FROM "MARINE MASTERS"
Water and wastewater managers negotiate underwater engineering services to extend asset life, avert regulatory violations, and increase operational efficiency.
Last year, an engineer-diver cleaned and assessed a covered, 2-million-gallon concrete drinking water reservoir owned by Rapid City, S.D. Here's a step-by-step description of the process.
The tank had efflorescence deposits adjacent to hairline cracks on the exterior walls, indicating leakage. Concerned about the asset's structural integrity, the Rapid City Water Division's Operations Management Engineer Dan Coon and Water Superintendent John Wagner requested proposals (RFP) from engineering firms to inspect the interior and exterior and submit a report on condition and proposed methods of remedial action.
The RFP specified that the internal inspection be done by a licensed professional structural engineer while the tank was full of water. This method was chosen because it would minimize the amount of time the tank would be out of service (three days total) and eliminate the need to drain the asset, providing both time and cost benefits.
Operations Management Engineer Dan Coon and Water Superintendent John Wagner selected Stanley Consultants Inc. of Muscatine, Iowa, to perform the inspections. The firm's in-house team of structural engineers, who also are certified commercial divers, mobilized their self-contained trailer to the tank location with all the equipment necessary to inspect and document the tank's interior.
A local contractor provided equipment to lift the underwater diving and inspection equipment to the top of the tank in preparation for the interior inspection. A clean laydown area was prepared for staging any equipment that would later enter the water system.
Before the engineer-diver enters the tank, all equipment is cleaned and disinfected.
After the diver is suited up, a final inspection is made of all systems including installation of hoisting equipment required for confined-space entry.
Water samples are taken from the tank, before and after the inspection, to test for chlorine residual.
One of the tasks is cleaning the tank's bottom before the inspection. Cleaning was performed by a hydraulic vacuum pump. The water was discharged through a de-chlorination system before discharging to surface drainage.
Conditions found in the interior included rust on steel components such as the access ladder and inlet/outlet pipe and exposed reinforcing steel.
Visible cracks on the wall interior corresponded to the hairline cracks observed on the exterior.
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