Power up16 February 2017
Barnhart Crane & Rigging has developed lift and structural-support systems to remove faulty clarifier support laterals at a nuclear power plant.
Barnhart Crane & Rigging was tasked with providing a lift system and a structural-support system to aid in the removal of clarifier support laterals at a nuclear power plant in Louisiana.
The power plant operates with two water clarifiers to supply the plant with water for cooling. Built approximately 35 years ago, each clarifier is a 30ft-high circular concrete facility with an inside diameter of 178ft. Each core cylinder weighs 435,000lb and is held up by 16 support laterals.
One of the clarifiers went offline when the structural stability of several laterals became comprised from normal wear and tear.
Although the power plant remained operational because of the other operating clarifier, there was a need for a quick solution to the problem.
The removal process of the damaged laterals had to be accomplished from above, as the area inside the clarifier was deemed an overhead fall hazard.
This risk prohibited anyone from entering the inside tank of the clarifier for any reason.
Barnhart nuclear-project sales representative Gene Pugh highlighted that the plant-project and site-safety personnel were the most significant factors to take into account when developing the method for planning the lift.
“Our team came up with this concept to place some beams directly over the walls of the clarifier such that that we could rig to the clarifier, support it and hold it in a very secure way, allowing man baskets to be lowered down inside but not below the compromised structure while removing the laterals,” said Pugh.
Barnhart Engineering designed a lift structure to lift, travel and rotate the clarifier core. The structure including the safety factor was designed to provide 600,000lb-lift capacity.
The backbone of this structure featured Barnhart’s 8ft girders, spanning 180ft and weighing up to 225,000lb. These were set to the clarifier walls with tandem lifts made by a Demag CC1800 in Super Lift and a 365t Liebherr hydraulic crane. The remaining girders and pull-up gantries were then carefully positioned to the specifications of the lift plans.
Additionally, Barnhart custom designed and fabricated several items deemed necessary for the safe execution of this lift plan. David Trawick, Barnhart engineering team member, added that the stability kickers for the 8ft girders on the wall were designed for anti-tipping.
Load-spreading rockers were needed for the contact points of the various girders due to the deflection in the 180ft spans. Finally, various spreader bars were fabricated to accommodate the complicated rigging design challenges.
With all the rigging in place, the clarifier was lifted and held in place while the lateral supports were removed. After the laterals were cut out and removed, the clarifier had to be slightly rotated and recentered.
Once the laterals were removed and the overhead fall hazard was mitigated, only the lift system and the clarifier core remained.
Pugh said: “We mobilised four more pull-up gantries with 5ft girders and were able to safely position this shoring system underneath the clarifier core.”
Once the shoring system was placed underneath the clarifier, the Barnhart team was able to disassemble the entire upper support structure.
This lift system had to be removed before the lateral support replacement scope of work could proceed. Once they are all replaced, the clarifier will go back online.
Pugh said another significant challenge this project presented was that they had to work with tight deadlines. This was due to financial and safety risks in the event that the other operating clarifier needed to go offline for maintenance.