Dry-Docking: Preparation, Procedure & Survey

1. Preparation — A Few Months Before

• Information about the dry-docking (estimated time and duration) is taken from the Chief Engineer.
• A preliminary repair list is collected and arranged by instruction of the Chief Engineer.
• Repair work is divided into two categories — dock work and ship's staff work. The repair list must contain:

• Number of sea suction chests and grids and their size.
• Number of sea suction valves and their sizes.
• Number of ship-side discharge valves and their size.
• Number of scupper valves and their sizes.
• Number of piping to be repaired, including length, inside/outside diameter, bends, number of flanges and PCD (with detail drawing).
• Number of engine room valves to be renewed or repaired and their specifications.
• Main engine overhauling preparation: spares, tools, measurement and instruction manuals.
• Auxiliary engine, boiler, pump and other machinery preparation for overhauling.

• The exact location of items to be repaired is marked with tag labels according to the repair code.
• Tools, spares and stores are checked and prepared for use; requisitions for stores/spares to be used during docking are ordered.
• All tools must be marked with paint to prevent mixing between ship and shore.
• Organise engine-room staff for safety and assign work; distribute work assignments among staff group by group.
• Instruction manuals and drawings are kept ready for prompt work.

2. Preparation — A Few Days & Hours Before

A Few Days Before:

• Ballasting and transferring carried out with the Chief Officer to achieve the required trim and draught.
• Engine room bilge wells and tank top cleaned thoroughly.
• All tank soundings taken and recorded.
• All portable fire extinguishers ready at required places; fire-fighting fixed installation kept ready.
• Emergency generator, emergency fire pump, emergency air bottle and emergency air compressor kept in good condition.
• Shore cooling sea water supply for the refrigeration and air-conditioning plant ready to receive.
• Shore electrical supply arranged (note Phase, Hz, V, AC or DC at the dock entry).
• Main engine crankshaft deflection taken and recorded.

A Few Hours Before:

• Boiler shut down and all steam pressure released using the easing gear.
• All engine room personnel instructed that smoking is prohibited in the engine room while the ship is in dock.
• Main air bottle and emergency air bottle pressed up to full and closed tightly.
• Ready to stop the ship's generator as soon as the vessel enters dry dock; an electrical officer stands by the shore connection box when shore power is taken.
• Settling and service tanks topped up; after topping up, soundings of all tanks taken and recorded.
• Remind all engine room personnel of their duties and readiness for emergency use.

The Critical Period & Stability Engineering:

The critical period in dry-docking is the interval of time between the ship's stern post first landing on the keel blocks and the vessel taking the blocks overall throughout her length.

Upthrust Reaction Math:

The trimming moment exerted by the stern upthrust is: Trimming Moment = P × l. Equating this to the hydrostatic trimming relationship gives:

P × l = MCTC × t  ⇒  P = (MCTC × t) / l

where P = stern upthrust (tonnes), t = trim change since landing (cm), l = distance from Centre of Flotation (CF) to aft perpendicular (m), MCTC = Moment to Change Trim 1 cm (tonnes-m/cm).

Virtual Loss of GM:

Because the upthrust P supports a portion of the vessel's displacement at the keel (below the center of gravity G), it causes an effective upward shift of G (or virtual loss of metacentric height):

Virtual Loss of GM = (P × KM) / W   (or virtual rise GG₁ = (P × KG) / W)

Effective GM = KM - KG - (P × KM) / W

Warning — Critical Period Safety Mandates

• Upright Condition (List = 0°): The vessel must enter completely upright. Any initial list combined with the virtual loss of GM during the critical period will cause the vessel to loll violently, slide off the keel blocks, or trip the shores with catastrophic hull crushing!
• Small Trim by the Stern: A slight trim by the stern (typically 0.3 m to 0.7 m) is required so the stern lands first under directional control. Excessive trim is strictly forbidden: excessive trim generates an immense upthrust P at initial contact, drastically prolongs the duration of the critical period, and induces huge virtual losses of GM that can turn effective GM negative!
• Eliminate Free Surface: All double bottom tanks must be either pressed 100% full or completely stripped bone-dry to minimize free surface loss (ΔGM_FSE).

The shore changeover. The riskiest moment is the changeover from ship power to shore power. Check the shore supply (phase, frequency, voltage, AC/DC), stand by the connection box, and be ready to stop the ship's generator the moment the vessel is in the dock.

3. On Dock: Work Coordination & Bottom Survey

On Dock:

• Repair work is divided into two categories — one for ship's staff and the other for the dry dock (dockyard).
• Tools and spares checked and prepared for use.
• Take a daily record of work done by ship's staff and dockyard workers.
• Negotiate with the dockyard in charge about the work sequence and show the location of items to be repaired.
• At the end of every day, feed back the condition of work to the Chief Engineer.
• Examine all work done by the dockyard and engine crew; if unsatisfactory, inform the Chief Engineer and dockyard in charge.
• Instruct engine-room personnel to use stores and spares thoroughly without wasting.

Bottom Survey:

• Carried out together with the class surveyor, Master and Chief Officer.
• Assist the class surveyor's visit and subsequent survey, including:
  • Supervise works that cannot be overhauled at sea.
  • Act as witness for docking survey items.
  • Supervise docking repair items.
  • Measure stern tube wear-down, rudder wear-down and jumping clearance.
  • Check the propeller for cavitation, corrosion and any damage (crack, deformation).

The surveyor's key measurements. The classic docking-survey measurements are stern tube wear-down (propeller drop), rudder wear-down (trammel gauge) and jumping clearance (rudder jumping stops) — plus propeller inspection for cavitation, corrosion and cracks/deformation.

Bottom Plugs Protocol & Flood-Up Disaster Prevention:

Double bottom tanks, cofferdams, bilge wells, and peak tanks are equipped with threaded bronze or stainless steel bottom drain plugs fitted into the bottom plating.

Danger — The Catastrophic Disaster of Mismanaged Bottom Plugs

If a bottom plug is accidentally forgotten, left loose, or cross-threaded when dock flooding commences:

• Internal Progressive Flooding: Sea water rushes uncontrollably into empty fuel oil bunker tanks, engine room cofferdams, or dry cargo holds.
• Severe Contamination: Heavy fuel oil tanks flooded with seawater produce thousands of tons of contaminated sludge, disabling the propulsion plant and requiring weeks of dock delays for de-bunkering.
• Catastrophic Loss of GM & Capsizing: Sudden ingress of free-surface liquid into double bottoms creates asymmetrical flooding, causing the ship to develop a severe list, slip off keel blocks, crush side shores, or jam against dock walls as water rises!

4. After Dry Docking

• Price negotiation — man/power and material per the repair list.
• Dock charge damage claim where applicable.
• Report — record the completed work and findings.
• Closely check sea valves — confirm correct operation and line-up before and after flooding up.
• Performance of repaired machinery — verify that overhauled/repair items perform correctly.
• Take the main engine crankshaft deflection again after docking.

Why repeat the crankshaft deflection? Crankshaft deflection is recorded before docking (to know the hull's condition) and after docking (to confirm the hull and engine bed have not distorted after repairs, ballasting and re-floating). A change indicates a possible hull/engine-bed alignment problem.

Surveyor asks: "Explain dry-docking preparation" and "What surveys/measurements are done in dry dock?" Structure your answer by timing: months before → days before → hours before → on dock → after docking, and name the stern tube/rudder wear-down and propeller checks.