A strange-looking contraption that could represent a new frontier in clean energy wallowed in the water alongside a coal dock here in a bay in northwest Spain. This floating windmill with a tower about 600 feet high was sheltering in the harbor. After waiting out the rough winter seas and the disruption from the coronavirus pandemic, it was towed in late May to join two others anchored in the Atlantic in 330 feet of water 11 miles off Viana do Castelo on the northwest coast of Portugal.

The idea is to try to stake out a vast new area of the oceans for the wind power industry. Generating electricity from wind began on land, but developers, led by Orsted of Denmark, started venturing into the sea in the early 1990s as they sought wide-open spaces and to escape the objections of neighbors to having a twirling monster next door. Three decades later, offshore is now the fastest-growing segment of the wind business, but marine wind farms have been limited to water shallow enough to allow turbines to sit on piles or other supports on the sea bottom. About 200 feet in depth is the outer limit for such devices, people in the industry say.

If platforms could be put almost anywhere at sea, “we can go to areas where we have never before harnessed the wind,” said José Pinheiro, the project director of WindFloat Atlantic. Mr. Pinheiro’s machine floats on three partly submerged columns, each about 100 feet long. Steel catwalks bridge the gaps between the giant cylinders. Sensors signal to pumps to add or remove water from the columns to keep the platform at the right level for optimal wind generation. In a gentle sea in the bay, the vessel, which weighs thousands of tons, seemed remarkably stable.