SEATTLE – Consultants investigating an accelerated corrosion problem on steel piles for Washington State Ferries’ new terminal at Colman Dock have recommended a solution estimated at $ 10 million.
This solution would be designed to protect 173 piles that were driven into Elliott Bay for the project in the fall of 2017. Approximately 500 piles in total were installed as part of the massive reconstruction effort on the Seattle waterfront.
In 2016 and 2017, with valuable engineering efforts aimed at reducing costs at Colman’s wharf, project managers decided to add additional thickness to the steel piles of the wharf to achieve a targeted lifespan of 75 years. and not to use a marine coating system, which must be maintained, Ferries Terminal Engineering Director David Sowers said in an interview with the Kitsap Sun. The coating system is used on piles at the agency’s other locations across Puget Sound, he noted.
“We thought this value engineering approach would be successful and also save us from having to do this maintenance step in 30 years,” he said.
In the spring of 2018, corrosion had germinated on the uncoated piles to a level that worried those responsible for the project. Engineering contractor WSP USA hired corrosion experts to assess the problem, and project officials chose to revert to the marine coating system on the remaining piles.
According to a report prepared for Washington State Ferries, 17 of the unpaved piles were measured at various points from December 2018 to December 2020, and consultant Tinnea & Associates found that nearly 60% of the locations measured had experienced “high” effects. or “serious”. corrosion rate. Applying the corrosion rates found at the site, the firm estimated that it would take 6 to 13 years for an average pile to reach a thickness below a minimum allowable threshold.
Low and moderate corrosion rates are expected in any structure built in seawater, the report notes, but unexpected high and severe rates are cause for concern.
“It was a real eye-opener for Washington State Ferries that this microbial-influenced corrosion was actually occurring at an accelerated rate beyond normal corrosion levels in the marine environment,” said Sowers.
In their report, the consultants recommended a solution that would add a “wrap” around the unprotected piles and an anode sled system, which would introduce a small electrical charge into the piles, concentrating the corrosive activity away from the piles, said Sowers.
Tinnea recommended that the repairs be completed by the end of 2027 and estimated they would cost between $ 7 million and $ 10 million. The company’s suggestions are conceptual at this point. Final designs will need to be worked out for repairs and funding will need to be secured, Sowers noted.
“This is something that we did not anticipate,” he said. “There are these unknown unknowns that can arise. I think, above all, that we have a good solution to fix it.
So what caused the problem? Tinnea pointed to “microbial-influenced corrosion” (MIC), claiming that a variety of different bacteria worked together to influence the rate of corrosion on the dock.
“MIC is increasingly recognized as a contributor to marine corrosion around the world and is an emerging problem in Puget Sound,” the company report said. “The bright orange corrosion product first seen at Colman Dock was later observed by Tinnea & Associates staff at other facilities. Rising temperatures and the acidification of the oceans are creating new environments and niches for bacteria to grow, and the waters of Puget Sound are experiencing microbiological changes that appear to promote bacteria involved in the sulfur and iron cycles that influence the corrosion.
The larger reconstruction of Colman Dock is on track for a 2023 completion date, Sowers said. More than $ 455 million in federal, state and local funding has been allocated to the work.
“We obviously have a wonderful facility underway that we plan to open in the spring of 2023,” Sowers said. “The pilings, the installation, the terminal, everything will be designed for a level of intensity of a thousand-year earthquake. This facility is going to exist for literally the next hundred years. “
Nathan Pilling is a reporter who covers Bainbridge Island, North Kitsap and Washington State Ferries for the Kitsap Sun. He can be reached at 360-792-5242, [email protected] or on Twitter at @KSNatePilling.
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