Dry Docks

Dry docks are structures which are used for the repair, maintenance, construction and launching of vessels.They are termed the ''workhorses'' of ship repair facilities and sometimes used in place of traditional shipbuilding methods.

There are four types of such docks in use based on how they provide ''dry'' access for work.

1. Basin dry docks which are basically excavations with a gate at one end opening to the waterway which when flooded buoys the ship structure from the keel blocks.
2. Marine railways consisting of cradles on rail running on inclines which launch and retrieve vessels.
3. Vertical Lifts which are ship elevators designed to be lowered into water and then lift it clear from the water surface in the basin.
4. Floating dry docks are "vessel" structures having their own stability which are capable of sinking and receive a vessel to lift it out of water.

SOME FLOATING DRY DOCKS BASED ON SHAPE AND MODULAR STRUCTURE

Although each of these docks have their own uniqueness in terms of being a facility for construction and repair, some factors needing consideration for the desirable operation of the dry dock regardless of the type are:

• Adequate space in and around the dry dock ease of movement of materials and people.
• Fast access to and from the vessel  and dry dock especially for vehicular traffic is a an advantage.Some of these arrangements are known as travelling stagings and 'Dock Arms'.
•  Good lighting and ventilation.
• Electrical facilities(outlets) and machinery.
• For heavy items, cranes and material handling systems need to be present.
• Efficient system for moving vessel in and out of dry dock.(tensioning winches,capstans,other line handling hardware.)
• Proper Block Arrangement.(Use of keel and bilge blocks).

Shipbuilding dock with an intermediate gate

Most of these blocks are made of composite construction having concrete or steel bases and 'cap pieces' made of rubber or timber for providing cushioning against ship hull.

 Removal of these blocks provide access to hull areas in case repair.This is accomplished by use of sand frames set on the top of the block's concrete base.

Sand in the frame supports the top pieces of wood and access is obtained by using water jets to remove sand which allows the wood to drop so that block can be removed.Although other sophisticated methods exist today for the removal of blocks but this simple arrangement is the most preferred and perhaps quite successful.

SELECTING A DRY DOCK:

Selection of a dry dock is influenced by many factors like:

• Dimensions, weight and characteristics and general features of vessels to be serviced by the dry dock.
• Available land area,available area in water,proximity to navigable channels or open water.
• Purpose of the dock(new building/long term vessel repairs/short term repairs or a combination of all of these).
• Near and far term goals and the possible future extension of facilities.
• Financing(Generally depends upon the type of dock and its features).

We shall discuss some basic features of basin and floating dry docks.

BASIN DRY DOCKS:

Basic features of a basin dry dock as shown in the photo include floor, sidewalls,head wall and dock gate.The structures may be gravity structures,ground anchored, and under drained (pressure relieved).

Basin dry docks are suited for large vessels. Advances in material sciences have continually expanded the dimensional limitations of dry docks.

The operation of this kind of dry dock is very simple.Once, the ship and docking blocks have been properly prepared, the basin is filled with water until the level of water reaches the depth with keel blocks above/below sill.

These docks are prone to silting and scouring at dock entrance.This factor needs to be considered as excessive silting would mean high costs from maintenance dredging.

Tides and waves at the site have a direct impact on construction,as they influence the elevation at the top of the gate and along the coping.

Soil types and groundwater levels also influence the structure suitable for the site,the depth to which it can be economically built and the structural design of floors and walls.

Also, supporting shop facilities should be in close proximity and sometimes crane services are essential for moving material in and out of basin.

The key dimensions of the basin depend on the type of ships to be docked and whether the basin is to be used strictly for construction,repair or a combo.There are allowances in these dimensions for providing working space .Most basin dry docks today have an effective length-to-width ratio between 5:1 and 7:1,in approximate proportion to large contemporary vessels.

A Basin Dry Dock

FLOATING DRY DOCKS:

As vessels grew larger and heavier, floating dry docks followed suit with a variety of designs.Maintenance of dry dock by virtue of a self docking capability was an important feature.Today floating dry docks continue to be important parts of many ship repair facilities.Modern floating dry docks are most often built as non-self-docking,one-piece steel units.

Floating dry dock works on the principle of Archimedes principle,by displacing a volume of water equal to its own weight.The dimensions and arrangement of structure,presence of flooding/pumping system permit the weight of the dry dock to vary which allow the dock's pontoon deck to be submerged or undock a vessel.Ballast tanks are present which allow sinking of the dock to desired water depth.The vessel is then placed/or removed from the dry dock. 

Capacity of such dry docks are determined by their buoyancy,stability and structural strength.

Important terms in the ship-dock stability calculations.

The dimensions of the wing walls and compartmentalization of the pontoon provide necessary stability of the ship-dock system.Intact stability for all phases during docking operation.Until the pontoon deck breaks the water's surface, the stability is at minimum.The dimensions of wing walls and ballasting should be co-ordinated with the dock's design vessel to ensure positive stability characteristics.

Compartmentalization of the pontoon provides for more precise control of the dry dock, in addition to enhancing the stability.Floating dry docks have the initial advantage that they can be built and fully equipped in shipyard and factory conditions. LSD

A Floating Dry Dock

Article By: Sudripto Khasnabis

Author's Note: This article is intended to let the readers know about dry docks and how they function in the shipbuilding process .The figures and photos do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com