DRY DOCKS AND DOCKING PROCEDURES All ships during their working life will find themselves entering a dry dock for survey work, routine maintenance or from the effects of an unscheduled accident. In any such case the choice and type of dry dock will often be dictated by the physical dimensions of the vessel, especially the draught and the geography reflecting the location of the ship in relation to the position of the dockyard. Clearly owners will also be influenced by the overall costs of one dock compared with another for the intended repairs. To this end the economic factors could well win the day on the eventual choice of dock. The fact that there are now several types of dry docking operations possible, this may also influence the choice depending on the nature of the required services TYPES OF DOCKING - Docking operations can take place in any of the following scenarios.
The Graving Dock Usually stone built, with stepped sides known as alters the graving dock has an access from the seaward, navigational channel. This is closed off, once the ship has entered, by a dock gate known as a caisson. Once aligned the water is pumped from the dock allowing the vessel to take the pre-arranged blocks aligned on the floor of the dry dock. Docking Slip Usually an exposed slipway with a cradle arrangement that can transport the smaller vessel up the slip by mechanical means, effectively dragging the ship onto the shore line. Once the tide drops the vessel is left exposed, high and dry on the slipway. Docking work and repairs can be carried out and the vessel can be walked back down the slip into the water following completion of docking. Floating Dock An effective tanking system that is flooded and allowed to sink.This allows the water line to be increased, permitting the vessel to be docked to float in and over the docking platform.The tanks are then emptied and the buoyancy allows the dock to rise bringing the vessel with the platform above thewaterline. Floating docks are particularly useful in the case of a damaged vessel because the dock itself can be listed to accommodate any adverse list that a damaged vessel might have acquired. Synchro-Lift A mechanical system capable of docking vessels upto about 10,000 grt.This system is a mechanical platform which is physically hoisted by a series of dock side winches. Once heaved up clear of the water, the vessel is pushed forward and heaved sideways on railed trackways into a docking bay. Such a system allows numerous ships to be docked with a single docking system, a distinct advantage over the single entry graving dock.
HYDROLIFT DOCKING SYSTEMS In December 2000, the ‘Lisnave Shipyard’ at Setübal, in Portugal opened a platform hydrolift, docking system for ships.This new concept allowed for the docking of three Panamax sized ships simultaneously. The system works in conjunction with a wet basin which is entered via a caisson at the seaward end. Once the inward bound ship is established in the basin, the outer caisson is closed.The water level is then increased in both the basin and the designated platform dock by pump operation.The platform dock can alternatively be filled by gravity from the basin area. Once the basin is full and level with the dock space, the ship will have been elevated sufficiently to clear the sill of the platform dock.This position allows the dock caisson to open and permit the transfer of the vessel from the basin to the dock platform. The dock caisson is subsequently closed and draining of the dock is then allowed by gravity. INWARD AND OUTWARD PROCEDURES FOR HYDROLIFT DOCKING SYSTEMS The docking method by means of the hydrolift system operates in a similar fashion to a vessel passing through locks. A prime example of this is seen in the Panama Canal passage, when vessels pass from the Caribbean Sea through to the Pacific Ocean. Log Book Entries – Entering Dry Dock (Assumed Graving Dock) A typical, routine docking operation would expect to include log accounts of all activities leading up to and including the docking procedure: • Tugs engaged at rendezvous position. • Vessel proceeding towards open lock (usually under pilotage). • Line ashore forward. Line ashore aft. • Tugs dismissed. • Moorings carried up Port/Starboard. • Stern clears gates.
• Vessel stopped making headway inside the dock. • Dock gates closed. • Moorings adjusted to align ship fore and aft. • Moorings checked to hold vessel. • Dock pumps commenced pumping out dock water. • Block contact made and vessel enters critical period. • Vessel sewn on blocks fore and aft. • Side shores passed to port and starboard. • Residual water cleared from dock. • Gangway access landed between shore and ships side. • Gangway walkable. • Pilot dismissed. • Pumping of the dock complete and dock floor walkable. Any additional tasks that are undertaken while the dock water is being pumped out would also be noted in the log book,e.g. cleaning and scrubbing round the hull by work punts in the area of the waterline, as the level falls. A separate record of tank soundings would be recorded in the tank sounding book for a complete set of ‘on the block soundings’. These would be taken as soon after the vessel is ‘sewn’ on the blocks. NB. Although the gangway access is provided by the Dock Authority, the responsibility of rigging and ensuring safe access remains the prerogative of the ship’s Master. Chief Officer’s Duties Preparation and precautions for entry 1. All hatches and beams should be in the stowed position to ensure continuity of strength throughout the ship’s length. 2. All derricks and cranes should be down and secured, not flying.
