Loss Prevention - Stability

Pre Load Requirements
Persons responsible for loading a vessel or barge, must ensure that they are made aware of the cargo weights to be loaded and the height of their centres of gravity. This information, wherever possible should be determined before loading operations are commenced, so that a safe load sequence can be calculated beforehand and that no nasty surprises are encountered at the last minute.

Notwithstanding any pressure placed on the vessel by the shore terminal, the responsibility for loading remains with the Master alone.

Free Surface Effect (FSE)
The free surface effect of any liquids on board has a marked impact on the vessel’s stability by reducing the effective GM (or conversely by effectively increasing the KG). Some of the claims the Club has dealt with have highlighted the fact that either no account of FSE had been taken, or if it had, the data was incorrectly applied.

Ideally, ballast tanks should either be pressed up full or completely empty so there is no free surface effect to consider. However, when this is not possible, it is best practice to initially allow the maximum FSE for each and every slack tank in the stability calculations. If the stability condition is then noted to be critical for any stage of the voyage, the actual free surface moments can be applied to the calculation in order to obtain an accurate assessment of the vessel’s condition.

It is essential that the FSE is always calculated and applied correctly and Masters should be given clear guidance on the Member’s requirement in this regard. It should also be borne in mind that free water on the decks has the same effect and when the stability condition is critical it can have a major impact.

Estimating Centre of Gravity
Masters are reminded of the need for accurate estimation of a cargo’s centre of gravity. Errors can accumulate if incorrect assumptions are made which can then compromise a vessel’s stability. Estimations should always err on the side of safety (i.e. it is better to estimate too high rather than too low). The centre of gravity of a container should always be assumed to be at mid height unless it is known to be different (some Classification Societies use 0.4 x container height).

Container Heights
Consideration should be given to ensuring the correct container heights are used when calculating the VCG. Whilst the actual difference between an 8’ 00”, 8’ 06”, 9’ 00” or 9’ 06” (Hi Cube) high container is not significant when considered individually, a large number of incorrect heights can have an adverse effect on the final VCG if not allowed for, particularly on smaller vessels.

Container Weights
The incorrect declaration of container weights is a problem encountered throughout the container shipping world and can manifest itself more in the local trades that our Members operate in rather than the main line trade.

Unfortunately this problem is one that is generally outside the control of Masters and shipowners. Visually there is no means to assess the weight of a container and the Master has to take the manifest or bill of lading at face value. Because this cannot be totally relied on, it places more emphasis on the need to monitor the vessels actual drafts during loading. If discrepancies arise they can be investigated further or allowed for by assuming the worst case scenarios.

Another problem with not knowing the weights of containers loaded, is the possibility that heavier units can be loaded on top of lighter ones with the subsequent reduction in stability. This problem may also occur when for the sake of economy and time, the number of container lifts are kept to a minimum and heavier containers are placed in an unsuitable location i.e. on top of lighter ones.

The Club has dealt with one claim where it was found that the overall difference between actual and declared weights was 10%. In the extreme, containers declared as being empty were found weighing in excess of 20 tonnes.

Although this problem can cause unacceptable situations, it is largely out of the control of the Master, however the potential for associated problems should always be kept in mind.

Draft
During cargo operations, it is important that the draft is observed visually, forward, aft and amidships on both sides at regular intervals and a comparison made to the calculated or expected draft. Any variances must be investigated. We have dealt with claims where little attention has been paid to the draft and vessels have subsequently been found overloaded which has contributed to a reduction in stability.

Cranes and Derricks
When ship’s gear is being used for cargo operations, the vessel’s centre of gravity always moves towards the weight loaded, away from a weight discharged or in the direction the weight is moved. When ship’s gear is used, the instant at which the container is clear of the deck, quay or wherever it rests, the weight is transferred to the point of suspension on the crane or derrick. As a result, the vessel’s vertical centre of gravity will be raised and moved in a direction towards the weight, effectively reducing the vessel’s stability. This can be a crucial factor during the final stages of loading and early stages of discharging when stability could be critical. Care must be taken when calculating the stability at such times and in particular, attention should be paid to the Free Surface Effect. It might be necessary to ballast the double bottom tanks in order to ensure adequate stability during the lifting operation.

