Mobile offshore drilling units (MODU) are classified into a number of structures, namely:

1. Drilling barges

They are mostly utilized for shallow water drilling in calm water conditions. Drilling equipment is placed on to the barges’ decks and towed to the site by tugs. Anchors hold the barges in the position where drilling is to be carried out.

2. Jack-up Rigs

These self-contained drilling structures are as good as the drilling barges only to the exception that these massive structures are positioned onto the site by lowering and rooting their three or four giant legs on the sea floor.

The barges are then lifted up above water and are more stable than the floating rigs to be able to drill up to 100 meters

3. Submersible Rigs

These rigs are pontoon-based which allow for water ballasting and de-ballasting in order to position, anchor and re-float the massive structure. All necessary drilling equipment is fitted on the platform’s deck, similar to the drilling barges and jack-up rigs.

The only conspicuous change in these structures is that the drilling and production facilities are rested on stilts that are raised to some height above the pontoons. Their operations are limited to shallow waters due to their submersible design.

4. Semi-Submersible Rigs 

A Semi-Submersible rig is a Mobile Offshore Drilling Unit better known as MODU, which is designed for offshore drilling in ultra-deep waters of the oil and gas-rich areas across the globe. They are partially submerged in water during drilling operations and are normally moored to the sea bed by anchors.

They are technically advanced drilling units which are designed to accommodate platform or decks containing heavy machinery and expensive drilling equipment and are supported by columns for support and stability in the treacherous waters. This design of partial submerging the platform to water is primarily conceptualized for stability reasons and chosen for the very same reason over another highly capable breed of drilling units known as the drill-ships.

5. Drill ships

These vessels are advantageous over the other drilling units such as the semi-submersibles due to their ultra-deepwater drilling capability and easy mobility. They are basically maritime vessels modified to drill wells in oil and gas fields. They are adapted to provide complete offshore drilling solutions to the clients across the globe. They are equipped with exhaustive mooring and positioning systems and are able to propel from one well to another without outside assistance.

are fitted with expensive machinery and drilling equipment which are similar to the ones fitted on any other drilling unit. Since they are required to work in ultra-deepwater areas ranging up to 3000 meters, all supplies and equipment are catered by the offshore workboats. The drillships have a drilling deck and are equipped with a moon pool, and are somewhat similar in design as the other drilling units. The drilling equipment that passes the moon pool is connected to the ‘sub-sea well’ via the ‘riser pipe’ (which is flexible to some extent).