Concrete gravity platform rig:

A rigid offshore drilling platform built of steel-reinforced concrete and used to drill development wells.

The platform is floated to the drilling site in a vertical position, and at the site tall caissons that serve as the foundation of the platform are flooded so that the platform submerges and comes to rest on bottom.

Because of the enormous weight of the platform, the force of gravity alone keeps it in place.

See platform rig.