Reply to: [PSUBS-MAILIST] ABS - 3/17.1.1 - Normal Ballast System

[Hugo Marrero <HMarrero@hboi.edu>]

Hi Vance,

I undestand that you used to work at HBOI as a tech, and not as a pilot, and
I want to clarify without getting into a contest, some of the comments that
you wrote in your posting, as some of the statements that were made are
inconsistent with safe design and operation practices. My intention is and
it will always be to educate everyone here and to make sure that whatever I
contribute to this site is with the purpose of helping others to be safe.
Please bear in mind that I will call a spade a spade and with this I don't
intend to offend you or to discredit you as a fellow PSubber.

As regard for your first comment:

(begining of your comment)

Ballast systems:

Interesting stuff. Someone from HBOI said that VBTs are for payload changes.
Well, sometimes yes, sometimes no.  The JSLs have 4 vertical VBTs built into
their main Tanks. Most PSUB sized vessels won't have that luxury, or that
requirement. That's a pretty cushy idea, but is monstrous hard on your air
supply. Which explains why they have all those fiber wound aluminum
T-bottles hung everywhere. Most PSUB sized vessels won't have that luxury,
or that requirement.


VBTs, strictly speaking, are variable BUOYANCY tanks, not variably BALLAST
tanks. They are supposed to be adjustable at sea (or lake, or pond) to 
accomodate minor or inadvertant changes away from neutral buoyancy. It
allows you to change the sub's weight without changing its displacement.
It's a handy little option. Everything from temperature changes in the water
to bottom mud in the battery box can make you wish you had one--or a bigger
one.

Major payload changes should be planned for so they can be done without
impacting the normal (read small volume) VBTs that most of us have. A few
chunks of 
ballast lead added to the design prior to construction can then be replaced
by new toys, goodies, science packages, girl friends, cases of beer and so
on 
WITHOUT impacting your ability to acquire and/or adjust for neutral buoyancy
with that dinky little old VBT you built.

MBTs are, as specified, for freeboard and safety. However, they work plenty
well as VBTs once you get down a hundred feet or so. You have to keep
adjusting 
the bubble, of course, but nothing moves your sub vertically quite as
quickly, unless you got horsepower fever, and more batteries than I do.

(end of your comment)

Yes!  it is indeed very interesting stuff!! And yes, Variable Ballast Tanks
can serve more than one purpose, however, their main purpose is to
compensate for differences in payload (read changes in net buoyancy) during
a dive. 

To clarify this, lets first  review the definitions of Ballast and Buoyancy:

1. ballast -- any heavy material used to stabilize a ship or airship. 

2.(a). buoyancy- The property of floating on the surface of a liquid, or in
a fluid, as in the atmosphere; specific lightness, which is inversely as the
weight compared with that of an equal volume of water.

2.(b) buoyancy- <physics> The upward pressure exerted upon a floating body
by a fluid, which is equal to the weight of the body; hence, also, the
weight of a floating body, as measured by the volume of fluid displaced.
Such are buoyancies or displacements of the different classes of her
majesty's ships. (Eng. Cyc)

3-buoyant force, buoyancy force: (also called Archimedean buoyant force.)
The upward force exerted upon a parcel of fluid (or an object within the
fluid) in a gravitational field by virtue of the density difference between
the parcel (or object) and that of the surrounding fluid.
Now let's break down your comments and analyze them.
Someone from HBOI said that VBTs are for payload changes. Well, sometimes
yes, 
sometimes no.  
This statement is correct. That someone was me. Sometimes you want to have
net negative buoyancy on purpose (when descending or when working at the
bottom), sometimes you want to maintain neutral buoyancy (to maintain
depth), and sometimes you want to maintain slightly positive or higher
positive net buoyancy (to prevent stirring up silt near the bottom, or
during ascent). 
Next Sentence: 
The JSLs have 4 vertical VBTs built into their main tanks.  
This is correct to the physical extent of it. Physically there are four
tanks mounted inside the Main Ballast Tanks. This is just the execution of a
design and the two systems are completely independent from each other. Each
side has a pair of tanks  joined by piping with no valves between them, this
makes them operationally speaking a single tank on each side.
Next Sentence:
They (the VBT's) are air compensated and operated at ambient pressure plus
whatever the operating pressure is set for to blow the system and operate
the valves (pneumatically controlled flood/vent valves). 
Now, in this comment it is implied that the variable ballast tanks in the
JSL subs operate at ambient pressure. This statement is incorrect.  The
VBT's are completely sealed from the enviroment by valves at the top and the
bottom of the tanks, these valves are designed to control the ingress and
egress of water from the system and once this is done they are kept in the
"closed" position throughout the dive and to isolate the interior of the
tanks from ambient pressure. If, and only if there is a need to get water in
or out of the tanks will these valves are to be opened during a dive. The
VBT system is therefore a closed system, completely isolated from the
external environment therefore the tanks are not exposed to ambient
pressure. 
What you may have missunderstood about the system is that the tanks are
internally compensated by an externally-mounted differential air pressure
regulator (In order to reduce the wall thickness of the tanks and to reduce
weight and costs) and for safety reasons (to prevent an explosion or
implosion) which is set to maintain aproximately 45 psi of positive pressure
inside the tanks relative to the ambient pressure. Whether the air pressure
increases or decreases inside the VBT (as a result of a change in ambient
pressure at different depths) both the net air volume and the net water
volume inside the tanks remains constant. Because it is a closed system as
the air gets compressed when the sub goes deeper  the ratio of air to water
remains constant and no net change in buoyancy results. As a matter of fact,
there will be a minor loss in net buoyancy due to the weight of the
compressed air added to the tanks as the sub descends, but for practical
purposes it is considered non-existent.  
The blow valve is only to be actuated when the vent valve is in the open
position as the blow line is directly connected to the main air manifold (
which could go as high as 4000 psig) otherwise the pressure in the tanks
will increase dramatically and in turn actuate the pressure relief valves
(Set to 50 psig) to prevent damage to the tanks.
In our case when we collect specimens or leave equipment underwater we have
to adjust the VARIABLE BALLAST SYSTEM to compensate for the net gain or loss
of weight (cause) which then produces a net loss or gain in buoyancy or
buoyant force (effect). It is also necessary to adjust the VBT's as both,
the air and oxygen systems decrease in pressure do to their normal usage,
there will be a loss of weight (and as a result, an increase in buoyancy) as
air or Oxygen is being used up. 

