Fathom’s Insight

PRESSURE

A fundamental consideration in the design of any vehicle transporting man or equipment underwater is pressure. Pressure may be resisted, as it is by the submersible’s hull, or it may be compensated, as is the case with many battery packs, propulsion motors, etc. Once the submersible’s operating depth has been established, the pressure at that depth will determine the dimensional and compositional characteristics of the vehicle and its components.

Pressure in the ocean is a function of depth, and for routine oceanographic calculations the 33-foot depth is equal to about 1 atmosphere (14.7 psi). To moderate depths, say to several thousand feet, seawater may be considered incompressible and the following expression is used:

• p = pa + wh

where p is pressure in pounds per square inch (psi), pa is atmosphere pressure (psi), w is a 1.0-ft head of standard salt water equal to a pressure of 0.4447 psi and his depth in feet; then

• p = 14.7 + 0.444h psi

At greater depths, the compressibility of water must be considered and, to obtain a more accurate value, the density of seawater may be taken as varying linearly from 64 pcf at the surface to 66.6 pcf at 30,000 feet (Fig. 2.1). Neglecting atmosphere pressure, the pressure at depth h then is approximately

p = 0.444h + 0.3 (   h   )² psi (ref.1)
                                1,000

Hence, at 6,000 feet, the pressure on the surface of a body is 2,674.8 psi acting normal to every exposed surface.

SEAWATER CONDUCTIVITY

Various devices in submersibles, e.g., motors, batteries, pumps, are immersed in a protective liquid which serves as an ambient pressure compensator and an insulator against loss of power to seawater. The intrinsic dielectric conductivity of seawater is approximately 4 mhos/m (milliohms/meter) or 4,000 times greater than that of fresh water; and, it increases with temperature, salinity, frequency of the propagating wave and pressure (1). A common cause of failure in electrical systems is contamination of the compensating/insulating fluid by seawater, where as little as 0.1 percent contamination reduces the resistivity of some fluids below recommended limits (3). Various forms of corrosion (pit, crevice, stress, layer, etc.) Attack metals in seawater. Protective coatings and/or sacrificial anodes should be considered in the initial design stage.

TEMPERATURE

The temperature of seawater (Fig. 2.1) has, among others, two important effects on submersible diving: 1) The occupants must deal with extremes of temperature caused mainly by loss or gain of heat through the pressure hull; and 2) the pressure hull material must be capable of retaining its desirable characteristics (crack arrest) under cold temperatures encountered above and below the surface.

LIGHT

Sunlight has been observed to penetrate the ocean to depths as great as 2,300 feet (4), but usable sunlight for detailed external viewing generally terminates at 1,000 feet even under the very best conditions. Consequently, the submersible user must rely on artificial light sources for external illumination. Because of the lateral and vertical variability of light transmission properties and the frequent blinding effects of backscatter throughout the oceans, lighting for each diving mission is approached on a case-by-case basis.

Fig. 2.1 Seawater density, salinity, and temperature as function of ocean depth. {From Ref. (2)}

CURRENT

Currents in the ocean and contiguous waters range in horizontal speeds from less than 0.05 knot (Pacific deep water) to 15.5 knots (Skjerstadt Fjord, Norway), and they fluctuate rapidly both spatially and temporally (5). Where currents are strong, the submersibles must be able to maintain control and headway to conduct its task and maneuver safely.

DENSITY

Since seawater density varies not only with depth (Fig. 2.1), but with temperature and salinity as well, vehicle buoyancy calculations must be based on the specific diving location. In some instances, underwater discharge of fresh or brackish water near the bottom has caused significant loss of positive buoyancy on a submersible working close to the bottom (6).

ACOUSTICS

Light and radio waves attenuate rapidly in the ocean. Depending on the frequency of the signal and oceanographic conditions, sound waves may travel for thousands of miles. Sound, therefore, is used for communications between ship and submersible, for tracking of the submersible from the surface and for a variety of data collection instruments. The velocity of sound in seawater varies from 4,775 to 5,150 feet/second and increases with increasing temperature, salinity and pressure (5). If sound is traveling vertically downward, the effect of refraction (bending) is relatively slight; as the beam direction approaches the horizontal, refraction may become quite great. The usual situation (Fig. 2.2) is for sound speed to decrease initially with depth as the temperature decreases; hence, the upper part of the sound beam travels faster than the lower part and a shadow zone, into which the sound beam does not penetrate, is left near the surface. Such refraction may occur at any depth in the ocean; its effects can control the ranges from which a submersible can be tracked from its surface support and still maintain voice contact.

SEA STATE

The operational limits of submersibles’ launch/retrieval devices are determined by wave height (the vertical distance from wave trough to crest) and period (the time interval between successive crests passing a stationary point); the condition is generally termed Sea State, and its boundaries are presented in Table 2.1. Sea state, as defined in the accompanying table, is misleading as a measure of the ability of a launch/retrieval apparatus, for it does not take into account wave period. For example, launch/retrieval may be ruled out in low sea states if the period is on the order of 8 to 10 seconds, but, if the period is doubled or greater, the frequency of the wave crest’s passage is less and time may be sufficient to complete the hook-up of lift lines between successive crests. One must be aware that the sea surface is rarely calm and is in a constant state of change. If a submersible system is to

SPEED OF SOUND, (FT/SEC)

Fig. 2.2 Typical variation of speed of sound with depth in the ocean.

be economically practicable, the ability to deploy and recover the vehicle safely under average weather conditions is just as important as pressure hull integrity.

BOTTOM CONDITIONS

The ocean floor ranges in composition from soft, fine muds to hard rock cliffs; a submersible can be expected to operate within these ranges. During operations requiring a vehicle to transit near the bottom, search missions for example, the pilot generally prefers to “fly” just off the bottom, a few pounds negatively buoyant. This procedure makes vertical control of the submersible much easier. Over a rough, hard bottom, rugged skegs or other devices (wheels, skids,) are used to protect the pressure hull and other components. On a soft bottom the submersible may accumulate sediment, the weight of which can become great enough to restrain the vehicle from surfacing. ALUMINAUT, for example, accumulated approximately 4,000 pounds of sediment in this manner during an operation off the coast of Spain.