DRYDOCKING 101

THE DRYDOCKING EVOLUTION

Most cases are not ideal dry docking situations. Very few ships have a uniform weight distribution along their length. The majority of ships will arrive at the dry dock with zero list, but few may arrive with zero trim or without hog or sag along the length of the keel. Because of these non-ideal conditions the ship will often land on one keel block and gradually load all the blocks.

The information in this chapter focuses on the actual drydocking of a ship. We stress the importance of “attention to detail.”

Preparation for Drydocking

Drydocking operating procedures shall be written in step-by-step detail and shall be followed verbatim. These procedures shall include dry dock check lists for use in prerequisite checks of dry dock systems (control systems, valves, pumps, lineup checks, etc.) status before the drydocking operations are initiated.

The Dock Master should be present at the time the dry dock basin is flooded and remain at the dry dock until all blocks are well covered. This is to make sure that no blocks come adrift or are misplaced. During the drydocking, the Dock Master should be on the dry dock rather than on the vessel being docked. This enables the Dock Master to be present during all preparations for the drydocking.

Prior to the arrival of the ship at the dry dock, the Dock Master shall have briefed the line handlers onboard the vessel and on the dry dock of the planned drydocking evolution and of their duties and responsibilities.

The Dock Master shall verify that the line handlers are properly stationed on the dockside and when dry dock lifeline stanchions can be removed or lowered (some lifeline stations cannot be removed or lowered). Dry dock lifeline stanchions should be removed or lowered only when there is water in the dry dock basin. Personnel have been injured or killed when they have fallen into a dry dock basin.

The Dock Master shall verify that all stations are adequately manned and that all necessary dry dock systems and equipment are ready for the drydocking.

Wind and storms are hazards to drydocking operations. Winds of high velocity can cause a serious handling problem for ships having large sail areas. A limit for acceptable winds should be established during the drydocking planning. In drydocking operations, the Dock Master and the entire dock crew must be alert to the first signs of abnormal conditions. Prompt recognition of an abnormal condition and immediate corrective action are essential to safe drydocking operations.

Critical Stages in the Drydocking Process

There are a series of critical stages in the drydocking process. The Dock Master must be aware of these and must be alert to the signs of abnormal conditions and be ready to take corrective action. A good axiom to follow is there is no such thing as a “routine drydocking”.

Once a ship had entered a dry dock, the Dock Master has responsibility for safety and control of the ship. The Dock Master should discuss with the ship, harbor captain or any other agencies involved in controlling the ship to determine when the responsibility crosses over to the Dock Master. It is important to discuss control to avoid fighting for control during active operations. The suggested time to transfer responsibility is the ship’s Captain/Master shall be fully responsible for the safety of his ship until the first extremity of the ship reaches the dry dock sill and the ship is pointed fair for entering the dry dock. The Dock Master shall then take charge of ship safety and complete the drydocking. The Dock Master shall remain responsible for ship safety throughout the lay period and until the extremity of the ship clears the sill and the ship is pointed fair for leaving the dry dock during the undocking, then the ship’s Captain/Master shall assume responsibility for the safety and control of the ship.

Ship Control: Control of the ship approaching and being maneuvered in the dry dock is essential to the safety of both ship and dry dock and is particularly critical because of the transfer of control to the Dock Master as the ship enters the dry dock. Wind, weather and currents pose special problems for the Dock Master in this regard and call for the application of sound judgment and good seamanship.

Problems related to ship control are usually averted by good planning, maintaining excellent communication among all parties involved in the drydocking, good judgment regarding drydocking under adverse environmental conditions, and through good training of the personnel assigned to ship control in the dry dock.

The Dock Master should carefully study the characteristics of the ship he handles in the dry dock and consult with other knowledgeable persons regarding the ship handling behavior under various conditions of loading, wind, etc.

Dry Dock Control: The dry dock systems must always be controlled such that at any stage of the drydocking, the process can be arrested and held indefinitely at that point. Successful, safe drydockings depend upon the ability to stop and hold the operation at any signs of abnormal conditions.

