When a vessel arrives in port after a lengthly sea passage it will have less stability than when it departed from the last port.
Fuel and Water may be consumed from below and the free surface effect caused by slacking the tanks will cause an effective reduction in GM (Metacentric height).
The minimum acceptable metacentric height on arrival, including allowance for free surface is 0.15m
The below diagram shows how a real shift of G parallel to the displaced wedge of water causes a virtual shift in G upwards as shortened GZ acts on the centreline.
When the vessel uses its own gear to lift a weight ‘g’ (the centre of gravity of the weight) shoots up to the top of the lifting gear and as a result there is a large loss of GM.
The vessel is now in a vulnerable position to engage in lifting operations.
The vessel is now tender and liable to roll wildly whilst along side. This video shows the result:
When lifting a large weight with ships gear remember to:
- Calculate the vessels liquid GM
- Read the lifting plan data on board
- Ensure the condition and certification of all equipment
- Hold proper briefing of all personnel and appoint officer in charge
- Complete a risk assessment and permit to work
- Ask all non essential personnel to go ashore
- Remove breast lines and tend moorings
- Remove passurel
- Rig large fenders at several levels on shore
- Inform port of the operation and ask them to place barriers to keep the public away.
- Warn vessels around you of your operation
- Have personnel tend the lines which steady the load
- UHF Radio communication to a minimum
If you have found that there is more to practical stability than you realised and you have further questions on any maritime matter talk to Tony for further advice or guidance.