This article originally appeared in the October 2019 edition of Water & Wastes Digest as "Quagga Mussel Invasion."
The Island Water Treatment Plant (WTP) is vital to the city of Toronto as the sole source for year-round deep lake cooling water. This critical supply determines both the time of year and working interval when even one of the three intakes can briefly be removed from service for cleaning.
During the fall of 2018, the city of Toronto initiated a project to clean all three 60-in. HDPE deep water intakes at the Island WTP, restore flow capacity to its original design specification, and reduce plant operating costs.
Quagga mussels, an invasive species of mollusk, arrived in the ballast water of ships navigating into the Great Lakes and have created catastrophic environmental impacts as it invades this new ecosystem. In 2014, remotely operated vehicle (ROV) inspections of the raw water intakes extending from Island WTP into Lake Ontario documented quagga mussel infestation. Fouling was heaviest at the mouth of the intakes and had created flow turbulence, reducing the volume of water delivered to the WTP and increasing pumping costs.
Scope of Work
Large diameter hydraulic pipeline cleaning is a somewhat unique specialty, and particularly so, under deep water conditions (250 ffw) as encountered off Toronto Island. In the summer of 2017, Montauk Services was contacted by engineers with the city of Toronto after their analysis of the pig-cleaning previously achieved at the Cornell University 60-in. deep water intake in Cayuga Lake. Toronto’s criteria, specifications, and budget for hydraulic pigging at the Island WTP reflected the procedures and results demonstrated at Cornell.
By June 2018, the city of Toronto had finalized specifications and bid requirements for this project. The specialized hydraulic pigging operations required support by a local marine contractor, therefore, Montauk teamed with Galcon Marine Ltd., which provided island transport, construction personnel and equipment and control of multiple subcontractor specialists, while also handling barge delivery, on-shore piping, pumping and marine support.
The contract scope of work included:
• Designing and fabricating a pig launcher with structural support capable of mating with the intake valve chamber and WTP piping system;
• Developing, documenting and providing a process design and procedure for hydraulic pig cleaning of the three WTP intakes consistent with the timing and utilization requirements imposed by the city. The movement of vehicles and personnel along Lakeshore Avenue on Toronto Island could not be significantly delayed or disrupted;
• Obtaining necessary regulatory permits and approvals;
• Performing an ROV inspection throughout the complete length of each intake before pig cleaning;
• Pig cleaning at three deep water intakes, including salvage of used pigs, which in view of environmental considerations could not be “lost at sea;” and
• Manual cleaning of the raw water suction well and the common inlet pipe.
Montauk Services handled the overall hydraulic layout, launcher design and hardware designation, as well as pig design, sequence determination, tracking and recovery procedures. Each pig was tracked leaving the intake valve chamber, entering Lake Ontario, and at the offshore mouth of the intake. Recovery procedures also were implemented to track and retrieve pigs discharged into the lake but then affected by bottom currents or adverse weather conditions.
Hydraulic Pigging Procedures
Pig cleaning pipelines are simple in concept, but success usually is determined by the experience, planning and preparation prior to actual pig runs. In Toronto, cleaning time was limited because even a single intake could not be removed from service without imposing constraints upon the system, and the operational consequences of any significant disruption such as a “stuck pig” were immense.
One significant concern when cleaning mollusks or removing similar hard pipeline debris is that a mass of material may accumulate ahead of the pig retarding or potentially stopping its forward movement. The design and sequence of pigs designated for Toronto was developed from nearly 50 years of experience at Montauk Services.
Foam pigs can be fabricated to channel approximately 10% of the propelling water around the pig. The volume of mussels or debris scoured off the pipe wall is thereby displaced and suspended ahead of the moving pig. Before a pig emerges from the intake terminus, a flow of murky water is first noted then followed by a trickle of mussels from the invert of the intake.
The discharge of mussels will increase in volume, creating a pile on the lake bottom, which is followed by a surge of black water, mussels, debris and the pig itself. The pig may be observable by the ROV video, but if visibility is obscured by debris, it can be tracked and located by sonar. Each pig was additionally fitted with an acoustic transponder which could be located at long distances to allow pig recovery even if it was transported across the lake bed by water currents.
Intake Cleaning Operation
It was fall by the time the field work for this project was underway, and contract specifications would not allow any interruption of intake flow before 15OCT18.
The WTP intake pipelines extended in length from 16,500 ln ft to 17,300 ln ft between the shore-side valve chamber and intake mouth installed at 250 ft of water depth in Lake Ontario. Each of the lines had an independent connection for the pig launcher so that cooling water supply through two intakes could be kept in service while the third was cleaned.
Pigs were propelled with water pumped from the shore-side of Toronto Island passing beneath a temporary bridge installed by Galcon to maintain traffic along Lakeshore Avenue. The pumps maintained uniform flow between 18,000 and 20,000 gpm. The pig run-time varied from 75 to 90 minutes, and each was tracked as it left the launcher and again as it crossed the beach into the lake.
If any problem had developed during pig transit, gauges and data-loggers would have indicated a pressure surge identifying the location of the pig. Each of the three Toronto intakes (west, center and east) was successfully cleaned by only three sequential pig runs. A fourth cleaning run was by contract available if desired at the request of the city engineers.
The Lost Pig
Cleaning at the west and center intakes progressed like clockwork with each pig array run and all pigs retrieved without issue. Contract specifications, however, required this cleaning to be undertaken between 15OCT18 and 14DEC18, which is not a convenient time interval for offshore operations on Lake Ontario.
The second pig did not exit from the east intake, and November weather suddenly developed high winds and a sea state, which terminated offshore operations. The support vessel could not safely leave the dock for a week to track and retrieve this lost pig. After a week of delay, conditions had subsided allowing the vessel to maintain station above the east intake and deploy the ROV. The lost pig was discovered resting happily in the truncated mouth of the intake structure at the end of the HPDE pipeline, and the ROV manipulator attached a haul-line from an auxiliary boat to pull the pig clear of the intake. The final cleaning pig was then shot and retrieved before adverse weather was again encountered.
Intake Cleaning Results
The pig cleaning operations progressed flawlessly, other than weather challenges. Immediately after cleaning the first intake, Montauk Services and Galcon stood by while the city engineers verified the Hazen Williams C-Factor achieved by three pig runs. They also determined if an optional fourth pig run would offer benefit. Three pigs had successfully cleaned the line and yielded a C-Factor, indicating that the pipeline was restored to a hydraulic condition better than new.
All of the Toronto intakes were cleaned with only three pigs and C-Factor analyses performed by the city in each case also documented water flow better than when the pipelines were newly installed. When the pipelines were built in 2004, the original C-Factors were calculated to be 137 to 140.
Prior to cleaning, the C-Factor had been reduced to approximately 110, but after 2018 cleaning operations, all three intakes at the Island WTP had C-Factors of 149 to 152 as calculated by city engineers. Pipeline cleaning and restoration by hydraulic pigging was validated during this project with results exceeding expectations. Work was completed on time and on budget while fulfilling a critical obligation that the Island WTP remain in operation throughout the work.