I'm halfway certain that there is sorcery from Middle Earth involved in building this humungous chain. But it's pure human ingenuity. The chain is meant to be used on Shell's Prelude project (a floating liquefied natural gas platform) which is basically some ginormous platform that's permanently moored offshore.
The whole video is interesting but Reddit found the ship chain being made (each link is 500 lbs!) and it totally looks like those chains are made in the pits of hell.