LESSON TOPIC: 4.6 TITLE: HULL DAMAGE ASSESSMENT/ LIST IN STABILITY
Contact periods allotted this LESSON TOPIC:
Classroom: 2.0 Test: 0.0
Trainer: 0.0 Total: 2.0
MEDIA: Classroom lecture with visual media
TERMINAL OBJECTIVES:
6.0 EVALUATE shipboard stability by analyzing weight and moment considerations. (JTI 3.2.1, 6.0, 6.1, 6.2)
ENABLING OBJECTIVES:
6.33 DESCRIBE the relationship between metacentric height and roll period.
6.34 DEFINE floodable length, angle of maximum roll, angle of semi-permanent heel, and reserve dynamic stability.
6.35 DESCRIBE the three possible causes for list after damage and the indicators for each.
6.36 Given a DC book section II(a), CALCULATE danger angle.
6.37 DESCRIBE the survivability design considerations outlined in NAVSEA DDS-079-1 and NAVSEAINST 9096.3(series) which enable the ship to withstand external, dynamic forces and damage.
6.38 Given a written summary of damage, a description of the ships behavior and list, EXPLAIN the cause of the list and the procedures necessary to eliminate or reduce the list.
6.39 DESCRIBE the intended method of disabling a ship as it applies to: homing influence torpedoes, impact (WWII) torpedoes, anti-ship missiles, influence and contact mines.
6.40 Given a ship's beam and average roll period in calm water, use the Roll period equation to CALCULATE the ship's metacentric height.
6.41 DRAW a righting moment curve with a heeling moment curve that identifies angle of maximum roll, angle of semi-permanent heel, and reserve dynamic stability remaining.
SITUATION ESTIMATE AFTER DAMAGE
Two possible situations exist following the infliction of damage:
Damage is so extensive that the ship never stops listing, trimming, or settling, and goes down within minutes
or
The ship stops heeling, changing trim, and settling shortly after initial damage.
Experience has shown that the loss of ships lasting several hours after damage and then sinking is directly traceable to progressive flooding.
CAUSES OF LOSS
1. Flooding
a. Bodily Sinkage - The addition of weight (flooding water) has caused the force of gravity to exceed the force of buoyancy.
b. Capsizing - Loss of transverse stability: the inclining moment exceeds the righting moment (IM > RM) and the ship rolls over.
c. Plunging - Loss of longitudinal stability: the trimming moment exceeds the longitudinal righting moment (TM > RM) and the ship sinks by the bow or stern.
2. Breaking Up - Caused by strength member failure
a. Excessive stresses from hull damage or improper distribution of liquid (failure to follow the ship’s Liquid Loading Instructions (LLI).
b. Violent longitudinal whipping (ex: grounding at high speed, underwater detonation.)
c. Fire: Extreme thermal effects on structural members.
METACENTRIC HEIGHT AND STABILITY
In lesson 4.01 the relationship between Metacentric Height (GM) and righting arms (GZ) was discussed. For small angles of heel (0°-7/10°) GZ is proportional to GM. Therefore, GM can be used as a representation of initial righting arms. These basic rules apply:
If GM is large, the ship has large righting arms and will have stiff, fast rolls.
If GM is small, the ship has small righting arms and will have tender, slow rolls.
If GM is very small, the ship is apt to hang at the end of each roll before starting upright.
If GM is slightly negative, the ship will loll (stay heeled at the angle of inclination where righting and upsetting forces are equal) and flop from side to side.
If GM is negative, the ship will capsize when inclined.
SHIP’S ROLL PERIOD
The ship's roll period is directly related to the Metacentric Height of your vessel. Should you desire an estimate of your Metacentric Height, the following method works well when underway.
The ship must be making way, approximately 8-10 knots on a relatively calm day. Have the OOD cycle the rudder, from right full to left full or vice versa. Using a stop watch, determine how long it takes to go from 0° inclination to full inclination to starboard, back to 0° inclination, to full inclination to port and back to 0° inclination. This is the ship’s roll period. Repeat if necessary to ensure that the time is as accurate as possible.
