Damage Control TrainingStability and Buoyancy Lessons
Damage Control TrainingLESSON TOPIC: 4.5 TITLE: DOCKING/UNDOCKING AND GROUNDING/STRANDING
Contact periods allotted this LESSON TOPIC:
Classroom: 2.5 Test: 0.0
Trainer: 0.0 Total: 2.5
MEDIA: Classroom lecture with visual media
TERMINAL OBJECTIVES:
6.0 EVALUATE shipboard stability by evaluating weight and moment considerations. (JTI 3.2.1, 6.0, 6.1, 6.2)
ENABLING OBJECTIVES:
6.25 DESCRIBE initial actions required to preserve stability during an unintentional grounding with respect to ballasting, weight additions, weight shifts, and jettisoning.
6.26 CALCULATE the effect on the ship's center of gravity from docking, beaching, or grounding.
6.27 DESCRIBE the hull stresses created and the appropriate actions to alleviate them when docking, beaching, or grounding.
6.28 DESCRIBE and CALCULATE critical draft.
6.29 DESCRIBE the contents and usage of the Docking Plan, Hull History, and Hull Penetrations Drawing when planning a drydocking.
6.30 DESCRIBE the Docking Master's responsibilities to the ship, and the crew's responsibilities to the Docking Master regarding the addition, removal, or movement of weights while in drydock.
6.31 DESCRIBE the problems associated with the unavailability of the ship's firemain system while in the drydock.
6.32 STATE various compartments which must be sounded or observed during docking and undocking.
DRYDOCKING
Situations in which drydocking may be required for your vessel:
OVERHAUL - Scheduled overhauls as established by CNO. The Fleet Modernization Program (FMP) has extended the overhaul cycle which varies from ship to ship, sometimes as long as 7 years.
EMERGENCIES - Serious hull damage following a collision, grounding, or battle damage. Often necessary to prevent the ship from sinking.
REPAIRS TO UNDERWATER FITTINGS - Any underwater work beyond the capacity of divers.
REMOVE FOULING OF THE HULL - Marine growth resulting in loss of speed, greater fuel consumption, and reduced plant efficiency. Due to funding, this is now the least common reason for drydocking. Navy Divers have the capability to clean the hull while the ship is still afloat.
6 MONTHS PRIOR TO DOCKING
1. Remind your CPOs that docking will be a good opportunity to overhaul or replace the skin valves in your division’s compartments.
2. Order any replacements for skin valves, be sure to get requisition numbers from Supply.
3. Route a memo to the MPA, AUX-O, ASWO, and WEPS so that they can do the same, but make it clear that they will be responsible for obtaining and replacing their own skin valves.
4. Ensure all jobs required to be done to your systems and gear are in the ship’s CSMP file so that they will be picked up in the contract.
DOCKING PLANNING CONFERENCE
The DCA is responsible to ensure the following services are written into the contract:
60 Hz, 450 VAC
250 VDC
CHT Connections
LP Air
Firemain
Sea water service for diesel or A/C plant
DOCKING ARRANGEMENTS
All details are worked out in advance by the Docking Master, SUPSHIPS representative, and the Commanding Officer. Although the following details may not necessarily be your responsibility, they are considerations for docking:
1. Time and date of docking
2. Tugs and pilot to be used
3. Whether bow or stern enter the dock first
4. Proper conditions of list and trim
5. Handling of lines
6. Record of tank soundings before the ship is drydocked
7. Gangways to be used
8. Utilities to be furnished to the ship, such as electric power, steam, and water
9. Sanitary services to be provided
10. Garbage and refuse disposal facilities needed
11. Drydock safety precautions
12. Pumping plans or other instructions or operating directives for ballasting/deballasting floating drydock with or without ship in basin.
DOCKING INFORMATION
The Commanding Officer shall furnish the Docking Master or SUPSHIPS representative with the following information:
1. Place and date of last docking
2. Last docking position
3. Date and file number of last docking report
4. Number of days underway since last docking
5. General itinerary of ship movements (if not classified)
6. Paint history for last complete painting
7. History of touch-up painting
8. Ship weight distribution (including tank sounding
report)
9. Offload supplies and hazardous stores
10. Lock screws in drydock position
11. Have 0° list and no excessive trim as per NSTM 997
PRIOR TO DOCKING
1. Ensure Dry Docking Bill is completed.
(Details in OPNAVIST 3120.32A SORM pg. 6-65)
a. Provide last plan to Docking Officer
b. Ship has no List
c. Ship has less than 1% Trim
d. Retract all moveable hull appendages
e. Minimize Free Surface Effect - all tanks full or empty
f. Deliver list of all hull fittings below the waterline to the Docking Officer.
