Why do ships need to dry dock?
Ships dry dock so that surveyors can examine the parts of the hull and machinery that are under water, and so that the owner can carry out work that is impossible afloat.
Two sets of rules drive the timing:
- SOLAS regulation I/10, for the cargo ship safety construction certificate, requires a minimum of two inspections of the outside of the ship's bottom during the five-year period, and "in all cases the interval between any two such inspections shall not exceed 36 months".
- IACS Unified Requirement Z3, applied by class societies, requires a minimum of two examinations of the outside of the bottom and related items in each five-year special survey period, one of them with the special (class renewal) survey, also with a 36-month maximum interval.
UR Z3 adds that compliance with class does not release the owner from SOLAS, which may require shorter intervals for some ship types.
What is examined during a dry-docking survey?
UR Z3 lists what the surveyor examines with the ship on blocks:
| Item | What the surveyor looks for |
|---|---|
| Shell plating, bottom and bow | Excessive corrosion, chafing or grounding damage, unfairness or buckling, bilge keel connections |
| Sea chests and gratings, sea connections, overboard valves | Condition and fastenings; valves need opening only once per special survey period unless the surveyor requires |
| Rudder, pintles, rudder stock and stern frame | Visible parts examined; rudder bearing clearances measured and recorded |
| Propeller and stern bush | Visible parts examined; stern bush clearance and oil gland efficiency recorded |
| Side thrusters and other manoeuvring propulsors | External examination, blade and shaft sealing |
Propeller shaft surveys follow a separate requirement (IACS UR Z21), so check when the tail shaft is next due and whether it falls in this docking.
When can an in-water survey replace dry docking?
An in-water survey (IWS) can replace the intermediate bottom examination in some cases, but not the one at the renewal survey under most flag rules, and not for older ESP ships.
| Rule | Source |
|---|---|
| Bottom examinations are normally done in dry dock; an in-water survey may be considered as an alternative | IACS UR Z3.1.3 |
| Special consideration is needed for ships of 15 years or over | IACS UR Z3.1.3 |
| ESP ships (bulk carriers, oil and chemical tankers and others under the Enhanced Survey Programme) of 15 years and over must be examined in dry dock | IACS UR Z3.1.3 |
| The IWS must give the information normally obtained from a docking survey, including rudder and stern bush clearances | IACS UR Z3.3.1 |
| It is done in sheltered water with enough visibility, by an approved diving or ROV firm, under a class surveyor | IACS UR Z3.3.2 to Z3.3.4 |
| If damage needing early attention is found, the surveyor may require dry docking | IACS UR Z3.3.6 |
| UK example: one of the two inspections must be out of the water and should coincide with the safety construction renewal survey | UK MCA, MGN 546(M) |
An IWS needs preparation: proposals submitted to class in advance, and permanent markings on the hull plating where the surveyor needs them to orient the divers or ROV. Ask your class society and flag before planning a docking cycle around in-water surveys.
Can the 36-month interval be extended?
Rarely, and not by class alone. UR Z3 allows class to extend the bottom examination by up to three months in "exceptional circumstances", defined as unavailability of dry-docking or repair facilities, essential materials or spares, or delays to avoid severe weather. For the statutory survey, Lloyd's Register has told owners that no extension to the 36-month SOLAS interval can be agreed by the class society alone; the flag administration must agree first. Plan the docking window so you do not need an extension.
How far ahead should a dry dock be planned?
Planning should start well before the window opens, because yard slots, long-lead spares and owner approvals take time. The table shows a typical sequence; adjust it to your fleet and trade.
| Stage | Main work |
|---|---|
| Survey status review | Confirm due dates and windows for class renewal, bottom survey, tail shaft, boiler and statutory items, and any outstanding conditions of class |
| Work list | Gather defects that need dry dock, PMS jobs that can only be done out of the water, owner's upgrades and regulatory retrofits |
| Specification | Turn the work list into a yard specification with quantities, drawings, photos and standard units |
| Enquiry and yard selection | Send the specification to shortlisted yards, compare quotes like for like, and agree the contract, dock days and payment terms |
| Procurement | Order owner-supplied spares, paint and equipment with delivery to the yard |
| Pre-docking | Crew briefings, tank cleaning and gas-freeing plan, enclosed space and hot work arrangements, safety meeting with the yard |
| In dock | Daily progress meetings, change orders, class attendance, quality checks |
| Close-out | Final invoice reconciliation, class and statutory certificates, updated PMS and drawings, lessons learned |
What goes into a dry dock specification?
A good specification is specific enough for yards to price the same job. It usually covers:
- General services: docking and undocking, dock days, shore power, cooling water, fire watch, garbage, gas-freeing certificates, cranes and staging.
- Hull: high-pressure washing, blasting and coating by area, steel renewal (normally priced per kilogram with a provisional quantity), anodes.
- Sea valves and sea chests: number of valves, sizes, overhaul scope.
- Rudder and propeller: clearances, polishing, tail shaft withdrawal if due, seals.
- Machinery: main engine, auxiliary engines and other items that need the ship out of service.
- Tanks: cleaning, inspection and coating repairs, especially in ballast tanks.
