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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.
Types
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.Military
The U.S. Civil War warship USS Monitor is considered to be the earliest semi-submerged 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. As it did not change its depth by admitting or expelling ballast water, is not truly a semi-submersible vessel.The USS Spuyten Duyvil (1864) was truly semi-submersable, as it was a weapon system that could ballast down by flooding tanks to form a low profile and stealthy attack craft, and like the Monitor contained an extensive number of innovative features, including a directable, extensible, and reloadable spar for manipulation of its weapon, a slightly boyuant naval mine to be released beneath its target.
The presentation of a small cross-section at the waterline can produce a very stable watercraft. 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). (See Small Waterplane Area Twin Hull - "SWATH " for additional information.)
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, or deckship, 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.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.Navybook, NO HIGHER HONOR: Photo: USS Roberts Aboard Mighty Servant II
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.
In 2004, the Blue Marlin also carried the world's largest oil platform, the BP Thunder Horse from a shipyard in Korea to a shipyard in Corpus Christi, Texas.[http://www.dockwise.com/?sid=33&press=46 Dockwise, ARRIVAL OF THUNDER HORSE PDQ A NEW MILESTONE AND ERA FOR THE OIL & GAS INDUSTRY]
Many of the larger ships of this class are owned by the company Dockwise, including the Mighty Servant 1 and 3, the Blue Marlin, and the Black Marlin. In 2004, Dockwise increased the deck width of Blue Marlin, to make it the largest heavy transport carrier in the world.
Offshore drilling
A different type of semi-submersible is found in offshore drilling. Shell’s Bruce Collip is regarded as the inventorOcean Star, Bruce G. Collipp. When drilling moved into ever deeper waters in the Gulf of Mexico, first jack-up barges were used. Later on, platforms were used that would be towed to location and submerged to the seabed. 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 propellers with dynamic positioning.
Crane vessels
After the drilling industry, the advantages were soon recognized for offshore construction. In 1978 Heerema introduced the Balder and Hermod. These semi-submersible crane vessels (SSCV's) consist of two lower hulls (pontoons), three columns on each pontoon and an upper hull. After that J. Ray McDermott and Saipem also introduced SSCV's, culminating in the workhorses DB-102 (now Thialf) and Saipem 7000, capable of lifting respectively 14,200 and 14,000 tons.During transit a SSCV will be de-ballasted to a draught where only part of the lower hull is submerged. During lifting operations, the vessel will be ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swell. High stability is obtained by placing the columns far apart. The high stability allows them to lift extreme high loads.
Floating dry dock
Some dry docks, used to repair and maintain ships, are semi-submersible.References
External links
- Dockwise website - Info on many large semi-submersible transport ships
- Photos of Mighty Servant 2 hauling USS Samuel B. Roberts (FFG 58)
Navire semi-submersible | Half-afzinkbaar schip | Statek półzanurzalny
"Semi-submersible"