Dan Zienty is a project design leader for Short Elliott Hendrickson Inc., St. Paul, Minn.
According to George Bernard Shaw, "You can’t make a silk purse out of a sow’s ear" (Man and Superman–1903). There is a good chance that the recent restoration of the 5-million gallon Yankee Doodle Reservoir, located in Eagan, Minn., might never have taken place had Mr. Shaw been a decision-maker for the City.
Over the years, time and temperature extremes wreaked havoc on the tank’s exterior paint system. The tank showed signs of the type of extreme deterioration commonly associated with aging and long-term exposure to the elements. Major maintenance was needed on the tank. Under ordinary circumstances, the City of Eagan would contract for engineering services and through the normal bidding process, find a qualified contractor to complete the work. This project was not ordinary.
With the tank’s exterior in an advanced state of deterioration, a high probability existed that the interior also would present a significant challenge to repair. Normally, the City could choose from many options to determine the tank’s interior condition. However, it had just completed construction of a new water tower and was not yet sure how its addition would affect the overall operation of the system. Therefore, taking five million gallons of water temporarily out of that system was not a viable option.
Wanting to proceed with plans for maintenance of the tank during the 1999 construction season, the City of Eagan contracted to have the tank’s interior inspected in November of 1998. Another company was hired to evaluate the exterior. Liquid Engineering, Billings, Mont., provided a dive inspection of the tank’s interior surfaces, permitting the City to keep the tank in service during the investigation. AEC Engineering, Minneapolis, Minn., evaluated the tank’s exterior. This evaluation included a report on the condition of the coating system, the tank’s design compliance to American Water Works Association (AWWA) guidelines and compliance with current OSHA safety guidelines. AEC also evaluated the information from the dive inspection and incorporated it into a full report that included maintenance recommendations and estimated project costs.
Recommendations
The report to the City revealed that complete tank reconditioning was in order. The tank was constructed in 1971 and the original interior and exterior paint systems were still in place. The interior system was a coal-tar epoxy and the exterior system consisted of a three-coat epoxy/urethane finish. The system’s manufacturer could not be determined.
The report recommended removal of the interior paint system to an SSPC SP-10 cleanliness standard for immersion followed by the application of a polyamide epoxy coating system. The exterior would be prepared to an SSPC SP-6 cleanliness standard and then coated, as before, with epoxy/polyurethane. The report also recommended adding two 24*-diameter hinged roof manways, a 24* shell (side wall) manway and a frost-free vent to enhance ventilation within the structure.
The tank was built with 12 sealed vertical pilasters (10* in width, 24* in depth) to stiffen the shell wall. Pinholes discovered in the welded seams permitted condensation to form and reform freely. Coupled with a continuous freeze/thaw cycle over time, the condensation caused minor damage, buckling the lower pilaster plate sections. Repairs were recommended to prevent continued degradation.
The report’s final recommendation involved removing the existing cathodic protection system and replacing it with a new submersible system.
Engineering Service Contract
The next step for the City was to contract for engineering services. These services would include providing plans and specifications based on the report’s recommendations, assisting the City with the bidding process and providing construction administration and qualified onsite inspection. A project of this scope typically requires a year of advanced planning in order to receive bids from the greatest number of qualified contractors and attract the best price. However, an engineer was chosen by February 1999. The contract was awarded to Short Elliott Hendrickson, Inc. (SEH) St. Paul, Minn., and work began immediately to meet the demands of a now-accelerated schedule.
With a construction start date of July 6, the first order of business involved a meeting with City staff and the project engineer to gather input on implementing the report’s recommendations. This meeting also established a clear line of communication and a full understanding of project expectations for both parties. The initial meeting identified each side’s special concerns.
City
• Water demand during tank down time.
• Proximity to main traffic arteries.
• Attached communications antennas.
Engineer
• Availability of as-built drawings and specifications.
• Interior paint chips.
SEH offered the additional service of system modeling to the City. Modeling would show the impact of the new tank and the effect of taking this tank out of service. Initial (actual) system tests showed that the new tank was not completely filling. However, before any modeling began, SEH suggested a check of the associated valves. Investigation by the City identified a valve closed during construction of the new tank. Once opened, the operational problem with the tank was eliminated as well as City concerns for tank draining during reconditioning.
Exterior paint chip sampling conducted during the evaluation determined that the existing paint system did not involve either lead- or chromium-based substances. Fugitive dust emissions and possible paint drift became the remaining concern for the project. Air emission in Minnesota is regulated by the Minnesota Pollution Control Agency (MPCA). In the time since the tank originally was constructed, the City of Eagan grew to a population of nearly 60,000. Located just south of St. Paul and with east access to the St. Paul/Minneapolis International Airport, the City’s growth is expected to continue. However, the tank is situated in an area that is zoned light industrial, allowing for sectional containment measures to be taken.
