Engineering:Submersible

Short description: Small watercraft able to navigate under water

Retired modern submersible Star III of Scripps Institution of Oceanography

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. 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 (33,000 ft) 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 to understand.

a. 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 a diver used such a gauge underwater, it would indicate only the pressure exerted by the water. For example, if the gauge reads 120, the measured pressure is actually 120 bar above atmospheric pressure, equivalent to 121 bar

b. Absolute Pressure: Before a diver descends beneath the surface, they are already under a pressure of 1 bar, or 103000 n/m2, which represents atmospheric pressure. As the diver descends, the pressure on them increases by 0.1 bar for every metre of depth. The total pressure on the diver at any given depth is the sum of the pressure of the water at that depth and atmospheric pressure. This total pressure is known as absolute pressure, and the relationship is as follows:

Absolute Pressure (bar abs) = Gauge Pressure(bar) + Atmospheric Pressure (1 bar)

To calculate absolute pressure, add the atmospheric pressure to the gauge pressure using the same unit. Working with depth rather than pressure is often 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

it is crucial to consider the pressure the diver experiences rather than the linear measurement of depth. Depth measurement in diving is standardized to overcome variations in water density.

To express the depth of a diver in water or in a compression chamber accurately, the measurement should be in meters (m). A change in depth of 10 meters corresponds to a change in pressure of 1 bar. The unit “meters of sea water” (msw) should not be used to avoid confusion and ensure consistent depth measurement.

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.

Another technology called ambient pressure submersibles maintains 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 scuba carried by the vessel.

Understand the Science of Floating

When an object is submerged in a liquid filled container, the level of the liquid rises. This intriguing phenomenon occurs because the object displaces the liquid, pushing it out of the way. Before the object come into contact with the liquid, it exits in a state of equilibrium, with the weight of the liquid above being balanced by an upward force called up-thrust.

However, once the object is partially immersed, the up-thrust, which previously opposed the weight of the displaced water, starts acting on the object itself. Consequently, objects submerged in liquids appear to weigh less due to this buoyant force. This fascinating 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 plays a crucial role in determining whether an object floats or sinks in a liquid. The relative magnitudes of weight and up-thrust determine the outcome, leading to three possible scenarios.

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

Neutral Buoyancy: if the weight of an object equals the up-thrust, the remains in its current position without 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 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

Ictineu 3 is a crewed submersible with a large semi-spheric acrylic glass viewport and is capable of reaching depths of 1,200 m (3,900 ft).

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 (36,000 ft) 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 6,963 meters (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 metres (14,800 ft). 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 10,908 metres (35,787 ft).

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.

MROVs

Small uncrewed submersibles called "marine remotely operated vehicles," or MROVs are widely used today to work in water too deep or too dangerous for divers.

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.

See also

• Engineering:Bathyscaphe – Free-diving self-propelled deep-sea submersible
• Engineering:Bathysphere – Unpowered spherical deep-sea observation submersible lowered on a cable
• Engineering:Diving bell – Chamber for transporting divers vertically through the water
• Engineering:Diving chamber – Hyperbaric pressure vessel for human occupation used in diving operations
• Company:Hawkes Ocean Technologies – American marine engineering firm
• Pisces V – Deep-submergence vehicle
• Engineering:Semi-submersible
• Engineering:Submersible drilling rig – Vessel floated to site and lowered onto the seabed for drilling
• Earth:Underwater acoustic positioning system – System for tracking and navigation of underwater vehicles or divers using acoustic signals

External links

• Video of a dive with a tourist submersible, shipvideos.net