Why it is important to check the stacking weights of a Containership stowage plan

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Why it is important to check the stacking weights of a Containership stowage plan

Container stacks are containers which are stacked vertically and secured horizontally by stackers, lashing etc. Prior loading cargo, stacking weights of containers must be checked against the allowable stack weights on board the vessel both on deck and under deck. Neglecting above may cause serious damage to ships structure, hull and eventually overall stabilty of ship may get affected. Maximum allowable stack weights of Tank tops, Hatch covers and Decks shall not be exceeded at any time.

Container stack load – Hatch covers for Panamax ships have stack weights up to 90 tonnes/20ft units and 120 tonnes/40ft units. Post-Panamax vessel could have 100 tonnes/20ft and 140 tonnes/40ft units.

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Stacking Weight Restrictions

Reasons For Stowfall

It is essential to avoid loading heavy containers over light, or at the top of a stack in a deck stow, unless specifically permitted in the Cargo Securing Manual. This is because the securing system would normally have been designed on the assumption that light containers are stowed on top. Stowage may allow for ‘heavy-heavy-light’; however, loading ‘heavy-medium-medium’ may result in the same stack weight but would produce different strain on the securing system, especially if the GM is high.

Container securing arrangement

If any stacks are found to be exceeding the allowable stack weights, Terminal planner / Central planner is to be informed and cargo stow plan appropriately modified. The most common mistake made when stowing and lashing containers is to load heavy containers over light or to load so that the maximum permissible stack weights are exceeded. Heavy on light can only be accepted when specifically permitted in the Cargo Securing Manual.

In case such anomaly is noticed after containers have been loaded, the master shall notify all concerned parties and have the condition corrected at the earliest prior departure.

Container stowage in holds

The standard 40ft containers (FEU) are stowed in cell guides without any lashing devices. If there is also a certain number of 20ft units (TEU) available at all time, the creation of a hold with 20ft guides may be considered. In most cases, 40ft cell guides are installed exclusively and TEUs are stowed in the 40ft bays. This, however, needs securing job. There are basically 3 systems to stow TEUs into 40ft cell guides:

Side support stowage system. The containers are connected with double stacking cones in a transverse direction. At the longitudinal bulkhead, you have either a fixable installed guide rail or foundations for buttresses that take up the load. This is the most conventional system. The containers can be loaded/removed only tier wise.
Stowage with anti-rack spacers. The containers are connected longitudinally with so called “anti racking spacers”, thus creating out of two 20ft containers one 40ft-unit. This system avoids side supporting, that means a 20ft stack can neighbour a 40ft stack and there are no foundations or rails in the longitudinal bulkhead. The disadvantage is that the containers have to be loaded/ removed also tier wise.
Mixed stowage The third system becomes more and more popular with regard to the a.m. disadvantages: the mixed stowage. Starting from the tank top you can stow from one to four tiers 20ft containers (secured only by single stacking cones) and top them up with at least on FEU. This system allows stack wise loading/discharge. The only disadvantage is that the stackweight of the TEUs is reduced a bit to about 60 ton. The new size containers make another problem. Most easily they can be stowed on deck installing additional foundations on hatch covers.

Container stowage on deck

Ship motions impose heavy loads on the deck cargo which would soon move unless secured in place. Containers carried on deck may be secured by twistlock alone. The integrity of the stow then relies on the racking strength of the containers.

Up to approx. 50t stackweight, the lowest container is able to carry the transverse and longitudinal forces on top by itself. With higher stackloads, the stack must be “ reinforced” by lashing bars: single or double cross lashings depending on the stackweight. The effectiveness of double lashings is 1.5 times that of single lashing, unless a load-equalising device is fitted. The general principle is that securing in the forward part of the ship are designed to be suitable for forces increased by 20%, unless a breakwater or similar substantial protection is fitted.

The 40ft ends have a lashing gap of minimum 700mm, so that lashing can be applied. If two 20ft containers are positioned on one 40ft place, the gap is 76mm wide and it is not possible to use lashing and the stack weight is limited to 50t. It is a rare practice to link adjacent stacks of containers. Skiiping the job substantially simplifies loading and discharging operations. It is, however, sometimes necessary to link an outboard stack to the adjacent stack to help resist wind loading. At a wind speed of 90 miles per hour, the wind force is about 2 tonnes on the side of a 20-foot box and about 4 tonnes on a 40-foot box.

Generally, container stacks do not depend on each other for support. However, they do provide protection to each other from wind and waves, so stowage in isolated stacks, especially in outboard locations, should be avoided.

Container ships are likely to be driven hard in order maintain very tight operating schedules. As a consequence, thousands of containers are lost overboard every year and continue to be the source of substantial claims for P&I clubs.
Source : Wärtsilä Encyclopedia of Ship Technology

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

Containership operation : Cargo Securing
There are six degrees of motion at sea that a ship may have to encounter in a voyage. However, pitching, heaving, and rolling are three major forces that impact most on a containership's lashing arrangement. Lateral rolling motion factors the greatest challenge for piles of containers. If containers are to be carried safely on the deck of a container vessel, they must be tightly connected to the ship. It is done with the aid of devices known as twist locks. .....

Containership operation : Common reasons for stowfall
Container stows often fail due to container stacks being too heavy and too high overall, exposing the lower containers to excessive transverse racking and compressive forces due to the tipping effect. Such an anomaly may occur if the ship is unable to calculate the forces acting on stow with precision. The Cargo/Container Securing Manual is limited in this respect as the examples of container weight distributions shown may not cover all permutations and eventualities. Software programs have the advantage of taking into account all known variables........

