What Is Dry Docking and Why Is It Mandatory for Ships?
Dry docking is a shipyard operation in which a vessel is removed from the water in a controlled manner — or the water in the dock is pumped out — so that the parts of the ship that lie below the waterline (the underwater hull) can be accessed. The ship is set down on blocks pre-positioned on the dock floor, making the entire hull accessible for inspection and maintenance.
For ships, dry docking is not merely an option but, in most cases, a mandatory requirement:
Class requirement: Classification societies require the underwater portion of the hull to be inspected in dry dock at regular intervals. Typically, two dry-dock examinations are prescribed within the five-year special survey cycle.
Structural safety: Corrosion, pitting and plate wastage on the bottom of the hull can only be measured and assessed accurately once the vessel is out of the water.
Performance and efficiency: Biological fouling on the hull increases drag and fuel consumption; in dry dock the hull is cleaned and the anti-fouling coating renewed.
Underwater equipment maintenance: The propeller, rudder, sea valves and cathodic protection anodes receive thorough attention in dry dock.
In a single sentence: dry docking is a fundamental maintenance operation that must be repeated at regular intervals so the vessel can maintain its legal validity, structural safety and operational efficiency.
How Is the Dry-Docking Process Planned at Shipyards?
Because dry docking is a high-cost and critical operation, its success depends on the quality of the planning:
1. Docking specification: Every task to be carried out — including inspection, maintenance, repair and class-driven items — is listed item by item. This specification forms the basis for shipyard quotations and job tracking.
2. Shipyard selection: A shipyard is chosen with a dock suited to the vessel’s dimensions and appropriate in terms of technical capacity, location and cost.
3. Scheduling: Dry docking is planned with regard to the vessel’s commercial programme (voyage schedule) and the class/survey calendar. The aim is to minimise off-hire.
4. Docking plan: The layout of the blocks on the dock floor is determined through engineering calculation based on the vessel’s weight distribution. This plan is critical to the ship’s safe seating in the dock.
5. Materials and spare parts: Long-lead-time parts are procured in advance so that dock time is not extended waiting for parts.
6. Labour and subcontractor coordination: The shipyard team, specialist subcontractors and, where required, manufacturer service teams are coordinated.
Good planning shortens dock time (and therefore cost), allows tasks to run in parallel and minimises surprises.
What Stages Make Up a Dry-Dock Operation?
A dry-docking operation consists of defined stages from start to finish:
| Stage | Content |
|---|---|
| 1. Dock preparation | Positioning the blocks according to the plan, preparing the dock |
| 2. Docking | Hauling the vessel into the dock and seating it on the blocks |
| 3. Dewatering | Pumping out the dock water, fully exposing the vessel |
| 4. Initial inspection | First assessment of the hull, propeller, rudder and bottom valves |
| 5. Surface preparation | Hull cleaning, blasting / water jetting |
| 6. Maintenance and repair | Steel renewal, equipment maintenance, coating application |
| 7. Inspection and testing | NDT, measurements, class surveys |
| 8. Flooding | Refilling the dock with water |
| 9. Undocking | Floating the vessel out of the dock |
| 10. Post-docking checks | Leak-tightness checks, sea trial (where required) |
Each stage depends on the successful completion of the previous one. The docking and undocking stages in particular are the most critical moments, directly affecting the safety of the vessel, and demand careful engineering.
How Is a Ship Docked and Undocked?
Docking and undocking a vessel are manoeuvres that require precise planning and coordination:
Docking:
- Block layout: Blocks calculated according to the vessel’s weight distribution and bottom form are positioned on the dock floor.
- Positioning: The vessel is hauled into the dock with lines and tug assistance and brought into exact position against the centreline references.
- Seating: As the dock water is progressively pumped out, the vessel gradually settles onto the blocks. The moment of seating is the most critical, as the weight must transfer evenly onto the blocks.
- Full dewatering: The water is pumped out completely; the vessel is secured laterally with side shores.
Undocking:
- Readiness check: All underwater openings (valves, plugs) are closed and leak-tightness is verified.
- Progressive flooding: The dock is filled with water in a controlled manner and the vessel begins to float again.
- Flotation and stability check: The vessel’s trim and stability are monitored and a leak check is carried out.
- Removal from the dock: The vessel is taken out of the dock with lines and tug assistance.
The dock master takes responsibility for these manoeuvres. Because incorrect block layout or uneven seating can cause serious structural damage, these stages are conducted with great care.
What Maintenance Work Is Carried Out During Dry Docking?
