SHIP DRY DOCKINGWALKTROUGH THROUGH THE BASICS OF DRY DOCKING , PREPARED USING THE KNOWLEDGE GAINED AS AN INTERN AT A PUBLIC SECTOR SHIPYARD IN INDIA

Jishnu Saji ,Dept. Of Naval Architecture & Shipbuilding ,SNGCE , Kerala , India

What is dry docking ?Well simply put, it is docking the vessel in such a way so asto expose all the wetted area for inspection , repair andmaintenance. Obviously this will also be a good time toundertake the maximum repairs and inspection jobs. Thisis a period when the vessel is docked in a facility which isbest equipped for all hull and machinery jobs andtherefore all major inspections are also scheduled duringthis operation

WHY DRY DOCK THE SHIP ?The dry dock is not required by SOLAS , MARPOL ,TONNAGE OR LOADLINE conventions. It is actually aCLASS society requirement. This is to conduct thedocking survey as per the classification society. Thedocking survey is part of the periodical surveyconducted by the classification society in order tomaintain the vessel in class. The docking surveys areconducted by the class societies surveyors.

The durations to conduct dry dock

• For ships of up-to 15 years age (before the 3rd specialsurvey ) 2 dry docks in each window of 5 years.

• The duration between above two surveys will be at-least2 years and maximum 3 years.

• For ships over 15 years of age (beyond the 3rd specialsurvey ) dry dock every 2 years.

MAIN AREAS OF INTEREST IN DRY DOCK INSPECTION• Shell plating• Side shell coating • Stern frame• Rudder• Propeller• Sacrificial anodes / ICCP arrangement• Logs and echo sounder transducer• Anchor and cable• Chain locker• Sea suctions and chests• Bilge keel and fittings

Personnel involved • Ship’s officers• Manager’s superintendent• Ship repair manager ( from Dry dock) • Class society’s surveyor / surveyors• Repair foremen• Paint company’s representative (the company who’s paint you will use on your hull)• Sub-contractors representative ( eg. Radio or automation company for specific jobs )

In case the dry dock is following a damage or accident then it will be necessaryfor a H&M underwriter and the owners P&I correspondent to also attend.

Generally following is the sequence followed:-

• Vessel will maintain a running defect list.• The defects which warrant attendance in the dry dock will be moved to a dry dock file.• The contents of the dry dock repairs is updated to include all the particulars and plans

relevant to the repair or maintenance.• Basis of the current contents of the dry docking file , a consolidated initial Dry dock

specification will be generated.• This copy is considered both by ship’s officers as well as the manager’s superintendent.• With due review of the specs, they are finalized and forwarded to various dry docks for

raising quotations.• Basis of the vessels operating area and availability of dock ,,also most importantly ,,the

tenders submitted by the various dry docks ,the vessel is booked for dry dock in a particular period.

• Charterers are informed and the vessel is arranged to go out of employment for the period it is to be in the dry dock.

• It is common for most ship owners to maintain patronage with a particular group of dry docks and try to arrange the employment of their vessels in such a fashion that they may disengage and proceed for dry dock without wasting too much time in transit.

• Ship’s plans are forwarded to the dry dock in advance.• A docking plan is discussed.• Keel blocks are laid as per the vessel’s docking plan.• Stability calculations are prepared prior arriving at the dry dock.• In consultation with the dry dock the docking trim is agreed upon.

Structural features of the ship are considered.

Common dry dock jobs

Hull cleaningHull paintingHull markings renewalAnode replacementStructural repairsPropeller cleaning and polishingRudder and shaft drop

measurementInspection of transducers

Inspection of bottom plugsAnchor ranging and markingChain locker demuckingBallast tank demuckingFuel tank inspectionClose up surveys of cargo and ballast

spacesStructural modificationsAll other jobs which are labour and

facility intensive

Common services required when in the dry dock

• Docking assistance• Access to vessel (as own

gangway will be unusable)• Shore power (as generators will

be overhauled)• Shore crane for heavy items • Oxygen and gas lines for gas

cutting• Welding connections for steel

renewals• Fire main to remain pressurized

(own pump unusable)

• Portable eductors and pumps for emptying unpumpables

• compressed air for miscellaneous jobs• communication facilities• medical and safety facilities• garbage reception facilities• Shore water supply• Sewage connection or reception

facilities• Ballast water prior dock flooding on

departure.

