Dry Docking

Hull inspection by a Senior Engineer
IACS Dry-docking requirement

The purpose is to inspect and carry out a repair as necessary for those underwater parts which are otherwise inaccessible during afloat conditions. Also, dry docking is mandatory in terms of classification society requirements.

All the vessels are required to carry out a special docking survey not exceeding a five-yearly period.
An Intermediate docking survey is also required in between the 5-yearly survey.
An "In-Water survey" in lieu of the intermediate docking survey may be carried out on ships where special high resistance paint have been applied. It is to be carried out under the surveillance of a surveyor with the ship at a suitable draught in sheltered water, with in-water visibility to be good and the ship's underwater surface reasonably clean. Diving and in-water survey operations are to be carried out by firms recognized by society.

Preliminary Preparation - prior to Dry dock

Drawings of the shell expansion plan, stern gear, rudder gear, propeller, ship-side valve & gratings, framings are to be studied and kept ready for dry dock.
Reports of last two dry-docks to be checked- Tail shaft, stern tube bearings, shaft seals, poker gauge reading, rudder drop, pintle clearance etc. noted.
Inventory of spares (for machinery to be overhauled), Tools ( spanners for propeller pilgrim nut, rudder palm bolts, drop gauge, poker gauge, trammel gauge, intermediate shaft coupling bolts, rudder pintle nut bolts etc.) are to be taken. Inventory of stores. eg pipes, bends, flanges, valves etc.; are also to be taken for probable use.
Continuous survey of Machinery items identified - for Dry dock (Tail shaft, propeller, rudder, underwater valves, sea chests, Bow thruster, stabilizer etc.)
Identification of defects and preparation of defect list, to be carried out during dry dock, in consultation with all engineers.
Preparation of Spares and Stores indent both imported and indigenous and to be sent to the office for procurement before the dry dock.
Requirement of any specialized service of equipment manufacturers such as automation and control equipment, to be informed to the Engineer Superintendent.
Job allotment to ship staff, shipyard, specialists identification.
Job Timing schedule to prepare to ensure satisfactory completion of all jobs within the dry-docking period.
Maintaining Safety - Readiness of safety equipment during the various stages of dry-docking Prevention of flooding of Double bottom tanks and Engine room.
Tank Plugs - Engine room D.B. tank plugs to be identified by frame numbers and to ascertain which tank to drain via plugs in dry dock and which plugs not to be touched, such as oil tanks.

Preparation before entering Dry dock

Shore Electrical Supply - To check whether dry dock supply is compatible with the ship's voltage, frequency and phase sequence and to arrange connection after checking interlock.
Utility Services - In the event of a ship's generator, domestic fridge, auxiliary condenser or sanitary system required to be used, in the dry dock, then arrangements must be made to pipe shore water into those systems and discharge to the shore facility tank.
Fire Protection - Fire connection to be made to the International Fire Connection main and to sprinkler system connected to it.
The impressed current system, Fresh Water generator, Oily bilge separator to be put off.
Information required :

Gas free certificates must be obtained for any empty fuel tanks etc. that are to be worked in the dry dock.
Before entering the dry dock, all tanks and bilges are sounded and recorded. Any transferring considered necessary should be carried out in advance. Since tank soundings just prior to dry docking are logged, any fuel and water consumed in the dock and its effect on trim and heel may be determined, from the tables of hydro-static data and loading table.

Precaution while Entering the Dry dock

The ship should be upright and trimmed slightly by the stern, empty of Cargo, with a minimum free surface in the tanks.
The ship is lined up over the keel blocks by permanent marks on the dock gates and head or by plumb lines.
As the dock water is pumped out, the ship should land on the after blocks and the reaction on the stern of the ship increases until the ship is just about to touch all along the blocks. side shores are fitted aligning with bulkheads or frames.
All the tank soundings are taken again to ensure there is no damage to any tanks due to the sitting on the Keel block.
Shore electrical connections and other service connections are made.