2. Hull Board will meets prior to both docking and undocking
a. Hull Board members - CHENG / 1st LT / DCA / OPS / ASWO
b. Review Docking plan, Hull History, and Hull Penetrations Drawings
DOCKING
1. Responsibility for the ship shifts from the Commanding Officer to the Docking Officer when the first part of the ship crosses the plane of the drydock sill.
2. Once the ship is positioned in drydock, dewatering of the dock begins. As the ship just touches down on the blocks, pumping is stopped. Divers will verify that the ship is properly resting on the blocks, and that the blocks are in the correct location. Upon verification, dewatering will continue.
3. When the dock is pumped dry, members of the hull board conduct an inspection with the Docking Officer.
a. Ensure ship is positioned properly in the dock
b. Ensure all shores in place
c. Note condition of propellers, rudders, overboards, intakes, and other projections
d. Note condition of zincs/cathodic protection anodes
e. Note details of any known or observed damage
4. NSTM 997 Section 2.11 requires the Docking Master to ensure adequate shoring and side blocking is installed to resist earthquake or hurricane forces.
WHILE IN DRYDOCK
1. DCA will maintain Dry Weight Log, a log of all weight shifts, additions, and removals in excess of 500 lbs.
2. Ensure all removed skin valves are replaced with blank flanges and that no liquids are discharged to the dock without consent of the Docking Officer.
UNDOCKING
1. Prior to undocking, the Hull Board will:
a. Inspect compartments and tanks below the waterline to verify tightness.
b. Ensure all valves below the waterline are secured.
c. Thoroughly inspect hull and projections.
d. Inspect drydock for chemicals or debris which might pollute the environment, clog intakes, or cause other damage as the ship is refloated.
2. The following spaces are continuously checked for flooding as the ship is refloated:
a. Spaces in contact with the keel and side blocks
b. Tanks and voids
c. Any space with external hull fittings
STABILITY CONSIDERATIONS
When a ship is drydocked or aground, there is a profound effect on stability. As the water level decreases, the keel will rest on the blocks or sea floor. A percentage of the ship’s displacement is now supported by these objects. Stability is affected as if removing weight from that point of contact. When weight is removed from the keel, there is a virtual rise in the ship’s center of gravity.
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1. As the waterline moves down, the center of buoyancy moves down. 2. As buoyancy moves down, the ship’s metacenter moves up. 3. As the ship’s keel rests on the blocks (a weight removal low), gravity moves up. |
Since the center of gravity always rises faster than the metacenter, the two stability points will eventually be in the same position. This results in neutral stability, where no righting arms are being produced. The draft where GM = 0 is called the ship's critical draft.
Calculating the ship’s critical draft is very important. When in drydock, dewatering of the dock stops just before critical draft is reached so it can be verified that the ship is properly supported by the side blocks. When aground, knowing the range of tide will determine if the ship might reach it’s critical draft.
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CALCULATING CRITICAL DRAFT
When aground or in drydock, contact pressure is applied at the keel. This has the same effect as removing weight at the point of contact. Looking at the KG1 equation:
Since weight is being removed at the keel, kg = 0 and (w x kg) = 0. The equation changes slightly:
Where, KGV = the new height of "G" after going aground
W0 = ship's original displacement (before grounding)
KG0 = ship's KG before grounding
WA = apparent displacement after grounding (from draft readings)
Using the Stranding Calculation Sheet and the draft diagram and functions of form, the ship's critical draft can be calculated.
a. Displacement b. KM 3. Solve for KGV using the equation. 4. GM = KM - KGV 5. If GM > 0, choose a draft 3 inches below the last draft and complete steps 2 through 4 until GM < 0. 6. Plot GM values (top) for drafts (left) on graph. Curve will cross zero at critical draft. |
EXAMPLE
: The ship is aground at high tide and the range of tide is 2 feet. Will the ship reach it's critical draft, and if so, at what draft? Initial conditions: KG0 = 18.51 ft Draft = 15’3"The Stranding Calculation Sheet will look something like this:
|
FT |
IN |
WA |
KM |
KGV |
GVM |
|
14 |
3 |
3400 |
22.35 |
20.42 |
1.93 |
|
14 |
0 |
3300 |
22.40 |
21.03 |
1.37 |
|
13 |
9 |
3200 |
22.45 |
21.69 |
0.76 |
|
13 |
6 |
3100 |
22.50 |
22.39 |
0.11 |
|
13 |
3 |
3000 |
22.60 |
23.14 |
-0.54 |
Using the graph to plot GVM values, critical draft is 13’5".
GROUNDING / STRANDING
In most stranding cases, the following considerations will ordinarily constitute good procedure:
1. Attempts SHOULD NOT be made to refloat the ship under her own power if wind and sea conditions indicate the possibility of the ship working harder aground, pounding, or broaching to sea.
