Thought bout them being anchors after I posted that , but didn't know what they were for .
It takes a lot of work to anchor those rigs solidly, a crew of chain handlers are needed on separate ships to move and set/unset the anchors when a rig needs to be moved but the rig itself hauls in the chain once the anchors have been freed from the ocean floor. The rigs have massive chain lockers in the upright tubes on the rig`s corners and it takes real powerful winches to pull all that chain in and let it out. I have several links of that chain that were cut out of a chain to allow for length, each one weighs a hundred lbs.