List and Stability: In planning and executing the drydocking, stability and control of list are major considerations. The list of the ship being drydocked must be carefully monitored both by the ship and from the dry dock throughout the operation.  The most critical period of stability is from the time the ship first lands on the block system until the sideblocks or shores are all in place touching the hull on both sides of the ship. In the case of fixed side blocks it is not as much of a concern. If a list starts to develop during this stage of the drydocking, the operation should stop at once. Taking additional water off the hull can decrease the stability of the ship and further increase the list. The cause of the list must be ascertained before drydocking is resumed.

Note that a growing list could be a sign of marginal or negative GM of the ship. Landing weights on deck to correct the list will only worsen the situation as adding high weight decreases GM. Such a correction could result in the ship assuming a much larger list in the opposite direction. Another cause for a growing list could be an off center interference between ship and dry dock. This situation cannot be corrected by adding weights or continuing the drydocking process.

If the ship develops a list during drydocking:

  1. Stop the dewatering process.
  2. Determine the cause of the list and correct the situation.
  3. Do not correct by adding off center weights.
  4. Do not proceed further without having taken corrective action.

In the case of emergency drydockings, it may be required to accept a ship that has or will develop marginal or negative GM during the drydocking. Steps can be taken to improve stability so that such a ship can be drydocked. One method is by fitting wale shores between the side of the ship and the side of the dry dock to hold the ship upright until sideblocks are in position. If the ship has trim, the wale shores are placed first at the point of landing aft and gradually worked forward as the ship knuckles.

The cause of the marginal or negative GM must be corrected while the ship is drydocked, for it cannot be safely undocked in that condition.

Monitoring: Continuous monitoring of ship drafts and dry dock water depths is essential. In cases where there is interference between the ship and the blocks, the usual sign is a temporary landing of the ship. If not noted and dewatering continues; the interference, ship structure, and/or blocks may be damaged.

Summarized actions to be taken for a problem during a drydocking:

– Follow the plan and procedures for a drydocking.

– Monitor ship list, trim, drafts, depth of water over blocks.

– Arrest the drydocking process at indication of abnormal conditions.

– Obtain information regarding the abnormal condition.

– Evaluate the information so that the Dock Master knows what has happened or is happening in the dry dock. If further information is needed, continue to hold and investigate further.

– When the situation is fully understood, plan the steps needed to correct the situation and carry them out. These actions may range from refloating the ship and rescheduling the drydocking, to proceeding without further delay after a minor drydocking interference has been cleared.

In all cases, the Dock Master must take time to develop a well-informed plan. It is imprudent to press ahead in a drydocking when the Dock Master has no or limited knowledge about an abnormal condition.

The Deflection Plane (floating dry docks)

A floating dry dock, like a ship afloat, will deflect according to the balance between its total loading and the buoyant forces of support. Most vessels are designed to handle severe sea conditions and ship’s Masters/Captains do not normally concern themselves with deflection measurements. Some very large cargo ships and tankers do monitor vessel deflections., but experience has taught them that when operating in very high seas it is prudent to reduce speed and/or change heading to avoid over-stressing the vessel and the possibility of catastrophic structural failures. A floating dry dock is not normally exposed to hazardous sea conditions. The loads imposed during a drydocking operation or even when adjusting ballast with no vessel in the dry dock can result in very high stresses in the dry dock structure. These stress are often a result of improper operation of a dry dock that can lead to a failure of dry dock structure. The stress levels set up in a dry dock structure are proportional to dry dock deflections. It is important that the Dock Master has a clear understanding of the deflection limitations of the dry dock, and the methods available to monitor the dry dock deflections.

Drydocking a short, heavy ship that leaves the ends of a dry dock unloaded will result in large internal stresses in the dry dock. Drydocking ships off centerline or multiple dockings can impose eccentric loads and twisting strains. To adequately control the stresses generated during such drydockings requires a carefully worked out pumping plan but the overall dry dock alignment to be checked while making the block set up and at frequent intervals during the pumping and flooding process by observing dry dock deflections.

