Submersible



A submersible is an underwater vehicle which needs to be transported and supported by a larger watercraft or platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent operation at sea.

There are many types of submersibles, including both human-occupied vehicles (HOVs) and uncrewed craft, variously known as remotely operated vehicles (ROVs) or unmanned underwater vehicles (UUVs). Submersibles have many uses including oceanography, underwater archaeology, ocean exploration, tourism, equipment maintenance and recovery and underwater videography.

History
The first recorded self-propelled underwater vessel was a small oar-powered submarine conceived by William Bourne (c. 1535 – 1582) and designed and built by Dutch inventor Cornelis Drebbel in 1620, with two more improved versions built in the following four years. Contemporary accounts state that the final model was demonstrated to King James I in person, who may even have been taken aboard for a test dive. There do not appear to have been any further recorded submersibles until Bushnell's Turtle.

The first submersible to be used in war was designed and built by American inventor David Bushnell in 1775 as a means to attach explosive charges to enemy ships during the American Revolutionary War. The device, dubbed Bushnell's Turtle, was an oval-shaped vessel of wood and brass. It had tanks that were filled with water to make it dive and then emptied with the help of a hand pump to make it return to the surface. The operator used two hand-cranked propellers to move vertically or laterally under the water. The vehicle had small glass windows on top and naturally luminescent wood affixed to its instruments so that they could be read in the dark.

Bushnell's Turtle was first set into action on September 7, 1776, at New York Harbor to attack the British flagship HMS Eagle (1774). Sergeant Ezra Lee operated the vehicle at that time. Lee successfully brought Turtle against the underside of Eagle's hull but failed to attach the charge because of the strong water currents.

Operation
Apart from size, the main technical difference between a "submersible" and a "submarine" is that submersibles are not fully autonomous and may rely on a support facility or vessel for replenishment of power and breathing gases. Submersibles typically have shorter range, and operate primarily underwater, as most have little function at the surface. Some submersibles operate on a "tether" or "umbilical", remaining connected to a tender (a submarine, surface vessel or platform). Submersibles have been able to dive to over 10 km below the surface.

Submersibles may be relatively small, hold only a small crew, and have no living facilities.

A submersible often has very dexterous mobility, provided by propeller screws or pump-jets.

Technologies
Different basic technologies used in the design of submersibles.

Atmospheric pressure
The Earth's atmosphere exerts a pressure on its surface, similar to how water exerts pressure due to its weight. Unlike water, however, the atmosphere is compressible because it is composed of gases. Consequently, the density of the atmosphere varies with height, with highest density at sea level. As a result, the maximum atmospheric pressure is experienced at sea level, gradually decreasing with increasing altitude.

To calculate atmospheric pressure, we consider the pressure exerted by a column of seawater 10 meters in height. Under average condition, the atmosphere can support such a column, resulting in an atmospheric pressure of 103,000 Newtons per square meter (N/m2).

Gauge pressure and absolute pressure When measuring gas pressures, including those experienced underwater, gauge pressure and absolute pressure are essential concepts.

Gauge pressure: A pressure gauge is typically calibrated to read zero when the gauge is at atmospheric pressure. This gauge records only the “difference of pressure” between the measured pressure and atmospheric pressure. If such a gauge is used underwater, it will indicate only the additional pressure exerted by the water. For example, if the gauge reads 120 bar, the measured pressure is actually 120 bar above atmospheric pressure, equivalent to 121 bar relative to vacuum.

Absolute pressure: At sea level the atmosphere exerts a pressure of approximately 1 bar, or 103,000 N/m2. Underwater, the pressure increases by approximately 0.1 bar for every metre of depth. The total pressure at any given depth is the sum of the pressure of the water at that depth (hydrostatic pressure)and atmospheric pressure. This combined pressure is known as absolute pressure, and the relationship is:

Absolute pressure (bar abs) = gauge pressure(bar) + atmospheric pressure (about 1 bar)

To calculate absolute pressure, add the atmospheric pressure to the gauge pressure using the same unit. Working with depth rather than pressure may be convenient in diving calculations. In this context, atmospheric pressure is considered equivalent to a depth of 10 meters. Absolute depth (m) = gauge depth (m) + 10 m.

Depth measurement: Pressure monitoring devices

The pressure the is more important for structural and physiological reasons than linear depth. Pressure at a given depth may vary due to variations in water density.

To express the linear depth in water accurately, the measurement should be in meters (m). The unit “meters of sea water” (msw) is a by definition a unit for measurement of pressure.

Note: A change in depth of 10 meters for a change in pressure of 1 bar equates to a water density of 1012.72 kg/m3

Single-atmosphere submersibles have a pressure hull with internal pressure maintained at surface atmospheric pressure. This requires the hull to be capable of withstanding the ambient hydrostatic pressure from the water outside, which can be many times greater than the internal pressure.

