DRYDOCKING 101

SPECIAL DRYDOCKING SITUATIONS

There is wide variety of special situations that will inevitably develop when one is involved in drydocking ships over any reasonable period of time. The individual nature of ships and of ship repair requirements assures that these special circumstances will arise.

As the Dock Master has already learned, every drydocking is “special” in that it requires attentive, individual care in both planning and execution. The Dock Master will find that the special drydocking situations will tax his ingenuity, his capacity for original planning, and especially his knowledge of and ability to apply the basic principles that ensure the safe operation of a dry dock. This chapter is aimed toward provoking thought and questions about serious problems every Dock Master may have to face when handling these situations.

Damaged Ships

Much of the work of dry docks involves the repair of damaged ships. The drydocking of damaged ships is a challenging problem as each case is new and unique to itself. Problems arise relating to a ship’s hull form, hull strength, list, trim, stability, and the dry dock capabilities.

The docking of a damaged ship presents to the Dock Master his most challenging assignment:

1. Each case of damage is different.
2. Standard docking plans serve as a guide but may require modification.
3. Standard ship weight distributions may be incorrect.
4. Ship stability must be corrected for free surfaces and for inclined centerlines if the damage involves flooding.
5. Ship hull structure may be both grossly distorted and weakened.
6. The ship may be unable to enter the dry dock until divers clear away damaged side and/or bottom plating.

Ships with compartments open to the sea should be landed slowly, allowing flood water to drain from the ship as it is landed.

Flood water in holds, tanks and other compartments which have progressively flooded or have been flooded during fire fighting may not drain as a ship is drydocked. If damage control dewatering has not been effective, it may be advisable to have a diver open the compartment to the sea to allow the water to drain as the ship is landed.

Because of weakened ship hull girder strength, local weight concentrations in the ship may be abnormally concentrated on the dry dock. This can result in excessive block loading or loading on the dry dock. For this reason, it may be advisable to partially or totally dry dock the ship, affect an immediate hull girder strengthening, and then carefully redock the ship for a final repair.Damaged ships are often unstable. Wale shores are frequently used to provide the needed stability upon drydocking. Sideblocking and spur shores can be carefully fitted after the hull shape has been determined.

Often, the drydocking of a damaged ship is an emergency situation. Planning time is short and advance information is sketchy. The Dock Master must plan and improvise as the situation develops. This will call upon all his expertise and knowledge of the drydocking operation, and especially upon his ability to foresee contingencies.

Hull damage may result in changes in hull form calling for blocking arrangements substantially different from those of the standard Docking Plan. Diver inspection of the hull is important to survey the extent and nature of damage and to determine the location and extent of any protruding members that could interfere with entering the dry dock or landing on the blocks. After landing on the blocks, careful diver surveys should again be made to assure that the blocking conforms to the hull as planned, and that further adjustments and additional blocks are not required.

Hull damage may so weaken the hull that the ship’s overall longitudinal strength has been seriously impaired. In cases of very serious damage, it is sometimes necessary to consider the forward and after portions of the ship as separate sections and plan the simultaneous docking of the “separate” parts so as not to impose further load on the remaining structure.

In case of flooding, the stability of the ship may be impaired. A very careful stability analysis is required. If adequate stability is retained, landing the ship will dewater flooded compartments, reduce free surface effects, and will normally result in a lowering of the center of gravity.

Special Design Ships

Special design ships may have hull characteristics that pose unusual problems to the Dock Master. Each case requires careful and detailed study when it arises. Many naval hull forms with little or no flat bottom and a high dead rise require very high bilge blocking and are unusual in that sense.

Tugs and trawlers also present unusual forms. The operational requirements for both result in a design with a large screw and often a large designed drag. Like many naval ships, many of these ships have no flat bottom, high dead rise, and a very rapid rise of buttock lines aft; all of which lead to high blocking for their size. Many, especially among the trawlers, are fitted with a bar keel rather than a flat keel. This requires special consideration of the keel block design.

Hydrofoils and Surface Effect Ships literally fly over the water, supported not by buoyant forces but by hydrodynamics and aerodynamic forces. Major portions of the hull cannot handle concentrated local hull loads. Special blocking arrangements must be designed to support the craft. Particular care should be taken to follow the Docking Plans for these craft in all required details. For all very light scantling ships, including fast patrol boats, special care must be taken to avoid local structural damage from block loading.

Cycloidal Propellers, Special Oceanographic Facilities have unusual long structures protruding beneath the keel of the ship. Docking Plans for such ships call for unusually high blocking or may require the provision of a recess or pit under the special fitting. The drydocking is further complicated by the need for special planning to move the ship into place over the high blocking.

As ship design changes and advanced ship concepts are introduced, the Dock Master may be confronted with the need to dry dock ships with markedly different underwater forms. For example, the unusual hull features that may be encountered include:

1. Extreme bulbous bows (on tanker classes).
2. Extreme below keel bulbous bows or sonar domes.
3. Hydrofoil craft.
4. Surface effect ships.
5. Ships with vertical axis propellers.
6. Ocean exploring and research vessels.

In most cases, adequate Docking Plans are available. In the case of major, below keel level protuberances such as sonar domes, it has been found necessary in some cases to modify dry docks to permit handling of these ships.

In the case of hydrofoil craft and air cushion vehicles, the lightweight hull construction will require careful analysis. Blocking support can only be provided at points designed to absorb the block loading, or must be widely distributed to limit local loading and pressure.

In the case of hydrofoils, ships with vertical axis propellers and other ships with long, below keel line protuberances, very high blocking may be required. This presents the following problems:

1. Structural stability of the high blocking must be assured. This is usually accomplished by cribbing.
2. Adequate clearance between ship and blocks during drydocking and undocking must be assured.
3. The stability of the ship/dry dock combination during docking and undocking must be checked (floating dry docks).

Since the center of gravity of the ship may be high, the situation may be more critical than that for the routine docking of normal hull forms.

In the case of hydrofoil ships, the beam of the ship in way of the foils is unusually large as compared to the beam of more normal hull forms having about the same displacement. This requires the use of a wide dry dock. In addition, dry dock loading concentrations require study. If the foils are non-retractable and the ship must be landed on the foils alone, the concentration of the load on the dry dock may be extremely high. Construction of special cradles to distribute this load and to partially support the ship by its hull is advisable. If work is to be done on the foils, then hull support is mandatory.

As mentioned above, damaged ships are special cases in the sense that the underwater hull form is unknown. Here, examination by divers can determine the arrangement of blocking that can best be used. At times, it has been found advantageous to bring the damaged ship into the dry dock and lower the water level so it is several feet below the ship’s keel. Divers can then use the dry dock as a working platform while they examine the hull and take the measurements needed to plan the block arrangement. The ship is then removed from the dry dock to erect the desired blocking.

Special Work in Dry Dock

Replacement of sonar domes, sensors, shafting, rudders and hull plates are a few examples of work which can call for repositioning of blocks or for unusually high blocking during a drydocking and may present unusual or unique problems to the Dock Master. The experienced Dock Master will carefully note the work planned and recognized and anticipate special needs.

Multiple Ship Dockings

To efficiently employ ship repair facilities multiple ship drydockings are commonly carried out. A multiple ship docking presents the Dock Master with all of the problems associated with the drydocking of each individual ship, plus the complications that are added because of the multiple ship operation. For each individual ship, all of the drydocking requirements previously discussed must be met. Some of the additional complications of multiple ship operations are discussed below.

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