The dry dock offers an intensive maintenance opportunity on the parts of the vessel that are only accessible once it is out of the water. The main tasks are:
Underwater hull: Cleaning, corrosion removal, plate thickness measurement, steel renewal where necessary and application of anti-fouling coating.
Propeller and shaft line: Propeller inspection, polishing/balancing, renewal of stern tube seals, measurement of shaft bearing clearance.
Steering system: Inspection of the rudder blade, bearing clearance measurement, leak-tightness testing.
Sea water systems: Sea chest cleaning, maintenance of overboard valves, grating renewal.
Cathodic protection: Renewal of depleted anodes and, where fitted, inspection of the impressed current system (ICCP).
Tank work: Ballast tank inspection, cleaning and renewal of protective coatings.
Class-driven work: Rectification of deficiencies identified during survey.
Above-water work (machinery overhauls, superstructure painting, etc.) is usually carried out in parallel during the same dock period, so that as much work as possible is completed within a single off-hire period.
How Are Dry-Docking Intervals Determined?
Dry-docking intervals are not arbitrary but are set by class and regulatory rules:
Class rule: The traditional rule is that two dry-dock examinations are carried out within the five-year class cycle, and that the interval between two dockings typically does not exceed 36 months.
Extended interval: For vessels meeting certain conditions (generally non-passenger ships of a certain age and condition), the docking interval may be extended by accepting an in-water survey in place of the dry-dock examination. This depends on class approval and the vessel’s eligibility.
Age factor: For older vessels, class may require more frequent and more extensive inspection.
Vessel type: For types requiring a high level of safety, such as passenger ships, the intervals are shorter.
Operational factors: Coating performance, the waters the vessel operates in and the vessel’s condition also influence the practical docking schedule.
Managing the interval correctly ensures both the preservation of class validity and the avoidance of the cost of unnecessarily early docking. Owners plan docking by optimising it between the class calendar and the commercial programme.
How Do Classification Societies Oversee the Dry-Docking Process?
Classification societies (DNV, Lloyd’s Register, Bureau Veritas, ABS, etc.) are the independent technical oversight authority for dry docking:
Dry dock survey: The class surveyor inspects the underwater portion of the hull on site, assessing the condition of the hull, propeller, rudder, bottom valves and plating.
Oversight of thickness measurement: The results of ultrasonic thickness measurements (UTM) performed by approved firms are presented to and assessed by the surveyor.
Repair oversight: Steel renewal, welding and structural repairs are carried out under the surveyor’s oversight and approval; weld NDT results are checked.
Deficiency management: Recommendations or conditions of class are raised for identified deficiencies and closed out as they are rectified.
Certificate validity: When the survey is successfully completed, the validity of the vessel’s relevant certificates is maintained or renewed.
A docking completed without class oversight does not safeguard the vessel’s legal and commercial validity. For this reason, the class surveyor’s schedule is an integral part of docking planning.
What Hull Maintenance Tasks Are Carried Out During Dry Docking?
The hull is the most intensively worked area in dry docking. The tasks follow a defined sequence:
1. Inspection and mapping: The hull is examined visually and by ultrasonic thickness measurement (UTM); areas of corrosion, pitting and deformation are mapped.
2. Surface preparation: Rust, old paint and fouling are removed by blasting or high-pressure water jetting.
3. Steel renewal: Plates that have fallen below the regulatory threshold (excessively wasted) are cut out and removed; new plates of the appropriate standard are welded in place.
4. Weld and crack repair: Cracks are repaired using the appropriate method; welds are verified by NDT.
5. Structural reinforcement: Reinforcement members are added where required.
6. Protective coating: The repaired and prepared surfaces are protected with a primer and top-coat system.
The quality of hull work is safeguarded by welder qualification, material suitability and NDT control. Structural repairs require the oversight and approval of the class surveyor, because hull integrity directly determines the vessel’s seaworthiness.
How Is the Underwater Hull Cleaned?
Cleaning the underwater hull (the hull surface below the waterline) is one of the key tasks of dry docking, being a precondition for both performance and coating application:
The fouling problem: The algae, mussels and marine shellfish that accumulate on the hull over time increase hull drag, raising fuel consumption and emissions. This is both an economic and an environmental cost.
Cleaning methods:
- High-pressure water jetting (hydroblasting): The most common method; it cleans fouling and loose paint with the force of water. Ultra-high pressure (UHP) can also remove old coating.
- Abrasive blasting: Used in areas requiring deeper cleaning and exposure of bare metal; it also provides surface roughness.
- Mechanical cleaning: Local scraping and brushing.
