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Containership Cargo Securing Arrangement -Design,Stack weights & Metacentric Height Factors

The design criteria for a coastal vessel's container stowage arrangement differ entirely from the requirements for a mega containership, and so do their container lashing systems. However, there exist some standard devices widely accepted in most ship types. These to include; Twistlocks, Lashing bars, Turnbuckles, Extension hooks, Stacking cones (single and double), Twist Stackers, Lashing D rings & Shoes/Sockets for base twist locks

Besides, it should be remembered that the container frame itself is an integral part of the securing system. The stow will remain secure only if the frame is in good condition and if the loads acting on it remain within safe limits.

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Details of the securing system and its constraints are set out in the vessel's approved Cargo/Container Securing Manual. In the event of any amendments, re-approval will be required Securing systems take into account the various forces triggered by violent motion during adverse weather, including those caused by the six degrees of freedom. Within these limits, each item of equipment is designed to function within its predetermined safe working load.

However, the calculations assume that the equipment is properly maintained and positioned. Should some of the items not be deployed as specified in the Cargo/Container Securing Manual or be allowed to deteriorate, the safe working load of the remaining equipment may be exceeded.

Stack weights and stack weight distribution

It is often thought that by keeping the total stack weight within prescribed limits, the securing system will not be overloaded. It is not necessarily the case, particularly when containers are loaded on deck. It is important to recognize that the total stack weight limit is merely the weight that can be supported safely by the deck or hatch cover, as applicable.

The calculations are based on the ship being upright in calm conditions (i.e., in port) and taking into account the static weight of the stack due to gravity. The figures also consider the anticipated dynamic stack loads acting on the deck or hatch covers in adverse weather due to the various ship motions described earlier.

Container lashing pattern
Parallel lashing rods and twist locks

However, stack weight limits do not ensure that the dynamic loads acting on the securing container system will remain within the margins of safety during heavy weather. Loading the cargo so that each stack does not exceed its total permissible weight is relatively easy to achieve. It is far more difficult to optimize the stack in terms of weight distribution, port rotation, and estimated forces, ensuring that the secure working load of the securing equipment is not surpassed.

Therefore the Cargo/Container Securing Manual must be consulted for guidance. The manual gives illustrations of typical safe stack weight distributions and total safe stack weights based on the ship's securing system's design limits.

Metacentric height (GM)

The higher the GM, the greater is the righting moment when the ship is rolling. Consequently, the transverse acceleration forces acting on a container deck cargo and the securing system intensify with GM's rise. Such forces are at their highest at the extremity of each roll. It is always important to consider the effect of GM on the securing system when estimating the forces, bearing in mind that GM may not remain constant throughout the voyage.

Lashing pattern of outboard containers
Lashing pattern of outboard containers

Estimating the forces

Examples of GM values are featured in the Cargo/Container Securing Manual and acceptable weight distributions in each case. This information should be compared with the actual figures as calculated by the ship. If there are any significant differences, stack weights and/or securing equipment should be adjusted until both are deemed to be within prescribed safe limits.

For reliability and accuracy, many vessels are now equipped with a container securing software package. There are several programs of this type on the market, most of which are designed to interface with cargo management or loading computer software.

Securing programs will calculate the applicable forces for any given stowage plan, comparing the data with the safe working criteria as specified by class. The effect of wind strength on the outboard stacks is also taken into account, allowing the corresponding load on the securing system to be adjusted accordingly. Most programs will warn if any securing components or individual container frames are likely to be overloaded, and the result of applying additional lashings can be shown. Inspection, Inventory, and Maintenance The securing equipment used on board depends on the ship's design, capacity, and trade. Accurate records regarding the quantity, location, inspection, maintenance, repair, and replacement of such equipment should always be kept.

In general terms, there will be a need to:
  1. Ensure that the amount of securing equipment and lashing material retained on board is sufficient to secure the maximum number of containers to be carried properly. There should also be an adequate safe margin of spares and replacements.
  2. Inspect all portable securing equipment (e.g., turnbuckles, lashing rods, twist locks, twist stackers, etc.) for distortion, buckling, corrosion, cracking, and/or excessive wear. As far as practicable, such checks should be carried out as routine before the equipment is reused.
  3. Lubricate turnbuckles and twist locks frequently. The spring, which holds the twist-lock in the closed position, should be examined regularly as it may become less effective over time, causing the device to unlock if the ship or stow begins to flex on passage.
  4. Examine all fixed fittings (e.g., container sockets, elephant's feet, dovetail connections) routinely for wastage, cracks, distortion, and/or general deterioration, including base plates where applicable. Particular care should be paid to D rings' condition, mindful that wastage of the main shaft may be concealed by the retaining sleeve.
  5. Maintain a file of all certificates of approval relating to the portable securing equipment carried on board.
  6. Suspect fittings or equipment should always be removed from service when found, and repaired or replaced as necessary.

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