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.
See also R/P FLIP (FLoating Instrument Platform) for another research ship that re-orients vertically by flooding its stern.
The USS Spuyten Duyvil (1864) was truly semi-submersible, 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.
During WW2 Britain developed the Welfreighter a semi-submersible designed to carry combat swimmers.
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.
A 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.
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.
The U.S. Navy has also chartered other heavy lift ships to carry smaller craft, usually mine-countermeasure craft, or other patrol craft. Since there are no US-flagged heavy lift/flo/flo ships, the U.S. Navy normally relies on its Military Sealift Command to charter them from the world commercial market.
Many of the larger ships of this class are owned by the company Dockwise, including the Mighty Servant 1, 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. Two of the company's vessels have been lost in recent years: the Mighty Servant 2 which capsized after hitting an uncharted underwater obstacle off Indonesia in November 1999, and the Mighty Servant 3 which foundered after unloading the drilling unit Aleutian Key offshore Angola in December 2006.
A 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.
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 an 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.