Semi-submersible
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A semi-submersible or semisubmersible is a watercraft that is designed with a large portion of its volume (bulk) underwater. Unlike a submarine it never is entirely underwater in normal seas.
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Types and Applications
Tourist underwater viewing
An example of this type is the "Capitan Nemo's Submarine Ride" at the Disneyland amusement park. This consists of a surface craft resembling a surfaced submarine, in which the passengers board and descend to seats below the water. Their viewing ports thus present an underwater view, with the intended illusion during the ride to be that of riding in a submerged craft. Similar craft have been used commercially to view natural oceanic reefs.
Deep-sea ocean study
A tilting craft was designed to form a stable platform in rough seas. This was constructed with a conventional bow and stern (forward and after parts) of a ship, but separated and joined by a tubular structure. Unlike a conventional ship, the stern part 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.
Military
The U.S. Civil War warship USS Monitor is considered to be the earliest semi-submersible ship, having very little freeboard (the distance between deck and the water) and containing all of its propulsion machinery, fuel, and crew accommodations below the waterline. Only a small pilot station, cylindrical two gun turret and a smokestack protruded above its flat deck.
The presentation of a small cross sectional area at the waterline can produce a very stable watercraft, less subject to the effects of waves and swells. It has also been observed that submarines can be very efficient craft when operating underwater, as they do not create a bow wave. Craft have been proposed and prototypes constructed that consist of two submarine-like structures underwater, with streamlined pylons supporting a superstructure. This offers both high efficiency and stable operation in rough seas (up to a limit determined by the size of the vessel).
Lockheed Corporation designed and built a craft, the Sea Shadow, in many ways similar to the above description, although rather than mounting the superstructure on pylons the superstructure was joined to the hulls with continuous members, with sloped sides, forming a stealthy structure less visible on radar.
Heavy-lift
A semi-submersible heavy-lift ship 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. Such a ship has ballast tanks that can be flooded to lower the well deck below the water's surface, allowing oil platforms, other vessels, and other cargo to be floated into position for loading. The tanks are then pumped out, and the well deck rises to shoulder the load.
The U.S. Navy has used such ships to bring two damaged warships back to the United States for repair. The first was the guided missile frigate USS Samuel B. Roberts (FFG 58), which was nearly sunk by a naval mine in the central Persian Gulf on 14 April 1988. The frigate was towed to Dubai, then floated home to Newport, Rhode Island, aboard the Mighty Servant 2.
Twelve years later, the MV Blue Marlin transported the U.S. guided missile destroyer USS Cole (DDG 67) from Aden, Yemen to Pascagoula, Mississippi, after the warship was damaged in a bombing attack on October 12, 2000.
Offshore drilling
Semi-submersible rigs are used for offshore drilling. These oil platforms are typically towed into position by tugboat, but are also carried by semi-submersible heavy-lift ships.
See also
One form of dry dock — a floating dry dock — is also semi–submersible. Dry docks are used for the repair and maintenance of ships.