Most relatively short one-piece floating docks are designed with sufficient longitudinal strength to handle various ship loadings and the normal differential loadings imposed by variations in pontoon and wingwall ballast tank water levels. These loadings can be handled safely within acceptable dry dock structural stress limitations. This is not to say that well thought out pumping plans to accommodate vessel trim and minimize dry dock and vessel stresses can be neglected, but the need for monitoring dry dock deflections can often be safely eliminated. The discussion that follows concerning dock deflection is directed primarily at the operation of sectional or longer floating dry docks.

The floating dry dock deflection plane and a means of monitoring distortion of this plane with changing dry dock loading provides the Dock Master with a very important safety device. Operation of sectional or longer floating dry docks should not be attempted, whether empty and changing ballast or with a ship in the dry dock without monitoring deflection on a continuous basis.

Monitoring of the deflection plane is accomplished by use of a deflection gauge system mounted on or near the top of each wingwall. The system consists of a high-powered telescope or transit with cross hairs and leveling screws and series of targets mounted on each dry dock section such that the target’s horizontal center lines lie in the deflection plane or in a plane parallel to the deflection plane when the dry dock is in an unstressed condition.

Targets or deflection indicators are located at or near the longitudinal midpoint of each section’s wingwall. It is desirable to mount them as high as possible on the side of the wingwall, above the maximum draft of the dry dock in a line that affords an unobstructed view of all targets along the length of the wingwall. Suitable distinctive markings on the targets allow determination of dry dock deflection, normally in inches (cm). Lighting of the targets is required for night operation of the dry dock.

Prior to commencing a drydocking operation as a means of checking or placing the dry dock in an unstressed condition for setting the blocks, the targets are aligned such that each target zero line coincides with the horizontal cross hair of the scope. The horizontal cross hair must always pass through the zero line of the nearest and farthest targets. This line is the datum line.

When the dry dock is in operation transferring ballast or drydocking a ship, the scope cross hair will tend to depart from this datum line. It is necessary to continually adjust the scope’s focus to maintain this datum line. With the datum line maintained, the amount of deflection of any section above or below the line can be read from the position of section’s target zero line relative to the datum.

During drydocking operations, a trained operator should be stationed at the deflection gauge on each wingwall. The operator’s readings of the gauges are transmitted to the control room in order to permit continual adjustment of the pumping or flooding process.

An experienced control room operator will keep the dock deflection to a minimum. In drydocking a damaged vessel or one for which no adequate weight distribution figures are available, dry dock deflections can become large. By slow and deliberate operation, any condition tending to introduce stresses or bending moments into the dry dock structure can be corrected. By pumping a section which is low or flooding a section which is high, any deflection in the dry dock can be eliminated. The dry dock should never be operated with a deflection beyond the dry dock’s design deflection limits.

The longitudinal girder strength of a vessel on the blocks will reduce or eliminate deflection in that length of the dry dock on which the vessel rests, but any vessel will hog or sag a slight amount if subjected to severe bending moments. For example, if the dry dock has no deflection as a vessel is landed on the blocks and continues in this condition until the vessel is lightly supported by the dry dock, it is a general indication of proper blocking and proper pumping. However, if the vessel is further loaded on the dry dock and a slight hogging or sagging is noticed in the dry dock under the vessel, it is an indication of improper pumping. Forces within the dry dock are applying a bending moment to the ship.

As a vessel is being landed on the blocks, if a sudden downward deflection or change of deflection is noted in the dry dock in a certain area which cannot be accounted for by the pumping, the control room operator should immediately stop all operations. This is an indication that something is wrong. Divers should be sent into the dry dock and under the vessel to note any misplaced blocks, ship projections contacting the blocks, or irregularity of bottom plating which could cause the dry dock to become deflected. By careful use of the deflection gauges the dry dock in single ship, multiple-centerline ships, or even echelon drydockings can be taken to any necessary and desirable trim or list without excessive deflection and consequent danger to the dry dock or ships being drydocked.

