To add to above: In the previous barge landing they did weld the landing legs to the barge using some kind of bracket. But they discovered that the centre of gravity is low enough to not need any welding. During the previous attempt the sea was heavier than this time.
Maybe they still weld it this time, but that is done by having someone go to the barge and do it.
Actually no they did not weld anything to the legs. Elon said that they realize it wasn't needed. They did use octojack though to support the weight off the legs.
Plus the platform has four computer controlled engines on basically universal joints to stabilize it. A crew then boards the barge and puts 'shoes' over the landing legs then welds them to the deck.
