ship handling

Home page|| Tanker Notes || Container Ship Operations || Ocean Navigation ||



Container ship cargo Securing arrangement -Design,Stack weights & Metacentric height factors

Securing equipment will vary depending on the type of ship but is likely to include;

Twistlocks Lashing bars Turnbuckles Extension hooks Stacking cones (single and double) Twist Stackers Lashing D rings Shoes/Sockets for base twistlocks

In addition 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.

Design

Details of the securing system and its constraints are set out in the vesselís 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. This 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.
Handling heavy weight
Fig :Handling heavy weight

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

However, stack weight limits do not ensure that the dynamic loads acting on the container securing 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 at all times the safe working load of the securing equipment is not surpassed.

Therefore it is vital that the Cargo/Container Securing Manual is consulted for guidance. The manual gives illustrations of typical safe stack weight distributions and total safe stack weights based on the design limits of the shipís securing system.

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 any rise in GM. 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.


Estimating the forces

Examples of GM values are featured in the Cargo/Container Securing Manual together with 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 type of securing equipment used on board depends on the shipís 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:

Ensure that the amount of securing equipment and lashing material retained on board is sufficient to properly secure the maximum number of containers to be carried. There should also be an adequate safe margin of spares and replacements.

Inspect all portable securing equipment (e.g. turnbuckles, lashing rods, twistlocks, twist stackers etc) for signs of distortion, buckling, corrosion, cracking and/or excessive wear. As far as practicable, such checks should be carried out as a matter of routine before the equipment is reused.

Lubricate turnbuckles and twistlocks frequently. The spring which holds the twistlock 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.

Examine all fixed fittings (e.g. container sockets, elephantís feet, dovetail connections) routinely for signs of wastage, cracks, distortion and/or general deterioration, including base plates where applicable. Particular care should be paid to the condition of D rings, mindful that wastage of the main shaft may be concealed by the retaining sleeve.

Maintain a file of all certificates of approval relating to the portable securing equipment carried on board.

Suspect fittings or equipment should always be removed from service when found, and repaired or replaced as necessary.



Related articles

Containership operation : Cargo Securing

Containership operation : Common reasons for stowfall

Basic guideline for Container Ship Operation

Containership operation: Cargo hold ventilation

Containership operation: Safety of personnel

Containership operation: wet damage in cargo hold




Related articles

Reefer cargo Handling In Port

Reefer cargo care at sea

Commodities Shipped In Reefer Containers

Reefer Cargo Temperature Recording

Reefer Cargo Maintaining Records

Reefer Cargo Defrosting

Basic check item prior stowing Reefer Cargo

Preventing Reefer Cargo deterioration



Container handling additional guideline:

Containership cargo stowage and planning

Stacking Weights Restrictions

Lashing strength calculation

Dangerous goods stowage and segregation

Reefer Container Stowage

Out of Gauge Container Stowage

Special Container Stowage

20 or 40 or 45 feet Compulsory Stowage Locations

Irregular Stowage of Containers

Over-stow of Containers

Hatch Cover Clearance (High cube containers Under Deck )

Other matters regarding cargo stowage as necessary

How to load maximum number 20 feet container on deck ?

What are the extra precaution should be taken prior loading a 45 feet container on deck ?

Container damage in ''2 in 1'' cargo Operation

Modern containership & loading of various container types

How to load containers coming in different forms/sizes



Our additional pages contain somewhat larger lists of resources where you can find useful informations






Other info pages !

Ships Charterparties Related terms & guideline
Stevedores injury How to prevent injury onboard
Environmental issues How to prevent marine pollution
Cargo & Ballast Handling Safety Guideline
Reefer cargo handling Troubleshoot and countermeasures
DG cargo handling Procedures & Guidelines
Safety in engine room Standard procedures
Questions from user and feedback Read our knowledgebase
Home page




ShipsBusiness.com is merely an informational site about various aspects of ships operation,maintenance procedure, prevention of pollution and many safety guideline. The procedures explained here are only indicative, not exhaustive in nature and one must always be guided by practices of good seamanship.

User feedback is important to update our database. For any comment or suggestions please Contact us
Site Use and Privacy - Read our privacy policy and site use information.
//Home //Terms and conditions of use

Copyright © 2015 www.shipsbusiness.com All rights reserved.