DRYDOCKING 101

LAY PERIOD OF SHIP IN THE DRY DOCK

Dry docks are located in various locations convenient for ships to utilize in order to accomplish maintenance which cannot be performed waterborne. Such maintenance may be planned, as in the case of interim drydocking, or may be corrective to make specific repairs. Since interim drydockings are scheduled during upkeep periods, a great deal of maintenance will be performed in dry dock that could be done just as easily alongside the pier. Most drydocking routines require inspection of the underwater hull and hull appurtenances and provide for corrective action based on the inspection.

It must be recognized that the extent of services provided by the dry dock will vary from drydocking to drydocking depending on the type of dry dock, the class of ship drydocked, the anticipated duration of the drydocking, the work scheduled to be performed, and the ship’s systems to be incapacitated during drydocking.

Assigned Responsibilities

  1. Dry dock personnel:
  2. Drydocking and undocking the ship
  3. Providing dry dock services while in the dry dock
  4. Installation of temporary connections.
  5. Installation and maintenance of gangways.
  6. Rigging.
  7. Crane services as requested, rigging and unrigging brows, etc.
  8. Dock Master:
  9. Regularly walk-thru of the dry dock basin.
  10. Regularly check for blocking and shoring stability, ensuring there have been no drastic changes.
  11. Observe that safety precautions are being utilized by all personnel in and around the dry dock and ship.
  12. Calculation of weight shifts, additions and removals to ensure the vessel stays within safe limits of stability.
  13. Responsible for the safety of the vessel until the completion of the undocking evolution.
  14. Ship personnel:

The ship’s preparation for interim drydocking starts well before the drydocking date with the submission of work requests for various services required in dry dock. The latest Docking Report and the latest corrected Docking Plan must be furnished to the Dock Master with information on the desired docking position in sufficient time to prepare the basin prior to the drydocking.

Once in the dry dock, the ship is responsible to ensure that temporary lifelines, webbing, stanchions, etc., are promptly and properly installed topside. Besides the ship’s underwater surface, the following items normally are inspected at interim drydockings:

  1. Rudder and stern plates
  2. Shaft(s), bearings and propeller(s)
  3. Main shaft seal
  4. Main ballast tanks and free flooding spaces
  5. Sea valve and sea chest hull strainer plates
  6. Zincs (impressed current cathodic protection system anodes)

In addition, ship personnel are usually required to furnish line handlers during drydocking and undocking; and to help connect services.

Everyone:

Personnel working over the side, aloft, or on staging are required to wear safety belts and harnesses and all tools used topside must be secured by a lanyard.

As further protection to personnel working in the dry dock basin, hard hats are required to be worn by all personnel who enter the basin.

Safety goggles are required for all personnel who use or work in the vicinity of power tools. They are also required for all personnel in the dry dock basin during sandblasting and painting operations. Personnel engaged in such operations must also wear respirators.

Handrails, safety lines and stanchions on the dry dock must be rigged at all times other than during the actual operation of drydocking and undocking ships.

Areas of Great Concern

Propellers and Retractable Gear:

Operation of any external equipment, such as propellers, rudders, masts, antennas, anchors, or the secondary propulsion units must be carefully coordinated to ensure that personnel are alerted and kept well clear while the unit is operated. This requires first passing the word throughout the ship and the dock basin that the evolution is soon to commence, with the warning that all hands should stand clear.

Communications must be established from the operating station to the external unit which is to be moved. The concurrence of the Dock Master must be obtained prior to operation of any external equipment.

Flammable Liquids:

No fuel, oil, gasoline, or other flammable liquid shall be pumped or drained into the dry dock.

Paint must be considered a flammable liquid, and fundamental precautions are established to avoid igniting it during its preparation and application to the hull. The amount of flammable material in the dry dock basin shall be the minimum required for immediate use. Smoking is not allowed in the dry dock topside or basin while spray painting is in progress. Additionally, all hot work including welding, burning, chipping, grinding, sandblasting or the use of spark producing tools on the hull exterior is prohibited during this time.

Environmental Considerations:

Dry dock work has an inherent potential for contaminating the atmosphere as well as the water around the dry dock. Inadvertent discharge of oil into the dry dock basin could be injected into the sea. It is imperative that the Dock Master be informed and appropriate corrective action be taken at once if any oil is discharged into the dry dock basin. Self-compensating or self-ballasting fuel, oil, and contaminated oil tanks present an obvious pollution potential if inadequately flushed prior to drydocking.

Spray painting in the basin can contaminate the atmosphere with toxic/flammable fumes. Sandblasting in the basin produces substantial amounts of dust and grit. It must not be disposed of into the surrounding waters. At the completion of work, all such industrial waste must be shoveled into skip boxes and removed to a suitable dump site.

