduce this risk, hydrofracture calculations were performed and used to design the bore to ensure it achieved a sufficient depth at which point the drill crosses beneath the runway and taxiway. At the taxiway location, the bore was designed to be approximately 60 feet below the ground surface. At the runway location, the bore was designed to be approximately 65 feet below the ground surface. On the other hand, the soils within the immediate vicinity of the entry and exit locations were at an even higher risk for hydrofracture due to the low depth of cover. The HDD contractor was required to have adequate equipment and materials on site to immediately respond to and cleanup any inadvertent returns to the surface. The HDD contractor was contracted to install approximately 1,750 feet of 16-inch HDPE IPS DR-11 pipe. The HDD was designed to have an entry angle of 16 degrees to allow the bore to achieve greater depths before reaching the taxiway; the first critical crossing location of the Camarillo Airport. Conductor casing was not required on the project. During pullback operations, the string of product pipe was required to be loaded onto pipe rollers to fully support the pipeline as the product pipe was not allowed to be dragged across the ground. Additionally, a bridge was required to support the pipeline as it crossed the County flood control channel during pullback. One of the most challenging parts of the project was utilizing a drill tracking system that could adequately track the location of the drill bit while drilling across an active airport. The tracking system must be able to continuously monitor and record the drill s location at least once per drill pipe length. A walkover wireline system was considered too disruptive to airport operations as the wireline would need to be strung across the entire length of the HDD to allow continuous monitoring. The Camarillo Airport would not allow this because stringing the wireline across the taxiway and runway was a risk to airport operations. The final design specified the use of a Paratrack-2 system, with a combination of wire coils near the entry and exit points, and an AC beacon tracking system. A Paratrack-2 system is first established by setting up a grid around both the entry pit and exit pit work areas. With a Paratrack-2, a coil is setup around the perimeter of the pits and down the designed centerline of the bore. The wire coil is wrapped around wooden stakes to create a complete loop, a grid, which establishes a magnetic field and runs an AC current. The Paratrack-2, in conjunction with the AC beacon, reads the induced magnetic field created by the wire coil to locate itself for performing a land survey. The land survey established the known points of the magnetic field at both pit locations. Figure 2 shows the AC beacon and the magnetic field being established by the wire coil. Paper MM-T

5 Once the magnetic field is established at both pit locations, the Paratrack-2 is loaded into a non-magnetic drilling collar (the monel) and orientated into the orientation sub housing, which was then connected to the monel. A wireline was also strung from the back of the monel and through each drill pipe that was added to the drill string to allow the location of the bit to be tracked continuously. After each drill pipe was advanced, the steer hand took a survey shot of the bore, which gave the azimuth, inclination, and away distance of the bore. Prior to drilling, the drill bit, directly in front of the monel, was surveyed to ensure that it was oriented in the direction of the designed bore path. The rig was then be adjusted according to the surveyed data and the entry angle was established. The tracking data was monitored using two programs; Rivcross and AutoCAD. Rivcross is a software that integrates tracking and survey data from the Paratrack tooling to provide real time steering guidance. The steer hand then determined the location of the Paratrack using the azimuth, inclination, and away distances read by the Paratrack, as interpreted in xyz coordinates by the Rivcross software. The data was then plotted in relation to the design using AutoCAD. The steer hand then coordinated with the driller on the steering required to follow the HDD design path. Figure 2: AC beacon tracking system with the survey equipment in the background (left); Wire coil strung around the perimeter of the entry pit area to establish the magnetic field (right). One of the more challenging parts of the tracking system design, comes from the restrictions of the AC beacon. The AC beacon has a maximum range of approximately 300 feet radially, meaning it needs to be moved after every 600 feet of drilling. Once the drill passes the taxiway, the bit will be out of range from the AC beacon, meaning the driller will need to steer using true magnetic north of the Earth for orientation. The magnetic field was only established at the entry and exit pit locations, so the infield, the area between the taxiway and the runway, was not surveyed. The AC beacon, which was used to verify the bore path, needs to be moved to the infield along the centerline of the bore path, such that the Paratrack-2 navigation system could contact the tracking software to ensure an accurate bore path. Each day as the drill advanced closer to the exit pit, the AC beacon in the infield needed to be moved further down the bore path to ensure the bore was accurately drilled. 