How does an oil rig stand on the sea bed?

Answer 1

They are fabricated on land. Typically they mobilized (moved) to location by barges. A jackup rig may be assembled in pieces, with the jacket or the rig structure being installed first, and then the topsides (all housing and equipment that rests on the structure) is installed second, by large cranes. See related link.

A semi-submersible, like the Deepwater Horizon, may be towed by tugboats to its location. See second related link.

Answer 2

A MODU (Mobile Offshore Drilling Unit) can be either:

Jack Up = 3 jackable legs are lowered from the rig that. is towed to location. Once they hit the seabed they then lift the rig out the water.

Semi Submersible = Floating rig towed to location, some propelled, either anchored in position or maintain position using dynamic thrusters.

Drill Ship = Monohull Vessel, self propelled, either anchored in position or maintain position using dynamic thrusters.

Production Platforms can be:

Conventional fixed platforms, steel jacket or concrete caisson.

Gravity Base, steel jacket or concrete caisson.

Compliant tower.

TLP, tension leg and mini-tension leg platform.

Spar.

FPSO, Floating production, storage, and offloading facility

Semis

Or, subsea completion, i.e rigless.