Restraint
As mentioned earlier the cargo may have to be restrained during the loading operations particularly if it is on a low friction device such as air or water skids. Once stowed in position lashing. Securing and tomming must be out to prevent the slightest movement of the load. All lashing must be set up tight and wooden tomming must be secured in such a way that it cannot be dislodged by ship vibration, working or movement. As a rule of thumb, a total lashing on one side of a load should have a combined breaking strain of at least ½ times the total weight of the load to be restrained. Tomming and ehocking would be extra to this.
All lashing and tomming should utilize the appropriate and approved point on a load to give the support. Where necessary extra securing point will have to be welded or riveted to ship frames and decks.
Lashing and securing should follow the approved securing diagrams and gear used should be of the specification detailed.
The higher the centre of gravity the more the heavy lift will tend to lift in a seaway, so sufficient downward lashings are required.
Where possible, weld bracing to the deck to hold the load in position and any additional methods of securing that can be welded direct to the deck/tank ceiling should be used. Always remember safty rules when welding over tanks! Wooden tomming is insufticient on really heavy loads and steel restraints should be used.
After completion of discharge remove all welded securing devices. Grind smooth and coat to avoid damage. Remember the underside of thank ceiling/deek plate will also have been damage and need recoating.
Loading
The ship should be upright when loading (or discharging) a heavy lift and have adequate stability. The double bottom tanks must be either pressed up or dry. If ship’s gear is being used it must be remembered that the effective centre of gravity of the load moves to the derrick head as soon as the lift is floated.
Lifting operations should be interrupted to carry out check on the slinging arragements:
a. As soon as the weight is taken by the slings.
b. When the load has been lifted a short distance.
The whole system should be checked through to ensure that no under strain is being imposed on gear, equipment, or the load itself.
Clear lines of communication should be established between all those involved in the operation. with one person in charge. It may be necessary to discontinue other operations close by on ship or shore to allow those involved with the heavy lift to hear and understand the control signals.
Discharging
Similar precautions should be taken to those described under “loading”. Furthermore, it may be the vessel, being at the end of a voyage or passage. Has less bottom weight in the form of fuel and/or water. This will have to be taken into account when considering the stability requirements. It should also be borne in mind that when a heavy lift is hoisted from its place of rest the centre of gravity of the weight is transferred to the derrick head. Again it is important that one person only should be in charge of the operation and that all activities are pre-planned with appropriate stops to check equipment and gear.
Occasionally havvy lift cargo has to be offloaded straight to the sea. This might be a requirement of semisubmersible vessels or a requirement of the cargo itself e.g. lighters, launches, cylindrical and rectangular tanks. Etc. in such cases, provided that the pieces are of sufficiently strong and robust construction to withstand the stresses and falls, they are carried on deck and launched over the side at destination.
Cylindrical objects, which must be perfectly watertight and free from small protruding parts, should be mounted on a strong launching structure built of timber, the inboard end of which rests on top of the adjacent hatch and the other on the bulwark, adequately supported at intermediate points. The frame of the structure should incline about 2 in the direction of the ship’s side.