Where:
B = Maximum Beam of the Ship (FT)
T = Period of Roll (Seconds)
GM = Metacentric Height (FT)
LIST IN STABILITY
HEEL - A heel is a "temporary" inclination of a ship, caused by outside forces such as winds, waves, or during a ship's turn.
LIST - A list is a "permanent" inclination of a ship, caused by one of the following conditions:
1. Off-Center Weight (99% of the time) 2. Negative GM (1% of the time) 3. Combination of Off-Center Weight and -GM |
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LIST CAUSED BY OFF-CENTER WEIGHT
Whenever the center of gravity of a ship is moved off centerline, the ship will create an inclining moment. If no external forces are present, the ship will assume a list. This was discussed in section 4.02 where the cosine correction was applied to the static stability curve.
Righting arms are significantly reduced when the center of gravity is off-centerline. This reduction results in the ship having a negative righting arm (upsetting arm) at 0° inclination. Where the corrected curve crosses the horizontal axis, positive righting arms are achieved. This is the angle of list, the ship will roll about this point.
Possible Causes
1. Unequal distribution of weight on either side of center line due to loading.
2. Shift of weight transversely.
3. Addition or removal of weight unsymmetrically about center line.
How to Recognize
1. Vessel assumes a permanent list to one side only.
2. Vessel has an average roll period about this permanent list.
3. If known weight is in excess on one side.
4. If a positive GM is known to exist.
Corrective Measures
1. Determine Cause of list first.
2. Shift weight transversely to higher side
3. Add weight to high side or remove weight from low side.
*** NEVER attempt to correct list by the above methods unless you are certain that GM is positive ***
LIST CAUSED BY NEGATIVE GM
When a ship's center of gravity moves vertically upwards and slightly above the Metacenter, the ship will develop a list (or possibly capsize.) The vessel may also "flop" over, developing the same list to the other side.
Possible Causes
1. Removal of low weight
2. Addition of high weight (ice, volcanic ash)
3. Moving weight upward
4. Free Surface Effect
5. Free Communication Effect
How to Recognize
1. Vessel will not remain upright and will assume a list to either port or starboard.
2. Vessel "flops" to port or starboard.
3. Vessel will have a very long, slow roll period about the angle of list.
4. A small GM is known to exist plus any of the above.
Corrective Measures
1. Eliminate Free Surface and Free Communication Effects.
2. Add low weight symmetrically about centerline.
3. Remove high weight symmetrically.
4. Shift weight down symmetrically.
LIST CAUSED BY OFF-CENTER WEIGHT AND NEGATIVE GM
The vessel's stability is reduced by both an increase in the height of the center of gravity and movement from centerline. A negative GM condition exists, represented by the "uncorrected" curve. An off-center weight, represented by the cosine curve, is added and a larger list develops.
Possible Causes
1. A combination of the previous causes of list.
How to Recognize
1. Vessel will assume a permanent list either port or starboard (vessel will not flop).
2. Very slow roll period about this permanent list.
3. The known off-center weight isn’t proportional to the ship’s list.
Corrective Measures
1. Correct Negative GM first.
a. Eliminate Free Surface and Free Communication Effects.
b. Shift weight down, add weight low, or jettison weight high.
2. Correct for Gravity Off Centerline
a. Add weight to higher side
b. Remove weight from lower side
c. Shift weight to higher side
*** ALWAYS correct Negative GM prior to shifting weights transversely ***
FLOODABLE LENGTH
Bulkhead Deck - The uppermost deck to which the transverse watertight bulkheads extend (usually the Damage Control deck.)
Margin Line - An imaginary waterline located three inches below the Bulkhead Deck.
Floodable Length - The maximum distance within the ship that can be flooded without submerging the margin line.
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FLOFFLOOD FLOODABLE LENGTH (continued)
There are two different forms that Floodable Length is discussed in the Damage Control Book.
