15 August 2002
caissons, compressed air is introduced to expel ballast water, in lieu of
After the discharge valves are closed and secured, and the machinery is
shut down, the caisson is warped out of the graving dock entrance
channel limits by capstans and/or tugs. The caisson weather deck is fitted
with chocks, bitts, and cleats for line attachment.
Closing the Entrance. To close the entrance, the opening operation is
reversed, except that ballast tanks are filled by opening flooding valves to let in water.
For air-operated caissons, compressed air must be released in order to admit the
ballast water. During submergence, the caisson must be positioned correctly with
respect to seats, and must have negligible list or trim.
Materials of Construction. Modern caissons are built of welded steel.
Some spare caissons are constructed of reinforced concrete.
Machinery. The main machinery of a floating caisson consists of
dewatering pumps or compressed air system and flooding, dewatering, and equalizing
valves to control the water ballast. Design the ballast control system so that complete
control of trim may be maintained during submerging and raising operations. The
pumps are driven by electric motors powered through cables attached to shore
connections located near the caisson seat at coping. Compressed air, when used, is
also obtained from shore connections, through air hose.
Auxiliary Machinery. Auxiliary machinery may include electric fans,
motorized drydock flooding valves, and an air compressor. When a drydock is flooded
through the caisson, two valves in each flooding tube are required for double valve
DESIGN OF BOX TYPE FLOATING CAISSONS
Shape. Since construction of caissons is not sufficiently repetitive,
definitive drawings have not been prepared. However, detailed design drawings of
modern caissons are available in the Naval Facilities Engineering Service Center
(NFESC), and may be obtained on request.
Elevation. In elevation, the shape of a caisson must conform to the
shape of the seats in the drydock walls, which have been dimensioned to provide the
required ship clearances.
Height. The elevation of the caisson weather deck should be
approximately the same as the drydock coping.
Cross Section. For a typical caisson crosssection, see Figure 9-1.
Determine beam by strength and stability requirements. Slope the bottom and sides
from the stems and keel at an angle of approximately 45 degrees, to meet the beam