When designing an ROV, it is common to use light weight components to keep the entire submersible from exceeding the desired weight limit, so aluminum or other lightweight materials are used. The weight of the submersible consists primarily of the following components: subsystem components, loads, and the buoyancy system used to establish the desired operational specific gravity.
The normal operating procedure is to give the submersible positive buoyancy to ensure that it can move freely in the water and return to the surface in the event of a power system failure. This positive buoyancy is usually less than 2.3kg for small submersibles, 5-6.8kg for larger submersibles, and in some cases up to 22.7kg. Another reason is that when the submersible is working close to the bottom, it can be lifted without propulsion power, which stirs up the bottom sediments, and avoids the need for continuous reversal of propulsion. operation, and very large submersibles have blown air ballast tanks to regulate underwater buoyancy.
The stability of a submersible is measured by evaluating the moment required to change the pitch angle of the submersible, which can be represented by the following equation:
m = (W) BG Sinm = Moment = (w)(d) w = Gravity d = Moment arm W = Weight of the submersible BG = Distance from the center of buoyancy to the center of gravity = Pitch angle, or cross-swing angle
Obviously, the units chosen must agree; if W is in pounds, BG is in inches, and m is in inch-pounds. If the BG is large enough, which can easily be the case when the weight is small and buoyant, the submersible will be very stable. External forces acting on the submersible while it is submerged can easily reduce the BG. e.g., when propelling the submersible down the vertical thruster forces will act as an added weight on the submersible and raise the center of gravity, so the submersible will rock in the pitch and cross-swing directions.
Most ROVs are designed to be as stable as possible during actual use. When designing an ROV, it is common to place heavier components such as the motor as low as possible and to place the float (fiberglass and composite foam) at the top end of the submarine to maintain a high degree of stability.
Ballast can be categorized as fixed ballast or adjustable ballast. Fixed ballast may be composite foam, lead. Adjustable ballast may be open, blown chambers called 'soft chambers' or sealed chambers that can be pumped or blown and have full access to diving pressure called 'hard chambers'.
The fixed ballast (positive fixed buoyancy) of a submersible is used to bring the submersible to the desired specific gravity by means of a pressure-capable float tank, composite foam and lead. Most submersibles place a block of composite foam on top to achieve positive buoyancy.
There are two different types of composite foam. One is a large number of plastic and glass particles bonded together and the other has only one type of particle. In general the composite type is only used in shallow water, the single particle material is suitable for deeper water. Obviously the smaller the particles the greater the ability to withstand pressure, so as the density of the foam increases so does the cost, and of course the working depth. So it is necessary to balance the cost, weight and pressure resistance to get a comprehensive design.
Submersibles constructed using sealed tubular frames for buoyancy can be damaged during operation. It is therefore common to have multiple compartments in the frame to ensure that a significant amount of buoyancy is not lost in the event of damage. Foam filling of the frame also maintains buoyancy in the event of damage.
Depending on the depth requirements, the use of a pressure vessel to provide buoyancy is a very good option, but this technique is rarely used on commercial ROVs, and is usually used on AUVs, which have a large pressure vessel as a major component.
The fixed load on the submersible is usually in the form of several lead blocks. The equipment can be adjusted by replacing these lead blocks without changing the foam on the submersible.
Adjustable ballast allows the submersible to grab objects and maneuver on the seafloor without the need for thrusters to push it down, and also allows the ROV to be heavy enough to remain stable in high current conditions. A typical soft ballast system should consist of one or more 3,000 lb. submersible bottles, a decompression regulator, a surface control solenoid valve, and a thin-walled tank with a large opening at the bottom. One disadvantage of this soft ballast is that the volume of air in the tank changes as the depth of the submersible changes.
The load can be adjusted by injecting or discharging buoyancy tanks. Adjustment by injecting hard buoyancy tanks when a weight is released from a submerged submersible is a simple and effective technique; venting of the tanks can be accomplished by squeezing water out through the air when a valve is opened or water is pumped.
Most ROVs are not differentially buoyant, but hybrid submersibles are usually differentially buoyant because they must be neutrally buoyant for some operations and heavy enough for some seabed work (e.g., cable and pipeline burials, repairs, etc.).