Closing Hull Openings at Night:

A plan has to be ready at all times, in the case of an emergency arising requiring the ship in the dry dock to be put afloat, encompassing measures to regain the ship’s watertight integrity below the waterline.

Storm Preparations:

Measures to resist high winds should be undertaken when it appears advisable due to any of the following circumstances: past experience at that time of year, weather reports, or storm warnings. The possibility that disruptive weather conditions may develop while a ship is drydocked makes it prudent for all concerned to stay current on the weather situation, particularly during storm season. If time permits, work on the ship may be expedited to make the ship seaworthy, thus providing the option of undocking prior to the onset of violent weather.

The Dock Master should be intimately familiar with precautionary measures required in preparation for hazardous or destructive weather conditions applicable to the area in which his dry dock is located. These measures should be determined in advance of the necessity for employing them, and must be tailored to the location, situation, and capabilities of the individual dry dock.

On the approach of a large storm, the Dock Master should consider preparatory measures to be taken.

Modern meteorological techniques provide, in most instances, ample warning of severe storm conditions, allowing time for precautionary actions such as those identified below:

If a ship is in dry dock determine whether the ship should remain in dry dock or be removed from the dry dock, based on the condition of the ship and the time required to place the ship in a safe condition for undocking. As a general rule, if a ship can be undocked in a condition that allows the ship full use of its navigation, ship control and propulsion system it is prudent to undock the ship. 

*Undocking a ship that lacks these capabilities, however, can present the ship with a significant risk unless adequate tugs, line handling and pier facilities are readily available. It was observed during severe Western Pacific tropical typhoons in World War II that ships remaining in dock, with proper measures to resist severe winds, survived without major damage to either the dry dock or ship.

Where it is necessary that a ship remain in the dry dock for the duration of a storm, every effort should be made to reduce, if possible, the areas exposed to the forces of the wind. For example, a floating dry dock can be safely flooded down to reduce sail area to the extent that approximately 50% of the ship’s weight continues to be carried by the dry dock blocking. With no ship in the dock basin, it is prudent to flood the dry dock down as deep as possible, paying close attention to the potential for grounding due to sea surge or the lack of water depth as a result of the storm driving water out of a harbor or bay coincident with the time of low tide. Care should be taken, where docks are moored with spuds, to determine that the clearance between the spud and spud keeper that retains the dock in its proper fore and aft position is not excessive. Excessive clearance in this fixture can allow the dock to surge and gain sufficient momentum to destroy the keepers and transfer the fore and aft load too, and cause the failure of, the transverse retaining spud system since this system is normally not designed to accommodate loads in the fore and aft direction. Consideration should be given to the use of wedges or spacers in the fore and aft keepers to correct excessive clearance problems and/or the use of spring lines to the pier to reduce the dry dock’s fore and aft motion.

*Lowering the floating dry dock’s freeboard can provide two benefits: (a) reduced sail area, and (b) in the case of dry docks moored to the pier with spuds, prevent the possibility of sea surges and/or high water lifting the dock and causing disengagement of spuds and keepers.

  1. Similar to the case of seismic loadings, care must be exercised to ensure that adequate blocking is installed to resist transverse rolling and thrust motions that can be generated by severe winds.
  2. Casualty Procedures

The most likely casualties to occur while a ship is being worked on in dry dock are fire and flooding. In either case, the protection of the drydocked ship is of primary importance. The prompt and effective control of damage and recovery from casualties is essential to the safety of the ship in the dry dock.

In almost all cases, the casualty will have been restored in the short time it takes the Fire Department to respond. In the exceptional case, the Fire Department will be on the scene promptly with their special equipment and expertise, prepared to keep the casualty from becoming a catastrophe. Dry docks which are not supported by readily available emergency assistance must be prepared to control the “worst case” damage or casualty with shipyard resources.

Dry dock personnel should be trained in all of the dry dock’s systems and capable of making informed decisions about what valve to shut or switch to turn to abate a casualty situation before it gets out of hand.

Weight Control Accounting:

Stability of a ship is dependent upon the location of its center of gravity and the shape of the ship’s underwater buoyant volume, and it is important to recognize that there exists a close interrelationship of weight, stability and buoyancy.

Major work on a ship in a dry dock often includes structural and equipment alterations resulting in the addition or removal of weight. The Dock Master must evaluate these weight changes so that he can accurately predict the stability, drafts, trim, and list resulting from the changes. It may be necessary to order compensating changes in weight to permit a successful undocking of the ship. In any case, in support of undocking preparations, the ship’s condition with altered weight must be predicted in detail. Accurate predictions are essential since failure of a ship to undock as predicted is an important warning of something is amiss.

Particularly important, stability effects can result from suspended weights, free surfaces of liquids in tanks, and, in the case of damaged ships, compartments open to the sea. The Dock Master must be able to assess the impact of all these situations.

