When workers started pulling apart the three largest hydroelectric units in North America — capable of supplying more than enough power for all of Seattle — they found the damage far worse than expected. They encountered large cracks, worn-out bearings and a defect in a critical weld that, if left in place, could fail, unleashing catastrophic flooding inside the powerhouse that risked killing workers and destroying the 7 million-pound generator-turbine units. That last discovery halted work for 10 months to give engineers time to come up with a fix that would ensure a crucial covering would hold fast. “How do we deal with the unexpected?