5. Construction Before anyone could work on the Camarillo Airport property, everyone, including the contractor, inspectors, and any subcontractors, were required to pass an airport driving training course that focused on how to drive on an active airport. Restrictions included no one being allowed to take a step onto the taxiway or runway without the approval from air traffic control. No one was allowed to walk within 100 feet of the runway without permission. If anyone were to step onto either the taxiway or the runway without consent, they were subject to immediate arrest. The Camarillo Airport took these regulations very seriously. Ventura Directional Drilling (VDD) employees were required to carry walkie-talkies at all times so they could be in constant contact with air traffic control since the airport was open during all hours of construction. The first week of the project consisted of fusing together the HDPE product pipe. The pipe was fused together at the north end of the airport in three separate sections; two 750 foot strings and a 300 foot string. VDD elected to locate their drilling equipment at the south end of the site. VDD drilled the project with a Ditch Witch JT100-Mach 1 HDD Paper MM-T

6 Rig. VDD hired InRock as their steering subcontractor to provide tracking of the drill. Figure 3 shows the HDD rig used on the project. While VDD was mobilizing their equipment, InRock began setting up their survey and tracking equipment. InRock began by surveying the coil grid using a theodolite and known markers. InRock surveyors repeated these steps at the exit side. The surveyors then needed to move to the infield to mark the centerline of the HDD bore path. To do this, VDD coordinated with air traffic control prior to crossing the taxiway. Next, InRock repeated the steps for establishing the coil at the exit side. InRock then installed the probe into the monel with the orientation sub and the wire line was attached and installed into the monel. Since the stakes had been surveyed and the wire line was attached to the monel, the wire coil was then wrapped around the wooden stakes that bordered the entry pit side and was connected to the AC beacon to establish the magnetic field. The magnetic field could be turned on and off as needed. Figure 3: Ditch Witch JT100-Mach 1 HDD Rig used by Ventura Directional Drilling, Inc. After all surveying was completed, the site was prepared for drilling. The first phase of drilling, the pilot bore, took eight days to complete. The bore was drilled a distance of approximately 1,685 feet from entry to exit. During the pilot bore, several issues manifested. One of those issues occurred on the first day of drilling. During drilling, the drill bit began sinking beyond the design path and continued to sink the further VDD was drilling. Originally, VDD chose to use a mud motor in anticipation of dense soil conditions; however, the down hole assembly (DHA), consisting of the monel, orientation sub, and mud motor, was causing over-excavation, resulting in sinking of the pilot bore. The HDD was designed to have a bore path tolerance of plus or minus 5 feet. Within six drill pipes, the bore had sunk 4.75 feet below the designed bore path. After leaving the site and coming back the next day, VDD was informed by the steer hand that if they continued to drill on the path they were on, they would exceed the design tolerances and would be approximately 40 feet deeper than designed. Because of this, VDD changed their tooling by removing the mud motor and changing their drill bit. They switched from a 6.5-inch mill tooth tri-cone drill bit to a 6.5-inch spade dirt bit because the soil encountered along the bore did not require a mud motor for excavation. Figure 4 shows the two bits used on the project. In addition, they changed the designed bend radius from 1000 feet to approximately 562 feet due to the steep bore path. The design team was consulted on these changes and was concerned about the depth of the new borepath. The geotechnical borings on the project terminated at a depth of 78 feet, but with the proposed drilling plan, the bore would be drilled to a depth of 75 feet. VDD was allowed to proceed but agreed to proceed at their own risk should they drill deeper than the designed maximum depth of 63 feet. Paper MM-T

