And last one for tonight ....

Getting It Up.
.... And Keeping It Up!    :-)

Lift Bags

At some point in their careers almost all commercial divers' have employed
lift bags in salvage or lifting operations.  Some have undergone formal
training in their use under the experienced eye of an instructor.  Others
have been left to fend for themselves.  Usually with varying degrees of
success.

This article is an overview intended to clarify some key points for those
with little or no experience in the use of lift bags.  One that may also,
perhaps, remind those who are experienced in their use of some of the
problems that can arise (lift-bag pun!).

Types and Models
There are several companies around the world who specialise in the
manufacture of lift bags.  Although sizes normally range between bags with a
50 kilo to 10,000 kilo lifting capability, larger and smaller bags can
always be made to order for special applications.
The two most common types of lift bag in use today are the Parachute and the
Enclosed.  Although each of these types feature models of various shape and
design, all lift bags essentially fall within this basic scope.

Parachute Lift Bags
The Parachute Lift Bag is the most common type used by diving contractors.
Their main use is to raise objects from the seabed - and in some instances
to support items on the surface.  The design is, as their name suggests,
similar to that of a parachute.  They provide buoyancy by virtue of the fact
that the air that is used to fill them is trapped at the top of the bag
while the bottom of the bag is open to allow excess, or expanded, air to
readily escape.

The material used in the fabrication of this type of lift bag is a heavy
denier (850 denier and upwards in thickness), abrasion resistant material
that offers air tight integrity.  Other qualities include resistance to
weathering and deterioration caused by oil and grease as well as U.V.
stability.  Commonly made from polyester materials the seams are usually hot
welded together.

The smaller, parachute-type lift bags, up to about 500 kilos, have webbing
lifting straps that are usually stitched to bag material pads which are then
welded to the sides of the Lift Bag.  Smaller bags in this range usually
have two straps while the larger bags have up to five straps to ensure the
added strength required for heavier lifts.

Larger parachute-type lift bags - 500 kilos and more - employ a heavier
denier material and more straps.  The webbing-type straps on the larger lift
bags usually run up the entire side of the bags through special welded on
pockets and secure together at the bag's crown.  This give enormous strength
for heavy lifts and large bags can have up to ten or more of these webbing
type straps.
Parachute Lift Bags are usually fitted with a manually operated dump valve
controlled by the diver.

The size of this valve increases with larger bags to ensure that the air can
easily be vented.  The valve is connected to a rope long enough to come down
to the skirt of the bag, where the diver is carrying out his connection to
the job.  This allows the diver to pull on the rope and activate the dump
valve so that the bag can be trimmed as needed.  Parachute Lift Bags should
never be used without a dump valve unless there is a very special
application and the diver is made aware of this modification.
There are two usual methods of filling the parachute lift bag with air:
Either from the open end, or skirt, of the bag or, in some instance, from an
air inlet control valve fitted to the crown (top) of the bag with a
connection that allows a hose connection.  This is an ideal way to fill the
bag and allows the surface team to take over control when necessary.

What Size Bag To Select For A Lift?
This is where experience counts.  It is of the utmost importance that the
correct calculations are applied when selecting the right size bag for a
particular lift.  Choosing the wrong size of bag can result in the item not
lifting from the bottom or, and worse still, being lifted and rising to the
surface at an uncontrollable speed.

Before attempting a lift Diver's need to consider the following points:-
1. Size and weight of the item to be lifted?
2. What is its weight in water?
3. The depth of water that the object rests in.
4. What is the best lift bag to use?  Or the best chosen from those
available at the time.
5. Is there an adequate air supply available for filling the bag?
6. Have you got sufficient shackles, strops, ropes, chains, etc., to assist
with the lift?
7. Surface conditions and sea state.
8. DIVER SAFETY.
9. The securing point(s) on the item to be lifted.
10. The nature of the seabed, ie. silt, sand, rock or mud, and the amount
of suction to contend with.
11. Never 'ride' a lift bag to the surface.
12. All other important aspects - unique to each job - that relate to the
lift.