Just as a glass of water, a Variable Ballast Tank can be half-empty or
half-full, in this condition, you have the weight of the water plus the
weight of the air (which is almost negligible). Having a constant volume,
the change in weight is a defacto  change in density relative to the
external volume of  the tank, and therefore it produces as a result a
positive buoyant force equal to the weight of water displaced by the air in
the tank.  While diving, the initial purpose of this system is to allow
enough water to produce a net negative buoyant force while descending, close
to neutral buoyant force to maintain depth, and positive buoyant force to
ascend & to remain on the surface. That is why they are referred in the
books,as a BALLAST Tanks and not a BUOYANCY Tanks. "Buoyancy" is "a force",
a by-product,  the result , or effect of the proper  operation (or improper
operation for that matter) of the Variable Ballast System. Strictly
speaking, the system is designed as a ballasting / de-ballasting system, not
buoying and de-buoying system.

In addition, to finally clarify the issue. Right in front of me I have the
JSL Submersible Operations Manual Section 2.3, Page 5 Section b, describes
the VBT's as follows:

b: Variable Ballast Tanks (Please note that it says BALLAST and not
BUOYANCY)

Port Tank 135 lbs (61 kg) of Salt Water
Starboard Tank 135 lbs (61 kg) of Salt Water

The ABS book Titled  "Rules for Building and Classing Underwater Vehicles,
Systems and Hyperbaric Facilities 2002"

refer to page 101, Section 9 - Engineering Systems,  Chapter 7 - Ballast
Tanks Article  7.1 - Hard Ballast Tanks .  Please notice the term "Ballast
Tanks"

In my humble opinion, It is evident to the entire professional community of
submersible pilot's and engineers that the system is defined as a  "BALLAST
SYSTEM" and not a  "BUOYANCY SYSTEM", whether it is referring to Hard, Soft,
Variable, Air , water, mercury, or lead. Of course, each and everyone is in
his or her right to call it what they want, it just doesn't mean that is an
accurate statement.

Now to your next Sentence...

That's a pretty cushy idea, but is monstrous hard on your air supply. Which
explains why they have all those fiber wound aluminum T-bottles hung
everywhere. Most PSUB sized vessels won't have that luxury, or that
requirement.

This statement is completely incorrect and missinformed. The reason for us
having six air cylinders and a sphere is because most of it was there to
begin with. Originally, the JSL subs had a diver lockout chamber, and most
of the air volume was dedicated to pressurize the dive lockout compartment
and to provide Heliox gas mixture for two divers to its maximum operating
depth of 800'. Since we no longer use the dive lockout option, we just kept
the extra air as an added safety precaution. After all, it was already
there, we carry so many fiber wound aluminium t-bottles hung everywhere ...
because we can... 

Also, the JSL submersibles have a maximum operating depth of 3000 FSW, and
for that reason, they must carry enough air volume to fill the Main Ballast
Tanks at depth at least twice.  In fact the air volume used by the VBT
system is negligible compared to the volume used by the MBT system. If you
look at the Remora submarine made by comex, you will see that is in fact
much smaller than the JSL, and yet you can't even see where the air
cylinders are stored. 


And as for the last statement of your first comment....

MBTs are, as specified, for freeboard and safety. However, they work plenty
well as VBTs once you get down a hundred feet or so. You have to keep
adjusting 
the bubble, of course, but nothing moves your sub vertically quite as
quickly, unless you got horsepower fever, and more batteries than I do. 

Just because you can actually use the MBT system as a VBT system does not
mean that it is correct or safe way to operate them. Just because you can
drive your car in reverse at 100 mph in the freeway, doesn't mean that is
safe. 

