The boat had just lost sight of land when two delicate shapes appeared on the horizon, like needles sprouting from the sea. As the boat got closer, they seemed to grow — and grow — until they towered above passing container ships. Two wind turbines now rise higher than the Washington Monument off the coast of Virginia Beach, $300 million down payments on what state officials wager will be a new industry and a source of clean energy for the future.

The last 253-foot blade was attached to one of the turbines Friday by contractors for Dominion Energy, Virginia’s biggest utility and the owner of the project. On Monday, Gov. Ralph Northam (D) signed laws creating a state Office of Offshore Wind and setting a mandate for 5,200 megawatts of offshore wind energy by 2034.

When the turbines start spinning next month, they’ll be the proof of concept for what’s expected to be the biggest wind farm in federal waters in the United States — more than 180 such turbines on this patch of ocean 27 miles offshore should be in operation by 2026.

“Virginia has a chance to be one of the leaders . . . in renewable energy,” Northam said. Environmental advocates have hailed the development, which is also enshrined in an omnibus energy bill passed this year that commits the state to carbon-free power by 2045.

But some caution that the project exposes consumers to enormous potential costs. Dominion’s status as a regulated monopoly allows it to make guaranteed profits, and the General Assembly has ordered regulators to regard wind energy as “in the public interest” — limiting their ability to force the utility to cut costs or issue customer rebates.