A semi-submersible or semisubmersible is a watercraft that can put much of its bulk underwater.
With a relatively small area above the water's surface, the semi-submersible is less affected by the waves than a normal ship, but must be trimmed continuously. Unlike a submarine, such a ship never is entirely underwater.
Tourist underwater viewing
An example of this type was the former "Submarine Voyage" at the Disneyland amusement park, which featured a surface craft that resembled a surfaced sub. Passengers boarded and descended to seats below the waterline, where viewing ports presented an underwater view. This gave the illusion of riding in a submerged craft. Similar craft have been used for tours of oceanic reefs.
Deep-sea ocean study
A tilting craft was designed to form a stable platform in rough seas. It had a conventional bow and stern separated and joined by a tubular structure. Unlike a conventional ship, the stern was built with decks, ladders, and fittings suitable for use if the forward part of the ship was sunk, leaving the entire craft pointing vertically downward toward the ocean bottom, with the stern portion well above the water line. The narrow tubular portion had the effect of transmitting very small forces to the craft as large ocean swells and waves passed by, so the platform was extremely stable in comparison to a conventional ship.
See also R/P FLIP (FLoating Instrument Platform) for another research ship that re-orients vertically by flooding its stern.
MV Blue Marlin carrying USS ColeA semi-submersible heavy-lift ship, or also known as a "flo/flo" (for float-on/float-off), has a long and low well deck between a forward pilot house and an after machinery space. In superficial appearance, it is somewhat similar to a dry bulk carrier or some forms of oil tanker. Its ballast tanks can be flooded to lower the well deck below the water's surface, allowing oil platforms, other vessels, or other floating cargo to be moved into position for loading. The tanks are then pumped out, and the well deck rises to shoulder the load. To balance the cargo, the various tanks can be pumped unevenly.
The flo/flo industry's largest customer base is the oil industry. They have transported many oil drilling rigs (the flo/flo ships can carry the rigs from their construction site to a drilling site at roughly three to four times the speed of a self-deploying rig). This means major savings to the oil industry if you can get the drilling rig transported to the drilling site rapidly. They also transport other outsized cargo and yachts.
Bluewater 1A different type of semi-submersible is found in offshore drilling. Shell's Bruce Collip is regarded as the inventor. When offshore drilling moved into deeper waters of up to 100-feet, fixed platform rigs were built, until demands for drilling equipment was needed in the 100 to 400-foot depth of the Gulf of Mexico, the first jack-up rigs began appearing from specialized offshore drilling contractors such as ENSCO International. The first semi-submersible arrived by accident in 1961. Blue Water Drilling Company owned and operated the four column submersible Blue Water Rig No.1 in the Gulf of Mexico for Shell Oil Company. As the pontoons were not sufficiently buoyant to support the weight of the rig and its consumables, it was towed between locations at a draught mid way between the top of the pontoons and the underside of the deck. It was observed that the motions at this draught were very small and Blue Water Drilling and Shell jointly decided that the rig could be operated in the floating mode. Since then, semi-submersibles were purpose-designed for the drilling industry.
Semi-submersible rigs make stable platforms for seeking and drilling for offshore oil and gas. They can be towed into position by a tugboat and anchored, or moved by and kept in position by their own azipod propellers with dynamic positioning.