Dry Docking Flashcards
What should be included in a dry dock pre planning meeting with class?
- Survey status and ship information
- Survey records from previous bottom survey
- Any outstanding recommendations or known areas of damage
- Any known structural damage
- Structural repairs to be carried out
- Items to be surveyed, thickness measurements to be taken and known maintenance work
What items must be completed during scheduled dry docking?
Examination of shell plating, stern frame, rudder, sea chests and propeller
- Examine shell plating for corrosion or deterioration by ultra sonic thickness measurement
- Sea chests, gratings, sea connections and all overboard valves cleaned and examined
- Rudder, rudder pintles and rudder shaft examined
- Propellor examined and/or cleaned
- Thrusters examined and/or cleaned
What areas of shell plating are measured for thickness during survey?
Renewal survey 1 - Suspect areas only
Renewal survey 2 - Main deck plating samples
Renewal survey 3 - Transverse sections amidships, cargo hold hatch covers, Forepeak internal frames, all air pipes and ventilators
Renewal survey 4 - All main deck plating, Transverse sections, wind and water strakes (between ballast and loaded waterline), keel plates and bottom plates, forepeak and aftpeak internals
Over 20 years, most structure has been examined
What should always be included on the dry dock repair list?
- Shore provided gangways
- Shore fresh water
- Sewage lines connected to shore
- Protective coverings for internal alleyways and offices
- garbage skip with regular replacement
- Anchor cables ranged, broken, end for ended and cleaned/painted
- Gangways lifted off platforms and inspected
- Boats lifted ashore for maintenance/painting
- End for end and grease lifeboat falls/overhaul davits
- Portable toilets for shore workers
- Mud removal from double bottoms/inspections
- Clean and paint chain lockers
- galley uptakes steam cleaned
- Any galley fumigation if required
- Shore transport for crew in the yard.
- Fire main shore connection
- Shore power
- Security watch if required
- Yard crane use/clarify availability and cost
- Gas free certificates required? Atmosphere testing requirements.
What preparations are made for dry docking?
- Prepare vessel docking plan and other ships drawings/documents for docking master
- Prepare arrival stability condition
- Prepare procedure for pre docking condition (max ballast per m of length, minimum GM and max trim will be given)
- The stability condition when the vessel enters the dock must be recorded as it must match the undocking condition exactly
- Prepare any tanks to be inspected and make ready any other areas for yard work.
- The ship must be perfectly upright to ensure the centre of gravity is acting on the ships centreline in alignment with the blocks.
What is included on the docking plan?
- Plan of the hull structure to plan the blocks to support the hull
- The location of all transducers so they are not in the block positions
- Location of all sea chests and drain plugs so they are not in position of blocks
What is the P value when dry docking?
The P force is the upward force which replaces the buoyancy force acting through the initial point of contact with the blocks near the stern. The P force increases as the buoyancy force decreases.
What is the critical period during drydocking?
The period of time between the vessel keel first touching the blocks when the ships stability begins to decrease until after the the critical instant when the vessel keel is fully resting on the blocks and is fully supported.
What is the critical instant during dry docking?
The moment before the whole vessel keel is resting on the blocks when the ship is at its most unstable and the P force is at its highest.
What is the calculation for the P force?
P = Reduction in True mean draft x TPC
How does the P force relate to the change in draft during drydocking?
As the draft decreases, it is apparently the same as removing weight from the vessel. The P force is equal to this apparent removal of weight.
What is the formula to calculate the loss of GM?
Loss of GM = (P x KM) / Initial displacement
OR
(P x KG) / Initial Displacement - P
How does the vessel lose stability whilst dry docking?
As the vessel sits on the blocks, the P value acting upwards at the keel is equal to a weight removed from the keel which is equal to the weight of the displacement lost.
As this assumed lost weight increases, the ships centre of gravity moves higher and KG increases.
At first, B will move lower, countering the rise in G until the equilibrium is passed and GM becomes negative, increasing until the vessel is secure from capsize on all the blocks.
How can the loss of GM be limited during dry docking?
- Limiting the P force
- Maximum initial GM
How can the P force be limited during dry docking?
The P force is related to the change in trim so by initially docking with as little trim as possible, the P force is limited. The time in the critical period is also limited