Jobs to be carried out in Dry dock

To clean the hull of marine growth to reduce ship resistance, thereby increase speed and minimize fuel consumption. Anodes and reference electrodes are to be covered.
Docking survey of Hull and machinery as per the requirement of classification society to maintain 'class'.
Hull Inspection- All underwater parts of the hull are inspected for damage and corrosion. Distorts dent are observed and inspection of the inside of the ship is carried out to determine internal damage if any. Serious dents are to be repaired and thinned plate sections are to be strengthened by doubler plate or stiffeners. Distorted bilge keels to be faired and their attachment to the hull checked. All dent positions and repairs are to be logged.
Corrosion - All cathodic protection systems should be checked for the security of attachment. A worn sacrificial anode should be renewed if necessary. Particular attention is to be paid at the forefoot, at the aft end in way of the stern frame and on twin-screw ships around propeller 'A' brackets for corrosion and cracks. Edges of weld to check for corrosion attack.
Rudder- To check rudder drop (maximum about 19 mm) between the rudder stock and carrier bearing. To check pintle clearance, security of palm coupling and condition of stock (crack), To remove the rudder drain plug to check if water has gone inside. After repair, it is tested by a water head of 2.5 metres above the rudder top surface. An internal oil coat is given before closing the drain plug.
Propeller- Propeller to be examined for erosion due to cavitation, cracks and bent blades, CP blade bolts and cone connection to be checked. Blades to be polished, edges to be rounded.
Stern Gear- Stern tube bearing wear out and oil seal condition to inspect.
Underwater Valves, Sea chests- All underwater valves (injections and discharges), Valve grids, and ship's boxes should be examined from the dock bottom to ensure the grids are clear and secure. The valve body, valve lid, valve seal and spindle to examine and to overhaul and replace damaged parts.
Anchors and Cables- Lowered on the dock floor for a special survey. Corrosion and wastage if greater than 11%, then defective length to replace as also in the case of cracks in any link Joining shackles are parted and checked. When reassembling, the pins require to be re-sealed with a lead pellet.

Checks before leaving the Dry dock

Just before flooding the dock, all underwater fittings are checked to see they are satisfactorily boxed up and all double bottom tank drain plugs checked to ensure they are replaced and properly tightened.
Before flooding, tank soundings are taken, so that any adjustments to heel and trim requires (due to modification, fuel burnt or flood water cleaned etc.) can be made, to bring the ship in Stable condition.

On flooding, constant checks have to be made to the following for any leakage or abnormality -

All underwater valves, shipside valves, sea chests gratings fittings.
Stern tube to watch for leaks. Stern gland, rudder stock gland to be equipped, if leak shows.
Impressed current system anodes, the position of hull repairs, fuel tanks, D.B Tank soundings etc. to be checked to ensure everything is all right.

In-water survey- preparation, operation and procedure

Preparation:- To facilitate the underwater survey, plans must be submitted showing the external features
of the hull below the sheer strake together with a key plan indicating the location of frames, bulkheads, weld lines, openings etc. in order that the surveyor can plan his action beforehand.
Cleaning:- The hull plating surface is to be cleaned prior to the in-water survey by a particular system called 'Brush Kart'. It is a hydraulically operated vehicle with three brushing heads, It is drawn by a diver to clear the plating surface of all forms of marine fouling.

Survey Vehicle (Scan) - It is a self-propelled, steerable survey vehicle fitted with the following-

A long-range low light TV Camera to aid steering and check for hull distortion;
A close view high-resolution colour TV Camera, to give a true picture of the state of the hull coating, and for inspections of weld seams;
A 35mm still Camera;
An ultrasonic probe to measure plate thickness;
A depth meter;
Speed indicator;
An umbilical cable from survey vehicle to survey Boat for electrical power supply to the hydraulic motor, and information transfer;
Buoyancy spheres and compressed air bottle for floating of the vehicle.

Survey Boat - It houses the surveyor and consists of-

i) A control console containing TV monitors
ii) Plate thickness print-out;
iii) Audio Cassette recorder;
iv) Video Cassette recorder,
v) Playback unit;
vi) Diver Communication System;
vii) Vehicle Control system and all associated instruments.

Operation:-

The survey vehicle is taken to the datum line by a diver,
With the aid of TV monitors and using the shell expansion plan as a map, the vehicle may be guided, from the control console, over the bottom and sides of the hull by following weld-runs and other features such as inlets and tank plugs. Pictures and navigational information are relayed back and video recorded along with plate thicknesses giving the surveyor an integrated visual record of all relevant information. The vehicle will also provide pictures of the stem frame, rudder, propeller, bilge, keels and hull openings.
Divers are also used for measurement of stern bearing wear down, pintle clearance and to inspect stern seals, anodes and rudder stock palm coupling bolts.
The video recording, audio recording of the conversation between the surveyor and the diver and thickness printouts are analysed to obtain a clear knowledge of the ship's underwater condition.

