Damage Stability Flashcards
When Dry Docking a ship must
Have a positive initial GM
Be upright
Be trimmed slightly, usually by the stern
Immediately the ship touches the
blocks aft, this denotes the start of the
critical period.
The upthrust afforded by the blocks at the stern is termed
P Force
After settling on the blocks fore and aft, water continues to be pumped from the
dock and the draught reduces at the same rate forward and aft. The upthrust P became?
uniformly distributed along the ship’s length
When the dock becomes nearly empty and the ship is fully dry,
the
Upthrust P = Ship’s Displacement
used to accommodate ships while they
are fitting out, loading or unloading
Wet docks
used to enable the ship’s bottom
and underwater fittings to be inspected and worked on.
Dry (graving) docks
If tidal, there must be sufficient depth at low tide to enable the ship to remain afloat.
Otherwise, it must be checked that the ship can be allowed on the dock floor without damage
Wet docks
Essentially large holes in the ground, lined with
masonry, and provided with a means to close off the entrance once the ship is in the dock so that the water can be pumped out.
Dry Docks
Example of Dry Docks
Graving Dock
Floating Dock
Shiplifts
A profile indicating points to which shore supplies of electricity, hydraulic power, cooling water and so on can be run. Ideally these will be aligned with the corresponding shore supply positions
Deck plans
Docking Plan
Sections of the ship at which breast shores can be set up, usually at transverse bulkheads where the hull will be better able to take the forces exerted by the shores. Details of projections that might foul the dock entrance or the blocks. For instance, the propeller may project below the line of the keel and bilge keels must be allowed for.
Deck plans
Ranging from small docks with a lift capacity of less than 500 tonnes to the ones capable of lifting ships up to 100,000 tonnes
Can be taken to ports/harbours which have no graving dock facilities.
Can be heeled and trimmed to match a damaged ship’s condition and provide partial support while assessments are made of the damage
Floating Dock
U-shaped box structure with side walls mounted on a base pontoon. The large part of the structure is devoted to ballast tanks which are free flooded to sink the dock so that the ship can be moved into the correct position
within the dock.
The dock, with the ship, is then raised by carefully controlled pumping out of the ballast tanks.
The dock stability, transverse and longitudinal, is high when it is at its operating freeboard with the deck of the pontoon above the water level.
Floating Dock
Devices providing a means of lifting ships vertically out of the water to a level where they can be worked on.
Shiplifts
Main Elements of Shiplift
An articulated steel platform, generally wood-decked arranged for end on or longitudinal transfer
Wire rope hoists along each side of the platform, operated by constant speed electric
motors
Load-monitoring system to ensure a proper distribution of loads so as not to cause damage to the ship or the platform
Cradle configured to suit the ship’s hull form
Ships are not intended to ground. When they do, the hull and underwater fittings may be damaged.
Grounding
Extent of Damage depend upon number of Factors
Nature of the seabed
Speed and angle of impact
Sea state and tide at the time of grounding and up until the ship can be refloated
Area of ship’s hull which impacts the seabed
A more serious situation arises if the seabed is rocky and the ship’s outer bottom is punctured allowing water to enter and leading to further stability changes and structural damage.
Grounding on a Rock
Main parameters involved in Grounding of a Rock
Grounding location and width of damage transversely
Height of rock penetration
Shape of rock
defined in terms of areas of the outer bottom (OB) and inner bottom (IB)
Grounding Damage Index (GDI)
produced by plotting the ratio of ultimate longitudinal strengths fro the damaged and intact ships against the GDI
residual strength/damage index (R-D) diagram
All types of ships and boats are subject to the — if they — whether by
risk of sinking, lose their watertight integrity
Collision, grounding and internal accident such as explosion
Formula for grounding on a Rock
Formula of GDI
The most effective protection is provided by —- by means of —-
internal subdivision , watertight transverse and/or longitudinal bulkheads, double bottoms and watertight flats
Two principal consequences for flooding of a ship’s hull
Loss of buoyancy and change of trim - sinking or foundering
Loss of transverse stability or build-up of such an upsetting moment - capsizing
has a strong influence on probability of survival.
The length and depth of damage and its location relative to transverse bulkheads
specify assumptions to be made regarding extent of damage and indicate how each standard deals with the question of possible damage on a bulkhead.
International subdivision standards
The results indicated that damage may vary from
1 or 2m to over 30m (100ft) in longitudinal extent
the draft will change so that the displacement of the remaining unflooded part of the ship
is equal to the displacement of the ship before damage less the weight of any liquids which were in the space opened to the sea
Change of Draft
Effects of Flooding
Change of Draft
Change of trim
Heel
Change of Stability
Change of Freeboard
Loss of Ship
the ship will trim until the center of buoyancy of the remaining unflooded part of the ship
lies in a transverse plane through the ship’s center of gravity and perpendicular to the equilibrium waterline.
Change of Trim
if the flooded space is unsymmetrical with respect to the centerline, the ship will heel until the center of buoyancy of the remaining unflooded part of the ship lies in a fore-and-aft plane through the ship’s center of gravity and perpendicular to the equilibrium waterline
If the GM is the flooded condition is NEAGTIVE, the flooded ship will be unstable in the upright condition
Heel
Initial Metacentric Height Formula
GM = KB + BM – KG
flooding changes both the transverse and longitudinal stability
Change of Stability
usually increases the transverse moment of inertia of the
undamaged waterplane, and vice versa.