The Dock Master should be aware of the deflection limits established for his particular dry dock and insure that during all dock operations deflections are carefully monitored and kept within the set limits. Experience with floating sectional docks has established various rules of thumb relating to deflections that may be useful to the Dock Master. For example, when pumping or flooding a dry dock, it should be possible to maintain hogging or sagging deflections to within 1 inch per 100 feet of dry dock length (3 cm per 30 m of dry dock length). This rule of thumb does not override the dry dock design limits. The dry dock should never be operated with a deflection beyond the dry dock’s design deflection limits.

For a floating dry dock, the list of the dock itself must be controlled throughout the drydocking operation and must be kept essentially at zero. A few minutes list is a large list angle during a drydocking operation.

Typical Drydocking Procedures

The Dock Master should recognize that actual procedures will differ from dry dock to dry dock. Also, for any specific dry dock, there will be variations in procedure to account for the ship to ship differences.

As stated in Chapter Three, the use of check-off lists is strongly advised. Generic check-off lists are provided in the Course Workbook. The Dock Master should be thoroughly familiar with the check-off lists for the dry dock he controls, and must fully understand each of the procedures involved. Such practices free the Dock Master to concentrate upon the execution of the ongoing operation and the next immediate steps of that operation, and avoid the “inadvertent omission” found all too often in casualty and accident reports.

  1. Drydocking Planning
  2. Determine date and time:

The date of the drydocking hinges both on scheduling of ship’s and dry dock’s availability. The time of drydocking is of major importance to the Dock Master and may depend critically on the state of the tide, currents, and expected weather. The importance of tide, currents, and expected weather can vary widely depending upon the location of the dry dock.

Check the readiness of the supporting systems:

The dry dock crew reports on the readiness of all dry dock systems needed to support the drydocking. This is an important check point. The redundancy of systems, or components within systems, is provided to assure that each drydocking can be conducted safely and reliably even in the event of a failure of one system or equipment. Undertaking a drydocking with the knowledge that a dry dock’s redundancy is impaired should be evaluated with great care.

Check of the dry dock basin

The Dock Master must check the readiness of the dry dock basin before any water enters. This includes checking that the block systems are properly secured and dogged and that debris has been removed. The final check is needed because work on the dry dock or in the basin may have produced debris or disturbed the block setup since the time the dry dock basin blocks were prepared and checked out.

Ballast down or flood the dry dock basin

Water should be added to the dry dock basin with care. Too rapid flooding of the basin can cause currents in the basin that may damage or distort blocks. The Dock Master should carefully watch the block system during flooding to see that nothing is damaged or comes adrift in the block system. Additionally, careful watch should be kept for debris which may enter the dry dock during the flooding process. Many harbor areas are badly littered with debris, often waterlogged wood which has only slight or neutral buoyancy. Such debris can enter a dry dock during flooding of the basin which can cause problems.

Ready to Receive Ship

Presumably the Dock Master has been accurately advised of the expected drafts of the incoming ship. Any calculations on ship displacement and stability have been based upon these expected drafts. Additionally, the water levels must be calculated to insure adequate clearance for the ship over the blocks.

A significant discrepancy can cause a delay in the drydocking. An unanticipated draft could be caused by a variety of actions on the part of the ship, for example, the filling of double bottom tanks expected to be empty. This could cause an issue of clearance over the blocks. Additionally, calculations will change if the drafts are different than expected. Expected tide levels can change due to weather effects. Thus, as the ship enters the dry dock, the Dock Master must check the actual ship drafts and level of the water in the dry dock basin.

