From an answer on Engineering SE:
It's very likely they are jacket foundations for offshore wind turbines. The excellent 4C Offshore has a database of such vessels used on offshore wind farms. It also has a list of all offshore windfarms under construction by country, and tells you which vessels are working on it, so given that you know where the photo was taken, you may able to work out which farm it's supplying.
But why is such a foundation a "jacket" foundation? I looked up the meanings of "jacket" in Merriam-Webster, and none seems to fit. I know that usually the word is used in industrial texts to describe some kind of cover. Example: "jacketed vessel." But in this case, it's just a frame supporting the wind turbine.
A picture containing a "jacket":