1. The Curve of Floodable Length: To use this curve, draw a line parallel to the forward line (60^{o} to vertical) and place it at the baseline of the forward most bulkhead where flooding exists. Project this line up to the Curve of Floodable Length. Draw a line parallel to the aft line (60^{o} to vertical) which intersects the Curve of Floodable Length at the same point. The baseline distance between the forward and after lines is the Floodable Length.
2. A List of Floodable Length Compartment Groups: The Damage Control Book will list all groups of consecutive compartments which constitute Floodable Length. Example, for the FFG-7:
Stem - Frame 100
Frames - 32-140
Frames - 64-180
Frames - 100-212
Frames - 140-250
Frames - 180-292
Frames - 212-328
Frames - 250-368
Frames - 292-Stern
General rule of thumb for Floodable Length:
If the ship's LBP is > 300 FT : 15% of LBP (3 spaces **)
< 300 FT : 2 spaces **
< 100 FT : 1 space **
** A space is the area between two transverse watertight bulkheads from keel to waterline, skin to skin.
Example: For a two compartment ship, flooding any two adjacent spaces will cause the ship to reach it’s Floodable Length.
DYNAMIC STABILITY
Dynamic Stability is the ship's ability to resist external heeling forces. Prior to launching any ship, it’s Dynamic Stability has been tested, and the results graphed in the Damage Control Book. As DCA, understanding these Dynamic Stability curves is very important.
The curve, called the Righting Moment curve, begins as the Righting Arm curve. A righting moment is simply a force (W_{F}) acting through some distance (GZ). Since displacement remains constant through all angles of heel, the Righting Moment curve retains the same shape as the Righting Arm curve. The vertical scale is changed by a factor of displacement.
The area under this curve is the Righting Energy the ship possesses, or the ship’s ability to right itself. Heeling Moment curves can be projected onto this curve to determine the maximum beam winds and seas the ship can withstand. A beam wind curve looks very similar to the cosine correction curve because the ship’s surface area (or sail area) decreases when the vessel is inclined.
DYNAMIC STABILITY (continued)
SEMI-PERMANENT HEEL |
Angle to which the ship will heel with a constant beam wind. |
MAXIMUM ROLL |
The largest angle to which the ship will roll in the case of an instantaneous beam wind. |
AREA A |
Area where the Heeling Moment is greater than the Righting Moment. (Energy that the wind has, but the ship does not) |
AREA B |
Area where the Righting Moment is greater than the Heeling Moment. (Energy that the ship has, but the wind does not) |
RESERVE DYNAMIC STABILITY |
Righting Energy which enables the ship to withstand any additional heeling moments. |
When a ship is exposed to heeling moments (100 kts beam wind) the ship inclines. If the heeling energy is imparted instantaneously, the ship must be able to overcome that energy or it will capsize. The ship will roll over to the angle of maximum roll, then right itself to where the righting moment equals the heeling moment (angle of semi-permanent heel). If the wind stays constant (100 kts), the ship remains at this angle of heel. Reserve Dynamic Stability is "leftover" righting energy which allows the ship overcome any additional heeling forces (usually there will be some sea state with 100 kts of wind.)
DESIGN CRITERIA
All Navy and Coast Guard surface ships are designed to withstand certain criteria. The "Law of 15's" is a rule of thumb that sets list/heel angle limits for various operating conditions. For the following situations, a ship should not list or heel past 15^{o}.
1. Design Beam Winds and Seas
2. Hanging Heavy Objects over the Side
3. Crowding of Personnel to one Side
4. List after Damage
5. Full Speed, Full Rudder Turn
6. Flooding 15% LBP (Floodable Length)
LIMITATIONS TO DESIGN CRITERIA
Should any of the following four limitations be violated, the ship will not meet it’s designed resistance to damage.
1. Do Not Submerge Limiting Draft Marks |
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2. No Abnormal Topside Weights |
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3. Follow Liquid Loading Instructions |
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4. Watertight Integrity is Maintained |
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SURVIVABILITY OF THE SHIP
If the ship lists to the Danger Angle (½ the angle of maximum righting arm) within 10-15 minutes after damage, the ship will probably capsize. If the ship lists past the angle of maximum righting arm, it will capsize. Under normal conditions (average wind and sea state,) the ship should survive damage which results in a list to the danger angle.
