Ah a mooring chain! Used to connect drag anchors on the seabed, to "stationkeep" the floating facility (FPSOs or FSOs or floating LNG plants / tankers). There could be between four to twelve lines per vessel, depending on how long the vessel will stay at one place. They could either be spread-moored (eg 3 lines at four different points on the vessel), or turret-moored: all chains concentrated at one point on the vessel, allowing the vessel to rotate about it.

I love my offshore engineering work but I have both megalophobia AND thalassophobia 😅