Dock Master assumes control of the ship When the ship is fair for entering the dry dock; the Dock Master assumes control of the ship. The Dock Master will advise the pilot with regard to ship position and alignment. Upon assuming control and starting the ship across the sill of the dry dock, the Dock Master has relieved the Captain/Master of the responsibility for control of his ship. At this time, the pilot receives further orders from the Dock Master and, in most cases, controls tugs as needed for the Dock Master. In case of winds across the face of the dock, it is often necessary to use tugs to keep the stern (or bow) of the ship fair with the centerline of the dry dock while the bow (or stern) is being held in position by lines handled by the dry dock’s personnel.

Haul-in of the ship into the dry dock

Once the ship has crossed the sill, it should be brought safely and smartly to position. As the ship enters the dry dock, pass additional handling lines and haul ship into dock.

In the ideal case, the dry dock would be wide enough with enough water over the blocks that the clearance between the ship and ship appendages and the side of the dry dock and blocks would not cause concern as the ship is being placed in position. Nearly all real cases are not ideal. The route to final position has to be carefully checked in the planning stage and carefully executed during the drydocking.

In planning the drydocking, the Dock Master will have decided how the ship will be brought into the dry dock. For some ships with unique underwater features or damaged ships with distorted hull plating, it may be necessary to bring the ship in offset from the dry dock keel block centerline. When in position longitudinally, the ship is then moved sideways into position over the blocks. By using scale drawings of the cross sections of the ship and of the dry dock, the Dock Master can plan exactly how he wants to move the ship into the dry dock and then into final position.

It should be clear from the above that the ship’s position must be under complete control as it is moved into the dry dock. The ship striking the blocks, the underwater structure, or sides of the dry dock during the positioning process is not acceptable and can cause damage to a ship and/or the dry dock. The ship contact may also disable that portion of the keel block or side block system and result in unequal distribution of loading between the ship and the dry dock. Fenders or bumpers can keep the ship from contacting the dry dock.

Line Handling Systems

Ships to be drydocked are brought into position, and held in position, by tugs just outside the dry dock sill. At this point, lines are rigged from the dry dock to the ship. Hauling the ship into the dry dock commences as the first extremity of the ship crosses the sill, the Dock Master takes over control and responsibility for the safety of the ship until such time as the last extremity of the ship crosses the sill during the undocking process.

Considerable skill and good judgment are required to properly handle drydocking lines to guide the ship safely into dry dock and to position the ship in the desired location. Years of experience with handling ships into dry docks have established the proper positioning of capstans, cleats and/or bollards needed for the line handling operation. The number, position and power of capstans and the number, size and position of cleats will vary with the size and configuration of the dry dock, and size and configuration of the types of ships expected to be drydocked.

Capstans: Ships coming into dry dock are often dead ships, that is ship’s propulsion power cannot be used. Additionally, with near zero speed rudders will not provide steerage way. Thus it is necessary to use dry docking lines and capstan power to haul a ship into the dry dock and capstans along with cleats to control the ship’s transverse position in the dock basin.

Cleats: Cleats for securing and/or controlling lines are spaced at intervals along the sides of the dry dock. The space of cleats is such that line handlers can shift lines while the line immediately forward or aft maintains control using a cleat until the line being moved has a proper lead from a cleat to provide control by running the line as necessary. Cleats can provide control only when a line is in tension. Always remember the basic axiom of line handling: “YOU CANNOT PUSH ON A LINE.”

Record Drafts

A recording of ship drafts should be made when the ship is in position and before dewatering of the dry dock basin has started. An exact knowledge of draft is important for the determination of the water level when the ship should land on the blocks and for necessary checks that the ship has not landed on a fouled block or projection.

Removing water from the dry dock basin

As the water is removed from the dry dock basin, the positioning of the ship begins. Removal of the water will change the tension in the lines. Thus, constant monitoring and adjustments must be made to the lines tension and to the ship’s position. Some shipyards will switch from normal ship handling lines to wire ropes because they do not stretch significantly with increased tension.

