Submersibles for Deep Sea Exploration

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Abstract

This article will describe submarine technologies aiding marine exploration, ECE concepts applied in the submarines, and discuss the future trends. Currently, most exploration is performed in manned or remote-controlled submarines, with unmanned submarines performing surveillance. The next step is for autonomous submersibles with enough power sustainability to perform exploratory missions.

Introduction

Key Terms

Submarines and submersibles both are used to describe vehicles that can be operated underwater and designed for research and exploration.

Background

With human curiosity constantly driving the urge for exploration, the vast oceans have always been a source of interest- from explorers sailing the high seas and seeking new lands, to divers exploring the depths of the oceans. With the gradual development of submarine technology, deep-sea exploration has blossomed, with scientists able to travel deeper and longer.  Updated tools such as more powerful batteries,  more accurate sonar, and the addition of laser imaging have enabled researchers to able to advance their studies in fields involving deep-sea surveying, marine ecology analysis, and the harvesting of natural marine resources.

Marine exploration and analysis has proven integral for furthering the understanding of the world around us; for example, recent studies of the teeth of sea limpets have shown that its material is stronger than titanium and spider silk. Moreover, the discovery of hydrothermal vents in 1977 helped explain many natural occurrences, such as the formation of mineral deposits, and led to the discovery of various extremophiles, organisms that feed off the chemicals spewing from the vents.

The discovery of these novelties would have been impossible without submarines; the ultimate tool in undersea exploration, both manned and unmanned submarines permit scientists to view and analyze aspects of the deep-sea, from institutes in Woods Hole, MA, to Monterey, CA.

Woods Hole Oceanographic Institution (WHOI) is dedicated to understanding the oceans and its impact on the rest of the planet, especially on humans. Moreover, its scientists and engineers employ underwater vehicles to paint a clearer picture of the oceans, with projects that include studying the ocean’s impact on climate, chemistry of the ocean, ocean resources, and marine biology. WHOI is also involved in various global initiatives, such as Geotraces, an international program seeking to further the understanding of ocean currents and resources3.

Monterey Bay Aquarium Research Institute (MBARI) is another well-established marine research organization. Studying the oceans with a vast array of sensors, like an UV spectrophotometer, biophysical sensor, and carbon dioxide sensor, enables MBARI to perform many monitoring functions: observing the organic carbon flux in the Monterey Canyon and its effects on marine life, determining chemical fluxes impacting certain regions in the ocean floor, and tracking the ocean currents and physical conditions. Currently, MBARI’s main focuses on biogeochemical cycles, ecosystem processes, ocean visualization, and the continuation of deep sea exploration.

Each institute has varying focuses, but one common theme remains: the use of submersibles. The variety of submersible technology employed by these institutions enables greater exploration and analysis of the ocean depths; from manned submarines to automated submersibles, the scientists can select the vehicle depending on their task, with manned submarines more suited for deep sea sampling, and automated submersibles more adept at monitoring conditions. These two examples are a small sample of the fleet of submarines utilized by the marine research institutions, which also use remotely operated submersibles. The range of submersibles available shows how submarine technology has continued to change and improve in order to better understand the vast oceans.

Manned Submersibles

Operated by a pilot, with some models able to hold other passengers and/or co-pilots, manned submarines are now able to dive to maximum depths of 6,000m.