The ship’s transverse stability is always a matter of concern during drydocking. Initial list and the sources of list, and the effect of these sources upon ship stability must be thoroughly understood.

The concurrence of the Dock Master is required for all anticipated weight shifts, including operation of any of the equipments listed in the preceding paragraphs, shifts of liquid loads, or taking onboard unusually large loads of provisions. The ship is responsible for recording all permanent weight changes and reporting them to the Dock Master prior to undocking. A form for recording tank level and weight changes is furnished to the ship as an enclosure to the Drydocking Notice. Limiting fueling or defueling operations while in Dry Dock is a wise safety precaution.

The Dock Master must keep an up-to-date log of the weight shifts, removals, additions, calculations, and current ship stability.

Before the Dock Master gives the “go-ahead” for any weight movements on the drydocked ship, the list calculation should be completed. If attention is not given and calculations are not performed, the safety of personnel and the ship are in jeopardy. The ship could crush the sideblocking and fall during the lay period. If control of the ship’s transverse stability was forfeited during the ship Lay Period in the dry dock, the undocking evolution will call attention to the fact; as soon as the vessel is afloat, it will fall to one side or the other. REMEMBER: A ship will capsize transversely before it will go stern-over-bow.

The following information is required of all weight shifts, additions, or removals:

  1. Item or Compartment Number
  2. Which Side of the Vessel the Weight is Being Moved, or Removed from or Added to?
  3. Weight of the Item
  4. Location of the Weight in Reference to the Keel
  5. Distance from Centerline
  6. Distance from longitudinal center of floatation (LCF)

The Dock Master must also accurately calculate any trimming moments that might affect the vessel, and to what amount in inches (cm). Longitudinal stability of the vessel is very important, but not as critical as the above-mentioned transverse stability. Once again, the Dock Master must ensure any of the longitudinal movement of weights on board the drydocked vessel has their results calculated before approval of the change. During the Lay Period, if trimming moments are not carefully watched or exceed the weight that can be supported by the blocking, tragedy can strike. An increase or decrease of vessel’s trim and/or its displacement will change the “Lift off draft” at time of undocking.

The Dock Master must remain steadfast in his resolve with regard to causing the ship’s crew to keep an accurate accounting of all weight shifts. On occasion, a value will be assigned to determine weights under a certain value for which accounting is not required. This process of assigning a maximum weight value before accounting for a weight can often encourage and lead a ship’s crew to estimating weight shifts, additions, and removals just under the required reporting weight. Remember, all movements will result in cumulative values and moments.

Any time during the lay period that major weight changes are going to occur, it is advisable to perform stability calculations (prior to the change) to ensure the change will not adversely affect the stability of the vessel. Calculation of the vessel’s stability is not only crucial while on the blocks, it is extremely important prior to floating the ship. A severe or unexpected list or excessive trim, or change in “Lift-off” could cause a catastrophe.

Affects of Weight Additions:

A weight being added to the ship affects:

  • the side of the ship it is being added to. For example, a weight added to Port creates a Port Inclining Moment.
  • the end of the ship it is being added to. For example, a weight added forward of the ship’s LCF creates a Forward Trimming Moment.
  • the KG of the ship. All weight additions create Added Vertical Moments.
  • the displacement of the ship. It increases the ship’s weight, thus causes Parallel Sinkage.

Affects of Weight Removals:

A weight being removed from the ship affects:

  • the opposite side of the ship. For example, a weight removed from the Port side creates a Starboard Inclining Moment.
  • the opposite end of the ship. For example, a weight removed from the bow creates an Aft Trimming Moment.
  • the KG of the ship. All weight removals create Removed Vertical Moments.
  • the displacement of the ship. It decreases the ship’s weight, thus causes Parallel Rise.

Affects of Weight Shifts:

A weight being shifted from location onboard the ship to another location onboard the ship has two actions: a weight removal followed by a weight addition. Since the weight was already considered in the ship’s displacement, the ship’s weight does not change. It still, though, can cause:

  • an Inclining Moment. For example, if a weight was shifted from Port to Starboard, a Starboard Inclining Moment is produced. Always remember, the Inclining Moment affects the side of the ship that gets heavier.
  • a Trimming Moment. Example, if a weight was shifted from Aft of the ship’s LCF to forward of LCF, a Forward Trimming Moment is produced. Always remember, the Trimming Moment affects the end of the vessel that gets heavier.
  • a Removed or Added Vertical Moment. A removed or added vertical moment will affect GM.

Final Note

“Prior to undocking, the commanding officer/master of a ship shall report to the Dock Master any material changes in the amount and location of weights on board which have been made by the ship’s force while in dry dock, and shall ensure, and so report, that all sea valves and other openings in the ship’s bottom are properly closed. The level of water in the dock shall not be permitted to rise above the keel blocks prior to receipt of this report. The above valves and openings shall be tended during flooding of the dock.”  (…more)

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