DRYDOCKING 101

THE UNDOCKING EVOLUTION

Personnel in the business of drydocking and undocking vessels have come to believe that the undocking of a vessel is a simple evolution that can be performed by nearly anybody. Nothing could be further from the truth. The undocking can be a most perilous operation if not performed properly. Preparations must be as absolute as those for drydocking and shall be carried out with the same meticulous attention to detail.

“Docking accidents should not happen, but lack of communication between the Dock Master and the supervisor of repairs caused a Navy tug to capsize in a dry dock during the refloating operation. The engines of the tug were removed and not replaced during the lay period of the vessel. Unaware of this, the Dock Master began the undocking evolution and attempted to refloat the tug. Since the vessel’s KG was increased due to the removal of the engines, the tug’s GM became negative; the vessel became unstable and capsized.” Hence, the undocking of a ship following alterations can be more hazardous than the actual drydocking, particularly when the current status of the stability condition of the vessel is unknown.

One of the principal concerns of the Dock Master during the undocking evolution is that all weight changes, additions, removals, and shifts have been accurately recorded and new refloating data with regard to GM, list, trim, lift-off draft, etc., has been calculated.

Often ships that have experienced significant weight changes during their lay period will have developed a list, critical trim, or even marginal to negative GM. Any one of these developments could cause significant damage to the ship and the dry dock and severe injury or even death to personnel.

Once again, the steps are for a “typical” undocking procedure and the Dock Master should recognize that actual procedures will differ from dry dock to dry dock and from ship to ship. The Dock Master should be thoroughly familiar with the check-off list for the dry dock he controls, and must fully understand each of the procedures involved. Such practices free the Docking 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.

Prompt recognition of an abnormal condition and immediate corrective action are essential to safe docking operations. The same careful planning and preparation, the same skillful coordination of personnel, and the same exactitude in execution required for the docking operation is again called for at the time of undocking. In many ways, undocking appears to be merely drydocking in reverse; yet it has singular features that require special attention.

The Undocking Notice and the Undocking Conference serve the same purpose as did the notice and conference before docking. They provide clear communication among the responsible organizations involved.

Weight Control Accounting

One purpose of the Docking Notice and Docking Conference was to notify the ship of the restriction on weight changes and shifts during the ship’s stay in dry dock. As called for by yard work, changes may result from transfer of liquids, emptying or filling tanks, removal or addition of machinery, changes of sonar equipment, etc.

All such weight changes must be accounted for in detail so that, prior to undocking, the Dock Master can compute the net change of weight, the net change in the center of gravity and thus the change in draft, list and trim of the ship between condition at the time of docking and the scheduled undocking. In cases of major underwater body modification, the buoyancy characteristics of a ship can be altered.

With the data available, then, the Dock Master can predict the drafts at which the bow will lift, the stern will lift, and the list the ship will assume as it clears the blocks. If a list is predicted, action should be taken to reduce the list to zero. Necessary weight transfers or additions should be made to achieve this condition, keeping in mind the impact of such action on the ship’s stability.

Pre-undocking Checks

In the typical drydocking situation, work on the ship continues almost to the time of undocking. For many cases, it appears to be a race to see if the work will be done before the ship is to be undocked. The responsibility for the work, the inspections and tests are shared between the shipyard and the ship. For this reason, it is important that the Dock Master become familiar with the routines involved. In fact, it is prudent for the Dock Master to conduct informal checks that items are not left incomplete, which would cause a last-minute postponement of the undocking.

Examples of such items of work are: work on propellers, shafting, bearings, rudders, hull repairs, underwater body preservation, installation of zincs, etc.

The Dock Master’s experience with ship overhauls and concerned attention to overall progress can be valuable assets in avoiding delays, if proper and timely notice of a needed action is provided to those responsible for undocking.

While it will vary from dry dock to dry dock, there are certain tasks and work performed by the shipyard which requires timely accomplishment. Some typical tasks are:

1. Removal of industrial work support equipment from the dock basin
2. Removal of staging
3. Removal of debris and trash and receptacles from dock basin
4. Disconnection and removal of dry dock services
5. Final Check of the Dock Basin

The Dock Master should always do final checks of the dry dock basin prior to flooding the basin.

Items checked include:

1. All debris is removed.
2. All blocks, wedges and piers securely dogged.
3. All block caps securely nailed in place.
4. Shores removed from the dry dock (if applicable).
5. Shores that are to remain in the dry dock rigged so that they can be maintained under control and will not foul hauling blocks or the ship after liftoff (if applicable).
6. All hauling sideblocks and hauling keel blocks checked to ensure that operating mechanisms are cleared.
7. Final Preparation for Undocking

Assume that upon undocking it will be necessary to immediately redock the ship; therefore, all equipment such as centering chains, sighting devices and spacers, alignment battens, weights to correct list, grip hoists, etc., must again be ready for use. It can happen that a ship must be redocked immediately after lifting off.