Alvin was one of the first deep sea submersibles that could carry passengers. In use since 1964 and operated from Woods Hole, MA, this submersible’s first un-tethered expedition dove to a depth of 35 feet. With upgrades to its hull and life support system, Alvin can now dive to a depth of 14,700 feet, and can remain submerged for up to 72 hours. This long-lasting submersible boasts over 4,000 dive expeditions and has carried more than 13,000 passengers to the ocean deep. To keep Alvin up-to-date, new technology such as improved lighting and imaging systems, as well as battery upgrades, were equipped. All in all, the Alvin draws upon the use of video cameras, hydraulic manipulator arms, sediment and water samplers, and chemical sensors. The arms and samplers can collect biological and geographical samples, while the chemical sensors provide real-time readings of the environment. More specifically, to integrate brushless motors for Alvin‘s propulsion, ballast, and hydraulic systems, engineers upgraded the 60/30 VDC to 120/28 VDC. VDC stands for voltage direct current, and 120/28 means that the battery now delivered 120 Volt direct current, with a secondary user voltage of 28 VDC. An example of Alvin‘s expeditions involved traveling through a submarine canyon in the central Oregon subduction zone, where Alvin displayed fluid vents that suggested vent and thrust faulting previously not thought to be within the canyon. Additionally, Alvin found similar vent formations in the Gulf of Mexico, employing 3-D seismic surface imaging to indicate potential sites of interest. Further research revealed how carbonates at hydrocarbon seep sites created the solid bottom floors.

Another submersible, DeepWorker 2000 is a single-person, battery-operated submersible; with a length of only 8.5 ft and a height of 5.75 ft, the DeepWorker 2000 is a light and nimble submersible that can perform a variety of tasks. Its array of instruments include an electronic monitoring within the cockpit, VHF multi-channel radio for communication with the surface, sonar, HD cameras, and a laser measuring system. A common feature now in the 21st century, a touch-screen is used for controlling the lights and monitoring the depth, among other functions. One useful feature of the DeepWorker 2000  is the combination of its sonar and laser systems, which can depict the environment beyond the pilot’s range of visibility. Like Alvin, the DeepWorker 2000 has arms that can hold various objects, whether it is a container for wildlife samples or a coral analyzer. However, contrasting with Alvin’s arms is the fact that the DeepWorker 2000’s arms are customizable, with different instruments able to be mounted on the arms for specific tasks. With a life support duration of 80 person hours, DeepWorker 2000 is a versatile submersible capable of performing a variety of tasks.

Similarly, Mir I and Mir II are three-person submersibles that can operate at a depth of 20,000 ft, meaning they have the capability of accessing 98% of the ocean floor. Despite having windows for gazing upon the ocean floor, the main data collection comes from the videos and manipulator arms, which can be used for biological or geological sampling. Since the video-capturing capabilities of the Mir twins is its greatest strength, movie directors and scientists alike use these submersibles for gathering footage: James Cameron used the twins for his movie Titanic, while scientific use includes the surveying of World War II ships, and documenting the North Pole seafloor. Unlike the DeepWorker 2000, which can be transported with a trailer, Mir I and II are not as portable, requiring a specific docking boat to deploy from. This hindrance is offset by the tandem capability of the twin submersibles: one can act as a backup or rescue to the other, as well as providing complementary support like additional lighting.

A pair of submersibles specializing in ping-localization are the Pisces IV and V. Used by the Hawaiian Undersea Research Laboratory, the Pisces duo are like the other manned submersibles with its HD cameras and mechanical arms, but stand out due to their pinger receiver system. With this system, these submersibles can detect signals from 8 to 80 kHz and use this capability to track one another, locate lost instruments, or relocate bottom monitoring sites marked with pingers or transponders. With this specialized equipment, the Pisces submarines have recovered lost devices valued over 5 million dollars, but invaluable content was the data, some collected over a year. Moreover, Pisces has discovered historical wreckages of Japanese aircraft carriers and submarines from World War II. The pinging system has made the Pisces stand out as great tools for locating objects.

Remotely Operated Submersibles

Remotely operated submersibles, or vehicles (ROV), are submersibles tethered to and operated from shore. They are unoccupied and hold various instruments for data collection. The advantages of ROVs include greater safety, and a larger array of instruments, but these ROV’s tend to be bulkier. Also, because many ROV’s carry a large amount of devices on-board, more than 1 remote operator may be required.

