Calgary Mechanical Pipe Plug with Strongjack

Petersen Products engineered and built a neutrally buoyant mechanical stop and integrated strongjack restraint that made it possible for the City of Calgary to replace critical 60-inch valves at the Bearspaw Water Treatment Plant without a system shutdown. This “complex and tricky” dewatering solution was delivered, reviewed and approved by third-party engineers, stress-tested, and deployed under tight time and access constraints. 

Challenges

Calgary needed to isolate two 60-inch steel-lined/prestressed concrete mains feeding the plant’s junction chamber to facilitate multi-week valve replacements, but plant-wide outages longer than 8–12 hours were infeasible due to constant city demand. 

Constrained access to the mains added to the challenge; after entry through a relatively cramped hatch, the dive contractor had to work carefully through about 60 meters of flooded passages with multiple bends. 

Available space only allowed one diver to work at a time, so equipment mass, buoyancy, and maneuverability became critical design factors. Additional complicating factors included a pipe-to-wall 5° angle causing compound loading, plant concerns about applying excess circumferential pressure to large diameter pipe, and a prohibition on permanent anchoring to the chamber structure. 

Why a Mechanical Stop?

Because personnel would be working downstream, the isolation had to be a strict mechanical stop with no inflatable component that would impart hoop loads—i.e., circumferential tensile forces in pipe caused by internal pressure. Instead, the dewatering solution would have to be reliant on known access chamber integrity.

Line-stop alternatives braced across the chamber were rejected as too heavy and complex to move and assemble within the cramped assembly chamber and limited time windows, and diver constraints including safety concerns.

Compartmentalizing the work to one 60-inch line at a time preserved plant capacity, and reduced operational risk while meeting the maintenance schedule. 

Petersen’s Engineered Solution

Petersen Products’ designer Henri Kinson engineered a one-piece mechanical disc plug sized for the 60-inch line, designed to be neutrally buoyant so a single diver could precisely maneuver an approximately 1,000 lb. out-of-water assembly weighing about half a ton. Offsetting buoyancy was achieved by adding sealed drums and expanding boat foam to the plug body, validated so final in-water force was near-neutral and manageable by one diver. Four temporary set screws allowed installers to hold the plug face in plane during alignment prior to engaging the external restraint system. 

“Assembling and ‘tuning” this massive 60-inch mechanical stop for neutral buoyancy was a challenge!” says Kinson. “Not to mention the design of the incorporated external restraint and the requirement for extremely precise alignment—all in all this is one of the most complex and tricky mechanical stops I’ve worked on in my career at Petersen.” 

Strongjack Needed

Given Calgary’s desire to minimize pipe hoop loads in this situation, Petersen designed an external strongjack bracket system that transferred axial and shear forces back into the square junction chamber rather than into the pipe wall. Welded shims were used to pre-tension the assembly and to resolve the 5° misalignment, ensuring the compound vertical and horizontal load components were satisfied and documented to third-party standards. To accommodate staging area constraints the bar, boot, and linchpin connections were designed so that a single diver could slide components into place on chamber I-beams and lock the restraint without permanent anchors. 

Design Review & Verification 

Petersen collaborated with a Canadian engineering firm to review and stamp the system, addressing local code requirements such as minimum hole edge-distance. A full-assembly stress test to approximately twice rated conditions verified the integrated plug-and-strongjack assembly performance prior to shipment, with documentation coordinated across firms. Procurement to shipment for the first system was completed in roughly 16 weeks including design, review, fabrication, and testing, after which a second identical system was produced at Calgary’s request.  

Precision Onsite Execution

Diver operations were planned in task-level detail with shift management and decompression protocols, minimizing underwater time while ensuring all torque and verification steps were completed.

Installation for the first isolation was performed during a tightly controlled 8-12-hour time window, with real-time coordination among dive supervisors, crane operators, and the surface team to move and place the neutrally buoyant assemblies. Decommissioning reversed the procedure; removal required roughly half the time of installation, reflecting the stability and predictability of the Petersen’s design and fabrication. 

 

Successful Outcomes

Both 48-inch butterfly valves from the 1980s were replaced safely and efficiently and the system was reconfigured with additional valves for improved future maintainability, de-risking a major operational vulnerability. The city avoided the costs and public impacts of a prolonged plant shutdown while maintaining service reliability, validating the isolation over multi-week work windows for each line. 

“Henri (Kinson) was extraordinarily responsive, engaged, and professionally and technically competent, says City of Calgary Senior Process Engineer Patrick Chan. “He took calls at 3 a.m., was happy to discuss very fine constructability details, and clearly cared about our success.”

“We evaluated other options, but Petersen’s neutrally buoyant mechanical stop and restraint were the only practical way to meet diver, schedule, and safety constraints inside our tight time windows.” 

“This was a uniquely difficult project,” says Kinson. “One diver, a thousand-pound isolation plug, a 5° offset, and zero room for error—so we designed and fabricated a precision assembly with neutral buoyancy and integrated strongjack restraint, stress-tested it before shipment, and were able to obtain third-party engineer approval.

Why it Mattered in Calgary

  • External restraint that re-routed forces into engineered chamber structures enabled safe and effective isolation of large-diameter lines without excessive hoop loading, advancing necessary maintenance without citywide shutdowns.
  • Neutrally buoyant, single diver-installable assemblies expanded what was achievable in flooded, constrained spaces, reducing risk and installation times.
  • Collaborative engineering support—design, FEA, code compliance, and stress testing—de-risked this novel isolation method for owner acceptance and third-party approval. 
  • Calgary’s Bearspaw water treatment plant was able to continually supply safe drinking water, with flows, storage, and process controls continuously managed, and no system shutdown.

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Angus Stocking is a former licensed land surveyor who has been writing about infrastructure since 2002.