Cranes and Derricks

Overloading
Following plan approval and periodic loadline surveys, all vessels are issued with a Loadline Certificate by the Flag State (or issued by a Classification Society on behalf of the Administration). This document alone is the overriding authority governing the minimum freeboard a vessel is permitted to load to. The Club has known cases whereby information from an unapproved stability manual was used for loadline purposes and this was found to be incorrect.

Loadline

A vessel is automatically considered unseaworthy if she puts to sea with a freeboard less than that permitted. Masters should be made aware of the fact that if a vessel is overloaded the P&I cover may well be invalidated.

Reductions in Freeboard
The Club is aware of instances whereby the freeboard of a vessel has been reduced (with the agreement of the local Authorities) because it is trading in coastal or local waters. If a reduction is being considered then it is imperative that a study of the vessel’s revised stability conditions is carried out by a Naval Architect to ensure they still comply with the regulations. A reduction in freeboard to permit a greater cargo carrying capacity for the vessel will result in a loss of reserve buoyancy and this consequently will reduce the dynamic stability of the vessel and the ability to resist external forces.

Failure to Confirm Vessel's Condition

It is imperative that at all stages of a vessel’s cargo operations the vessel maintains a stability condition that complies fully with the stability criteria for that vessel. This requirement is equally important for all stages of the voyage as well, and consideration has to be taken for the consumption of fuel, water and stores and the free surface effects this might introduce. It might be necessary to ballast the vessel to compensate for these consumables being used. If this action is necessary, then the free surface effect of water being introduced into the ballast tanks must be taken into account before any ballasting operations are carried out. It is not uncommon for ballasting a tank to initially make the situation worse before an improvement in the stability condition is achieved.