ROPOS is an ROV that can operate in both shallow and deep water; for depths beyond 350m, controllers lower ROPOS to the desired depth housed inside a steel cage, and the ROV exits the cage and explores while attached to a 300 ft tether. With the combined weight of the ROV, the cage, and the 5,000 m optical cable totaling 14 tons, the docking ship must be large, making expeditions with the ROPOS time-consuming and more costly. However, ROPOS was designed to maximize research effectiveness, and is equipped with numerous instruments: besides the standard tools of cameras and arms, the ROV also houses sonar, a telemetry system, and additional tools such as a hot-fluid sampler, chemical scanner, rock-coring drill, and a laser-illuminated camera can be attached. Therefore, ROPOS’s versatility helps scientists perform a wide range of tasks and respond to sudden occurrences in the deep sea.

With all of its tools, ROPOS has a plethora of information constantly needing to be processed, and thus four or more operators are required for full control of the submersible. A scientists guides the pilot and manipulator operator through data-collection tasks, and provides commentary for the video log. The manipulator operator handles the observational tools, and a data logger highlights significant values and events to reduce data processing time after the dive. All the personnel needed to operate ROPOS makes this submersible expensive in person-hours and cost, but the expenses are offset by the range of tasks that ROPOS can perform.

Another ROV is Hercules, initially designed to find and study artifacts on the ocean floor, its responsibilities have grown to include analyzing marine biology and geology. Hercules is equipped with the standard instruments found in the other submersibles, but one distinguishing factor is its ability to run for 24 hours. As a result, scientists can gather more data for a longer time, with teams of scientists, pilots, and operators rotating for others to rest. Another interesting capability of the Hercules involves its telepresence. In conjunction with an antenna and data transmission system, Hercules can send HD video and data to the research facility on shore, allowing for scientists and students alike to partake in the research endeavors of the ROV without having to travel to sea.An issue encountered by ROV’s, as seen with the Hercules, is how area mapping at a high resolution is impeded by sensor constraints and capabilities. Because light traveling underwater is affected by greater attenuation and backscatter, hundreds or thousands of images are required to adequately cover large areas. To address this, methods such as self-consistent bathymetric mapping, which essentially uses sonar to provide acoustic imaging in addition to the visual images reduce the lack of visual clarity by adding other forms of visualization. As a result, regardless of the distorted light, optical images combined with acoustic ones are able to depict the objects more clearly.

MBARI employs multiple ROV’s for study of the undersea Monterey Canyon, comparable in size to the Grand Canyon and home of great biodiversity since it is found 2 miles beneath the ocean surface; with ROV’s, MBARI is able to study elusive and secretive creatures such as the vampire squid and barreleye, also known as the spook fish.

MBARI employs strictly unmanned submersibles due to the convictions of David Packard, who believed the similar capabilities but assurance of human safety provided by ROV’s qualified them as the ideal tools for deep sea exploration and analysis. ROV’s employed in MBARI’s arsenal include the Ventana and Doc Ricketts.

The ROV Ventana, named for acting as a “window” to the ocean depths, collected its first sample in 1988. Since then, it has undergone a plethora of modifications, having gained tools to measure the physical conditions of the water at depths of up to 1700 meters in addition to the standard devices (i.e. arms, cameras, sonar). These tools include a transmissometer, to measure the deep sea water’s extinction coefficient of the atmosphere (similar to measuring the absorption of light from the surface), flowmeters, and oxygen sensors. With these tools, the Ventana can both perform sampling of biological and geological aspects as well as provide thorough analysis of water conditions in the deep ocean.

Another ROV, Doc Ricketts, is noteworthy for its ability to dive up to 4000 meters, or 2.5 miles. This permits scientists to study the environment at much deeper levels than most available ROV’s, but its instruments are similar to the other ROV’s, with standard manipulator arms and HD cameras.

Overall, ROV’s are widely used because of their ability to carry a wide arsenal of analytic tools while basically eliminating the risks for piloted submarines.

