Stability Flashcards
What causes neutral stability?
No GM, vessel will remain at whichever angle it is diplaced
How does the free surface effect affect GM
Fluid will move to the downhill side, moving G to G1
Where a vertical line from G1 cuts the centre line, is the new Gv
or virtual G, This reduces distance from G to M
And reduces the righting moment from Gv to B
called the vitual raise of G
When a weight is picked up by a crane, how is G affected?
As soon as the weight lifts, its gravity acts on the head of the crane. Moving G up and off centre
What is a list?
An incline caused by forces within the vessel:
loading of cargo, lifting tenders, uneven fuel/water distribution
To correct, add weight/pump to the high side
What is Heel?
An inlince caused by external forces: eg wind
What is Loll?
When the vessel is unstable when upright
Achieves some small GM/stability when on one side or the other
if an external force changes this state, it will float at the same angle on the other side
How to fix loll?
Lower the centre of gravity, starting on the low side
Press tanks on the low side, initally making the loll worse, but then creating a list
And/Or remove weight from up high
Drydocking, preparations to be completed:
*Normal checklist for dep/arrival to be done, good briefing and checks of equipment before the critical manouever
* Stow all hatches and beams to ensure continuity of strength throughout the vessel
*Secure all derricks and cranes
*Remove as much free surface as possible, press/empty tanks
*stability calclations taking into the rise of G when taking the blocks
*Communicate to the Port any projections from the hull as per the drydock plan
* Sound all tanks once the vessel has taken the blocks, to ensure a similar state when leaving
* Disable toilets before entering
*Adequate fenders and ‘Dock shores’ are adequately positioned
* Inform shipyard of extra dock shores to take account of cargo or extra stresses aboard
*Obtain Fire Main, electricity, sanitation connections as soon as possible
What is on the Docking Plan?
Profile, plan drawings of the hull.
Positions of frames and stringers, so that Dock shores/blocks can be lined up with them
What is the critical moment in dry docking?
why does it occur?
What should be done prior?
Just before the Vessel takes the blocks
As it touches the blocks, the bouyancy is transfered from the water to the block, acting as an upthrust on the hull.
Causing a virtual rise of G!
*Check that the vessel has an adequate GM to allo the rise of G *All slack tanks should be pressed or pumped out
* The vessel should not be trimmed too much by the stern
What should be checked while the vessel is out of the water?
External condition of hull and fittings
Clean shell plate
cracking at welds or dicontinuities
Areas prone to vibration or slamming
dissimilar metals, props, valves
sacrificial anodes
grids and inlets/outlets
Propellers for pitting, bearings, glands
Preparations before refloating?
All bottom plugs are in, watertight
All underwater repairs completed and fittings replaced
All valves fitted and closed
Hull painted and anodes refitted
All necessary surveys completed
Sufficient crew on board,, briefed, deck power available, mooring lines out
Stability acceptable and the same as on arrival
Stability extracts from LYC
Righting lever should be?
When should max GZ occur?
What should be the initial GM?
The righting lever (GZ) should be at least 0.2m at an angle of heel of 30º or greater
Max GZ should preferably occur at an angle exceeding 30º, but not less than 25º
After correction for Free Surface, the Initial GM should not be less than 0.15M
LYC, Damage stability, What should the watertight bulkheads achieve?
Should be so arranged, that the free flooding of one compartment, will cause the vessels waterline to stay more than 75mm below the freeboard deck
In this damaged condition, there should be enough residual stability, that the angle of equilibrium does not exceed 7º from upright
How do we compensate for bad weather?
Inform Master, ECR, interior
Consider re-routing
Verify position
Update weather report
plot the position of the storm often
Engage manual steering
Reduce Speed
Check vessel security
Watertight integrity
Crew Safety
