A semi-submersible platform is a specialised marine vessel used in a number of specific offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They are designed with good stability and seakeeping characteristics. Other terms for these platforms include semi-submersible, semi-sub, or simply semi.
While drillships are another type of drilling rig that can drill in ultra-deepwater—drillships are capable of holding more equipment—semisubmersibles are sometime chosen for their comparative stability.[en]