A complicating factor for this project was the recent presence of antennas for mobile phone communications systems on the water tower. Many communities (including the City of Eagan) have opted to restrict antenna location to their water towers for aesthetic reasons. The problem is that unless design provisions were made at the time of installation, antennas and coaxial cables may be positioned too close to the tank surface to remove old paint properly. The City of Eagan and SEH worked with Aerial Communications and Sprint Spectrum to design and install new permanent brackets before the start of the project. By moving the cables away from the tank, complete surface preparation could be achieved without damage to the provider’s equipment (cables were wrapped by the contractor for protection) and without permanent system downtime.
Many concerns expressed by SEH could not be addressed until the tank was taken out of service for restoration. Though a dive inspection was a more feasible method of evaluation for the City, it had some limitations. Interior paint chips were not extracted, millage was not assessed, measurement of the roof’s structural components could not be made and weld deficiencies could not be viewed clearly. In addition, available as-built drawings were limited to general tank size and amenities.
Before the design phase began, final investigations resulted in establishing a clearer picture of the project details. City staff provided assistance in obtaining a few measurements of the interior roof structure. Oxygen Services Company (OSC) of St. Paul provided insight into pilaster repairs. OSC suggested identifying pinholes by putting each pilaster under low pressure and covering the weld seams with soapy water to identify all leaks. Following weld repair, existing couplings would be replaced and the entire pilaster (10* ¥ 2¢ ¥ 80¢) would be purged with an inert gas (nitrogen) to reduce further interior corrosion.
Developing project specifications encompassed the entire month of March and ended with Council approval. Special provisions incorporated into the specification included
• A contractor-provided personnel monitor for identification of ionizing radiation exposure. This is needed when working near active antennas.
• Collection and disposal of waste material generated by the coating removal process (as reviewed by MPCA and including containment and waste sampling).
• The addition of a submersible cathodic protection system. (Optional)
Following approval of the construction documents by Council, an Engineers Estimate was prepared setting the estimated project cost at $852,000. The public notice appeared and bids were accepted until the deadline of May 6, 1999. The seven bids ranged from $524,960 to $882,600. Swanson & Youngdale, Minneapolis, Minn., was the low bidder and, after review of submitted qualifications, was awarded the project.
By the Fourth of July holiday weekend, submittals had been reviewed and a preconstruction conference was held. New coax cable brackets had been installed and the City was draining the tank. Swanson & Youngdale began mobilization on July 7, hanging exterior rigging and removing accumulated silt from the tank interior.
Paint Removal
To achieve the SSPC SP6 degree of surface cleanliness specified by the city engineer, Swanson & Youngdale utilized an approved water-injected abrasive blasting system along with impermeable ground cover to remove the exterior paint system. Once the tank had been drained, paint chip sampling and analysis verified that the existing interior pain system was neither lead nor chromium-based. The project progressed as exterior and interior operations were carried out simultaneously. Swing staging was set up between pilasters to remove the old paint system using the approved blasting method. Holdtight was used as an additive to prevent flash-rust prior to prime-coat application by roller.
Interior removal operations began with abrasive blasting of all weld seams to identify deficiencies such as skips and spatter. Since the tank was not drained at the time of inspection, the amount of grinding required was determined based on past projects of similar scope. However, continual blasting of the weld seams revealed that their presurface preparation during the tank’s original construction ranged from minimal to nonexistent. Therefore, the number of hours for grinding was nearly twice the original estimate of 150. The importance given to the removal of weld deficiencies could not be minimized. These deficiencies often lead to early coating failures and substantial repair costs.
An engineered opening into the tank’s bottom shell plate was cut to aid in increased daily productivity and allow for ease of accessibility throughout the structure. This permitted the deployment of three boom-supported aerial platforms (JLGs) able to reach roof structural members and all shell plate surfaces. With the exception of floor work, presurface preparation (welding, grinding), dry abrasive-blast surface preparation, application of a three-coat polyamide-epoxy paint system and application of an elastomeric sealant (between small gaps above the waterline) were carried out using the JLGs in this manner. When the interior work was complete, the cut shell panel was welded back into place and radiography performed. Weld seams were prepared per the specification and the applied paint was force cured.
While operations continued on both the interior and exterior, City staff and SEH went to work on a design for the City’s planned logos. A design already existed on another of the City’s tanks but that structure did not feature pilasters. The existence of 12 exterior pilasters would interfere with the layout of the original design making the logos less visible. With the use of tank photos taken from different vantage points, SEH was able to use Virtual Reality Imaging (VRI) to give the City a variety of two-dimensional design options.
The total project had been scheduled for 14 weeks. However, with rain delays and additional work (removal and replacement of the tank access ladder, including platforms), the project was extended to the first week in November. By then, installation of the cathodic protection system was complete and the tank was rinsed and disinfected prior to placement back into service. The contractor was fortunate that Minnesota was experiencing its second mild fall in a row, a weather bonanza that would allow for successful completion well before winter’s first snow.
Summary
The similarity in all tank-reconditioning projects is that, in the end, they all are somewhat different. The project to restore the City of Eagan’s Yankee Doodle Ground Storage Reservoir was unique in some respects. Usually there are fewer unknowns going into a project and that tends to limit project stakeholders to the Owner, Engineer and Contractor. Other circumstances such as those found in this project require the input and expertise of a number of resources. This project brought together 14 different entities to produce a successful restoration that met the client’s expectations.