Operating a submersible in this way can lead to an uncontrolled ascent
condition (which has happened) and potentially to an accident. As you
yourself have indicated it is a tedious process as your buoyancy keeps
changing as you ascend or descend, and it gives a false sense of security.
In my opinion, ALL submersibles, regardless of how small, should have a VBT
system. An example of this is the Deepworker 2000... it is very very small,
and still has a VBT. 

The exeption to the rule is Graham Hawk's "Aviator" submersible. This is an
exceptionally intricate submersible built by an accomplished submarine
builder and inventor, who is always on the leading edge of technology. The
Aviator's submersible principle of operation is completely different to that
of the PSUBs mentione in this site and therefore is in a classification of
its own. 

As for your second comment....


Drop weights:

It all depends, like the man said. The JSLs drop a big box of railroad 
batteries and you'd best believe the sub leaves--RIGHT NOW--when it goes.
The Perry 
boat that sits right beside the Sea Links in their shop (PC-1204) has a big 
steel tray under the belly (usually) and it does, in fact, weigh the same as
the 
difference between flooded and dry in one pod (about 900# if memory serves).

 All of the above is correct, except that the lead weight in the drop tray
of the PC 1204 is at least 2400 lbs. which is the dry weight of a single pod
plus the weight of the batteries.

That lead is NOT used for payload changes (except when they sling
jettisonable payload packages in its place). 

The statement above is correct in the context that on the Perry PC 1204, the
amount of lead in the tray is fixed. The lead that I was referring to in my
previous posting is lead that can be positioned at different locations to
accomodate for differences in payload that are known previous to the dive
(such as the weight of passengers, and equipment added or removed), and that
we know by trial and error how this changes will affect the trim of the sub.

On the other extreme, the little Delta has no floodable spaces outside the
pressure hull itself, and so has a fifty pound chunk of lead up front they
can drop just for fun.

I don't know enough about the Delta sub to make a qualified statement as to
its accuracy, however, I assume that the air and oxygen tanks are mounted
externally, and as such, the largest cylinder would qualify as the largest
floodable volume outside of the pressure hull.

The thing to remember is that your sub's STABILITY is paramount, whatever it
is. You need to figure it in all configurations, including without your
dropweight if you have one.

This statement is accurate to the "T".

Now, as for this statement..... 

And by the way, the sub doesn't necessarily need to stay  upright when you
drop the weights. The PC-14s that Perry built were designed with a single
jettisonable battery pod instead of a drop weight per se. And they were
designed to ROLL OVER at the surface. You would pressurize the hull (if you
survived the rollover, one assumes) and open the conning tower hatch, which
would have been converted to a temporary diver lock out trunk by then. No
one ever had to do it, but ABS bought the design and certified the boats
that way. You got to think outside the box, some days. That's all there is
to it.

In my opinion, and without intending to offend you Vance, this is an
irresponsible and uninformed statement. I am quite a bit concerned about it
as there is nothing in the ABS rules books that I have (1990 & 2002) which
allows a sub to come inverted under any circumstance. I also do not know the
Perry PC-14, but I have worked with the Perry PC-1203, 1204, and 1205. In
addition I have personally seen the 1802 and 1805, and in both of these
subs, BOTH battery pods are jettisonable, and there is enough ballast to
keep the hatch upright upon surfacing. If the ABS rules allowed this
previous to the 1990 rules I do not know, but I will investigate. If that
was the case (which I seriously doubt), that rule would only apply to those
subs built (and therefore grandfathered) under that rule and not for new
constructions.

As a matter of fact, on section 3 of the rules, specifically Chapter 17,1.56
Titled Emergency and Damaged Condition , states the following:

"Submersible units are to have adequate stability under any possible
combination of dropped jettisoned masses. Under some emergency conditions,
the distance between  the center of gravity (CG)  and the center of buoyancy
(CB) may be reduced, but in no case is to be less than one-half of that
required in 3/17.1.5 above. Inverted surfacing is not permited."

Just think about the consequences of coming to the surface inverted. If
batteries are mounted internally like in the Delta sub, the weight of them
would make them collapse through the deckplates and fall over the occupants,
then the battery acid would spill inside the people. The matteries may even
fall in the hatch opening... then because you are inverted, all the air in
the Main Ballast Tanks is lost, and there you go back to the bottom... then
what??? The logic of this scenario shows the irrationality of it. It would
create more of a death threat to the occupants than staying in the bottom
and sending a buoy to the surface (which is an actual ABS rule). 

Vance, I hope that this information helps you to understand the physics of
ballasting a submersible. Proper ballasting under normal and emergency
situations must be thought out carefully, and completely understood. Then
and only then should they be disseminated to others. Remember that there are
people in this site who know nothing about submarines and they are here
because they want to learn, some may even be already building a sub inspite
of their lack of comprehension of the physics and engineering involved in
the design, construction, and operation of a submersible. What is to say
that someone some day will  sue you because a husband or son built a sub
based on innacurate information that you posted and died because of it?

Again I repeat, my intention is not to offend you or anyone here. I want to
see the day when personal subs are as common as runabouts. Until that day,
be safe and be informed.

with deep respect to all,

Hugo



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