15 August 2002
reactions carried to the walls by girders and to the floor by beams. These gates are
extensively used for small docks. Since their placement and removal must be done by
weight handling equipment, the sizes of cranes required for naval drydock closures
makes this type of closure unpractical.
Foster Hinged Gate. This type of gate is a patented commercial device
developed by C. J. Foster, Inc. It is a structure which folds and which does not require
transverse girders between the sidewalls of the dock. It can be designed for unlimited
widths and depths to suit dock conditions. Reaction due to water pressure on the gate
is distributed to the floor slab of the dock. Massive abutments are not required to
support the ends of the gate.
Sliding and Rolling Caissons. These types are built-in box shapes,
mounted on hardwood sliding surfaces or metal rollers that move them into or out of
place. They may be equipped with air chambers for buoyancy which reduce the work of
moving. They have some advantages of a floating caisson, but require expensive
recesses in dock walls for stowage. Cleaning and maintenance of the roller or slide
paths are difficult.
Floating Caissons. Floating caissons are watertight structures with
flooding and dewatering systems for operation. For design of hull, floating stability, and
all operational purposes, they are symmetrical both transversely and longitudinally.
The cost is reasonable since practically all of the hull elements function
A caisson may be handled easily and is seaworthy for towing to other sites
(to another drydock for repair) or from its construction site to point of use.
It may be used at more than one seat in the same drydock.
Its symmetrical form allows reversibility with either side toward the dock
chamber, permitting maintenance and repair on exposed side.
The time for closing and opening the dock entrance is slower than for
other types of closures.
Because of shore service connections (electric and sometimes air), it is
not operative too far from the caisson seat.
DESIGN OF FLOATING CAISSONS
Application. Comparison of the attributes of various types of entrance
closures leads to the exclusive choice of floating caissons for Navy drydocks. The