chapter  1
Dry Dock Types
Pages 18

The seaward end is closed o by a dock gate called a ‘caisson’. Once the vessel has entered the dock area the water in the dock is subsequently pumped out, lowering the vessel onto pre-arranged blocks. The term ‘graving’ originates from the practice of ‘graving a ship’s bottom’ – namely, the burning away of weed and sea water growth. The practice became dated with the advance in steel hull ships and development of anti-fouling protective paints. However, ships still need to routinely go into dry dock to complete survey work and carry out essential maintenance which cannot be carried out while the vessel is in a seagoing condition. The advantage of the graving dock is that it has permanency and with that goes all the facilities of dock administration, workshops, local labour source, and regular supplies by shoreside services. The disadvantage of this type of dock is that it cannot be trimmed, as say with a floating dock. As such it may not be capable of accepting every type of damaged vessel, which may not be on an even keel, onto horizontally laid blocks. Most graving docks generally are limited to accommodating only one vessel at a time, because of length and being serviced by a single caisson. The side walls of the dock are usually stepped and where distortion stresses may be encountered, or where the vessel being docked is narrow beamed, side shores can be deployed from the ‘side steps’ (known as ‘alters’) into the ship’s sides. The side shores (wale shores), made of timber baulks, are designed to prevent hull movement and the possibility of toppling from the keel blocks occurring. If used, these shores are tightened at the dock sides by wedges preventing any movement. The shipboard ends of shores are secured by gantlines passing over the ship’s gunwales and being secured on deck. The floor of the dock is flat, but is constructed on a slight incline. The flat surface is suitable for the use of vehicles such as forklift trucks or mobile platforms. The incline, known as the ‘Declivity of the Dock’, tends to assist drainage of the flood water when the dock is being pumped dry as well as reducing the critical period when a vessel is being docked. The use of graving docks in today’s shipping industry, where ships are being constructed much larger, has limiting market availability because of size restriction. Only the extremely large docks in both length and beam width can accommodate the large VLCC/ULCC vessels. Having said this, most docks seemingly find adequate work between routine, scheduled docking and accident repair work. A single working dock could expect to average around 200 ships per year.