D: Stress and Stability Flashcards
What are the intact stability requirements for Grain Ships?
1) Angle of heel due to grain shift: Max List Must be less than 12˚
2) Residual Dynamic stability (Max List – 40˚) .075 mr
3) Initial GM .30 m
4) Vessel upright before sailing.
What is Lightship?
Ligthtship is the ship there ready, before it has been loaded to do anything. Once you want it to do stuff you add fuel, crew, stores, freshwater, temporary ballast, cargo.
Determined in the inclining experiment.
The displacement (weight) of the vessel and superstructure, complete and ready for service in every respect;
including: permanent ballast, spares, lube oil, water in the boilers.
excluding: Cargo, fuel, freshwater, temporary ballast, stores, passengers crew and their effects
What is FWA?
How do you calculate it?
Fresh Water Allowance is the amount in mm the vessel will sink its mean draft (at its summer draft) when it moves from salt water density to fresh water density (or vice versa)
FWAmm = Summer Displ / 4xTPCsw
What is DWA?
How do you calculate DWA?
Dock Water Allowance (DWA) is the amount by which a ship’s mean draft changes when she moves from the sea standard salt water (SW @ 1.025 t/m3) to dock water (DW @ various t/m3).
DWA = FWA * (1.025 – density of DW) / 0.025
What is excluded in lightship weight?
Cargo, Fuel, Temporary Ballast, Stores, Personnel Passengers and their effects.
Anything that goes on an off the vessel - ie not a constant for it to sail.
What is an Angle of Loll?
- Vessel in unstable Position (-ve GM)
- External Force is applied, vessel is heeled over and develops a capsizing lever (-ve GZ / BF acting inside WF)
- Will continue to heel over until BF acts in same Vertical Plane as WF and GZ is at equilibrium.
- When inclined beyond this point, GZ is positive and vessel will develop righting moments to bring it back to an angle of loll.
What causes an angle of Loll?
G moving Above M:
1) Rise in G
- Free Surface
- Icing. or wet Timber
- Pumping out Ballast from low KG
- Use of Fuel, Lube, F.w from low KG
- Heavy lifts using ships gear
2) Reduction in M
- Loss of Buoyancy from flooding / loss of watertight integrity.
What actions to take for Angle of Loll?
1) Do not Assume it is a List (ie move weight to high side)
2) Reduce external dynamic forces - Head to Weather - Pitch instead of Roll - avoid rolling to other side and developing inertia.
3) Must crew to identify cause of loll and assist in rectifying it.
4) Determine Cause: change in G or loss of Buoyancy
5) Recalculate vessels stability - Check it is not a list - recalculate port/stbd moments
6) Check for Cargo Shift - Sound All spaces and tanks (fuel ballast LO) for free surface - Check Ballast missing or tanks are slack
7) Correct Loll by reducing vessels KG - Avoid Increasing KG initially - beware of adding new free surface effects
- D/C high weights - empty high slack tanks
- pump out bilges
- Fill low slack tanks
- If ballasting low tanks and adding free surface - chose least wide tank, smallest tank, fill quickly, one tank at a time, low tank, center tank, high tank.
8) Manually move weights below.
What is Free Surface Effect?
When liquid in a tank is not pressed up it is free to move.
When the ship is heeled to an angle of inclination a wedge of liquid transfers to low side of the ship.
The cog of liquid moves from g-g1.
The cog of vessel moves parallel and in the same direction to G-G1
This results in a reduction of the Righting Lever from GZ to G1-Z1.
Less righting moments as if there was a virtual rise in G.
What determines the magnitude of the Free Surface effect?
FSM(tm)= (LB^3×ρ_tank)/12
FSC=FSM/ΔFinal
Larger the free surface area – greater the loss of stability. Primarily the breadth of the tank. B3.L
Denser the liquid – greater the loss of stability
Greater the number of divisions – lesser the loss of stability FSM x 1/n^2
Greater the Displacement – lesser the loss of stability
What is the IMO Stability Criteria for Cargo Ships?
Initial GM 0.15
Max Righting Lever occurs 30* or highter (not less than 25)
GZ Lever >0.2m at 30 or higher
0-30* Area under GZ >0.055 meter radians
0-40* Area under GZ > .09 Meter radians
30-40* Area under GZ> .03 Meter Radians
VSMII Particulars
LOA 181 BREADTH 26 Draft 6.3 Lightship: 8,900 Deadweight: 8,000 Displacement, 16,900 Gross Tonnage 25,000 Net Tonnage 7,500
What is Deadweight?
The difference between full load displacement and lightship displacement.
It is the amount of cargo, stores, ballast, fuel, crew onboard the vessel.
Searoad Mersey II Summer Dead Weight is 8,000t