Trim by the stern
the increase in draft after flooding
results in a decrease in the amount of freeboard. Even though the residual GM may be positive, if the freeboard is minimal and the waterline is closed to the deck
edge, submerging the deck edge at small angles of heel greatly reduces the range of positive righting arm (GZ), and leaves the vessel vulnerable to the forces of wind and sea.
Change of Freeboard
where changes in draft, trim and/or heel
necessary to attain stable equilibrium are such as to immerse non-watertight portions of a ship, equilibrium will not be reached because of progressive flooding and the ship will sink either with or without capsizing.
Loss of ship
maximum portion of the length, having its center at the point in question, that can be symmetrically flooded at the prescribed permeability, without immersing the margin line, which is generally — below the
top of the bulkhead deck at the side.
Floodable length, 7.5CM (3INCH)
to decrease both sinkage and trim and
therefore to increase the length which may be flooded in so far as sinkage and trim are concerned
intact buoyancy
calculation of the related changes in draft, trim, heel, and stability as a result of damage to one or more specific compartments of a ship
Calculations of the intact stability and buoyancy necessary to attain any particular assumed equilibrium damaged condition, as well as to the stability characteristics in that damaged
condition
Damage stability
BONUS
The points on the floodable length curve are calculated for the actual lines of the ship, using Equations 2 and 3 to determine the volume and location of the flooding water that would immerse the ship to the margin line.
Direct Method of Calculation
proposed the method of Floodable Length Calculation
Dipl. Ing. F. Shirokauer
On a profile drawing showing the margin line and a number of transverse stations, Bonjean curves are plotted from a low draft to the margin line
Direct Method of Calculation
used to identify the points of the
floodable length curve
Trial and Error Method
Well-known and widely-used method of floodable length calculation
Undoubtedly easy to comprehend and suitable for manual calculation
BUT, this is often to be found tedious for manual calculation owing to the involvement of assumption and/or “trial and error” approach and not very simple to write computer programs
Shirokauer Method
Is calculated, inclusive of allowable permeabilities, either in order to place subdivision bulkheads or verify their compliance.
The ability of a vessel to survive flooding is determined by curves of floodable length.
Floodable Length Curves
These curves can be plotted very inexpensively
Traditionally, a ship is divided longitudinally into a number of watertight compartments to restrict the flooding to one or more compartments in case of damage.
This prevents progressive flooding (i.e. flooding across the entire ship’s length in case of a damage at any location).
The compartmentation is done by means of transverse watertight bulkheads.
Floodable Length Curves
Calculating the lost buoyancy due to a compartment/s being opened to the sea, and equating that lost buoyancy and its moments to the buoyancy gain and moments accompanying sinkage, trim, and heel of the remaining intact
part of the ship.
Lost Buoyancy Method
Calculating the flooding water up to this waterline, and subtract it and its moments from the total displacement and moments up to this waterline in order to obtain a corresponding before-damage condition
Trim-Line Added-Weight Metho
Calculating the flooding water up to this waterline, and subtract it and its
moments from the total displacement and moments up to this waterline in
order to obtain a corresponding before-damage condition
Trim-Line Added-Weight Method
A vessel’s displacement in the damaged condition is equal to its initial undamaged displacement less the weight of any liquids which were in breached tanks before damage
Lost Buoyancy Method
Not only is the virtual displacement different from the initial displacement, but the GM has a different meaning – since both KG and KM are different. However, the product of displacement and GM should remain unchanged.
Added Weight Method
SHCP divides the length of the hull into a user-specified number of equal intervals (minimum length LBP/40), and taking each of these points in turn as a center of damaged length, calculates by an iterative process the resulting equilibrium draft and trim, varying the damaged length until the equilibrium waterline just touches the margin line
Floodable Length
Most widely used program in calculating the
subdivision and damage stability
Ship Hull Characteristics Program (SHCP)
Objective is to calculate the righting arm curves for the ship after damage and flooding of various compartments and combinations of compartment
Damage Stability
Damage lengths determined represent points on the floodable length curve.
Plotter routines are available which quickly plot
curves of floodable length at any number of varied permeabilities.
Floodable Length
- The correction of the righting arm values for the actual KG of the ship versus the pole coordinate.
- The plotting of the corrected righting arm curve and a
determination of the residual heeling angle after
damage in the equilibrium condition. - The graphical determination from the righting arm curve of the residual GM at the equilibrium angle.
- The plotting of draft and trim against heeling angles and the determination of final draft and trim at the equilibrium heeling angle.
- The manual check of the final equilibrium waterline against the lines plan to determine minimum freeboard.
Damage Stability – Residual GM, Heel, and Freeboard
Where the survival criteria call for minimum range and amplitude of righting arms and minimum values of righting energy – areas under the righting arm curve, a plot of righting arms from the computer printout provides all the information required to determine compliance or non-compliance with the criterion, except for the location of any point of down flooding, which must be checked manually
Damage Stability – Residual Range and Amplitude of Righting Arm