SPECIFIC GUIDANCE ON HULL MAINTENANCE

When asking for quotations for hull fabric maintenance it is necessary to make an accurate assessment of corrosion on the side shell and also the area of various sectors as follows:-

Formulae for Accessing area

underwater area = [ (2X Keel to waterline + Breadth ] X LBP X block co.eff

also :- Wetted area = 2.58 X displacement X LOA

Underwater Vertical sides area = (2 X Keel to waterline) X LCB X (Breadth / 2)

Dry vertical side area = 2 X Freeboard X { LOA +( Breadth/2)}

Boot top strip = 2 X Bw X {LBP + (breadth /2)}

Flat bottom = u/w area – wetted vertical sides (as calculated above)

In contemporary use and practical application it is common forvessel to be provided with a paint manual which will include allabove information about the vessel and same may be referred forraising quotations.

When noting the area to be grit blasted a set of reference cards areused. Each card has a picture of a surface with corrosion. It ismarked as 5% surface corroded, 10% surface corroded, 20%surface corroded etc..

The officer breaks up the surface of the ship’s hull into sectors

Aft topside

STERN

Mid-ship top side Forward topside

BOW

Boot top area ….forward 1/3rd , mid 1/3rd , aft 1/3rd

Vertical side …. forward 1/3rd , mid 1/3rd , aft 1/3rd

Assessing corroded surface area on the hull

Once the area corroded is established we do the

following:

• Area corroded + 10% = area to grit blast

• If area to blast is > 65% of total area then might as well grit blast the whole with almost sameexpense.

• One of the major expenses during a dry docking is the hull cleaning, grit blasting and coating. Forthis purpose the owners engage a particular paint company. It is usual for the company to send apaint manager to attend to the whole process. His job is to monitor surface preparation , cleaningand painting process. It must be ensured that panting is not done in conditions of excessive relativehumidity (air temp should ideally be 3 to 5 degrees above the dew point of the air )

• The area which has been blasted will be touched up with a first coat which will overlap up-to 10%beyond the blasted area. The second coat will overlap 20% of the blasted area. The whole processof mixing and preparing paints and also application renders about 10 to 20 % wastage and thisshould be accounted for when ordering paints.

• The “touch dry “ and curing times of various layers must be well understood.

STABILITY CONSIDERATIONS ON DOCKING AND UNDOCKING

When the vessel takes to the dry dock keel blocks itexperiences an upthrust on the point where the hullmakes contact with the block. This upthrust can beconsidered as the discharging of weights from that point.You can understand that if we discharge weights fromthe bottom of the vessel we shall cause the centre ofgravity of the ship to rapidly rise. Due to this rise in COG,there is a virtual loss of GM, which may render the vesselin an unsafe , unstable condition if due regard is notgiven to anticipate this loss of GM.

Vessel must have :-

• Adequate initial GM so that despite the loss of GM (apperant loss ) the vessel shall still have residual positive GM and remain in stable equilibrium )

• Must have minimum trim (preferably by stern ) to ensure not too much upthrust is generated ( upthrust = trim at time of taking blocks X MCTC )

• Vessel must arrive upright to avoid any problems when taking to the blocks• Appreciate that RM = (W-P) X GM sin heel – P X KG sin heel ) hence have min

GM required to have adequate RM to keep vessel upright • Pre-empt all intermediate GM and drafts fwd and aft for the critical period.• Pre-calculate the draft at the time that blocks are takes all over.

Excessive trim = excessive up-thrust = excessive load on the stern frame = excessive loss in GM.

Hence a minimum or moderate trim must be arrived at for purpose of docking. Alsodisposition of ballast for refloatation must be such that the trim on refloatation is also notexcessive to avoid the same problems. Here you must appreciate that ballast has to be putback in the tanks by shore facilities to allow refloatation trim to be achieved. As this is apaid facility owners will press you to refloat with the minimum ballast. In this case theminimum ballast to take will be such that the Main sea suction must be submerged onfloatation so that M/E cooling is achieved and further ballasting may be done by vessel.The distribution of such ballast will again take into account the trim on refloatation.

This presentation gives verylimited knowledge ,butcovered the very basics ofdry docking procedures.

DO NOT REPRODUCE/EDIT THIS DOCUMENT ,

THANK YOU