Automated Underwater Vehicles

Finally, automated underwater vehicles (AUV) are the third major type of submersible utilized in marine exploration. AUV’s, like the name implies, are programmable submersibles that operate without piloting and instead can drift, drive, or glide through the ocean depths. In addition to being cheaper than the aforementioned submersibles, AUV’s eliminate the need of a pilot, supervising scientist, with its smaller size permitting AUV’s to travel farther and longer as well; the biggest disadvantage, however, is that the AUV’s lack as large an array of instrumentation as other previously mentioned submersibles, in order to reduce power needs and size. AUV’s can be used to survey and map the ocean floor, and return measurements of the ocean water conditions such as temperature and salinity. AUV’s in use today include the Remote Environmental Monitoring UnitS submersibles (REMUS), the Sentry, Spray Gliders, and the Slocum Glider.

The REMUS AUV’s use a propeller and fins for steering and diving, and acoustic navigation and sensors to survey and record data. The different REMUS submersibles have varying functions, and therefore some have specialized equipment, such as the Sharkcam that has cameras and tracking instrumentation to follow tagged marine animals, like the great white shark.

The Sentry is a versatile AUV that can produce depth analysis and magnetic maps of the ocean floor, as well as taking photographs of deep-sea geological features like deep-sea vents. A durable submersible, the Sentry can operate in extreme conditions, like volcano calderas. To navigate, the Sentry has a Doppler velocity log and inertial navigation system, all coordinated using an acoustic navigation system.

Spray gliders are utilized for measuring water conditions like temperature, salinity, pressure, and turbidity. Moreover, Spray gliders can measure the speed of ocean currents as well as provide the chemical decomposition in the water. For example, three years of Spray glider data collection in the southern California Current System (CCS) have aided numerical analysis of the poleward currents (water flowing towards the poles).  Programmed to dive to 500 m during a dive that lasts about three hours, the Spray gliders are attached with a conductivity temperature-depth sensor, an acoustic Doppler profiler, and a cholorophyll a fluorometer to map the water depth and water flow. The temperature-depth senor demarcated the direction of warm and cold currents, while the fluorometer gauges the algae content in the water, and can highlight its movement since it flows with the water. This project with Spray gliders highlights the utility of AUV’s in marine research.

The Slocum Glider is a long-range submersible that is well suited for sampling oceanic characteristics on a regional scale. Able to travel for weeks during one trip, the Slocum Glider saves time, money, and provides greater data than manned submersibles. However, constant exposure to salinity and oscillating temperatures can impede this AUV’s functionality and therefore labs such as the Centro di Taratura Oceanografico in Italy exist to perform simulations and experiments to evaluate the glider’s data quality in these conditions. The bath the glider is placed in circulates saline water where the temperature is cycled from 30°C to 0°C in 5° increments. This results in an environment comparable to the ocean conditions encountered by the glider during its expeditions. Thus, by analyzing the data gathered by the submersible in this laboratory setup, the scientists can analyze how these changing conditions affect the data quality of the AUV.

All of the AUV’s mentioned above provide an alternate research method to the other submersibles due to their programmability and ability to collect data for extensive periods of time, all the while virtually eliminating the need for human involvement during the trip.

Conclusions

With a variety of submersibles available for deep sea navigation, our understanding of the vast oceans has improved dramatically, with manned and unmanned, manual and automated submersibles all contributing to knowledge of the ocean conditions. From tagging and tracking wildlife to analyzing chemical compositions on the ocean floor, the ever-increasing variety of submersibles has helped paint a fascinating picture of the oceans covering our planet. In the future, AUV’s that are able to dive deeper to monitor the ocean floor can provide more sustained data, whereas currently the submersibles able to travel so deep have very short journey durations relative to the AUV’s. Imagining a 24 hour or even week-long surveillance is tantalizing to any marine scientist- this is just one future technology everyone can look forward to. For the people at home, maybe someday affordable personal AUV’s will become available, where divers will have another angle to document their underwater excursions, children will be able to swim closer to the fish while staying on shore, and professors will teach their marine biology classes in the ocean!

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