6.
Ballasting and Trim Systems pg.
279
- Weight and Volume Estimates: pg.
279
- How to estimate the weight
of the vehicle
Structures: pressure hull, exostructure,
fairing 65%
Propulsion and Electrical
Plants: Propellers,
thrusters, battereies; 13%
Communication and Control: Underwater telephone,
steering controls, radio; 3%
Auxiliary Systems: High pressure air,
ballasting, life support; 14%
Outfit and Furnishings: Hull fitting, chairs,
paint;
Crew and Instrumentation: Crew, scientific and operational
instruments, tools
- Size and placement
conditions:
1. Sum of all weights must
be equal to weight of the water di8splaced by all buoyant volumes
2. The resultant center of gravity of all weights must be below and in a vertical line with the resultant center of buoyancy
* check Rechnitzer and
Gorman (Source 1): how to calculate submerged displacement of various parts
* Final adjustment to attain
neutral submerged buoyancy made w/ fixed, positive ballast (like syntactic
foam) or negative ballast (like lead weights) on outside/inside of hull
6.
Ballasting and Trim Systems pg.
279
- Compressed Air and Deballasting: pg.
282
- Main way to empty variable
ballast tanks of water is compressed air
-
Only
practical where volume & pressure required to store the gas is practical
and it’s density under pressure provides effective buoyancy (tanks must
withstand ambient pressure)
-
Water
ballast usu used in shallow water submersibles (< 2,000 ft)
- Important terms:
-
Gage
Pressure: difference between pressure being measured and surrounding
atmospheric pressure (psig)
-
Absolute
Pressure: true pressure (psia)
-
STP:
volume of gas at 14.7 psia and 32 degrees F or 760 mm Hg (= 1 atm) and 0
degrees C
-
Normal
Temp and Pressure (NTP): volume of gas at 14.7 psia and 68 degrees F
- Two important Gas Laws:
-
Boyles
Law: if temp is constant, volume of gas will vary inversely as the absolute
pressure, and density will vary directly as the pressure
-
Charles’s
Law: If pressure kept constant, volume of a gas will vary directly as the absolute
temp
-
Combine
to make (P_1*V_1)/T_1 = (P_2*V_2)/T_2 ß pressure and temp are absolute in EQ
- Tanks, flasks, or bottles:
carry air supply for blowing water ballast
- Made from : aluminum alloy, steel, or “other special
materials”
-
Depart
of Transportation maintains regulations for design and manufacture of high
pressure cylinders (available from 1,800 psig to 5000 psig)
-
Color
code tanks for safety
-
Usu
both low and high pressure system used: low pressure for use on surface and a
high pressure sys for emergencies or when submerged (Ex. BEN FRANKLIN: 1,422
psi low pressure sys and 2,874 psi system for emergencies at 2,000 ft).
-
ALUMINAUT (depth of 15,000 ft): 4,500 psi supply of air to blow ballast to 4,000
ft in case of emergency
- Must find equation to figure
out capacity of ballast.
6.
Ballasting and Trim Systems pg.
279
- Ballasting Systems: pg.
285
- Marine Technology Society
recommends that main ballast tank capacity should not be less than 10% of
vehicles displacement at normal diving trim
I. Reversible Systems:
A. Main Ballast Tanks:
1. Functions:
a. Provide large changes in pos/neg buoyancy
b. Provide adequate freeboard for maneuvering
for entry/exit of people into the hull (?)
2. Operations:
a.
On
surface, MBT is empty. Vent valves located on top of tanks and flood valves at
the bottom.
b.
To
dive, vent valves opened by operator & seawater flows in thru flood
openings forcing air out thru top (light to indicate when full). Vent closed.
c.
Now
vehicle either buoyant or neg buoyant. If former, smaller capacity variable
ballast tanks are flooded or weights are added. Vehicle descends.
3. Location: straddle the pressure hull and located high on vehicle to provide stability when surfaced by raising center of buoyancy w/ respect to the center of gravity
4. Materials: Steel, fiberglass, aluminum…basically
anything that’s strong enough (must withstand “wave slap” ex. 1000 psf)
5. Examples: BEN FRANKLIN, All Ocean Industries submersibles
B. Variable Ballast Tanks:
1. Functions:
a. Provide small scale buoyancy adjustments
2. Operations:
a.
Hard
Tank: seawater put in tank using ambient pressure differential to get neg
buoyancy.
b.
Pressure
Resistant Tank (usu used in deep submersibles) also called Hard/Soft tank sys:
connected to collapsible (flexible) oil-filled bags.
i. On surface: Tanks are
partially filled w/ oil and air at ATM, and bags are collapsed
ii. Oil pumped into bags for
pos buoyancy when submerged (bags expand and displace seawater)
iii. For neg buoyancy: let
oil flow back into rigid tank.
3. Location:
a. No standard, sometimes in the pressure hull (saves expense of system exposed to seawater), sometimes external (saves space). Most VBT’s situated below behicle’s center of gravity to keep center of gravity low. If two, balanced on sides for front/back. (If like latter, can also be used as trim)
4. Materials:
a.
Soft/hard
tank should be spherical to withstand ambient pressure.
b.
Titanium
spheres:
5. Examples: ALVIN, SEA CLIFF, TURTLE, PISCES IV,
V