Hull inspection by a Senior Engineer

Q. Describe the hull inspection that you would carry out as the Senior Engineer of a ship in dry dock stating what inspection you would be made as the work started,
a) Large seawater inlet chest and valves.
b) Forward end of the ship.
c) Propeller and stern bush.

Ans. Preliminary preparation prior to dry dock:-

(i) Taking inventory of spares (for machinery to be overhauled), tool (spanners for propeller nut, poker gauge, rudder palm bolts, intermediate shaft bolts etc.) and stores (pipes, bends, flanges, valves etc.)
(ii) Items for a continuous survey of the machinery for the special survey are identified (e.g. Tail shaft, Propeller, Stern tube bearing, Rudder, underwater Valves, sea chests) and necessary spares and tools are kept ready.
(iii) Existing defects identified and defect list prepared.
(iv) Last dry dock reports checked - Tail shaft, stern tube bearing dimensions, poker gauge reading, clearances, propeller condition, sea chest gratings etc.
(v) Stern gear and Rudder gear drawings to check, Engine Room double bottom tank plugs identified by frame numbers, which tanks to drain via plugs in dry dock and which plugs not to be touched, such as oil tanks, sea chest positions and overboard valve position identified.
(vi) To inform Engineer Superintendent regarding any specialized services for automation equipment or any imported spare requirement at dry dock.

At dry dock:-

(1) Shore electrical connection taken after checking voltage, frequency and phase sequences.
(2) Drinking water and sanitary water connection/disposal, fire hose connection and dom Fridge condenser cooling water are taken if run.
(3) To keep the ship upright and slight trim by stern.
(4) Sounding of all tanks recorded before docking.
(5) Any abnormal noise to be noted while the vessel is grounded above the keel block.

Inspection:-

Large Seawater Inlet chest and valves-Sea chest gratings, valve grids are examined dock bottom for any damage and same repaired or replaced with new gratings or plates. Any corrosion damage was examined and plate thickness checked. Sacrificial anodes are to be replaced with new ones. Valves overhauled and checked with hose test for any leakage. Finally painted with primer and bituminous paint. While flooding all the sea chests and underwater valves are to be inspected for any leakage constantly till the vessel is fully afloat.
The forward end of ship-All underwater parts of the hull are Inspected for damage and corrosion

If any distortion or dent is observed, an inspection of the inside of the ship is to be made to determine internal damage and if serious, should be repaired. All sacrificial anodes are to be renewed or if fitted with impressed current cathodic protection, reference anodes and main anodes are to be inspected and covered during dry dock to protect against sandblasting and painting.
The structure forward of the collision bulkhead is built to withstand the high dynamic stresses imposed (panting and pounding), bulbous bow to reduce wave-making resistance to provide increased buoyancy forward. Any damage to plate floors, deep floors, panting beams and panting stringers are to be inspected. Any damage to the flare surface due to falling anchor is to be inspected. Water tightness of forepeak store, forecastle store, forepeak tank to be ensured.

Propeller and Stern bush:-

Propeller surface and blade tips checked for roughness and damage due to cavitation pitting. Any major pitting on the surface is built up and the entire surface is smoothened by grinding. Blade tips rounded off. For keyed propeller, keyway, to be checked for any surface crack by magnetic particle detection for the keyless propeller, any ship between the tapered shaft and propeller boss to be examined, and forward one-third of the shaft cone is to be examined by a magnetic particle crack detection method. You can read a detailed article for the procedure for the inspection of the rudder in a dry dock
Stern bearing wears down to be measured by poker gauge for oil-lubricated stern bearing, condition of oil glands and shaft liver to be examined. In case of previous records of oil leakage outside seawater leakage inside, the oil seal ring garter spring or worn seal ring is to be replaced. Groove on the shaft liner, if noticed, due to which leakage occurred, the position of seal ring has to be shifted by a distance ring.

After complete assembly and during flooding close examination is required for any leakages Crankshaft deflection, thrust bearing clearances are to be checked to ensure, alignment has no been disturbed.

IACS Dry-docking requirement

IACS = The International Association of Classification Societies, the group of leading classification societies.
Other than an emergency due to damage to the ships hull, propeller or rudder, the dry docking requirements fall under the ‘Periodical Survey of the Outside of the Ship’s Bottom and Related Items’ as per the class of a ship. Rules of Classification Societies as members of IACS are guided by the Unified Requirements (UR) and Recommendations of IACS.
“Requirements Concerning Surveys and Certification” and, in particular, ‘Periodical Survey of the Outside of the Ship’s Bottom and Related Items’.

