A semi-submersible is a specialised marine vessel with good stability and seakeeping characteristics. The semi-submersible vessel design is commonly used in a number of specific offshore roles such as for offshore drilling rigs, safety vessels, oil production platforms and heavy lift cranes.
The terms semisubmersible, semi-sub or just semi are also generally used for this vessel design.
Read more about Semi-submersible: Characteristics, Early History, Classification