The Dock Master will have to reposition the ship just as during the drydocking and conduct the landing of the ship again. Standard procedure should be to plan for this contingency.

Flooding to a “Safe Stop Level”

At the Undocking Conference, an agreement will have been reached to stop flooding at a water depth in the dry dock just less than that at which the ship will lift off. The Dock Master will flood the dry dock basin to this point. The dry dock water level will be held at this stage to permit a careful inspection aboard ship of the watertight integrity of the ship, particularly in way of any repairs or alterations that have been made during the course of the drydocking. If it is discovered that corrective action is needed on some underwater work, the ship is still fully landed on the blocks and the dry dock can be readily dewatered to undertake any necessary corrective action.

When the ship informs the Dock Master that all checks have been completed and conditions are satisfactory, the undocking operation is continued.

Lift Off of the Ship

During this process, the list of the ship should be watched with care. Presumably, the ship should lift with no list.

Be mindful…while the ship is partially resting on the sideblocks, a list could possibly begin to develop. The combination of improper weight distribution in the ship and the water’s buoyant effect on the ship’s hull could cause it to begin to heel. This would be a cause for concern and require investigation as to its cause by the Dock Master.

The Dock Master will have predicted the drafts at which the bow and the stern will lift. Just as in drydocking, if these occur at a markedly different draft, it is a cause for concern, for it indicates that the ship has a different weight distribution. This can indicate a change in the stability characteristics other than that which was originally estimated by the Dock Master for the time of the undocking. A lift off of the bow or stern at a draft significantly smaller than predicted is reason to stop undocking and to investigate the situation.

If all continues as planned, the dry dock basin is flooded until the ship has floated well clear of the blocks. As it will probably surge as it lifts off, the ship must be securely held by mooring lines at this time. At this stage, the ship is again held in position for a final check of watertight integrity by the ship. If a reason for redocking is discovered, the ship is still within the dry dock, certain essential dry dock services are being provided, and drydocking can be accomplished with a minimum of additional effort.

When the ship reports that all needed checks have been made, the dry dock basin is then flooded to exit draft.

Flood to Depth for Exit

Following the ship lifting off the blocks and completion of final shipboard checks, the dock basin is flooded and any dry dock services that remain connected are removed.

Sideblocks and hauling keel blocks are hauled to their extreme outboard position and any shores used in the drydocking are removed.

The Ship is Hauled Out of the Dry Dock

As the last extremity of the vessel crosses the still line, the responsibility for the ship passes from the Dock Master to the ship’s Commanding Officer or Master.

Deballast and Inspect the Dock

As soon as the ship is clear of the dry dock, dewater the dry dock basin for inspection.

When the dry dock is dry, a careful check is made of the dock basin. This is done to ensure that all blocks, all block caps, etc., can be completely accounted for. The purpose of this check is to assure that during the undocking; a piece of dry dock material did not become dislodged, possibly fouling the ship. Dry dock block caps and other materials have fouled ship’s keels, sea chests, propellers, etc.

Accounting for the material in the dock following undocking assures that such a situation does not exist. If dry dock material is missing, the proper authority should be notified so that a check by divers can be made to ensure that there has been no critical fouling of the undocked ship.

Dock List Control during Undocking (Floating Dry Dock)

Before concluding a discussion of undocking, a special comment should be made about the critical control of the floating dry dock list during undocking.

As the undocking process starts, the pontoon is clear of the water. The transverse stability of the dry dock depends upon the moment of inertia of the total area of the dry dock’s water plane, i.e., the area of the wingwalls on each side and the area of the entire dry dock floor.

As flooding proceeds, a point is reached at which the pontoon goes under the water. At this point, the moment of inertia contributing to stability is only that of the wingwalls, and the negligible contribution of the bilge blocks and keel blocks. The entire pontoon is now free surface, open to the sea. This condition leads to a sudden and dramatic decrease in GM of the floating dock. Depending upon the weight distribution in the dry dock, free surface effect in ballast tanks, and the height of the center of gravity of the drydocked ship, GM of the dry dock can decrease by a factor of 5 or even more. If the dry dock had a slight list just before this decrease in GM, the list will then be multiplied by a factor of 5 or more! If the dry dock develops a list, the list would rapidly grow to 1 degree or more. It is crucial to maintain the list of the floating dry dock to zero.

This is a critical time in flooding down a floating dry dock. Ballasting has to be very carefully controlled so that list is negligible as this sudden decrease of GM takes place.  (…more)