Fort McMurray sewage pipe project takes advantage of latest horizontal drilling technology
Two of the main components of the Forcemains Systems Project in Fort McMurray, Alta.-the installation of 8.4 kilometres of large-diameter sewer forcemain and the use of current oil and gas horizontal directional drilling (HDD) technology to drill twin boreholes that run beneath the bottom of the Athabasca River-were, on the face of it, a straightforward infrastructure upgrade.
Perhaps. But the $45-million project was the focus of an article last year in Trenchless International-the official publication of the International Society for Trenchless Technology-and was recognized as Northwest Trenchless Project of the Year by the North American Society for Trenchless Technology (NASTT). As well, the project received an Award of Merit from the Consulting Engineers of Alberta (CEA).
The citation for the CEA award praised the use of "innovation including state-of-the-art investigative and design techniques," and its detailed risk management program. The scope and size of the project, and its use of the latest HDD technology, were all part of what made it a unique project.
"It used the latest HDD techniques and successfully covered a lot of bases," says Todd Simenson, a managing principal at Stantec Consulting Ltd.
The project was just one small part of a vast program of infrastructure projects and upgrades on the books for the Regional Municipality of Wood Buffalo, worth a total of about $1.6 billion. The city is in acute need of infrastructure to support both recent and future growth. Fuelled by huge investments and development in the oilsands, Fort McMurray has been growing at a phenomenal rate, with projections of 12 per cent growth per year to 2016. Some projections estimate that the city will have a population of 250,000 by 2016.
The new forcemains provide additional capacity to the sanitary sewer system of Fort McMurray's lower townsite, allowing for new development there and to the south. As a project, it was anything but simple.
The project involved multiple stakeholders and construction consisted of six contracts, including upgrades to two pumping stations, three open-cut pipe installation contracts, plus the twin forcemains crossing the Athabasca River. The lower townsite, which is bounded by the Clearwater River at the north end, with new subdivisions sprouting on the higher ground across the river, is in a valley at the confluence of two rivers, the Athabasca and the Clearwater.
The 8.4-kilometre Forcemains pipe begins at Pumping Station 1B at the eastern edge of the lower townsite, runs along the north end, and, at the edge of the Athabasca, crosses a small channel of water before running along the western edge of an island where a golf course is located. At the north end of the island, the pipe is twinned where it crosses the Athabasca to the west side, and then runs north to the waste water treatment plant.
Before construction started, a detailed environmental assessment was done, with particular attention paid to where the river should best be crossed. Various methods and locations were considered, but it was concluded that HDD installation would have the least impact on the river and surrounding areas. Pipelines, gas utilities, power lines, the adjacent golf course, the bridge and highway reconstruction on the west side of the Athabasca added to the challenge of determining the best route for the pipe. It was a case of a lot infrastructure competing for a relatively small amount of real estate.
"There was a degree of complexity with utilities and highways competing for utility assignments, and this pushed for the pipe under the river as an option," says Darcy Elder, director of engineering for the regional municipality.
Much of the 8.4 kilometres of pipe consists of a single forcemain with an inside diameter of 750 millimetres. The section crossing the river has two pipes of 750-millimetre inside diameter each. The extra pipe is for operational redundancy and capacity for the future, Simenson says. The final section of the forcemain also has twin pipes, but of different diameters-one at 750 millimetres and the other at 900 millimetres.
The size of the pipes was determined by hydraulic modelling, which was also used for pressure parameters and pipe system configuration.
"This modelling was challenging due to the large flow variations during wet weather periods, the various hydraulic heads involved due to the system configuration and for system redundancy," according to a paper co-authored by Simenson and presented at the NASTT No-Dig Show 2010.
Part of the risk assessment process, which Simenson says included the identification of 42 risk items and development of 73 mitigation measures, involved doing some geotechnical drilling.
"A critical risk we found was that at the exit side, there was natural gas at a depth of about 30 metres that was quite close to the exit point," he says.
Both the entry and exit points of the horizontally drilled tunnel crossing the Athabasca, which was at a depth of about 45 metres below the riverbed, had to be set well back and at sufficient depth below the water to avoid what's known as a "frac out." This would occur if the ground above the HDD borehole gave way and resulted in drilling mud mixing with river water.
Says Simenson, "It would then involve the Department of Fisheries and Oceans and would be serious."
Apart from the twinned section crossing below the riverbed, which was high-density polyethylene (HDPE), PVC pipe was mostly used for the forcemain project. Simenson says that the advantage of using HDPE is that sections are fused together with no joints.
The lack of available land on which to set a surface tracking coil led to the use of a barge as a base for a solenoid beacon system to track and guide the drill head for the pilot borehole. For the second hole, a ranging wire was attached to the first HDPE pipe as it was being installed. This was used to provide guidance for the second hole to ensure that it was drilled at a sufficient distance from the first hole.
The issue risk for the horizontal drilling had to be handled differently than it would in the oil and gas industry.
Says Simenson: "When a horizontal directional driller provides a bid to Shell Ltd., it's based on production and time, but the producer may take on some of the risk. But a municipality does not. With a municipality, the cheapest one gets it, so we had to review bids carefully to ensure they didn't offload risks."