FULL VACUUM TANKER
This is an exciting project to develop a tanker
which will be far superior to double bottoms
in reducing spill volume in a grounding.
The system can used to replace the dangerous double bottom; or
if that proves politically infeasible,
incorporated in double bottom ships.
The concept is:
This will put the Neutral Level at about the main deck.
In other words, there will be no hydrostatic outflow
from any damage that does not break the vacuum in the top of the tank.
The resulting ship is equal to the Mid-Depth Bulkhead as far as side damage is concerned
and superior to the Double Bottom because the side tanks are wider.
Beef up the top of tank structure to take a full vacuum,
about 10 M water under-pressure.
Attached a vacuum pump or ejector to the IGS piping, exhausting up the stack.
Keep the IGS piping very close to the deck.
After loading pull all cargo tanks down to about the vapor pressure of the cargo,
e.g. about -.7 bar or -7 meters water gage for Arab Light.
The resulting ship is far superior
to the Double Bottom in bottom damage:
Much more importantly, unlike the Mid-Depth or the Double Bottom,
bottom damaged tanks will automatically pump themselves out
to undamaged tanks.
This transfer will begin immediately upon damage with absolutely no intervention from anybody.
Please re-read the last two sentences.
The increase in pressure at the bottom of the damaged tank
will push cargo thru the IG/vacuum lines to the other tanks.
On a typical big V carrying Arabian light (density 0.85 and vapor pressure 0.25 bar absolute),
the seawater will push an amazing 23 meters into the tank,
i.e. just about to the loaded waterline.
Assuming there is sufficient volume elsewhere in the system
and the vessel draft doesn't change, the equation for equilibrium is
Unlike the Double Bottom it doesn't depend on the bottom damage not penetrating
the inner hull which as a practical matter will happen in any major grounding.
The double bottom is only two to three meters deep
and the connecting structure nearly guarantees
that any substantial damage to the outer hull will involve the inner hull.
Unlike the Mid-Depth it doesn't depend on the bottom damage
not penetrating the Mid-Depth Bulkhead.
This of course in extremely unlikely unless the ship is totally lost
in which case all systems are the same.
Draft * 1.02 = (Vapor pressure Suction Head) + (Height - x) * SG + 1.02 x.
where vapor pressure is in meters gage
which will be negative, e.g for Arabian light, -10*(1 - 0.25),
Height is the height of the cross over to the other tanks above keel
(about 30 M for a U), SG is the density of the cargo,
and x is the equilibrium oil/water interface above keel.
In reality the vessel will sink as it floods
which will help matters assuming it is not overdone.
Obviously, it's essential that the damage doesn't break the vacuum,
i.e., the damage is below the waterline.
I've also assumed the vacuum doesn't decrease
which requires either a lot of unfilled volume or the
vacuum pump being still operational.
Not only does this automatic redistribution of cargo from the damaged tank(s)
eliminate the dynamic spillage argument (Live Bottoms just above the bottom will
generate some spillage from waves, current and vessel motion);
but it will have a tremendous impact on the Exchange Flow
that would have occured in either the Mid-Depth or Double Bottom from side damage.
It also goes a long way toward handling the one extremely unlikely situation
where the Double Bottom beats the Mid-Depth Bulkhead:
a grounding at high tide where there's lots of tide
which grounding penetrates only the outer hull of a double hull.
The Full Vacuum Tanker may be cheaper to build than either the Mid-Depth or the Double Bottom.
The mid-depth bulkhead on a VLCC is about 16 M above the keel in a 26 or 27 M depth vessel.
Therefore, the mid-depth bulkhead already has to be built
to take a 10 or 12 meter pressure differential.
Basically, what we are doing is moving the mid-depth bulkhead up to the main deck,
eliminating the structure that is already there.
The result is a more efficient structure than either the Mid-Depth or the Double Bottom.
The Double Bottom results in a very unbalanced structure due to the lowering of the neutral axis.
In the Mid-Depth Bulkhead all the mid-depth structure
contributes nearly nothing to section modulus
because it is very close to the neutral axis.
For the same steel, we will have a stiffer ship.
And the pressure/vacuum valves will be eliminated.
The Full Vacuum Tanker will be drastically superior to the Double Bottom
and significantly superior to the Mid-Depth
from the point of maintainability and safety.
Ballast tank coated area will be less than one-third that of the Double Bottom.
Like the Mid-Depth, the nightmare of maintaining
and inspecting the inner bottom space is eliminated.
We also eliminate the very substantial complexities associated
with inspecting and COW-ing the lower tank in the Mid-Depth.
The resulting ship is even superior to existing single hulls from an operational point of view.
It's important to recognize that this vacuum system
does not depend on monitoring and controlling the amount of vacuum
to avoid overstressing the structure.