Surface quality control: After cleaning, the surface is checked for cleanliness grade, salt/ion residue and roughness. This is critical to the adhesion of the subsequent coating layer.
Environmental measures: The waste generated during cleaning (paint particles, fouling) is collected and disposed of appropriately, and prevented from mixing into the dock water.
Well-executed hull cleaning extends the life of the new coating, improves fuel efficiency and restores the vessel’s performance to like-new condition.
How Is Propeller and Shaft-Line Maintenance Carried Out in Dry Dock?
The propeller and shaft line are the underwater systems that determine a vessel’s propulsion capability, and they can only be maintained thoroughly in dry dock:
Propeller maintenance:
- Visual and dimensional inspection; checking for cavitation erosion, edge damage and cracks
- Repair of damaged blade edges by grinding/welding
- Surface polishing — a smooth surface improves efficiency and reduces cavitation
- Static/dynamic balancing where required
Shaft-line maintenance:
- Renewal of stern tube seals
- Measurement of shaft bearing clearance (“wear down”)
- Shaft withdrawal (where required) and detailed inspection
- Alignment check
Integration with the rudder: Since the propeller and rudder are in the same area, they are usually assessed together.
The results of this work (particularly bearing clearance measurements) are presented to the class surveyor. Maintaining propeller efficiency delivers a directly measurable gain in the vessel’s fuel economy.
How Are Marine Coating Works Applied During Dry Docking?
Paint and coating form the most critical layer protecting the hull against corrosion and fouling; their success depends on discipline in application:
Systems by area:
| Area | Coating Type | Purpose |
|---|---|---|
| Flat bottom / vertical sides | Anti-fouling (SPC / hard matrix) | Fouling and drag control |
| Waterline (boot-top) | Abrasion-resistant coating | Resistance to wave/ice abrasion |
| Ballast tanks | Epoxy protective coating (PSPC) | Preventing in-tank corrosion |
| Above-water hull / superstructure | Epoxy primer + polyurethane top coat | Corrosion protection and appearance |
Critical stages of application:
- Surface preparation: Most of the quality is determined here; the appropriate cleanliness grade and roughness are essential.
- Climatic condition control: Air temperature, surface temperature, humidity and dew point are measured to verify suitability for application.
- Film thickness: After each coat, the dry film thickness (DFT) is measured to verify compliance with the manufacturer’s specification.
- Curing time: The over-coating intervals between coats and after the final coat are observed.
Anti-fouling technology is continuously advancing in line with environmental regulation (for example the TBT ban) and efficiency targets. A correctly selected and correctly applied coating delivers both fuel savings and a longer docking interval.
How Is Corrosion Controlled During the Dry-Docking Process?
Corrosion is the fundamental enemy of steel ships; dry docking is the most comprehensive opportunity to combat it:
Corrosion assessment: The rate of wastage is determined through plate thickness measurements (UTM); plates falling below the regulatory threshold are renewed.
Protective coating: A coating system applied to correctly prepared surfaces is the primary barrier against corrosion.
Cathodic protection:
- Sacrificial anodes: Zinc/aluminium anodes prevent corrosion by “sacrificing themselves” in place of the ship; they are renewed in dock when depleted.
- Impressed current system (ICCP): Provides active protection through a controlled electric current; it is checked and maintained in dock.
Tank protection: High corrosion-risk areas such as ballast tanks are protected with special protective coatings (PSPC standard).
Monitoring and mapping: Corrosion areas are recorded so that trends can be tracked at subsequent dockings; this enables predictive maintenance.
Effective corrosion control directly protects the vessel’s structural safety, class validity and economic life. Neglected corrosion turns into far more expensive steel renewal work at the next docking.
How Are Dry-Dock Costs Calculated and What Do They Cover?
Dry-docking cost has many components, and calculating it correctly is the foundation of budget management:
| Cost Item | Content |
|---|---|
| Docking fee | Docking, dock stay duration, undocking |
| Labour | Man-hour-based welding, machinery, painting and mechanical work |
| Materials | Steel, paint, anodes, spare parts, consumables |
| Equipment/services | Blasting, cranage, NDT, special testing, subcontractors |
| Class and survey | Class inspection and approval fees |
| Indirect costs | Passage to and from the shipyard, fuel, port, off-hire |
The main factors affecting cost:
- The vessel’s size and age (larger/older ship = more work)
- Scope of work (routine maintenance only, or extensive repair?)