2. Anchors to seaward should be quickly laid if possible to prevent the ship from working further ashore.
3. The ship should be weighted down, not lightened, in an effort to help keep the ship from working harder on the beach, and secondly, to prevent damage caused by working and pounding of the ship on the bottom.
INITIAL ACTIONS
When a ship goes aground, the initial reaction on the bridge is to back down using the engines. Before attempts are made, consideration should be given to:
- Depth of Water
- Sea Floor Composition
- Possible Damage to Propellers and Hull
Surface Ship Survivability, NWP 3-20.31, paragraph 5.5.1 states, "If propellers are reversed and there is no tendency of the ship to back away, no further attempts to move the ship by means of the screws should be made."
The primary reasons we do not continue to use propulsion in a grounding situation:
1. The ship's screws become less effective in shallow water and the ship may squat. Propellers may also be damaged due to contact with the sea floor.
2. Propeller wash will drive silt and/or bottom aggregate in and around the hull, possibly causing a suction when the ship is pulled from its location.
3. This silt and aggregate can be sucked into sea chests, fouling necessary cooling equipment required to maintain the ship's propulsion systems.
WEIGHT DOWN THE SHIP
If attempts at backing down fail, the ship should be weighted down to firmly fix the hull in position. This is especially important if the tide is expected to rise and adverse sea conditions exist; tides or heavy surf may drive the ship further aground or cause it to broach. Weighting down is accomplished by ballasting tanks and if necessary, flooding low compartments.
INVESTIGATE FOR DAMAGE
After the ship has been weighed down, a careful investigation should be made to sound all voids, check fuel tanks for leakage and examine the interior of the hull for signs of structural damage.
DETERMINE TONS AGROUND
Determine displacement prior to grounding using daily draft report and Draft Diagram and Functions of Form. Read drafts after grounding and determine new displacement. The difference is the amount of tons aground.
CRITICAL DRAFT CALCULATIONS
Use the Stranding Calculation Sheet to calculate critical draft. If the tide is receding, determine whether or not stability will become critical. If so, lower the ship’s center of gravity by adding more weight low, jettisoning weight high, or shifting weight down.
BROACHING
Ground tackle should be rigged and kedge anchors laid seaward as quickly as possible. This will help to keep the ship from broaching. When a ship is broached, scouring occurs. Sand and gravel under the hull is washed away by the action of the surf. Currents produced by the swells breaking against the ship sweep around the bow and stern with great velocity. These currents remove sea floor material from under the ship and build them up in a sand spit amidships on the inboard side. As the material is cut away from under the ship, an extreme hogging condition results that will eventually cause failure of the hull.
TAKE SOUNDINGS
The ship's boat should be launched to take soundings around the hull, determining the slope and nature of the bottom. These soundings should be continued in the direction toward which the ship is to be hauled off, in order to locate rock formations, coral ledges, or other under water obstructions. Currents which may effect the ship as she comes off should be noted.
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CHECK FOR HOGGING/SAGGING STRESSES
The drafts are also checked to ensure that the ship is neither hogging nor sagging. If the ship is aground at one end, sagging stresses are increased, resulting in the need to remove weight amidships and relocate it at the bow and stern. If aground on a ledge or pinnacle amidships, hogging stresses are increased. Weight should be removed from the bow/stern and relocated amidships. Irregular rock or coral formations or sharp changes in gradient produce concentrated pressures that can crush hull plating and result in flooding. This damage can be intensified if the hull works or shifts position.
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Hogging Situation Main deck: In Tension Keel: In Compression To relieve these stresses: - Jettison FWD and AFT - Ballast Amidships |
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Sagging Situation Main deck: In Compression Keel: In Tension
To relieve these stresses: - Ballast FWD and AFT - Jettison Amidships |
SALVAGE ASSISTANCE
A request for salvage assistance should be made immediately, not delayed while refloating attempts are made. Early mobilization and dispatch of salvage assistance might mean the difference between success and failure of the salvage operation. When a request for salvage assistance is made, the following information should be provided:
- An accurate position of the grounding site, including latitude and longitude, applicable chart numbers, and means of fixing the position.
- Ship’s draft at last port and estimated time of stranding.
- Drafts forward, amidships, and aft, following stranding with time taken and the state of tide.
- Soundings along the ship from bow to stern, corrected to the datum of the chart area.
- Course and speed at the time of grounding.
- Ship's heading after grounding with details of changes.
- Liveliness of the ship.
- Weather conditions, to include: wind direction and velocity, current weather at the grounding site, and any weather forecasts.
- Sea and ocean current conditions, to include: direction and height of seas and swells.
- Extent and type of damage to the ship.
- Location of grounding points and estimated ground reaction.
- Type of sea floor at the grounding site.
- Status of ship's machinery.
- Ship's cargo list or manifest.
- Amount and location of known hazardous materials.
- Help available at the scene or in the area, such as tugs, large boats, bulldozers, cranes, etc.
