DECONTAMINATION & DECOMMISSIONING | VESSEL CLEANING
Above left: Bottom view of the tank. One section between the baffles is mostly complete
Above right: Top of the tank after the scaffolding was erected showing the last bit of material left after 3D tool use and before the hand gunning to finish
Above: The Walco 3D tooling
Located at a facility in northern Canada, the dimensions of the vessel were 21ft x 29ft (6.4m x 8.84m). The task prompted Walco to carefully consider and evaluate two potential methodologies – the utilisation of robotics and the use of 3-D tooling. Given the brevity of the assignment timeline, the cost implications associated with decontaminating or replacing robotics, and the exposure potential while operating the robotics safely and efficiently, the company obtained approval to employ the 3-D tooling method for the bulk of the liner material removal. The 3-D tool, using 360-degree rotation and high-
pressure water jetting, is designed for many applications and is used in vessels such as the one in question regularly. Due to the size of the tank, the placement of the mixing blades, the baffle locations, and the speed at which an employee can position the equipment, the choice was
26 | February 2024 |
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made to use an arm length of 18 inches (45.72cm) with the tooling. This allows for a 3-foot (91.44cm) span reach. Given the effectiveness of water jetting from distance (depending on nozzle size), the optimal distance away from the walls of the vessel was no further than 6-10 inches (15.24-25.4cm). Moving the tool too far away from the walls diminishes the cutting power. From the top of the vessel, areas were mapped out in sections like a clock face for 6x6-inch (15.24cm by 15.24cm) access holes to be cut. Due to the four tank baffles within the vessel, positioned at the 12, 3, 6, and 9 of the ‘clock’, four sections were created, each with three access points. These access points, 12 in total, were designated for the half-hour marks and cut roughly 24 inches (60.96cm) away from the walls of the vessel. This design allowed the tool to drop in from the top through each of the three access holes in each 90 degree section while giving an allowable overlap between the 3-D tool rotations and still stay within 6 inches of the walls. The full rotation speed of the tool with the intended nozzle size was calculated at 45 minutes per rotation. Knowing the height of the tank at 29ft (8.84m) and the extension arms for the 3-D tool, each lift of the system was calculated to be 3ft (0.914m) to maximise efficiency in both speed and overlap. Despite the problematic issues of radiation exposure, it was determined that manual lowering and lifting of the tooling was the most efficient method even though the initial in-tank exposure readings were so strong that they would only permit a worker to complete approximately 15 minutes of work before they were at the allowable exposure limit. Manual lifting only exposed the worker to the higher radiation levels for two-to-five-minute intervals while the exposures for the surrounding area allowed workers to be deployed for 3-5 hours instead of 15 minutes.
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