Loss Prevention - Stability

During investigations into claims, we have found that there have been occasions when the senior officers responsible for cargo operations were not familiar with the vessel’s stability manuals or the onboard Class approved stability/loading instrument program. Members and Masters must ensure that all personnel involved in cargo operations make themselves thoroughly familiar with the contents of the stability manual and the operating parameters therein.

Stability Requirements
The IMO has issued minimum stability criteria for different types of vessel and these criteria are taken into account at the vessel’s design stage and when calculating the data for the stability book.

Sea staff and shore personnel involved with marine operations are usually aware of the minimum permitted height for the GM and can mistakenly use this as the sole measure of a vessel’s stability. However, this is only one single criterion, and compliance with this alone is not enough to guarantee adequate stability. There are other equally or more important factors which have to be taken into account to ensure that the vessel has sufficient positive stability for the voyage. In the Club’s experience these other limitations are not always fully understood or taken into account.

Typical Curve of Statical Stabiilty

Utilising the vessel’s stability data, the Curve of Statical Stability can be drawn and from this the vessel’s dynamical stability can be determined. Dynamic stability is the ability of a vessel to resist or overcome external heeling forces and is directly proportional to the area underneath the curve of statical stability. Thus the more dynamic stability a vessel has, the greater the ability to resist external forces.

The IMO sets down minimum requirements for a vessel’s stability (which vary according to
ship type) stipulating:-
■ Area under the curve from 0 to 30 degrees
■ Area under the curve from 0 to 40 degrees or the angle at which flooding commences
■ Area under the curve from 30 to 40 degrees or the angle at which flooding commences
■ Minimum Righting Arm at 30 degrees
■ Angle from 0 degrees to maximum righting arm
■ Minimum GM at equilibrium.

When undertaking manual calculations, the GM can be calculated with relative ease but the other criteria involve long and complex calculations. To overcome this, the requirement is for the stability book to provide the Master with an easy means to obtain a quick check to ascertain whether or not the vessel’s stability complies with all the minimum requirements.

This information normally takes the form of either a table and/or a graph indicating the maximum Vertical Centre of Gravity (KG) permitted for a particular displacement. Providing the vertical centre of gravity lies within the parameters laid down in the vessel’s stability book, the vessel’s stability complies with the minimum requirements stipulated by the IMO/Flag State for that type of vessel. (Note: Standard Loading conditions are usually included in the stability book as a guidelines).

Depending on the vessel type and the naval architect, the stability information can be presented in differing formats. It is therefore important that the persons responsible for the stability of the vessel are fully familiar with the information and how it is presented for their vessel.

The following is an example of a table found within the stability book for a barge type vessel.

In this case the limiting factor is LIM3 which is at the minimum requirement for all displacements. Providing the vessel’s VCG (KG) does not exceed the stated value for the relevant displacement (interpolating as necessary), the vessel’s intact stability lies within the acceptable limits.

The following graph shows the maximum VCG versus Displacement for a ship shape vessel when the Longitudinal Centre of Buoyancy (LCB) has to be taken into consideration. Providing the vessel’s VCG lies within the graph, the stability complies with the minimum requirements.

Providing the vessel’s VCG lies within the graph, the stability complies with the minimum requirements.

In this example, a vessel with an LCB of 21m forward of midships and with a displacement of 875mt, the maximum permitted Vertical Centre of Gravity (VCG) is 3.39m

This graph shows the maximum permitted VCG against displacement for a barge. As with the previous graph, providing the vessel’s condition lies below the graph line, all the stability requirements are complied with.

This graph shows the maximum permitted VCG against displacement for a barge.

In this example, for a displacement of 3550mt, the maximum permitted VCG (KG) is 8.0m.

For some vessels, the criteria is shown relative to the vertical centre of gravity of the cargo above the main deck and not the VCG of the vessel (the vessel’s KG related to the baseline). The following is an example of this.

For some vessels, the criteria is shown relative to the vertical centre of gravity of the cargo above the main deck and not the VCG of the vessel (the vessel’s KG related to the baseline).

In this example, for a draft of 2.40m the maximum verticle centre of gravity of the cargo above the main
deck is 4.4m.

In the following graph the governing limits for area under the GZ Curve, angle of heel due to wind and minimum range of stability are plotted individually. When the information is presented in this way, confusion can exist but in every case the minimum VCG must be complied with i.e. for some drafts one criterion might govern the maximum KG and for others it might be one of the other two criteria.