3. Any free surface in tanks should be removed or reduced to as little as possible, either by emptying the tank or pressing it up to the full condition. 4. Stability calculations should be made to ensure adequate GM to take into account the rise of ‘G’ when the vessel takes the blocks. 5. Consult dock authorities on draught of vessel and trim required. Generally a small trim by the stern is preferred, in normal circumstances. 6. Inform dock authorities in plenty of time of any projections from the hull of the vessel, as indicated by drydock plan. 7. Sound round all ship’s tanks before entering the dock, to be aware of quantities aboard. Note all soundings in sounding book. 8. Sound round all tanks once the vessel has taken the blocks, to ensure a similar stability state when leaving the drydock. 9. Lock up ship’s lavatories before entering the dock. Ensure adequate fenders are rigged for entry into the dock and that dock shores are correctly placed against strength members once the vessel is positioned. If it is the custom in the graving dock, arrange for fo’c’sle head party to position shores on one side and the stern party to deal with the other side. 11. If required, endeavour to have the vessel cleaned and scrubbed as the dock water is pumped out. When drydocking with cargo aboard 12. Inform dock authorities where to position extra shores or blocks to take account of additional stresses caused by the weight of cargo aboard. 13. Give cargo areas a lock-up stow whenever possible. When in dock
14. Obtain telephone/electricity/and water pressure fire line garbage and sanitation facilities as soon as possible. 15. Have documentation ready, inclusive of repair list, for dock personnel. 16. Should tank plugs need to be removed, sight their removal and retain the plugs for safe-keeping. Ensure that plugs are labelled after removal.
Duties of the Chief Officer prior undocking Ensure all the listed work is completed to a satisfactory standard. In particular that all 'survey work' is completed, prior to leaving the dock. To this end a final internal inspection of the vessel would be the order of the day. Carry out an external inspection of the hull and enter the Dry Dock. This final visit to the dock floor would also encompass the replacing of any tank plugs that have been drawn. This task should not be deligated to a junior officer as the Chief Officer must sight all the tank plugs being replaced. The Dry Dock Manager would accompany the ship's Chief Officer on final inspections and ensure that no vehicles, materials or personnel are remaining in the dock, prior to commencing any flooding operation. Inform the ship's Master of the expected departure time and the crew would be engaged in activities to make the vessel ready for sailing. These activities would include odering the Navigator to plan the ships movement from the dock, posting the sailing board and cancelling shore leave, placing the engine room and respective
personnel on standby, carrying out checks on all navigation equipment and making relevant entries into the deck and offical log books. Ensure that a full set of tank soundings have been taken and that adequate supplies of fresh water, fuel and lubricating oil are on board to suit the ships movement needs. These tank quantities would then be applied to a complete stability check to ensure that the vessel has an acceptable GM once she floats clear of the keel blocks. Stability checks are the sole responsibility of the ships personnel and comparison should be made between the entry soundings when the vessel was last afloat. All hatch covers would be closed up and the watertight integrity of the uppermost deck assured. Anchors and cables would be heaved up and stowed correctly aboard the vessel. All pipelines, power lines etc. would need to be disconnected and relavent manpower should be made available both ashore and aboard the ship in order to release these safely and at the appropriate time. Tugs, the marine pilot and linesmen would need to be ordered to standby for the time of departure. Ships crew would be placed on standby on the fore and aft ends to tend moorings. Finally, the chief officer would sign the Authority to Flood Certificate. This is provided that he is satisfied that the Dry Dock Authority has completed the docking specification and that the ship is in a seaworthy condition. This certificate should then be completed to allow the flooding of the dock to commence.
The Undocking Process
Before water is pumped into the dock, there are a few checks that must be made. Bottom plugs must be closed and sea chests should be in full working condition. Also, ballasting of the ship must be done. This is to ensure that the ship does not have an even keel draught (the aft draught is usually greater than the forward draught). The pump room, which is usually located at the forward end of the dock controls the amount of water being pumped out of the dock. This is also referred to as ballasting the dock. The gangway is lifted sometimes by means of a crane once the dock personnel have cleared the ship. Fire hydrants and all shore connections are disconnected. The forward and aft ends of the ship are attached to shore based mooring lines which are winch controlled. A crane lifts these lines and places them on the deck so that they can be attached. These help to control the movement of the ship as it leaves the dock so as to ensure it leaves smoothly. When the level of seawater in the dry dock reaches the sea level, the dock gates are opened. A tug boat attaches a tug line to the aft end of the ship and begins to pull the ship backwards (out of the dock). The shore based mooring lines help to guide the ship smoothly out of the dock. Another two tugboats are on standby on either side of the ship. Once the ship is halfway out of the dock, the aft shore based mooring lines are disconnected and the standby two tugboats attach themselves to the ship by means of tug lines.