- Deck: hatch covers, cranes, mooring equipment, piping.
- Regulatory and owner items: retrofits or upgrades the ship needs before its next cycle.
Link each specification item to its source: the class item, the PMS job or the defect report. That makes the yard bill easier to check and closes the loop in the PMS after the docking. Our planned maintenance guide explains why dry-dock-only jobs belong in the PMS.
How do you control dry dock cost and time?
Most overruns come from scope that was not in the specification and from quantities that were guessed. Practical controls:
- Survey before you specify. A pre-docking inspection of tanks, hull and machinery gives real quantities for steel, coating and repairs.
- Use provisional quantities with unit prices for steel and coating, so growth is priced at contract rates.
- Approve every extra in writing through a change order with price and time impact.
- Hold a daily progress meeting with the yard, superintendent and class.
- Track actual cost against budget while in dock, not only after the final invoice.
- Record what was found and done so the next docking starts with better data.
The condition found in dock is also useful evidence later. Buyers and their inspectors ask for the last docking report, coating records and steel renewal history; see our pre-purchase inspection service.
What safety risks need managing during a dry dock?
A ship in dry dock combines yard workers, hot work, open tanks and reduced crew. Plan for:
- Enclosed space entry by both crew and yard workers, with the ship's Enclosed Space Register and a gas-freeing plan; see our enclosed space entry guide. MSC.581(110) specifically recommends a schematic plan showing safe and unsafe spaces where large numbers of shore personnel work on board, such as in dry dock.
- Hot work in or near tanks and fuel systems, under a permit system.
- Fire safety while fixed systems are isolated, with fire watches and clear isolation records.
- Working at height on staging and in holds.
- Isolation of systems before yard work, and restoration before undocking.
Agree in writing which party controls permits for each area, and keep the ISM management of change process running for any change to equipment or procedures made in dock.
Dry dock planning checklist
Use this as a starting point and adapt it to your SMS:
- Survey due dates and windows confirmed with class and flag
- Outstanding conditions of class and memoranda reviewed
- PMS dry-dock-only jobs and deferred defects listed
- Tail shaft, boiler and other machinery surveys due checked
- Specification written with quantities, drawings and photos
- Yard quotes compared on the same scope
- Owner-supplied spares and paint ordered with delivery dates
- Budget approved with contingency for steel and coating
- Safety plan agreed with the yard: permits, enclosed spaces, hot work, fire watch
- Class surveyor attendance booked
- Undocking checks: sea valves closed and tested, draft marks, anodes, stability
- Close-out: certificates issued, PMS and drawings updated, final invoice reconciled, lessons recorded
Frequently asked questions
How often does a ship need to go to dry dock?
Most cargo ships need two bottom examinations in each five-year period, no more than 36 months apart, under SOLAS and class rules. One is normally in dry dock with the renewal survey. The other may be an in-water survey where class and flag accept it, so a ship docks once or twice in each five-year cycle.
What is the difference between dry docking and an in-water survey?
Dry docking takes the ship out of the water so the hull, rudder, propeller and sea valves can be examined and repaired directly. An in-water survey uses approved divers or ROVs, under a class surveyor, to gather the same information afloat. It is only accepted where class and flag allow, and not for ESP ships aged 15 years or more.
Can a ship of over 15 years use an in-water survey?
Sometimes, but not if it is an ESP ship such as a bulk carrier or tanker. IACS UR Z3 requires ESP ships of 15 years and over to be examined in dry dock, and asks for special consideration for other ships of that age. Flags set their own limits; the UK MCA normally applies 15 years.
Can the 36-month bottom survey interval be extended?
Only exceptionally. IACS UR Z3 allows class to grant up to three months for reasons such as no dry dock or repair facility being available, missing essential spares or weather delays. For the SOLAS interval, Lloyd's Register states that the flag administration must agree any extension; class cannot agree it alone.
What surveys are usually done during a dry dock?
The bottom survey under UR Z3 is the minimum: shell plating, sea chests and valves, rudder, propeller, stern bush and thrusters. Depending on the cycle, the docking may also include the class renewal survey, tail shaft survey, tank inspections, anchor and chain cable checks and statutory items due at the same time.
What should a dry dock specification include?
It should list every job with enough detail for yards to price it the same way: general services, hull blasting and coating by area, steel renewal with provisional quantities, sea valves, rudder and propeller, machinery, tanks, deck items and owner's upgrades. Each item should link back to a class requirement, PMS job or defect.
Sources
- IACS UR Z3, Periodical Survey of the Outside of the Ship's Bottom and Related Items (Rev.7, January 2018), copy hosted by BG Verkehr (German flag)
- Lloyd's Register, Class News 10/2013, Periodical surveys: interval between inspections of the outside of the ship's bottom (27 May 2013), quoting SOLAS regulation I/10
- IMO, Resolution MSC.204(81), amendments to SOLAS chapter I (copy hosted by USCG NAVCEN)
- UK Maritime and Coastguard Agency, MGN 546(M) Amendment 1, In-water surveys
- IMO, Resolution MSC.581(110), Revised recommendations for entering enclosed spaces aboard ships