Important:
Bringing items up from deep water imposes additional problems.  Proper
calculations are essential.  Divers should take note that using a lift bag
with too much lifting capability will - allowing for air expansion in the
bag - cause a rapid increase in the ascent rate.  This can result in
disaster, with the item eventually reaching the surface at great speed where
air is spilled from the bag allowing the item to rapidly descend back to the
seafloor. Calculate each lift.  Use small pilot bags if necessary to ensure
that there is MINIMUM spare volume in the main bag once the lift begins.
Have a small bag above the larger bags that take the final lift.

Enclosed Lift Bags
Enclosed Lift Bags, sometimes referred to as 'closed', or 'sausage bags',
usually come in sizes that range from 250 kilos and upward.  Primarily used
to support the flotation of items on the surface they are not used to lift
items from the sea-bed.

Although made from the same materials as the parachute-type, lift bags these
bags, being totally sealed, vary in design.  These bags have filling points
with valves that allow remote filling.  They are also fitted with
over-pressurisation valves on the bottom so that the bag does not
over-inflate and burst.  Most bags have at least two filling points, located
at each end of the top of the bag, and several over-pressure valves on the
bottom.

The webbing straps run all the way around the bag and are fed through
pockets.  For example, a 5,000 kilo enclosed bag has around seven straps.
These should be secured to a spreader bar to spread the load when lifting.

Because enclosed lift bags usually come into contact with the item that they
are supporting it is important to prevent damage to the bag.  Always remove
any barnacles, welds, jagged metal, rough paint, etc., from the contact
surface.  Chafing will badly damage the bag.  If need be cover the contact
area of the bag with canvas,  hessian bags or place marine ply between the
contact surfaces.  Chafing happens quickly.

Procedures To Consider:
Enclosed bags need to be well secured to the lift point.  It is equally
important that the top of the lift bag also be secured to the item being
supported to prevent a possible capsize.  When floating a vessel, for
example, the bags should not only be connected to the lifting points, but
they should also be secured fore and aft at the securing lugs on the top of
the bag and also secured inboard to prevent them from moving away from the
hull and causing a possible capsize.

Calculations for these bags are also important.  The use of Enclosed Lift
Bags should be avoided in lifting items from the seabed, especially at
depth.  The chance of exploding these bags due to rapid air expansion is
very real.  Use Parachute Bags to lift, and Enclosed Bags to support and tow
an item.  (Enclosed bags are ideal for towing purposes as there is little or
no chance of spillage from the bags, whereas there is a chance of spilling
air from open, parachute-type lift bags, when towing an item).
Summary of Lift Bag Use:
1. Be aware of the DANGERS when using lift bags.
2. Have experienced Lift Bag personnel on site.
3. Carry out full calculations on the item to be lifted or supported.
4. Ensure that you have the appropriate combination of bags.  And that  you
know which bags to use.
5. Ensure that you have plenty of support equipment: ie., shackles, ropes,
strops, chain, marine ply, canvass, compressed air, hoses, etc..
6. Check the strength of the lifting points on the item you are lifting or
supporting.
7. Use Parachute Bags for lifting.  Use Enclosed Bags for surface
flotation, especially when towing.
8. Be aware of the sea conditions, including sea state, currents, tides and
monitor the weather forecast.
9. Take your time, and remove anything that can damage the bags, ie.,
barnacles, jagged metal, etc..
10. And finally:
Be prepared for the unexpected!

Calculating The Lift
0.97 litres of air provides 1 kg of lift in seawater.
1 litre of air provides 1 kg of lift in fresh water.
How much air will be required to lift an object weighing 500 kgs from the
sea floor in 30 metres (4 ATA's) of water? (0.97 X 4 ATA's = 3.88 litres of
air will provide 1 kg of lift at 30 metres.
3.88 X 500 (weight of object to be lifted) = 1,940 litres of air)

---ENDS---