Artificial islands as modern military sea platforms

Are there modern sea platforms built for military defense and surveillance like converted oil platforms or earlier Channel forts?

China's artificial islands in the South China Sea are the example given.

China's artificial islands in the South China Sea are the modern example of offshore military platforms offered here. They provide fixed positions at sea for military use. Alex Clarke also criticises the quality of concrete at some installations. This separate allegation is made without naming a site or providing construction evidence.