Continue deballasting or dewatering until the ship is 1 foot (.3m) before touching the blocks. Final positioning of the ship will be completed before divers enter the water to accomplish block, ship hull, and positioning checks.

The use of divers is valuable insurance. It is possible that docking plans are not properly

updated and that blocking in accordance with the plan provided will result in a block interfering with a hull opening. Also, a check at this stage will detect debris which may have entered the dry dock. The debris can be wedged between the ship and a block or between the ship and the dry dock itself.

A careful check by divers is essential in the case of damaged ships where it may be necessary to modify or relocate some of the blocking. A careful check by divers is time consuming. In cases of routine drydockings of ships previously drydocked, diver checks are often made only in the case of indications of fouled blocks, problems, etc. It is better to take the time to be safe than to hurry and have a mishap

Centering and Alignment Systems

Monitoring the transverse centerline position of the ship relative to the block system centerline can be accomplished by any one of numerous techniques.

Centering chains or wires suspended between the dock’s wingwalls, at right angles to centerline of the block system. The “tee” in the forward chain is used to match the center point of the last keel block for transverse centering of the vessel’s bow, whereas, the “tee” in the after chain is used for transverse centering of the stern and longitudinal placement of the vessel over the blocking.

Transits or suitable telescopes mounted on the dry dock so as to coincide with the block system’s centerline plane can be used to monitor the ship’s centerline plane.

Battens mounted on the dry dock so as to define the block system’s centerline plane. The ship’s stern, transom and/or suitable targets mounted topside are then maintained aligned with the plane established by the dry dock battens.

Centering tackle can use a variety of mechanical devices that provide significant mechanical advantage or purchase power while at the same time provide a vernier control and a positive means of holding a selected position. Chain falls, wire rope block and tackle, “come-a-long” wire power hoists, ratchet driven turn buckles, etc., are examples of equipment that can be used for this purpose. Wire or chain should be used with such centering systems to avoid stretch or surge conditions that could result in an unstable positioning system.

Pairs of centering tackle are rigged from the dry dock sides to the ship, port and starboard, and just prior to the ship landing these are tensioned, as required to maintain the ships’ centerline plane coincident with the block system centerline plane. Similar pairs of tackle can be rigged that tend fore and aft to maintain the ship accurately in the proper longitudinal position relative to the block system.

Resume dewatering of the dry dock basin and land the ship

In the ideal case, the ship will not have any trim and the ship will land fore and aft on the keel blocks simultaneously.

Some dock operators prefer to land a ship with a small amount of trim. By so doing, they have controlled the point of first contact between the ship and the dry dock. This may make it easier to monitor the depths of water over the blocks and the drafts at which the ship first lands and lands all along the keel.

In the case of many ships, one or more of the ship’s draft marks do not correspond to heights measured from the reference plane the Dock Master uses to measure depth of water over the blocks. Propellers, rudders, sonar domes or other appendages may protrude below this plane and the draft marks may indicate depth to the extremity of the appendage. The Dock Master will have examined the docking plan with care to learn how the draft marks are located on the ship.

Notes: Especially on small ships, personnel movement aboard ship can provide substantial heeling moments. For this reason, personnel movement on such a ship must be reasonably restricted until after the sideblocks/shores are in contact with the side of the ship and the Dock Master so informs the ship.

Knuckle block loads

As the dry dock is pumped up, the ship with trim lands on a keel block near the deeper end of the ship. This is the “knuckle block” (sueing block). As the water is removed from the dry dock basin, the ship pivots about this block. The block or blocks upon which the ship initially makes contact exert an upward force on the ship, which reduces the trim while, at the same time, reduces the mean draft of the ship. If the knuckle block were infinitely rigid, the ship would contact it only at one end, and a concentrated point force would be applied only to that end of the one block. Actually, keel blocks are purposely constructed in part of timber and allowed to deform appreciably as the weight of the ship is applied. As more and more weight is borne by the knuckle block, the trim of the ship is decreased so the angle between the keel and the line of the top of the keel blocks decreases. At the same time, the knuckle block is more and more compressed, so that eventually Block #2 also begins to support the keel, then Block #3 and so on, forward along the line of the keel blocks (ship assumed down by the stern).

