General Securing Information
When a container securing plan (for the holds and the deck) is prepared, the following are some of the important considerations:
1. The stresses from the ship’s motions.
2. The stresses from the wind thrust.
3. Slippage and toppling.
The popular ways to carry the containers together is by:
● Stacking the containers in vertical guide rails
● Stowing them in stacks
● Stowing them in blocks,
The containers are connected together by twist locks, bridge clamps & double stackers etc and fixed to parts of the vessel by lashing rods, bottom stackers and bottom twist locks etc.
Securing in Vessel Holds with Cell Guides
Securing in vessel holds with cell guides is mostly by cell guides alone. Fully containerised ships are provided with cell guides with vertical guide rails as securing means. The Containers are not connected vertically. Since the containers are not connected together vertically, lateral stress is transmitted by each individual container to the cell guides. In such cell guides individual containers are not usually able to shift. The advantage of such stow being negligible damage to other containers or stack even if the corner posts of one of the containers at the bottom of a stack collapse under excessive pressure.
The containers are guided by these rails of the cell guides during loading and unloading. At the bottom they mainly have fixed cones, which engage in sockets welded into the tank top area, supported by stiffened structure below the tank top.
Securing in Vessel Holds without Cell Guides, by Conventional Securing and Stacked Stowage
A variety of securing pattern may exist in the holds of older, conventional general cargo vessels. Most common being the stacked stowage, combined with various securing methods. The lower containers stand on foundations capable of withstanding the stack pressures, which can be very high in certain loadings. Dovetail foundations, into which sliding cones fit, are provided to prevent slippage. The containers are connected together by single or double stacking cones or twist locks. The entire stack or container block is lashed using lashing wires or rods and turnbuckles. This system involves a lot of lashing work and material and, moreover, is less secure than securing in cell guides.
This securing method is found less and less frequently, but it is still found on some conbulkers and other multipurpose freighters. Containers are interconnected horizontally and vertically using single, double and possibly quadruple stacking cones. The top tiers are connected by means of bridge fittings.
To the sides, the containers are supported at their corner castings with ‘pressure/tension elements’.
This type of container securing suffers from the following disadvantages:
● If an individual container breaks, the whole container block is affected.
● Due to dimensional tolerances a large wear and tear is caused to the stacking cones. The entire block can move in rough seas, causing the intermediate stacking cones to break and an entire block may collapse.
Securing on deck using container guides
On some ships, carriage of containers on deck in cell guides or lashing frames has been experimented. On some ships, cell guides can be pushed hydraulically over the hatch cover as soon as loading below deck is completed and the hatches have been covered up.
Securing on Deck using Block Stowage Securing
This method was used a lot in the early days of container trading, but has been used less and less in recent years for economic reasons.
The containers in the bottom layer are positioned in socket elements or on fixed cones. Double stacking cones are used between the layers and the corner castings of adjoining containers are connected at the top by bridge fittings. The containers are held together over the entire width of the ship or hatch cover by cross lashings. The difficulty encountered being at the time of loading and unloading, where the choice and flexibility is curtailed when a particular container in the lot is to be reached. Diagonal lashings help resist racking. The usual arrangement is to fit one tier of lashings placed diagonally within the width of container securing the bottom of the second tier of containers. A second pair of lashing may be fitted reaching to the bottom of third tier of containers for additional strength parallel lashings may be use.
Securing on Deck using Stack Stowage Securing
The main advantage of this securing method on deck is the high cargo handling flexibility. The containers are stacked one on top of the other, connected with twist locks and lashed vertically. No stack is connected with any other stack. The container lashings do not cross over the lashings from other stacks. ‘wind lashings’ however must be taken on the outer sides of the ship. Containers on deck may be secured by twist locks alone provided the stack is not more than two high.
When containers are two high the twist lock alone will be sufficient if the containers are not heavy. The twist locks resist horizontal movement and lifting of containers in heavy weather. If the stow is twist lock alone then the racking strength of the containers is a concern, therefore for the stows of more than two high lashing rods are fitted.
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