Load handling operations

System stability

What is stability?

Stability is:

• the ability of the crane and vessel combination to sustain a changing loading condition without too much heeling or turning over, and
• its ability to maintain a constant list angle.

The stability and the static and dynamic behaviour of a crane on a vessel includes:

• the floating vessel
• crane
• intended loadings.

Why is stability important?

The stability and behaviour of these parts is important for the safe operation, installation and transport of the crane on a vessel.

The crane and vessel combination is a combined system. It should be checked and approved as an individual case by persons with knowledge and experience in naval design.

If loads applied go beyond class approval of the vessel, a naval architect needs to push the structural book for class approval before operation.

There are various factors to consider for this approval, including:

• the forces and loadings that will happen during lifting operations, transport and installation of the crane on the vessel
• environmental circumstances (like wind).

Suspending a load on the vessel

Lifting a load which is placed on the vessel will change the vessel's centre of gravity. This will also change the GM value.

Lifting a load which is placed on the vessel will change the vessel's centre of gravity. This will also change the GM value.

Here are some examples showing the effects of lifting a load from the deck of a vessel:

The image below shows a vessel with a land-based mobile crane and a load resting on the deck. The centre of gravity (CoG of the three components of the vessel system add up to a combined centre of gravity (GT)). This CoG is at a height just below the vessel deck. The Metacenter is high above the vessel deck.

Land-based mobile crane and load on deck

The image below shows what happens after lifting the load free from the vessel deck. Even though the load is only lifted a few centimetres above the vessel deck, the application point of the load is now at the boom head sheaves centre.

Lifting the load free from the vessel deck

The combined centre of gravity includes the crane, load and vessel.

In the first image, the centre of gravity is just below deck. The second image shows the load being lifted. After lifting the load, the centre of gravity shifts to about 18-20 metres above deck. This change happens in a matter of seconds and causes the Metacentric height (GT-M) to drop in a similar way.

This effect is the reason why the stability of the vessel and crane system should be checked before lifting any loads.

Repositioning the suspended load to a different radius (horizontal movement) will change the horizontal position of the combined centre of gravity. This effect is shown in the image below. Moving to other radii will make the crane heel. These effects should be considered.

Placement of suspended load

Recommended minimum GM value

The actual metacentric value (GM) should be positive during the entire operation. But there is a minimum GM value that is advised. The minimum GM value depends on different factors affecting the complete system. The system refers to the vessel, crane and load.

For the use of cranes on vessels, a minimum calculated GM of 10.0m can generally be used.

What if this GM value can't be adhered to? In that case, detailed calculations of the crane and vessel combination by a naval architect or marine engineer are required.

It is recommended that a naval architect or marine engineer should verify:

• the minimum GM value, and
• the maximum vessel inclination that will occur during operations.

Environmental circumstances (like wind) should also be taken into consideration.