The Dock Master can estimate the ship’s draft and the total force exerted upon the keel blocks at the time the ship lands fore and aft. The critical condition for block loading and for block stress occurs not at landing fore and aft, but earlier in the drydocking during the process of removing trim. At that stage, the force between the dry dock and the ship is concentrated at the knuckle block or on those one or two blocks immediately forward of the knuckle block. The loading and stress in the knuckle block exceeds that borne by single keel blocks after the ship has fully landed.

The nature of the composition of the keel blocks is a crucial factor determining how the load is distributed along the blocks as the ship pivots about the knuckle to land fore and aft. Any analysis of individual block loads, then, must carefully account for the load-deformation characteristics of the keel blocks.

Resume beballasting and land the ship (floating dry dock)

In the ideal case, deballasting is controlled so that the trim angle of the ship and the trim angle of the dry dock are matched. In such a case, the ship will land fore and aft on the keel blocks simultaneously.

Note that if the ship has zero trim, faulty deballasting can result in the dry dock having trim. As a result, the ship would land either forward or aft first and pivot about a knuckle block at that point until the ship and dry dock trim angles match.

Some dock operators prefer to land a ship with a small but definite relative trim with respect to the dry dock. By so doing, they have controlled the point of first contact between the ship and the dry dock. This may make it easier to monitor the depths of water over the blocks and the drafts at which the ship first lands and lands all along the keel.

In any event, it is imperative that the ship land so that the depth of water over the keel blocks at the point of landing equals the draft of the ship at that same point. In the case of a ship and dry dock with zero trim, using a horizontal baseline and the ship drafts measured from that baseline, the ship should land when the depth of water equals the ship’s draft.

In the case of a ship with trim being landed on a dry dock with trim, the question of draft at landing and depth of water at landing becomes more complex. If the trim angle of the dock matches the trim angle of the ship, the ship will land simultaneously fore and aft at the free floating drafts of the ship. As a general rule, the dock depth boards indicating depth of water over the blocks are not located at the same fore and aft position in the dock as are the draft marks on the ship when the ship is placed in its proper longitudinal position for drydocking. Since the dock has trim, the depth of water read at the depth board is not the depth of water at the point where the ship’s drafts are read. This means that in planning the drydocking, the Dock Master must calculate what these readings would be at the instant the ship lands. By so doing, the depth of water as measured at the depth board will be known such that when the ship lands, the water depth over the keel blocks will be equal to the draft of the ship at each point of landing. As the ship lands aft, it will start to pivot about the knuckle block. The stern beings to emerge from the water and the bow sinks deeper into the water. For the typical ship, the change in drafts at the bow and stern are roughly five times the change in mean draft. For this reason, the emergence of the stern and the sinking of the bow should be detected before there is any significant change in the mean draft. As this rotation of the ship is noted by the Dock Master, it will indicate that the ship has landed aft. If the rotation starts early, or otherwise than planned, the basin water removal should be stopped until the situation is checked, evaluated, and understood.

From the discussion so far, it is clear that it is important to determine when a ship has landed on the blocks. For typical ships, a very precise determination of landing can be made by using plumb bobs forward and aft. Consider the case of a ship which will land first aft, and then begin to rotate to meet the block forward. A plumb bob is suspended and tended over the ship’s stern, and so placed that its top is just 1/8 inch below the water surface. Forward, a bob is similarly suspended from the stern except it is placed so that its top is just 1/8 above the water surface. As the ship lands aft and starts to pivot, the aft bob comes above the water surface and the forward bob sinks below the water surface. This procedure can easily detect landing aft before mean draft has significantly changed.