When the ship has cleared the dock gates, the front shore based mooring lines are detached and the tug boats turn the ship around. Once the ship is some distance away from the dry dock, the tug lines from all three tug boats are detached and the tug boats move away from the ship. The propeller is then started and ship moves away on its own propulsion. Draught and Trim The vessel’s required draught and trim will be decided by the drydock manager and the declivity of the drydock bottom.A small trim of between 12 in (30 cm) and 18 in (45 cm) is considered normal but will be dictated by circumstances. If a floating drydock is to be engaged, the drydock itself can be trimmed to suit the vessel, especially if the vessel has sustained shell damage. Drydock Plan This is a plan carried aboard the vessel which shows recommended positions for keel blocks and shores. Normally the frames are numerically indicated from aft to forward, and the strakes lettered from the centre-line out and upwards. Indicated on this plan will also be the position of any external projections from the hull, namely, echosounder units, stabilisers, scoops for condensers etc. Either a separate plug plan will be carried or the tank drain plugs will be indicated on the drydock plan. Stability of Vessel This is the responsibility of the vessel, and should be adequate to cope with the virtual rise of G as the vessel takes the blocks.The vessel shouldnot be listed. Should damage be such that the vessel cannot counter an acquired list, then shoreside weights should be taken aboard to bring the vessel to an even keel.
Positioning of Shores and Associated Docking Stresses The docking procedure incurs many stresses on the hull from the shores placed in accord with the docking plan. Incorrect placing of shores can and do cause damage to the vessel when she ‘Takes the Blocks’.This is especially so in the case of specialised vessels fitted with additional appendages like azimuth thrusters, long bilge keels or prominent condenser scoops.To this end, before dock pumping is commenced, many dock authorities are now employing the services of divers to ensure that correct lineup has beenachieved and the ship will not incur additional damage during the critical period. Shore positions should be placed with care and should ideally be placed in way of strength members like the intersection of ‘Deck Stringers and Frames’. Bottom hull blocks, set for wide beam vessels especially, should be placed to coincide with intercostals and other similar longitudinal members to avoid ‘soft spots’ which could lead to hull indentation of the shell plate. Dry dock stresses occur because of the loss of support which is normally gained from the all round water pressure.The vessel will become subject to an upward thrust from below the keel position caused through the lower blocks on the floor of the dock.There will also be a tendency for the ship’s weight to cause a downward and outward stress action to the vessel’s sides while in the dock. Provided the ship is only docked for a short period of time, any permanent or extensive distortion through stress is unlikely and the ship should revert back to her normal lines once re-floating occurs. It is also possible to dry dock a vessel with cargo on board for a short period of time. This can be done successfully without incurring overdue stresses in the hull, provided the position of the cargo is known and additional shores can be deployed to prevent undue deformity caused by the cargo weight. The possibility of being able to complete the dry-dock specification without pumping the dock completely dry is also an option to relieve stresses on the hull. Though this option is not always
possible and restricts working on the hull, it remains an option, especially if the vessel is loaded or part loaded. Repair Lists It is normal practice to carry out repairs when entering drydock, these repairs may be expedited by detailed work lists covering expenditure limits, work monitoring, state of survey, maintenance of classification, and protection of owners’ interests. To Calculate the Virtual Loss of GM There are two methods for ascertaining the virtual loss of GM. In each of the two methods the force P must be known. Force P represents the upthrust at the stern at the moment the vessel touches the keel blocks. The time the keel first touches the blocks until the vessel has taken the blocks overall is considered to be the critical period (Figure 14.10).
where MCTC represents the moment to change trim one centimetre, t represents the trim in centimetres on entering the drydock, L represents the distance between the centre of flotation and the vertical line of action of the P force, in metres. The first method considers the movement of the metacentre (M):
The second method considers the movement of the centre of gravity (G):
Either of the two methods are acceptable when W represents displacement of the vessel. KM represents the distance between the keel and the metacentre, KG represents the distance between the keel and the centre of gravity of the vessel.