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Stability: Failure to observe basic principles
Pre Load Requirements Persons responsible for loading a vessel or barge, must ensure that they are made aware of the cargo weights to be loaded and the height of their centres of gravity. This information, wherever possible should be determined before loading operations are commenced, so that a safe load sequence can be calculated beforehand and that no nasty surprises are encountered at the last minute. Notwithstanding any pressure placed on the vessel by the shore terminal, the responsibility for loading remains with the Master alone. Free Surface Effect (FSE) The free surface effect of any liquids on board has a marked impact on the vessel’s stability by reducing the effective GM (or conversely by effectively increasing the KG). Some of the claims the Club has dealt with have highlighted the fact that either no account of FSE had been taken, or if it had, the data was incorrectly applied. Ideally, ballast tanks should either be pressed up full or completely empty so there is no free surface effect to consider. However, when this is not possible, it is best practice to initially allow the maximum FSE for each and every slack tank in the stability calculations. If the stability condition is then noted to be critical for any stage of the voyage, the actual free surface moments can be applied to the calculation in order to obtain an accurate assessment of the vessel’s condition. It is essential that the FSE is always calculated and applied correctly and Masters should be given clear guidance on the Member’s requirement in this regard. It should also be borne in mind that free water on the decks has the same effect and when the stability condition is critical it can have a major impact. Estimating Centre of Gravity Masters are reminded of the need for accurate estimation of a cargo’s centre of gravity. Errors can accumulate if incorrect assumptions are made which can then compromise a vessel’s stability. Estimations should always err on the side of safety (i.e. it is better to estimate too high rather than too low). The centre of gravity of a container should always be assumed to be at mid height unless it is known to be different (some Classification Societies use 0.4 x container height). Container Heights Consideration should be given to ensuring the correct container heights are used when calculating the VCG. Whilst the actual difference between an 8’ 00”, 8’ 06”, 9’ 00” or 9’ 06” (Hi Cube) high container is not significant when considered individually, a large number of incorrect heights can have an adverse effect on the final VCG if not allowed for, particularly on smaller vessels. Container Weights The incorrect declaration of container weights is a problem encountered throughout the container shipping world and can manifest itself more in the local trades that our Members operate in rather than the main line trade. Unfortunately this problem is one that is generally outside the control of Masters and shipowners. Visually there is no means to assess the weight of a container and the Master has to take the manifest or bill of lading at face value. Because this cannot be totally relied on, it places more emphasis on the need to monitor the vessels actual drafts during loading. If discrepancies arise they can be investigated further or allowed for by assuming the worst case scenarios. Another problem with not knowing the weights of containers loaded, is the possibility that heavier units can be loaded on top of lighter ones with the subsequent reduction in stability. This problem may also occur when for the sake of economy and time, the number of container lifts are kept to a minimum and heavier containers are placed in an unsuitable location i.e. on top of lighter ones. The Club has dealt with one claim where it was found that the overall difference between actual and declared weights was 10%. In the extreme, containers declared as being empty were found weighing in excess of 20 tonnes. Although this problem can cause unacceptable situations, it is largely out of the control of the Master, however the potential for associated problems should always be kept in mind. Draft During cargo operations, it is important that the draft is observed visually, forward, aft and amidships on both sides at regular intervals and a comparison made to the calculated or expected draft. Any variances must be investigated. We have dealt with claims where little attention has been paid to the draft and vessels have subsequently been found overloaded which has contributed to a reduction in stability. Cranes and Derricks When ship’s gear is being used for cargo operations, the vessel’s centre of gravity always moves towards the weight loaded, away from a weight discharged or in the direction the weight is moved. When ship’s gear is used, the instant at which the container is clear of the deck, quay or wherever it rests, the weight is transferred to the point of suspension on the crane or derrick. As a result, the vessel’s vertical centre of gravity will be raised and moved in a direction towards the weight, effectively reducing the vessel’s stability. This can be a crucial factor during the final stages of loading and early stages of discharging when stability could be critical. Care must be taken when calculating the stability at such times and in particular, attention should be paid to the Free Surface Effect. It might be necessary to ballast the double bottom tanks in order to ensure adequate stability during the lifting operation. Overloading Following plan approval and periodic loadline surveys, all vessels are issued with a Loadline Certificate by the Flag State (or issued by a Classification Society on behalf of the Administration). This document alone is the overriding authority governing the minimum freeboard a vessel is permitted to load to. The Club has known cases whereby information from an unapproved stability manual was used for loadline purposes and this was found to be incorrect. A vessel is automatically considered unseaworthy if she puts to sea with a freeboard less than that permitted. Masters should be made aware of the fact that if a vessel is overloaded the P&I cover may well be invalidated. Reductions in Freeboard The Club is aware of instances whereby the freeboard of a vessel has been reduced (with the agreement of the local Authorities) because it is trading in coastal or local waters. If a reduction is being considered then it is imperative that a study of the vessel’s revised stability conditions is carried out by a Naval Architect to ensure they still comply with the regulations. A reduction in freeboard to permit a greater cargo carrying capacity for the vessel will result in a loss of reserve buoyancy and this consequently will reduce the dynamic stability of the vessel and the ability to resist external forces. Failure to Confirm Vessel's Condition It is imperative that at all stages of a vessel’s cargo operations the vessel maintains a stability condition that complies fully with the stability criteria for that vessel. This requirement is equally important for all stages of the voyage as well, and consideration has to be taken for the consumption of fuel, water and stores and the free surface effects this might introduce. It might be necessary to ballast the vessel to compensate for these consumables being used. If this action is necessary, then the free surface effect of water being introduced into the ballast tanks must be taken into account before any ballasting operations are carried out. It is not uncommon for ballasting a tank to initially make the situation worse before an improvement in the stability condition is achieved. Top of Page
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Failure to observe basic principles