Once the ship has landed aft, the forward observer must keep tending the bob so it is just below the water surface. As the bow sinks deeper into the water, the bob is continually raised to maintain its initial position relative to the surface of the water. When the bow lands, the forward bob will come out of the water without the observer raising it. Again, landing forward can thus be detected within a small draft change.

Throughout the landing operation, the list of the ship should be carefully monitored by those aboard ship and by personnel on the dry dock. In a normal drydocking, no list should develop.

As the ship lands on the blocks, the blocks pick up some of the weight of the ship, exerting an upward force on the keel. The effect of this force is identical to the removal of weight from the ship at keel level, resulting in a virtual rise of the ship’s center of gravity (increasing the distance KG).

This increase in KG results in a decrease in the ship’s initial stability, since GM = KM – KG. If the ship were supported by the keel blocks alone and the dewatering process continued, at some point GM would reach zero and the ship would become unstable on the keel blocks and could topple over to assume a list at some equilibrium position. Before this point is reached, some form of side restraint must be provided.

To ensure complete control of “Holding the Vessel,” grip hoists may be used and/or place the lines in mooring configuration. Remember to watch the lines/wires for tension or slack. After landing on the blocks and diver inspection of the ship fit on the blocks, resume deballasting and level the dry dock. Note that the dry dock is maintained at the planned drydocking trim until after the sideblocks are in place. Only then is it brought to zero trim.

Dry dock control is better when the transition through the pontoon deck waterplane can be made with a slight dry dock trim.

Stop water removal when the ship has landed on all the blocks

If required, sideblocks must be hauled or shores installed well before the ship becomes unstable. Damage has been caused by hauling sideblocks before a ship with trim has landed firmly forward. The blocks, hauled too far inboard, result in the ship landing primarily on the sideblocks rather than the keel blocks at the forward end.

A 1 ft (.3m) change of draft may be dangerous for ship stability in the drydocking of tugs or other like vessels and sideblocks may have to be hauled earlier in the drydocking process.

To avoid the possibility of damage to the ship’s hull, it is important that this condition of full keel contact be achieved before the sideblocks are hauled. If the sideblocks are hauled too early, these blocks will bear too large a portion of the ship’s total weight. If the sideblocks are hauled too late, the ship, lacking the support of the sideblocks, will become unstable.

Drydocking a ship with trim introduces special problems with regard to the timing for hauling the sideblocks. If all the sideblocks are hauled too early when docking a ship with trim, the load picked up by the sideblocks at the forward end of the ship as it rotates down to make contact with the keel blocks could be great enough to damage the blocks, the ship, or both.

After sideblocks are hauled, they should each be checked and reported to be in the proper drydocking position. Failure of a sideblock to properly position can result from a malfunction of the block haul mechanism or may reflect a fouling of the block or hull which could result in damage. An improperly placed block should be of concern to the Dock Master and he should delay further steps in the drydocking until he is satisfied as to the cause and significance of the misplaced block.

After landing on the blocks and diver inspection of the ship fit on the blocks, resume dewatering.

Inspect Blocks

As soon as the water level permits entry into the dry dock, the Dock Master enters the dry dock to check the positioning of the ship on the blocks and inspects each block for proper fit and support of the ship.

It may be found that one or more blocks make a poor fit to the ship’s hull, in which case they should be shimmed or wedged to provide the proper support. If this arises because of a hull deformation, note should be made so that proper corrections can be entered on the Docking Plan.

The Dock Master may decide to add shores or further blocking to support overhangs, supplement blocks which appear to be damaged, etc.

When satisfied that the ship has been properly and safely landed and securely blocked, the Dock Master then completes the remainder of the drydocking process including securing and preparing the dry dock and ship for the repair process.

When a dry dock has completed a drydocking, the safety of the dry dock and the drydocked ship relies on the support of the dry dock and its system. Care must be taken immediately following dewatering to detect any leaks in the flooding system. Just as debris can foul a block in the dry dock, so too can small debris foul the seat of a flooding valve. For this reason, careful individual checks of the closure dry dock valves should be made at this point. (…more)

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