Z3.1 General
“Special Survey” = “Special Periodical Survey” = “Class Renewal Survey”

ZZ3.1.1 The Owner is to notify the Classification Society whenever the outside of the ship’s bottom and related items can be examined in a dry dock or on a slipway.
It means: Inform the Classification Society about the tentative dates and location for dry docking the vessel.

Z3.1.2 There is to be a minimum of two examinations of the outside of the ship’s bottom and related items during each five-year special survey period. One such examination is to be carried out in conjunction with the special survey. In all cases, the interval between any two such examinations is not to exceed 36 months.
It means: Ship has to be dry docked at least twice between two ‘Special Surveys”. The maximum span between two dockings is limited to 36 months.
The first Special Survey is to be completed within 5 years from the date of the initial classification survey and thereafter 5 years from the credited date of the previous Special Survey. However, an extension of the class of 3 months maximum beyond the 5th year can be granted in exceptional circumstances.
In this case, the next period of the class will start from the expiry date of the Special Survey before the extension is granted. For surveys completed within 3 months before the expiry date of the Special Survey, the next period of the class will start from the expiry date of the Special Survey.
For surveys completed more than 3 months before the expiry date of the Special Survey, the period of the class will start from the survey completion date. (In other words, it is not advisable to complete the special survey more than three months before the deadline.)
The Special Survey may be commenced at the 4th Annual Survey and be progressed with a view to completion by the 5th-anniversary date. When the Special Survey is commenced prior to the 4th Annual Survey, the entire survey is to be completed within 15 months if such work is to be credited to the Special Survey.

Z3.1.3 Examinations of the outside of the ship’s bottom and related items of ships is normally to be carried out with the ship in dry dock. However, consideration may be given to alternate examination while the ship is afloat as an In-water Survey, subject to provisions of Z3.3.Special consideration is to be given to ships of 15 years or over before being permitted to have such examinations. For ESP ships exceeding 15 years of age, such examinations are to be carried out with the ship in drydock.
It means: Subject to certain conditions, there is provision for alternate means of examination other than Drydocking. e.g. In Water Survey (IWS). Such options may not be available for ships of 15 years or over and definitely not for ships required to have Enhanced Survey Programme (ESP) (i.e. for bulk carriers and tankers)

Z3.1.4 The interval between examinations of the outside of the ship’s bottom and related items for ships operating in fresh water and for certain harbour or non-self-propelled craft may be greater than that given in Z3.1.2.
It means: For ships operating in freshwater, certain harbours and non-self-propelled crafts, the interval between drydocking may be extended.

Z3.1.5 Compliance with UR Z3 does not absolve the owner from compliance with the requirements of SOLAS as amended, especially when shorter intervals between examinations of the ship’s bottom for certain types of ship are required.
It means: Under certain circumstances, the interval between dry docking can be less than ordinarily permissible.

Z3.1.6 For Oil Tankers, Combination Carriers, Bulk Carriers, Chemical Tankers, Double Hull Oil Tankers, Double Side Skin Bulk Carriers and General Dry Cargo Ships, reference is also be made to Z10.1, Z10.2, Z10.3, Z10.4, Z10.5 and Z7.1 as applicable.
It means: Requirements vary with the type of ship and keep getting modified. One needs to keep a track of changes that are usually notified by the Classification Society.

Z10.1 Hull surveys of oil tankers Rev. 11 August 2003
Z10.2 Hull surveys of bulk carriers Rev. 16 Feb 2004
Z10.3 Hull surveys of chemical tankers Rev. 6 Oct 2002
Z10.4 Hull surveys of double hull oil tankers Rev.1 Oct 2002
Z10.5 Hull Surveys of Double Side Skin Bulk Carriers Dec 2003/Corr.1 Jan 2004
Z7.1 Hull Surveys for General Dry Cargo Ships Rev.1 June 2003

Z3.2 Scope of the survey

Z3.2.1 When a ship is in a dry dock or on a slipway, it is to be placed on blocks of sufficient height and with the necessary staging to permit the examination of elements such as shell plating, stern frame and rudder, sea chests and valves, propeller, etc.
It means: Arrangements to facilitate inspections need to be made.

Z3.2.2 The shell plating is to be examined for excessive corrosion, or deterioration due to chafing or contact with the ground and for any undue unfairness or buckling. Special attention is to be paid to the connection between the bilge strakes and the bilge keels. Important plate unfairness or other deterioration which do not necessitate immediate repairs are to be recorded.
It means: Guidelines for inspection of corrosion and deterioration.IACS Requirement for Drydocking.

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