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Cargo Work

 

Securing Cargo

 

 

Need for solid stow and securing of all cargoes

Cargo onboard a ship will tend to shift with the motion of the ship. This necessitates the cargo to be lashed (secured) to the ship structure. However the lashing with ropes/ wire ropes/ iron restraining bars is not very effective because of the fact that the tightened lashings have a tendency to work loose with the motion of the ship.

On shore any nut which is fitted tightly on a bolt works loose with vibrations as such - spring washers are used together with check nuts and split pins to prevent the working loose of such nuts. This is not practical on shipboard lashings - except for turnbuckles and bottle screws with restraint bars. Below deck lashings further are not attended to during sailing and if they work loose it is practically impossible to do a very effective job to re-secure them. Temporary measures are often adopted and these may not be very effective as stated earlier.

Thus the only way to prevent the lashings from working loose is to stow the cargo very close to each other and then to shore the cargo with timber. This would prevent the cargo from acquiring momentum while swaying with the ship and thus prevent to a large extent the working loose of the lashings.

 

For bagged cargo if the same is not stowed solidly and thus allowing too much of broken stowage, would tend to shift with the motion of the ship, thus shifting the centre of gravity laterally and inducing a list to the ship. This coupled with the heeling of the ship would make the weather deck of a ship too close to the water line and thus endanger the safety of the ship.

Bulk cargo on general cargo carriers are therefore saucered with the same cargo, in order to prevent the cargo from shifting to one side.

Deck cargo due to the high KG is especially vulnerable lateral shifting and the lashings work loose and also to part lashing. Especially since the transverse momentum gained by such cargo during the rolling of a ship is liable to part lashings. Thus all deck cargo has to be definitely shored and then also lashed to deny the cargo from gaining any momentum.

Deck cargo - Lashed

Deck Cargo - Shored and Lashed


Cargo liable to slide during rolling, such as steel rails, should be Stowed fore and aft

All long cargoes such as steel rails, pipes, long steel plates as well as steel coils are stowed with their ends in the fore and aft direction. This again is necessary due to the fact that most of theses cargo cannot be individually lashed they rather grouped into bundles and the bundles are lashed to make many small bundles of pipes or rails as the case may be. This prevents the individual pipes from sliding and since as mentioned the transverse momentum is quite large when the ship is rolling, and the pipes are thus prevented from damaging the sidewalls of the hold. This is severe since repeated banging has resulted in tearing holes in the shipside plates below the waterline and the ship capsizing due the inflow of water.

If the pipes / rails are stowed in the fore and aft direction this is prevented.

Bundling of long cargo (pipes/ rails):

This is the first tier. It is important to place the dunnage to spread the load as well as to facilitate the passing of slings at the disport. The lashing wires are also placed prior to loading the cargo. The size of the bundles should be to the capacity of the derrick/ crane that would be used to discharge the cargo. The number of lashing wires is dependent on the weight of the bundles as well as the length of the cargo.

As each bundle is completed the lashings are closed and tightened. And subsequently dunnage is again placed and the lashing wires again spread on top of the earlier cargo.


Stowage and securing for vehicles and trailers

Vehicle lashing on deck

Force parallel to and across the deck = 1.0 W

Force normal to the deck = 1.4 W

Force in the longitudinal direction = 0.3 W

The above forces are intended to represent the total force to be applied in each direction i.e., the aggregate of the static and the dynamic forces.

Case 1 Vehicle stowed in Fore and Aft direction:

The forces preventing tipping of the vehicle are the vertical downward force and the lashings holding the vehicle (FLT)

Taking moments about A (the outer edge of wheel i.e., fulcrum position)

FLT x L = (1.0 W x 2/3 H) (1.4 W X)

FLT x (X + Y) sinq = W (0.67 H 1.4 X)

FLT = (W (0.67 H 1.4 X) / ((X + Y) sinq)

Note the importance of the fulcrum position (A),

The height of the centre of gravity, normally taken as 2/3 H

q is the angle of inclination of the lashings

To examine the force causing the vehicle to slide sideways:

For this example a trailer is supported by wheels on the one end and with a trestle at the other end.