- Shipyard location and labour cost
- Unforeseen additional work (for example, more steel renewal than anticipated)
Calculation approach: A detailed item-based quotation (specification) is obtained; fixed-price and variable (man-hour) work is separated out; and a contingency budget is set aside for surprises. The most common mistake is to look only at the shipyard quotation and fail to account for off-hire and possible additional work; the true cost is the sum of all of these.
When Is Emergency Dry Docking Required?
Emergency dry docking comes into play in situations calling for unplanned and urgent docking:
Collision and grounding: Damage to the bottom of the hull that causes water ingress or threatens structural integrity.
Underwater damage: Serious damage to the propeller, rudder or shaft line that impedes navigation.
Leakage: Uncontrollable water ingress from the bottom of the hull or from underwater openings.
Loss of propulsion: Shaft- or propeller-related failures that render the vessel unable to propel itself.
Class/PSC-related emergencies: Deficiencies affecting the vessel’s seaworthiness that must be rectified immediately.
The challenges of emergency docking are limited dock availability, the short planning time and the high cost. For this reason, rapid access to the nearest suitable dock, damage assessment and class coordination are of critical importance. Since emergency docking is far more costly than planned docking, preventing emergencies through regular maintenance and monitoring is the most economical approach.
How Are NDT (Non-Destructive Testing) Methods Used in the Dry-Docking Process?
NDT (Non-Destructive Testing) is the fundamental means of assessing a vessel’s condition without damage during dry docking:
Ultrasonic thickness measurement (UTM): The thickness of the hull, tanks and structural members is measured by ultrasonic means. It is the standard method for assessing corrosion-related wastage and is the basis for the steel renewal decision.
Magnetic particle (MT): Detection of surface cracks in weld seams and structural joints.
Penetrant (PT): Making surface-breaking cracks visible; particularly in propeller and rudder inspection.
Ultrasonic (UT): Detection of the internal integrity of weld seams and of structural defects.
Visual testing (VT): The basis of all inspection; observation of corrosion, deformation and defects.
NDT provides the class surveyor with evidence, particularly in verifying structural repairs and weld quality. It is essential that the results be produced and documented by certified NDT personnel (e.g. ISO 9712). By “seeing the unseen”, NDT guarantees the vessel’s safety at the docking stage.
How Is Ballast Tank Cleaning Carried Out in Dry Dock?
Ballast tanks are the highest corrosion-risk areas because they are in constant contact with sea water; docking is the opportunity for their comprehensive maintenance:
1. Emptying and ventilation: The tank is emptied and cleaned, and enclosed-space entry procedures are applied (gas measurement, entry permit).
2. Cleaning: Mud, sediment and loose rust are removed; the surface is made ready for inspection and coating.
3. Inspection: Structural members, welds and coating condition are examined; plate thickness measurements are taken.
4. Surface preparation: Areas with degraded coating are prepared by blasting/mechanical means.
5. Protective coating (PSPC): A coating conforming to the protective coating standard set for ballast tanks (Performance Standard for Protective Coatings) is applied.
6. Anode check: The sacrificial anodes inside the tank are checked and renewed where necessary.
Because they are enclosed and confined spaces, ballast tank tasks are among the items most hazardous in terms of occupational safety; work is never carried out without gas measurement, ventilation and an entry permit. Well-protected ballast tanks directly extend the structural life of the vessel.
How Is Ship Stability Maintained During Dry Docking?
Stability is one of the most critical engineering matters in dry docking, because the vessel passes through delicate moments of equilibrium as it settles onto the blocks and as it refloats:
The critical moment during seating: In the phase from the instant the vessel touches the blocks until it is fully seated, the weight transfers progressively onto the blocks. During this transition the vessel’s stability can decrease; for this reason the moment of seating (particularly the “critical period”) is managed carefully.
The importance of the docking plan: Positioning the blocks to suit the vessel’s weight distribution and bottom structure ensures the load transfers evenly and prevents structural damage. Incorrect block layout can cause local overloading.
Side shores: Once the vessel is seated, it is supported laterally to eliminate the risk of tipping over.
Managing weight changes: Work carried out while in dock (cutting/adding steel, removing/fitting equipment, filling/emptying tanks) alters the vessel’s weight distribution. These changes must be assessed for stability, particularly before undocking.
Undocking control: The moment the vessel begins to refloat as the dock is flooded is once again a critical phase for stability; trim and equilibrium are monitored carefully.
The dock master and the vessel’s responsible officers manage this process together in the light of stability calculations. Neglecting stability can lead both to structural damage and to serious safety incidents.
What HSE Procedures Apply During Dry Docking?