7 Figure 4: First drill bit, 6.5-inch mill tooth tri-cone bit (left); Second drill bit, 6.5-inch spade dirt bit (right) Once the new drill bit reached the existing borehole where drilling was paused to switch drill bits and remove the mud motor, VDD began drilling its new bore path at the altered bending radius. The new drill bit s response to steering was greatly improved once the mud motor was removed. After drilling approximately 308 feet, the AC beacon was moved to the north side of the taxiway. VDD was previously drilling under the taxiway using true magnetic north for reference, with the guidance of InRock. Moving the AC beacon to the other side of the taxiway allowed the drill to be tracked via the InRock software, as the magnetic beacon used the wireline and the downhole signals to locate the drill bit. After moving the AC beacon, the steering hand adjusted the elevation of the bore as the beacon reading indicated that the bit was approximately 3 feet lower than calculated with true magnetic north. They continued drilling to move further from the taxiway and re-evaluated their location. The new drill bit was successful in regaining elevation. VDD continued drilling until they were approximately 560 feet out, at which point the InRock steering hand had to guide the bore off of only the azimuth and inclination readings. Since the steer hand was confident in performing the method, it was decided that they would continue to guide the bore via the azimuth and inclination until they reach the exit coil that was setup prior to the start of the pilot bore. In all, there were no other steering issues that occurred. In relation to the project s design, the completed pilot bore ended up being 3.1 feet to the left of the centerline and 4.5 feet short of the designed exit point (Figure 5). Another issue during the pilot bore resulted from the wireline being attached to the monel and being strung out through each drill pipe. Occasionally the wireline would cut out, resulting in the bore tracking being rendered useless. VDD would then trip back several drill pipes until the problem could be fixed. In some cases, VDD would need to trip back all the way to the monel to fix the issue. Since this issue continued to be a problem, VDD decided that after drill pipe #42 (~620 feet) they would add a built-in spider to the drill string. The spider acted as an area to store the wireline, such that if the wireline were to cut out again, VDD would only need to trip back to the location of the spider to regain connection. In addition, VDD switched their wirelines from zinc to copper wire in hopes it would also assist in reducing wire shortages. Paper MM-T

8 Figure 5: HDD designed exit point vs. actual drill bit exit point The biggest issue with the pilot bore was also one of the biggest risks the design team hoped to avoid. While drilling at 1,350 feet, the crew noticed a loss of circulation, as mud returns back at the entry pit were slim, if any at all. They believed the mud must have been going into a void or fissure of the existing soil because no inadvertent fluid returns, or frac-out, was found on the surface at the entry, exit, or infield locations. This made the crew slightly worrisome, so they monitored the surface for any fluid returns. Alas, after drilling 1,605 feet out, inadvertent fluid returns were found at the exit side, directly over the bore centerline, in very close proximity to the runway (Figure 6). The drilling mud covered an area measuring approximately 20 feet by 30, approximately 50 feet short of the designed exit point. Fortunately, the thickness of the runway was adequate to confine the drilling fluid pressures, keeping the drilling mud from escaping onto the runway or taxiway. After it was spotted, air traffic control was alerted. Air traffic control had to temporary stop all take-offs and landing for planes on the runway until they spill could be cleaned up and contained. VDD brought out vac trucks, while being escorted by Camarillo Airport personnel, to vacuum the inadvertent fluid returns. The cleanup caused a delay of approximately 30 minutes. Unfortunately, the location where the inadvertent fluids were surfacing from the ground could not be located. Figure 6: Inadvertent fluid returns area found right next to the airport s runway Paper MM-T