In the following graph the governing limits for area under the GZ Curve, angle of heel due to wind and minimum range of stability are plotted individually.

Example
With a draft of 2.60 metres this vessel is permitted a maximum VCG of 21.0 metres.

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Stability: Lack of Understanding of Stability Criteria
During investigations into claims, we have found that there have been occasions when the senior officers responsible for cargo operations were not familiar with the vessel’s stability manuals or the onboard Class approved stability/loading instrument program. Members and Masters must ensure that all personnel involved in cargo operations make themselves thoroughly familiar with the contents of the stability manual and the operating parameters therein. Stability Requirements The IMO has issued minimum stability criteria for different types of vessel and these criteria are taken into account at the vessel’s design stage and when calculating the data for the stability book. Sea staff and shore personnel involved with marine operations are usually aware of the minimum permitted height for the GM and can mistakenly use this as the sole measure of a vessel’s stability. However, this is only one single criterion, and compliance with this alone is not enough to guarantee adequate stability. There are other equally or more important factors which have to be taken into account to ensure that the vessel has sufficient positive stability for the voyage. In the Club’s experience these other limitations are not always fully understood or taken into account. Utilising the vessel’s stability data, the Curve of Statical Stability can be drawn and from this the vessel’s dynamical stability can be determined. Dynamic stability is the ability of a vessel to resist or overcome external heeling forces and is directly proportional to the area underneath the curve of statical stability. Thus the more dynamic stability a vessel has, the greater the ability to resist external forces. The IMO sets down minimum requirements for a vessel’s stability (which vary according to ship type) stipulating:- ■ Area under the curve from 0 to 30 degrees ■ Area under the curve from 0 to 40 degrees or the angle at which flooding commences ■ Area under the curve from 30 to 40 degrees or the angle at which flooding commences ■ Minimum Righting Arm at 30 degrees ■ Angle from 0 degrees to maximum righting arm ■ Minimum GM at equilibrium. When undertaking manual calculations, the GM can be calculated with relative ease but the other criteria involve long and complex calculations. To overcome this, the requirement is for the stability book to provide the Master with an easy means to obtain a quick check to ascertain whether or not the vessel’s stability complies with all the minimum requirements. This information normally takes the form of either a table and/or a graph indicating the maximum Vertical Centre of Gravity (KG) permitted for a particular displacement. Providing the vertical centre of gravity lies within the parameters laid down in the vessel’s stability book, the vessel’s stability complies with the minimum requirements stipulated by the IMO/Flag State for that type of vessel. (Note: Standard Loading conditions are usually included in the stability book as a guidelines). Depending on the vessel type and the naval architect, the stability information can be presented in differing formats. It is therefore important that the persons responsible for the stability of the vessel are fully familiar with the information and how it is presented for their vessel. The following is an example of a table found within the stability book for a barge type vessel. In this case the limiting factor is LIM3 which is at the minimum requirement for all displacements. Providing the vessel’s VCG (KG) does not exceed the stated value for the relevant displacement (interpolating as necessary), the vessel’s intact stability lies within the acceptable limits. The following graph shows the maximum VCG versus Displacement for a ship shape vessel when the Longitudinal Centre of Buoyancy (LCB) has to be taken into consideration. Providing the vessel’s VCG lies within the graph, the stability complies with the minimum requirements. In this example, a vessel with an LCB of 21m forward of midships and with a displacement of 875mt, the maximum permitted Vertical Centre of Gravity (VCG) is 3.39m This graph shows the maximum permitted VCG against displacement for a barge. As with the previous graph, providing the vessel’s condition lies below the graph line, all the stability requirements are complied with. In this example, for a displacement of 3550mt, the maximum permitted VCG (KG) is 8.0m. For some vessels, the criteria is shown relative to the vertical centre of gravity of the cargo above the main deck and not the VCG of the vessel (the vessel’s KG related to the baseline). The following is an example of this. In this example, for a draft of 2.40m the maximum verticle centre of gravity of the cargo above the main deck is 4.4m. In the following graph the governing limits for area under the GZ Curve, angle of heel due to wind and minimum range of stability are plotted individually. When the information is presented in this way, confusion can exist but in every case the minimum VCG must be complied with i.e. for some drafts one criterion might govern the maximum KG and for others it might be one of the other two criteria. Example With a draft of 2.60 metres this vessel is permitted a maximum VCG of 21.0 metres. Top of Page
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Lack of Understanding of Stability Criteria