The dry dock is one of the highest-risk working environments in the maritime industry; HSE is therefore an undisputed priority:
The main risks: Working at height, enclosed/confined space entry (tanks), hot work (welding/cutting), blasting, heavy lifting and simultaneous operations.
Core procedures:
- Permit-to-work system: Hot work, enclosed-space and working-at-height activities are subject to permit.
- Enclosed-space entry: Tanks are never entered without gas measurement, ventilation and continuous supervision.
- Hot work safety: Fire watch, fire-fighting provision and control of flammable materials.
- Working at height: Safe scaffolding, guardrails and fall-prevention systems.
- Lifting operations: Authorised operator, lifting plan and the rule against standing under a load.
- PPE: Mandatory personal protective equipment.
Simultaneous operation management: Carrying out more than one task in the same area (for example painting above and welding below) requires special coordination.
Training and culture: Regular safety training, toolbox talks and a shared safety mindset among all parties (shipyard, vessel, subcontractor).
HSE is not only a human imperative but also an economic necessity that prevents the downtime, penalties and loss of reputation that accidents would cause.
Why Is a Sea Trial Conducted After Dry Docking?
A sea trial is conducted after a comprehensive dry docking to verify, in real sea conditions, the work carried out on the vessel. Although not mandatory after every docking, it is of critical importance when significant work has been done on the machinery or propulsion system.
The reasons for and checks in a sea trial:
Propulsion system verification: If work has been done on the propeller, shaft or machinery, the vessel’s speed and propulsion performance are tested.
Manoeuvrability: If work has been done on the steering system, verification is carried out with turning and manoeuvring tests.
Machinery performance: The performance of overhauled machinery under full load, along with temperature and pressure values, is checked.
Leak-tightness verification: The absence of leakage after underwater work is verified while under way.
Vibration and noise: Vibration levels are measured after shaft/propeller work.
System function tests: Navigation, automation and safety systems that have been maintained are tested.
The sea trial results confirm that the vessel is in a fully operational and safe condition after docking. Where required, it is carried out under the oversight of the class surveyor and is the final approval stage for the vessel’s return to service.
What Are the Dry-Docking Technologies and Innovations at Modern Shipyards?
Dry docking is undergoing rapid technological transformation in line with efficiency, safety and environmental goals:
Underwater inspection robots (ROV/drone): The ability to carry out certain inspections in the water (in-water survey) without docking the vessel reduces the need for, and cost of, docking.
Aerial (drone) inspection: Safe and rapid inspection of high and hard-to-reach areas with drones lowers the need for scaffolding and the associated risk.
Robotic hull cleaning and painting: Carrying out hazardous and repetitive blasting/painting tasks with robots improves quality consistency and occupational health and safety.
Advanced anti-fouling technologies: Silicone-based, foul-release and low-friction coatings improve fuel efficiency and extend the docking interval.
Digital dock management: Sensor-equipped block systems, a digital docking plan and real-time load monitoring for safer seating.
Spare parts via 3D printing: On-site production of critical parts at the dock shortens waiting times.
Data-driven maintenance: Monitoring thickness measurement and coating performance data to optimise the timing and scope of docking.
These innovations shorten dock time, reduce off-hire cost, improve safety and enable decisions to be based on data. Shipyards that invest in technology offer the owner an advantage in both speed and reliability.
Looking for a Reliable Solution Partner for Your Dry-Docking Processes?
At Orionis Shipping, our experienced team of specialists is by your side in the planning of dry-docking processes, the preparation of scope and specifications, shipyard and class coordination, docking plan and stability oversight, monitoring of hull and structural work, paint and coating quality control, NDT and survey processes, and emergency docking coordination. From the propeller and shaft line to ballast tank maintenance, and from corrosion control to post-sea-trial confirmation, we support you across a broad spectrum in managing your vessel’s docking investment as efficiently as possible in terms of cost, time and quality.
Contact us for detailed information on dry-docking processes and solution partnership.
This content is for informational purposes only. In your technical and legal decisions regarding dry docking, maintenance and survey processes, please rely on the relevant classification societies (DNV, Lloyd’s Register, Bureau Veritas, ABS, etc.), your flag State administration, accredited surveyors and the current national/international regulations (SOLAS, MARPOL, etc.).
Tags: dry docking, ship docking, dry dock process, underwater hull cleaning, ship hull maintenance, propeller shaft maintenance, marine coating anti-fouling, corrosion control, plate thickness measurement UTM, ballast tank cleaning, ship stability docking, dry dock cost, emergency dry docking, NDT non-destructive testing, dry dock safety HSE, sea trial, dry docking technologies, class survey docking, Orionis Shipping