Since the structure can take zero pressure,
there's no way of pulling too much vacuum.
Even existing old ships suffer from being very sensitive
to overfilling or a P/V valve failing closed.
There is simply no way that this ship can be damaged by underpressure
(i.e. P/V valve fails during discharge)
and it is much less sensitive to overfilling (the overfilled cargo simply goes to the wrong tank).
Moreover, since there will be much less tank breathing (normally none),
by leading the vacuum pump exhaust up the stack,
we can create the same kind of deck we have on the LPG ships,
a totally closed system.
This means we can have pumps on deck, electric valve actuators, etc, etc.
There will be at least five objections to the Full Vacuum Tanker:
This project will undertake a number of design studies
relating to the Full Vacuum Tanker.
A leak in the vacuum piping could expose the tanks to an explosive atmosphere.
It will take a big leak to move us out of the too-rich zone but,
at a minimum, this will require careful monitoring.
At a maximum, it will require a double pipe system in which the annulus is inerted.
Running some of the piping inside the tanks is another possibility.
One thought is to inert the ballast tanks
and then run the vacuum piping inside the ballast tanks.
This is a legitimate concern and we need careful, conservative design here.
Damaged stability could be compromised
by flow thru the IG/vacuum lines to the low side
especially in the case of raking damage.
This also needs detailed study
but one obvious, if inelegant, fix is list actuated one-way valves.
At worst, it implies slightly larger or more compartmentalized ballast tanks
to meet the same floodable length requirement.
If the IG lines are the same height as they are now,
the situation will be no different than what we have now.
The system depends critically on the density and vapor pressure of the cargo.
High vapor pressure and high density hurt.
For a U in which the IG/vacuum line is 8 meters above sea level,
the worst cargos I've looked at so far are
Saharan Blend (very high vapor pressure but light)
and Mayan (heavy with a surprisingly high vapor pressure).
In both these cases, equilibrium occurs
after the water has pushed about 5 meters into the tank.
Of course, this is already much better than Double Bottom or Mid-Depth,
and the situation can be improved drastically by lowering the IG line slightly
at a cost of possibly exacerbating objection (2).
Sooner or later during salvage you will to break the vacuum.
True, but by then you can have salvage pumps in place ready to go,
containment and collection equipment deployed, etc.
You also will have had a chance to plug the damage
in a zero or negative pressure differential situation.
Finally, if the system really caught on,
devising a salvage pump system that doesn't break the vacuum would not be difficult.
The most difficult point for self-transfer will be at the beginning of loading
and at the end of discharge, where the ship is high in the water.
This issue too will require carefull study and may force additional ballast capability.
On the other hand, few major spills occur at the begining of loading and at the end of discharge.
The Full Vacuum Tanker functions best when the ship is loaded,
which is exactly when you want it to.
In reality, the environmentalists' primary objection will probably be
it depends on the owners not overloading the tanks.
This shows that they do not understand the commercial realities of tanker operation.
The amount of cargo is carefully checked
and thoroughly documented both at load port and discharge port
by the owner, the charterer, the receiver and the terminal.
Big dollars are at stake.
This involves not only measuring the volume in each tank to (an attempted) five significant figures
by an independent surveyor which measurement is witnessed by all the parties involved;
but also comparing the results with those of the discharging or receiving terminal
Some of these parties want the numbers high; some want them low.
So there are natural checks and balances.
Any mistakes/discrepancies cause at least one of these parties real money
in either cargo revenue or freight.
There is no way an owner can "over-load" a cargo tank
without it being caught at both ends of the voyage.
And of course, for those who cannot understand these realities,
we could easily implement sealed continuous, tank level recorders.
Same thing to document vacuum.
First and foremost, we have to determine how much extra
steel will be required by the requirement to take a 10 meter under-pressure.
Once the CTX has an operational hull finite element capability
per the HULL project.
This will by a fairly straightforward exercise.
Assuming the results are favorable,
then the project will have to address some of the above concerns.
The stability issues can be analyzed
with possibly some modest modifications.
The same thing is true of the start/load, end/discharge question
We will also have to size and design the vacuum system
and address the issue of how close to the cargo's vapor pressure
we can actually achieve and maintain.
This will almost certainly involve some kind of system
for recovering and reinjecting the petroleum in the vacuum system,
using the reliquification technology used on refrigerated LPG ships.
Several big tankers already have vacuum recovery devices fitted.
We will also have to confront the issue
of where will we find room for the transferred cargo.
One obvious place is the ballast tanks,
but this raises a number of safety and regulatory issues
which will have to be addressed.
Finally, we will have to address the safety problems
associated with air leaking into the tanks.
Email about this project should be sent to firstname.lastname@example.org.