Page 132 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

120 CHAPTER 5 Vehicle Design and Stability
FIGURE 5.14
Vehicle in equilibrium No vertical moment
Vehicle
specific gravity <1.000 Vehicle floats
Ambient water specific gravity = 1.000
Vehicle
specific gravity >1.000 Vehicle sinks
Specific gravity versus vehicle buoyancy.
More than 97% of the world’s water is located in the oceans. Many of the properties of water are modified by the presence of dissolved salts. The level of dissolved salts in seawater is normally expressed in grams of dissolved salts per kilogram of water (historically expressed in imperial units as parts per thousand, or “ppt,” with the newer accepted unit as the practical salinity unit, or “PSU”). Open ocean seawater contains about 35 PSU of dissolved salt. In fact, 99% of all ocean water has salinity of between 33 and 37 PSU.
Pure water has a specific gravity of 1.00 at maximum density temperature of about 4C (approx- imately 39F). Above 4C, water density decreases due to molecular agitation. Below 4C, ice crys- tals begin to form in the lattice structure, thereby decreasing density until the freezing point. It is well known that ice floats, demonstrating the fact that its density is lower than water.
At a salt content of 24.7 PSU, the freezing point and the maximum density temperature of sea- water coincide at 21.332C. In other words, with salt content above 24.7 PSU, there is no maxi- mum density of seawater above the freezing point.
Most ROVs have a fixed volume. When transferring a submersible from a fresh water environ- ment (where the system was neutrally ballasted) to a higher density salt water environment, the ROV pilot will notice that the system demonstrates a more positive buoyancy, much like an ice cube placed into a glass of water. In order to neutralize the buoyancy of the system, ballast weights will need to be added to the submersible until neutral buoyancy is re-achieved. The converse is also true by going from salt water into fresh water or between differing temperature/salinity combi- nations with the submersible (Figure 5.14).
Water is effectively (for our purposes) incompressible. At deeper depths, water will be at a higher density, slightly affecting the buoyancy of the submersible. The water density buoyancy shift at the deeper operating depths is partially offset due to the compression of the air-filled spaces of the submersible. This balance is more or less dependent upon the system design and the amount of air-filled space within the submersible.