 

 

 

 

In both cases sliding is resisted by the frictional resistance m between the tyre/ deck and the trestle/ trailer frame and also lashings (FLS).

Case 1 Effect at the trestle end of trailer.

Note: Assuming total forces act at each end of trailer then effective sliding force = 0.5 W 0.7 W x Ls (assume 0.2)

= 0.5 W 0.14 W

= 0.36 W then the force in the lashing resisting sliding = FLS = 0.36 W / cosq

 

Case 2 Effect at wheel end of trailer.

Effective sliding force = 0.5 W 0.7 W x m (assume 0.4)

= 0.5 W 0.28 W

= 0.22 W

then the force in the lashing resisting sliding = FLS = 0.22 W / cos q

Note the importance of m the coefficient of friction and q the angle of inclination of the lashings. In the above it can be seen near vertical lashing is great to prevent tipping but is useless for sliding whereas a near horizontal is great for sliding but is useless for tipping. So a correct angle of inclination should be fixed appropriate for the cargo.

In general the safe working load (S.W.L.) of lashing wires is taken as 1/3 the Breaking load.

If chain is used for lashing then:

If made of H.T. steel then the SWL would be 40% of the Breaking load.

And if made of ordinary steel then the SWL would be 33% of the Breaking load.

Efficient securing of cargoes is essential for the safety of the ship as well as the cargo

Securing of cargo is of prime importance not only for the cargoes themselves but also for the ship as a whole including the crew that sail on her.

Improperly secured cargo will shift in a seaway and can endanger the cargo as well as the ship.

In the worst cases the cargo may fall overboard and may endanger other ships such cargoes like logs and containers have been noted to have floated and come within the sea-lanes.

When a container falls overboard it must be remembered that it does so in spite of it being secured on the ship as well as the opposition to this being offered by the ship structure. Thus when it does go overboard it does after causing a great amount of structural damage.

There are many instances of cargo improperly secured breaking the lashings and punching a hole at or below the waterline and the ship having been lost with casualties.

Deck cargos if they part their lashings are liable to cause extensive damage, which can endanger the watertight integrity. Even minor movement of heavy cargoes has been known to shear off air pipes and sounding pipes resulting in water entering the tanks or other spaces below deck. Fire lines have also been damaged due to inadvertent movement of cargo.

Accommodation ladders as well as companionway can be damaged due to the cargo movement on deck in a seaway.

Even if the ship is not lost the damage such heavy cargoes can bring upon the structure of the ship is very heavy. Crew has often been sent to re-secure such cargo in rough weather with the crew suffering loss of limbs and other injuries.

Stowage and securing of deck cargo should be adequate for the worst conditions which could be experienced

Good stowage and good securing arrangement should be foreseen prior loading the cargo. If it is required extra lugs and eyes on deck have to be welded to provide lashing points for the cargo- this is generally done for heavy lifts or cargoes of odd sizes.

Securing should be always for the worst weather that would be encountered. Many a ship have suffered damage to cargoes and to their own structure by neglecting good and adequate lashing while on a short voyage, failing to take into account diversions and anchorage at open roadstead and cyclonic weather.

Hatches should be securely closed and cleated before loading over them

Once the cargo below deck has been loaded and all securing has been completed (securing can continue after the hatches are secured provided there is adequate space for the crew to enter and to lash), the hatches are closed and battened down and all cleats and centre wedges should be in place.

Only after the above have been completed should any cargo be loaded on to the hatch tops.

If this is not done, and the hatch is battened down after the cargo has been loaded on to the hatch tops the battening down and the fitting of the cleats as well as the centre wedges would be ineffective since the weight of the cargo would not permit the hatch covers to be correctly in place and the hatch would leak in a seaway or even in rain.