DCA DECISION FACTORS
Following damage, the DCA evaluates the overall situation and makes recommendations to the Commanding Officer based on the survivability of the ship. This includes:
1. Determine the corrective measures which will improve the situation.
2. If corrective measures will not improve the situation, inform the CO so he/she can decide whether to abandon ship.
Four Major Considerations
1. Ship's ability to extinguish fires and control flooding.
2. Ship's ability to reach a safe haven.
3. Ship's ability to float and remain upright.
4. Ship's ability to stay in action and repel attack.
Corrective Measures
Once initial recommendations have been given to the Commanding Officer, the DCA prioritizes the corrective measures.
Step One: Establish Flooding Boundaries.
Step Two: Dewater any space colored pink on the ship’s Flooding Effects Diagram.
Step Three: Size up the situation. Determine if stability is critical before taking further action.
There are four instances where stability is considered critical:
1. GM is very small or slightly negative
Indications of Negative GM
- Feel of the Ship:
Logy, Sluggish?
List With No Off-Center Weight?
Flopping From Side to Side?
- Large Areas of FSE or FCE?
- Large Amounts of Added High Weight?
If unsure, assume GM is negative!!
Corrective Measures for Negative GM
- Eliminate Free Surface and Free Communication Effects
- Jettison Top Side Weight
- Ballast Low Tanks
- Shift Solid Weights Down
- Restore Flooding Boundaries
2. Flooding exceeds Floodable Length
Indications that Floodable Length is exceeded
- Excessive Flooding (15% LBP)
- Minimal Freeboard
Corrective Measures for Floodable Length Exceeded
- Verify Flooding Boundaries
- Plug and Patch Damage
- De-water
3. List to the danger angle
Indications of Danger Angle List
- Ship’s inclinometer
Corrective Measures for Danger Angle List
- Determine the cause of the list
- If due to -GM, move G down
- Otherwise shift G back to centerline
4. High winds or rough seas combined with flooding damage
Indications
- The ship is damaged and adverse weather conditions are prevailing
Corrective Measures
- Repair Damage as possible
- Maneuver the ship for favorable weather conditions
Step Four: Eliminate or Reduce List
Methods to Correct List (With positive GM)
Indications
- Vessel has a list with positive GM
- Vessel has known off-center weights which correspond to the angle of list.
Corrective Measures
- Pump Out Off-Center Flooding
- Shift Liquids Transversely
- Counterflooding
- Shift Solid Weights Transversely
- Deballast Wing Tanks
Methods to Correct Excessive Trim (Greater than 1% LBP)
Corrective Measures
- Shift weight towards higher end (bow or stern); Add weight to higher end; Remove weight from lower end.
Methods to Relieve Hull Girder Stress
Indications of Excessive Hull Stresses
- Ship is in Hogging or Sagging condition as described in Section 4.5.
- Stress Fractures, Cracks, Panting Bulkheads, Sagging Decks and Stiffeners.
Corrective Measures for Hull Stresses
- For Sagging condition: Remove weight amidships and ballast fore and aft.
- For Hogging condition remove weight fore and aft, and ballast amidships.
- Shore up panting bulkheads and decks. Reinforce (where possible) cracks and areas where structural failure has occurred.
CRITICAL STABILITY
The following table is an easy reference to the four thumbrules of critical stability and actions to be taken:
Critical Thumbrule |
Actions to be Taken |
Negative GM |
FLB, Eliminate FSE/FCE, Shift Weight Down, Add Weight Low, Remove Weight High (Symmetrically) |
Flooding Exceeds Floodable Length |
FLB, De-Water, Shore Holes and Bulkheads |
List to the Danger Angle |
FLB, Determine Cause of List, Correct for -GM, Correct for Off-Center Weight |
Damage with Bad Weather |
FLB, Maneuver Ship Out of Weather, Repair Damage |