There are four basic types of off shore drilling rigs for exploration:

• Submersibles - These are not used often. They can be in shallow water locations then ballasted to sit on the bed of the ocean

• Jackups -  Usually used in waters around 160 metres deep. They are almost always towed to their location an lowered and setup in their location

• Drill ship.  These locations look like regular ships but have a derrick on the top which drills through a hole in the hole.  They are anchored at a certain position and uses it's computer controlled propellers to continually correct the ships drift. Often used to drill 'wildcat' wells in deep waters

• Semi submersible - These are mobile structures. Their superstructures are supported by columns sitting on hulls or pontoons which are ballasted below the water surface. They provide excellent stability in rough, deep seas.

The moveable offshore rigs are mostly used for exploratory purposes because they are less expensive to use than the permanent platforms.

 

Production platforms

Once oil or gas is discovered, the drilling rig is generally replaced by a production platform, assembled at the site using a barge equipped with heavy lift cranes.

Platforms vary in size, shape and type depending on the size of the field, the water depth