Ambient pressure submersibles maintain the same pressure both inside and outside the vessel. The interior is air-filled, at a pressure to balance the external pressure, so the hull does not have to withstand a pressure difference.

A third technology is the "wet sub", which refers to a vehicle that may or may not be enclosed, but in either case, water floods the interior, so underwater breathing equipment is used by the crew. This may be scuba carried by the divers, or a breathing gas supply carried by the vessel.

Buoyancy
When an object is immersed in a liquid, it displaces the liquid, pushing it out of the way.

Once the object is partially immersed, pressure forces exerted on the immersed parts are equal to the weight of water displaced, Consequently, objects submerged in liquids appear to weigh less due to this buoyant force. The relationship between the amount of liquid displaced and the resulting up-thrust is known as Archimedes' principle, which states:

"when an object is wholly or partially immersed in a liquid, the up-thrust it receives is equal to the weight of the liquid displaced."

Buoyancy and weight determine whether an object floats or sinks in a liquid. The relative magnitudes of weight and buoyancy determine the outcome, leading to three possible scenarios.

Negative Buoyancy: when the weight of an object is greater than the up-thrust it experiences due to the weight of the liquid displaced, the object sinks.

Neutral Buoyancy: if the weight of an object equals the up-thrust, the object remains stable in its current position, neither sinking or floating.

Positive Buoyancy: when the weight of an object is less than the up-thrust, the object rises and floats. As it reaches the liquid's surface, It partly emerges from the liquid, reducing the weight of the displaced liquid and, consequently, the up-thrust. Eventually, the reduced up-thrust balances the weight of the object, allowing it to float in a state of equilibrium.

Buoyancy control
During underwater operation a submersible will generally be neutrally buoyant, but may use positive or negative buoyancy to facilitate vertical motion. Negative buoyancy may also be useful at times to settle the vessel on the bottom, and positive buoyancy is necessary to float the vessel at the surface. Fine buoyancy adjustments may be made using one or more variable buoyancy pressure vessels as trim tanks, and gross changes of buoyancy at or near the surface may use ambient pressure ballast tanks, which are fully flooded during underwater operations. Some submersibles use high density external ballast which may be released at depth in an emergency to make the vessel sufficiently buoyant to float back to the surface even if all power is lost, or to travel faster vertically.

Deep-diving crewed submersibles
Some submersibles have been able to dive to great depths. The bathyscaphe Trieste was the first to reach the deepest part of the ocean, nearly 11 km below the surface, at the bottom of the Mariana Trench in 1960.

China, with its Jiaolong project in 2002, was the fifth country to send a person 3,500 meters below sea level, following the US, France, Russia and Japan. On June 22, 2012, the Jiaolong submersible set a deep-diving record for state-owned vessels when the three-person sub descended 22,844 ft into the Pacific Ocean.

Among the most well-known and longest-in-operation submersibles is the deep-submergence research vessel DSV Alvin, which takes 3 people to depths of up to 4,500 m. Alvin is owned by the United States Navy and operated by WHOI, and as of 2011 had made over 4,400 dives.

James Cameron made a record-setting, crewed submersible dive to the bottom of Challenger Deep, the deepest known point of the Mariana Trench on March 26, 2012. Cameron's submersible was named Deepsea Challenger and reached a depth of 10908 m.

Commercial submersibles
Private firms such as Triton Submarines, LLC. SEAmagine Hydrospace, Sub Aviator Systems (or 'SAS'), and Netherlands-based U-boat Worx have developed small submersibles for tourism, exploration and adventure travel. A Canadian company in British Columbia called Sportsub has been building personal recreational submersibles since 1986 with open-floor designs (partially flooded cockpits).

A privately owned U.S. company, OceanGate, also participated in building submersibles, though the company fell under scrutiny when their newest submersible imploded underwater with no survivors.

Marine remotely operated vehicles
Small uncrewed submersibles called "marine remotely operated vehicles," (MROVs), or 'remotely operated underwater vehicles' (ROUVs) are widely used to work in water too deep or too dangerous for divers, or when it is economically advantageous.

Remotely operated vehicles (ROVs) repair offshore oil platforms and attach cables to sunken ships to hoist them. Such remotely operated vehicles are attached by an umbilical cable (a thick cable providing power and communications) to a control center on a ship. Operators on the ship see video and/or sonar images sent back from the ROV and remotely control its thrusters and manipulator arm. The wreck of the Titanic was explored by such a vehicle, as well as by a crewed vessel.

Diver lock-out submersible
Class of submersible which has an airlock and an integral diving chamber from which underwater divers can be deployed, such as: