Design, Manufacturing, and Testing of an Improved Watertight Door ...

Design, Manufacturing, and Testing of an Improved Watertight Door
Figure 11: New
Watertight Door
Showing Holes in
Stiffeners along the
Neutral Axis Before
Spot Welding the
Face Sheet to the
Stiffeners
Figure 12: Automated
Welding
System Developed by
MDL under Subcontract
to ARL Penn
State
100 & 2010 #4
WATERTIGHT DOOR DESIGN
The stiffener arrangement in the panel and frame
of the new door is illustrated in Figure 11. Extensive
finite-element analyses were carried out to
ensure the structural integrity of the watertight
door. They resulted in increasing the stiffener
height to 1.3 in. and the face sheet thickness to
0.048 in. to increase stiffness and strength. A
compensating weight reduction was achieved by
laser cutting holes centered on the neutral axis of
the stiffeners.
The new watertight door was hydrostatically
loaded multiple times during prototype lab testing
to 15 psi pressure, an overload of 50%,
without permanent deformation, mechanical
failure, or any loss of functionality.
The new watertight door weighed assembly including
frame 213 lbs, a 27% reduction
compared with the 26 in. 66 in., 10 psi
NSWTD.
SEAL SYSTEM
Leakage at low rates ( 1–10 mL/min) was not
difficult to achieve with the hydraulically actuated
seal system, but the Navy requires zero
leakage at 10 psi design pressure. Although presenting
a great challenge, this requirement was
finally satisfied in laboratory testing. Important
issues contributing to this success included: controlling
the gasket length; forming a smooth
joint between the ends of the extrusion; uniform
positioning of the gasket in the corners of the
door; and smoothness of the gasket cavity. ARL
Penn State worked closely with its supplier,
Northwest Rubber Extruders, Beaverton, OR, to
address these issues.
A hydrostatic loading test matrix specified by
NSWCCD was successfully completed, including:
two frame/panel combinations from
different doors; two different gaskets; and two
loading directions (repeated three times) giving a
total of 2 3 3 5 24 successful tests. In each test,
the door was loaded to 10 psi and held for 20
minutes with no leaks whatsoever. The opening/
closing (pull/push on handle) force for the new
watertight door was o2 lbs.
MANUFACTURING
The automated manufacturing system developed
by MDL to manufacture the new watertight door
is shown in Figure 12. The door was attached to a
rotary stage after spot welding. Several welds
were made on one side and then the door was rotated
so that the thermal stresses and distortion
produced by these welds could be balanced by
making welds on the opposite side. This was continued
in a specified weld sequence until the
welding assembly was completed. In Figure 12,
the door is shown in mid rotation with five continuous
welds completed on one side.
The automated welding system has demonstrated
the capability of completing all
continuous autogenous laser welds, approximately
332 ft of welds, in 45 minutes. This
automation is critical to achieve the goal to reduce
the procurement cost to US$4,500 per door
assembly.
NAVAL ENGINEERS JOURNAL