making full
speed

Remanufacturing gives an old vessel a new mission.

This
article was prepared by staff writers in collaboration with outside
contributors.

Ships have been cutting through waves for
thousands of years. Long ago, hull design moved from art to science. Yet,
ship designers continue trying to find ways to get smaller vessels to sail
along on top of the waves instead of plowing through them.

One
concept, the well-known hydrofoil, behaves like an aircraft wing to lift a
hull out of the water. Another, the surface effect ship, or SES, rides on
a cushion of air generated by a big downward blowing fan.

Both have
drawbacks. Hydrofoils don't work well until the ship reaches high speed;
during medium- and slow-speed operations, the underwater planes just get
in the way. They add greatly to the ship's drag and can double its draft,
creating a hazard in shallow or rocky coastal waters. SES vessels, on the
other hand, tend to ride hard in heavy seas.

With a variety of
specially built research ships, the U.S. Navy has worked on these
challenges for three decades. Recently, the Office of Naval Research
shifted its attention to lifting bodies. They generate lift from buoyancy
as well as from their hydrodynamic shape. Hydrofoils, in contrast,
generate lift only by moving through the water. Thus, lifting bodies can
raise a hull out of the water at slower speeds than hydrofoils can.

Before going to the yard for its
latest remanufacturing, the surface-effect vessel SES-200 spent some of
its time under way at speeds in excess of 40 knots.

When the Office of Naval Research decided that it needed a vessel to
prove the concepts behind lifting bodies, it opted to save money by
converting an existing research vessel. By remanufacturing and upgrading
the decommissioned surface-effect vessel, SES-200, the Navy is saving
about $11 million from the $18.5 million cost of a new ship. The
conversion is now under way at a shipyard in Hawaii. Before the project
could begin, however, ONR had to be sure that the hull was up to the
task.

To do that, ONR contacted the National Center for
Remanufactured Resource Recovery at the Rochester Institute of Technology.
In the past, experts there had come up with ways to recapture value from
obsolete industrial and automotive components.

Sometime after her
original 1979 launch, the 110-foot SES-200 was cut in half and lengthened
by 50 feet. Her original propellers were replaced with water jets that
could drive the ship at more than 40 knots. At the time of her
deactivation in 1990, she was the Navy's only operational
SES.

According to Joel Berg, a senior staff engineer at the
remanufacturing center, the Rochester lab needed to determine if the
ship's modified hull could withstand the load passed to it through new
struts and lifting bodies. Among the most important analyses were those
focusing on the radically different stress and modal factors in the
redesigned hull. Conventional ships are designed for strength
longitudinally, from stem to stern. The main stress on such a ship comes
from cargo and the force of waves passing beneath the hull.

For
the converted SES, adding struts and lifting bodies changes the stresses
completely—in effect, rotating them 180 degrees. These new transverse
loads—known as squeezing or prying loads to distinguish them from the
hogging and sagging loads seen by a conventional craft—were likely only a
secondary consideration at the time of the original ship's
design.

A simplified model of the full
hull depicts the location and orientation of two of the four lifting
bodies that would be added eventually to the SES-200.

"This meant that the vessel's transverse bulkheads would have to be
redesigned," Berg said. "The hull stresses generated from the struts would
have to be reduced by linking the lifting bodies in some way." The struts
were considered in both 20- and 25-foot designs and, in each case, they
were about 30 inches thick.

The hull arrangement of the
reconfigured ship resembles that of a SWATH vessel, Berg said, which
stands for "small waterplane area, twin hull." This design uses lifting
bodies submerged beneath twin hulls to lower water drag on the vessel. But
where a SWATH vessel uses two lifting bodies, the modified SES-200 would
use four, Berg explained.

There is a drawback common to these
designs, Berg observed. "From a structural standpoint, all the loads are
concentrated in just a couple of regions of the vessel. A traditional hull
has its loads distributed more evenly," he said.

Using Mechanical
software from ANSYS Inc. of Canonsburg, Pa., Berg's group began modeling
the complete structure of the ship—its hull, decks, stringers, and
bulkheads, as well as the lifting bodies and their struts. "The work
became very complex geometrically," Berg said. But, analyses confirmed
that the hull structure designed for one purpose could be adapted
economically to something quite different.

This is the view of a lifting
body looking inboard (top) and outboard (bottom). Red indicates high
stress in the lifting body mounting and hull.

The first big full model took about 15 hours to solve initially, Berg
said. "We got better at meshing and learned to use half models and even
quarter models," he said. Quarter models could be solved in three or four
hours. "At first, we went overboard with tight meshing. Then we learned to
use larger meshes wherever we could," he said.

The full program
took two years. Most of the modeling was done in the first half of 2000.
The first set of models took three or four months to build. Revisions went
faster.
"We had never done an analysis of this size before," Berg
said, "and never modeled anything like a large marine vessel."

The
initial modeling was done with models of SES bulkheads and frames imported
from Pro/Engineer 2000 software from PTC of Waltham, Mass. The
remanufacturing center did a lot of modeling manually, as well, using
ANSYS products.

"Working from AutoCAD drawings, we dimensioned the
model, then added stringers. We skinned it, loaded it, meshed it, remeshed
it, rebuilt it, and so on," Berg said. "The second time around we got
smarter. We started with a skin of the hull as a closed volume created in
Pro/E. Then, we added the two dozen bulkheads as slices turned into planar
surfaces. This cut the model building time from a month to a week."
AutoCAD is the trademark of Autodesk Inc. of San Rafael,
Calif.

This simplified mesh of the aft
starboard hull includes bulkheads and stringers. A lifting body (in red)
can be seen below the hull.

The remanufacturing center did not come up with the basic design; that
was the Navy's. The geometry for the lifting bodies came from the Hawaiian
shipyard. Naval architects will refine the center's feasibility study and
structural analysis. "We were more of a litmus test with these analyses,"
Berg said.

The center is currently evaluating alternate
configurations of the lifting bodies and their attachment for stress. It
also conducted a modal analysis to identify natural frequencies that could
be excited by hydrodynamic loads.

While most of the SES-200 is
fabricated of marine-grade aluminum, Berg is analyzing composite
structures for the Office of Naval Research as well. The lifting bodies,
for example, while now aluminum, could eventually be made from composites
to shape them into the advanced geometries that are likely to develop.

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