Semi Submersible

Type of Modu

A semi-submersible rig is a type of floating offshore drilling unit designed for deepwater operations. Unlike jack-up rigs, which rest on the seabed, semi-submersibles remain buoyant and are stabilized by submerged pontoons and vertical columns.

Semi-subs operate in water depth that exceed the limitations of a jack-up and are used in depths from 4,00ft to 10,000ft

A semi-sub consists of submerged pontoons on which the main deck is attached through structural columns which keeps the main deck above sea level. The pontoons are ballasted down when the rig is in position which cause the vessel to be less affected by wave and sea conditions.

The semi-sub is able to maintain its position above the well by either running anchors or for more advanced version, using dynamic positioning (which is required for depths where anchors and not useable).

Main Advantages

- Can operate in deep water beyond the capability of jack-ups

- Relatively stable compared to normal vessels

- Can operate in harsh environments

Main Disadvantages

- Expensive compared to jack-ups

- Complexity of equipment is increased due to the well construction items being placed on the seafloor (subsea BOPs are more complex and expensive)