15 August 2002
Tunnels. This system is good if hydraulic requirements do not result in
tunnels of such size as to produce complex and high stress concentrations in the walls.
This is especially true of thin walls with stressed reinforcing steel and less true in gravity
walls. Even for the latter case, formwork is expensive. Long stretches of tunnel
increase cleaning and maintenance work.
Transverse Floor Type. Transverse floor culverts comprise a number of
openings, spaced across a dock floor above a wide culvert located parallel to, and from
3.05 to 5.5 m (10 to 18 ft) inboard of, the inner caisson seat; the inboard distance
depending on the dock size. The culvert ends rise in the sidewalls and terminate in the
Flooding Factor. In operation, the method of flooding through openings
of transverse culverts necessitates partial opening of the sluice gates for a sufficient
time to attain adequate water depth on the floor, so the flow velocity toward the head
end will not dislodge or damage the blocking.
Cost Factor. A properly designed system of this type will comply with
specified flooding times at lesser cost than the system of flooding through sidewall
Flooding Through the Caisson. Drydocks can be flooded through ducts
in an entrance closure caisson or gate. This method for flooding the drydock can be
used exclusively, or it can be used in combination with flooding intakes in the dock
walls. This is usually the most cost effective method of flooding the drydock for both
initial construction costs and O&M costs.
For this method of drydock flooding, multiple round tubes or ducts
penetrate the caisson shell plating through the caisson ballast tank. Tube diameter is
normally 762 mm (30 in,) and should not be larger than 914.4 mm (36 in.) The number
of flooding tubes is dictated by the desired flooding time and hydraulic characteristics.
However, utilize at least two flooding tubes for redundancy if the dock is to be flooded
exclusively through the caisson. Each flooding tube must have two valves installed in
series to provide "double valve protection" against inadvertent flooding. Additional
requirements are contained in Chapter 9.
Superflooding. Increased ship docking capability may be obtained by
installing a superflooding system that lifts ships by flotation from one level to another
within a drydock, similar to that of a canal lock. The operation consists of off-center
vessel entry into the dock chamber, placing entrance caisson in seat, pumping water
into the dock chamber thus raising the ship, breasting the ship over preset keel blocks,
and then dewatering the dock chamber.
The installation requires a special pumping capability to raise the water
level in the dock chamber above the level attained from normal flooding. It also requires
an inboard- and outboard-faced caisson seat, and securing devices for the entrance
closure to resist hydrostatic pressures caused by the raised interior water level. A
superflooding system has been used in several graving drydocks with inadequate water