In fall 2018, the City of Toronto initiated a project to clean three sixty-inch diameter HDPE raw water intakes at the Island Water Treatment Plant, restore intake flow to its original design specification, and reduce plant operating costs.
The Island WTP serves as the only source providing year-round deep lake cooling water for Enwave Energy, a private utility generating electricity. Hydraulic capacity of each deep water intake as determined by Hazen-Williams “C-Factor” rating had been markedly reduced during 14 years of service.
In 2014, Remotely Operated Vehicle (ROV) inspections of the deep water intakes extending from Island WTP into Lake Ontario documented quagga mussel infestation. Mussels adhering to the interior wall of the HDPE pipe created flow turbulence which diminished the volume of water delivered to the facility and caused increased pumping cost.
Quagga mussels adhere to any submerged surface and feed off plankton in the water. They propagate at a massive rate and lead to dramatic disruptions of the ecosystem. Tom Nalepa, emeritus research biologist at the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory, has stated “Without question, the quagga mussel represents the greatest threat to the Great Lakes of any invasive species.” Neither chlorine nor other chemical procedures have proven effective as a control mechanism.
Scope of Work
Large diameter hydraulic pipeline cleaning is a unique specialty, especially in deep water conditions. In 2017, Montauk Services was contacted by Engineers with the City of Toronto Water Division because they had completed a similar project on Lake Cuyahoga for Cornell University.
Montauk teamed with Galcon Marine Ltd., a Toronto based contractor with offshore expertise. Galcon provided island transport, support, and control for multiple subcontractors, as well as GC tasks including barge delivery, on-shore piping, hydraulic water supply pumping, ROV services, and marine operations for pig retrieval.
“Completing the project in the tight timeframe was a challenge due to poor weather conditions,” said Ryan Vogt, senior project manager of Galcon. “From a marine standpoint, the prevailing winds during Fall are from a predominantly bad direction making retrieval of the pigs, ROV operations, and drawing water for the operation very difficult.” Vogt said the timeframe chosen for the project was optimal for the City of Toronto because it was the lowest usage time for their clients.
- Design and fabricate a pig launcher and structural supports capable of mating with the Intake Valve Chamber and WTP piping system
- Develop, document, and provide a process design and procedure for hydraulic pigging of the three WTP intakes consistent with the timing and utilization requirements imposed by the City.
- Obtain necessary regulatory permits and approvals
- Perform ROV inspection throughout complete length of each intake before pigging
- Pigging at three deep water intakes, including salvage of used pigs which in view of environmental considerations could not be “lost at sea”
- Manual cleaning of the Raw Water Suction Well and the Common Inlet Pipe
Montauk Services handled the overall hydraulic layout, launcher design, pig design, sequence determination, pig tracking, and recovery procedures.
Hydraulic Pigging Procedures
Pigging pipelines is simple in concept, but success is determined by experience, planning and preparation. Pigs clean by compression of the foam body as they pass along a pipe propelled forward by hydraulic pressure created as pumps discharge into the line behind the pig. Think of pulling a very large sponge through a pipeline and scrubbing the wall surface as it moves. The Toronto WTP lines were almost 61 in. in interior diameter. The first pig shot was a soft foam material 58 in. in diameter.
One concern when cleaning mussels or removing similar hard pipeline debris is that a mass of material may accumulate and shoal up ahead of the pig retarding or potentially stopping forward pig movement. The volume of Quagga mussels displaced from the intake ahead of the pig was monitored in real time by submerged video cameras aboard an ROV stationed at the intake mouth. The amount of discharged material and the documented pressure profile provided the information needed to determine characteristics of the next pig to be launched.
The final pig shot through each intake was full sized with embedded plastic bristles to ensure that the pipe wall would be completely cleaned. The beauty of foam pigs is that they can be designed to channel approximately 10 percent of the propelling water around the pig. The volume of mussels or debris scoured off the pipe wall is displaced and suspended ahead of the moving pig. A few minutes before the pig is discharged, a flow of murky water was followed by a trickle of mussels dropping onto the lake bed from the invert of the intake mouth. The trickle became a flow creating a pile on the bottom followed by a surge of black water, mussels, debris and the pig.
“A few minutes before the pig is discharged, a flow of murky water was followed by a trickle of mussels dropping onto the lake bed from the invert of the intake mouth. The trickle became a flow creating a pile on the bottom followed by a surge of black water, mussels, debris and the pig,” he continues.
Toronto Intake Cleaning Operation
Each of the intake lines had an independent connection for the pig launcher so that cooling water supply from two lines could be kept in service while the third was cleaned. Each pig was propelled with water pumped from the shore side of Toronto Island passing beneath a temporary bridge to maintain traffic. Pumps were controlled to maintain flow uniform between 18,000 and 20,000 gallons per minute. Intake pipelines extended in length from approximately 16,500 lf to 17,300 lf between the Valve Chamber and intake mouth. Pig run times varied from 75 to 90 minutes.
The surface support vessel with ROV was stationed at the intake mouth at 250 ft depth. The pig was tracked as it left the launcher and again as it crossed the beach into the Lake. During pig transit, gauges and data-loggers were in place to indicate any pressure surges and allow the location of the pig to be estimated.
Deep Water & Weather Concerns
The cleaning was to be undertaken between Oct. 15 to Dec. 14, 2018, not ideal due to unpredictable Lake Ontario weather. Cleaning at the West and Center Intakes progressed like clockwork with each pig sequence run and all pigs retrieved without issue.
The good luck ran out and the second pig on the East Intake did not exit. High winds and sea conditions halted offshore operations. It was a week until the surface support vessel could safely leave the dock to track and retrieve this pig, which was the size of a minivan. The “lost” pig was discovered sitting happily in the truncated mouth of the intake structure at the end of the HPDE pipeline. It was a simple procedure for the ROV manipulator to attach a haul line for an auxiliary boat to pull the pig clear of the truncated intake section.
Intake Cleaning Results
Immediately after the first intake was cleaned, City engineers verified the Hazen Williams C-Factor rating achieved by three pig runs. Initial results indicated that the pipeline was restored to a hydraulic condition better than when it was originally installed. The quagga mussels and any other build up reducing water flow had been completely cleared. The remaining Intakes were cleaned with three pig runs.
C-Factor analyses performed by the City demonstrated water flow conditions better that when the pipelines were new. A perfect C-Factor for this type of pipe is 150. In 2004, C-Factors were calculated to be 137 to 140. Prior to cleaning, the C-Factor had been reduced to approximately 110. After the pigging operation, all three intakes at the Island WTP had C-Factors of 149 to 152 as calculated by City engineers.
Work was completed on time and on budget while the Island WTP remained in operation. This method of pipeline remediation is worthy of consideration for situations on the Great Lakes or in other areas with pipeline flow restricted due to quagga mussels, zebra mussels or other species of biofouling.
Client: City of Toronto, Island Water Treatment Plant
General Contractor: Galcon Marine Ltd., Toronto, Canada
Hydraulic Pigging Contractor: Montauk Services Inc., Hackensack, N.J.