Containership operation: Cargo hold ventilation
Cargo holds ventilation onboard a containership is very important as it minimizes the risk of harm or damage to cargo. A proper ventilation system assures the quality of the transported goods by preventing the formation of condensation in cargo spaces, reducing the harmful heating of the shipment, and removing potential hazardous gases from cargo spaces........

Containership operation: Safety of personnel
In port stevedores board the vessel for lashing, unlashing and cargo operations and their safety whilst on board is the vessels responsibility. It is important to understand that any injury caused to stevedores or shore personnel due to a condition on board being unsafe, can result in very large claims to the vessel. .....

Containership operation: wet damage in cargo hold
When water entered into a ship's cargo, hold it may cause wet damage to the cargo inside containers especially to those stowed on the bottom stack, unless the bilge water is drained in a proper and swift manner. The regular sounding of bilge well or monitoring bilge alarm must be one of the very important or rather essential routine jobs on board. However, this job requires special attention on board. All bilge alarm need to be tested regularly......

Reefer cargo care at sea
Unlike permanent cold stores or refrigerated ships, where robust equipment is under constant care by qualified personnel, the ISO refrigerated container may travel by several different modes and be in the care of many and varied people. Before being despatched to load refrigerated cargo (usually at shippers' premises), the container and its machinery should be subjected to a rigorous examination.......

Containership cargo stowage and planning
Master and officers of all vessels require a good working knowledge of the various kinds of cargo they are likely to carry their peculiar characteristics, liability to damage, decay, or deterioration, their measurement, and the usual methods of packing, loading and discharging, stowage, dunnage, etc., as the Master is responsible for the safe loading of his vessel and the proper storage of the cargo......

Stacking Weights Restrictions
rior loading cargo, stacking weights of containers must be checked against the allowable stack weights on board the vessel both on deck and under deck. Neglecting above may cause serious damage to ships structure, hull and eventually overall stabilty of ship may get affected. Maximum allowable stack weights of Tank tops, Hatch covers and Decks shall not be exceeded at any time......

Lashing strength calculation
Lashing strength of deck cargo shall be ascertained by using the appropriate lashing strength calculation software where provided. All resulting values for lashing strength must be within the tolerance limits prescribed by the vessels classification society......

Dangerous goods stowage and segregation
Clear guidelines apply to the stowage and segregation of Dangerous Goods and in some cases may require particular commodities to be carried in completely separate holds. The interaction of two cargoes will not occur if the packaging of that cargo remains intact. However, the Master must always consider the possible effect should the cargo escape for any reason and should not restrict his consideration to those cargoes which are listed in the IMDG Code......

Reefer Container Stowage
Reefer containers proposed for stowage must be accompanied by a reefer manifest. This reefer manifest should contain Container No., Stow position, Commodity, Temperature, and Ventilation status......

Out of Gauge Container Stowage
It is essential that, during out of gauge cargo operations, a careful watch is kept for any damage caused to the vessel, her equipment, or to containers. Notice of any damage must be immediately brought to the attention of the Stevedore's representative, the Port Captain/Supercargo and Charterers Agent. Damage reports must be completed in all cases giving the full and comprehensive details of damage caused......

Special Container Stowage
After receiving stowage plan ships, Chief Officer must ensure that all Deck Officers are aware of any specialized containers due to be worked, such as reefers, vents, over-heights, over-widths, flat racks, etc. and their unique requirements.....

20 or 40 or 45 feet Compulsory Stowage Locations
Most cargo securing manual provide a guideline for different container types. These stow positions of 20 feet,40 feet, or 45 feet are also incorporated in a ship-specific stowage planning software and highlight errors if any violations occur......

Irregular Stowage of Containers
The Terminal Planner shall present the pre-loading plan to the Chief Officer to obtain his approval/comments. The Chief Officer, in turn, enters the cargo data in the loading computer and must ensure that the required criteria, concerning stack weights, trim/stability/stresses/ visibility limitations, DG cargo segregation, and specialized container requirements, are met. He should allow the bunker/freshwater consumption during the voyage and all possibilities of ballasting / deballasting. The completed loading plan must be presented to the ship's Master for approval.....

Over-stow of Containers
With a closed roof, the hardtop the container offers the same reliable protection as provided by a standard box. Hardtop containers have more lashing points than other container types. This guaran- tees reliable and convenient securing of cargo. .....

Hatch Cover Clearance (High cube containers Under Deck )
Hatch cover clearance must be checked carefully in case of loading over height containers or high cube containers underdeck......

Other matters regarding cargo stowage as necessary
Bulk products carried in a closed container might include malt, grain, seed, polythene granules, chemically inert powders, brake fluid, detergent, fruit juice, wine, non-hazardous oils, sodium silicate, fatty acids and maple syrup amongst many others......

Stacking Weights Restrictions

Lashing strength calculation

Dangerous goods stowage and segregation

Reefer Container Stowage

Related Information:

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

Container handling more info pages:

Definition of various containers in containership
The exterior dimensions of all containers conforming to ISO standards are 20 feet long x 8 feet wide x 8 feet 6 inches high or 9 feet 6 inches high for high cube containers. Some of the most commonly used types are:Read more......

Dimensions of various containers

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