9 After the spill was contained and cleaned up, VDD decided that since they were only 50 feet from the exit point, they would drill the rest of the bore without mud. They believed that since the source of the surfacing drill fluid could not be located, drilling without mud would be the safest choice for not causing another frac-out. No problems occurred from drilling without mud for the remainder of the pilot bore. After the pilot bore was completed, VDD was ready to begin the reaming phase. VDD had hoped to push ream with a 24-inch barrel reamer and then simply swab the bore hole prior to pullback; however, the plan did not go smoothly. After push reaming for four days and advancing approximately 690 feet, VDD decided to use a different type of reamer due to very slow reaming times. They tripped the reamer back to the surface revealing that the reamer was packed with a mixture of clay, silt, and sand. They added an 18-inch fly cutter reamer to be placed in front of the 24-inch barrel reamer in hopes it would speed up the push reaming process. However, the new reaming plan failed after approximately 30 feet of reaming due to high torque. Since the previous attempts at push reaming failed, VDD decided to start over with a pull reaming process. VDD began with an 8-inch hole opener which took one day to complete. Next, they used a 12-inch terminator hole opener which took slightly over one day to complete. Once completed, VDD installed an 18-inch open type, threeblade reamer with carbon bits in front of the 12-inch terminator. The 18-inch pull ream took another day to complete. After completion, VDD then prepared for the final pull ream phase. They used the 12-inch terminator in front as the stabilizer and attached a 19/24-inch combo reamer behind it. The final pull reaming took a day and a half. Figure 7 shows the various reamers used on the project. Figure 7: Clockwise from upper left: 24-inch barrel reamer; 18-inch fly cutter reamer; 12-inch terminator hole opener; 18-inch open type, three-blade reamer; 19/24-inch combo reamer. Paper MM-T

10 After the reaming phase was completed, VDD prepared for pullback. They started by using the 19/24-inch combo reamer in front of a 22-inch diameter barrel reamer to swab pull the bore hole to prove it was ready for pullback. While this was happening, VDD also prepared the 1,800 feet of HDPE pipe for pullback by fusing the three sections together and setting it on rollers. The crew needed to be creative in ensuring the pipe could be supported during continious pullback operations. The County flood control channel, slightly north of the exit point, required the pipe to be continuously supported as it is lofted and pulled across the channel. The crew used pieces of wood and a back hoe to fully support the pipeline as it crossed the channel. Figure 8 shows the pipe being fully supported across the channel. Figure 8: 16-inch HDPE pipe being supported across the County flood control channel (left and right) For pullback, a 2-inch tremie pipe was strung throughout the 16-inch HDPE pipe and filled with water for buoyancy control to allow ballasting during installation. The 22-inch barrel reamer was attached in front of the swivel which connected to the 1,800 feet of HDPE pipe. In all, pullback was completed in one continious operation and took approximately six hours and thirty minutes to complete. 6. Lessons Learned Although the installation had some difficulties, VDD was able to successfully cross the active Camarillo Airport via HDD. Similar to other HDD projects, permits and coordination with public or private establishments are still required and inadvertent fluid returns are still a concern. In this case however, inadvertent fluid returns were seen as a higher risk because spilling onto an active runway is extremely dangerous due to the properties of the fluids. To improve upon this project, VDD should have had someone constantly monitoring for fluid returns on the exit side. The large spill area next to the runway would not have been able to get as large as it did if it was constantly being monitored during the entire pilot bore. In addition, a Paratrack-2 and AC beacon tracking system proved to be successful at tracking the HDD operation while crossing beneath an active airport without causing any major disruptions to airport operations. This project serves as an example for the Federal Aviation Administration (FAA) and other engineers as to how to directionally drill beneath an active airport. 7. References Bennett, D., and Ariaratnam, S. (2008) Horizontal Directional Drilling Good Practices Guidelines, North American Society for Trenchless Technology (NASTT), Third Edition, USA. Daily Inspection Reports and Photos, Camarillo Airport Project Earth Systems Southern California (2012) Geotechnical Engineering Report For Proposed Waterline Camarillo Airport Camarillo, California. Plans and Specifications, Camarillo Airport Project Staheli, K., Wetter, L. (2012), Trenchless Method Feasibility Camarillo Airport Water Improvement Project. Paper MM-T