Contatto Editoriale:
Paolo Lista,
Lista Studio srl®
Borgo Belvigo 33, 36016 Thiene Vi ITALY
tel/fax 0445,372479 o info@lista.it
Aquaculture - commercial fish farming in pen
enclosures - is becoming more prevalent around the world
as an important source of food. The United Nations Food
and Agriculture Organisation estimates that by the year
2000 some 25% of total fish landings will be products of
aquaculture. While fish farms inshore are commonplace,
offshore aquaculture was, until recently, a tricky
proposition. Recently, one company, with the help of
computers and the latest in desktop engineering software,
began modifying conventional trawling systems and
applying the concept to offshore farming.
NETSystems of Bainbridge Island, Washington, has
created an innovative open ocean pen system for raising
fish. NETSystems studied the problem of keeping
aquaculture pens stable in rough, open ocean waters for
years before hitting upon the "ocean spar"
concept: vertical steel buoys anchored to the sea bottom
that serve as floating fence posts for nets, keeping them
relatively stable through constantly shifting currents.
Greater stability to the net pens means the fish will be
less stressed, and the system itself will last longer.
NETSystems is the principal supplier of commercial
deep-sea fishing net products in the North Pacific,
including the Gulf of Alaska and the Bering Sea. The
company has diversified to reflect a greater market
demand for ocean farming, forming another company called
Ocean Spar Technologies (OST).
Not just for the landlocked
Gary Loverich, chief engineer and part owner of OST,
needed a method to test the stability of his net pen
system before deploying an expensive, large-scale
prototype in the deep sea. He saw ads for Working Model,
the motion simulation software from Knowledge Revolution,
but assumed that it was only suitable for landlocked
mechanical engineers, not for modelling floating objects
at sea. A colleague convinced him to order the
demonstration disk anyway, and Loverich discovered he
could model one of his net pen systems with just the demo
copy. His reaction: "Holy cow." In no time, he
purchased his own copy of Working Model for his Pentium
90 PC.
Whether they are designed by the user in a separate
CAD software or within Working Model's own drafting
environment, Working Model applies the laws of physics to
these models depicting the results of its motion analysis
with simulations and graphic data readouts. This
kinematics/dynamics analysis software will reveal whether
a design can behave as expected under certain conditions.
Properties such as mass, force, gravity and torque are
applied to any model and then simulations are run to
analyse the performance of both components and entire
systems.
Recently, a 1:9 model of the net pen system was
jointly tested by Ocean Spar Technologies and the MIT Sea
Grant. The model was instrumented so that Loverich could
gather hydrodynamic information on the system. He
incorporated some of this test data into his Working
Model simulations to optimise his sea farming system.
"There are different characteristics you may
enter in a Working Model simulation; for example, you can
enter load as a force or as a drag coefficient acting on
a certain area of the design. So far, I feel most
comfortable entering data as a force that I know is
acting on a certain centre of effort within the
model." Loverich makes the distinction in the
simulation between the centre of effort (where all the
force is concentrated) and the centre of gravity because
as a wave passes around one spar support of his net
system, the velocity of the water in the wave is maximum
at the surface, and decreases farther under the water, so
the majority of the force or centre of effort is not at
the mid-point of the spar but toward the surface. Without
this data, Loverich cannot make the best design decisions
that will help maintain the shape of these net pens,
especially under very strong currents (e.g., 2.5 knots).
Evaluating fatigue
Heavy currents and waves put a lot of fatigue on the
net farming system. The most critical points are the
loads on the synthetic mooring lines that retain the
nets' shape while securing them to the spars. "The
lines are constantly fatigued, from zero stress to
maximum stress, back and forth," said Loverich,
"and if you bend any wire enough times, it'll
break." The ocean makes this effect even stronger.
"Fatigue can happen at much lower loads than the
breaking strength of the material, because the stress
load cycles from zero to maximum so many times when it's
in the water." These loads were measured by Loverich
using Working Model.
"What I wanted to find out with these simulations
is the orders of magnitude. Are the waves causing a
10,000-pound load on these ropes, or a 2,000-pound load?
I don't need a precise answer, but I need to know if it's
plus or minus 500 pounds. That's as close as I want to
get, because I'm going to over-design my system anyway
for the forces I can't foresee, such as a boat impact.
Working Model can show me which wave frequencies are
going to be the most critical. It's not necessarily the
biggest waves - it might be the medium-sized waves out of
phase that might put on the biggest load."
Once he finds the proper load on the mooring lines,
and the acceleration of the spar, then Loverich exports
that Working Model data to his FEA program and tests the
system to make sure the hardware connecting the two
components is strong enough. Loverich uses COSMOS/M to
analyse this aspect of his system.
The clarity of its graphical user interface,
documentation, and Windows-based environment made Working
Model easy to get acquainted with, and he began to use it
immediately as part of his overall prototype analysis
work. "We've modelled and analysed our system every
way we can, using long-hand static equations, the
ninth-scale model, our FEA package, and now with Working
Model. All of these methods have yielded complementary
results, so we feel pretty confident that we understand
the load environment we're dealing with," said
Loverich.
Simulating wave force
One doesn't actually see waves of water moving across
the computer screen when Loverich runs a Working Model
motion simulation. Instead, a vertical spar or spars
float in a co-ordinate axis system. The wave force is
depicted by an arrow, which increases or decreases as the
wave passes. He measures the tension on the anchor lines,
and the velocity of the spar buoys.
"In Working Model I am able to move the
connections on the spar buoy to different locations to
enhance the stability of the system. I can move a ballast
weight, the attachment points, anchor lines, and the net,
because there are plenty of attachment points to the
buoys." While conducting his wave frequency
simulation, Loverich subjected two linked spars to phased
wave loading (i.e., where the load isn't hitting the
spars at the same time).
The waves forced the spars into certain periodic
motions, and once that motion became stabilised, Loverich
would end the Working Model simulation. Depending upon
the simulation, that might take just two or three wave
crests, or as high as 20 or 30 wave crests. "I
extend the time until I see some stability," he
said. Loverich then analysed the phasing of the waves and
how it affected the tension values on each mooring line
and attachment.
Loverich has also simulated the release of a single
spar into the ocean using Working Model, waiting for it
to oscillate to equilibrium where it will sway equally
around its centre of gravity. "Waves will cause this
type of forced motion so that it's repeated as long as
the wave operates." But Loverich is also interested
in a static simulation of a single spar floating in still
waters. "I have to know how it is going to float in
still water, because that will influence how it acts when
waves come by. I used Working Model to measure the static
forces on all the anchor lines. It's a way to calculate
the median load."
Deploying the net pen system safely
The single spar Working Model simulation helped
Loverich determine the safest way to lower the spars into
the water, and even answered his questions about how many
workers he can safely afford to fit atop a platform that
forms the top of each spar. He adjusts the ballast weight
to make the spar sink to the desired depth, what is
called "freeboard" (the amount of spar above
the water line) and "draft" (amount of the spar
below the water line) is at an optimum ratio. Adding
weight to the crow's nest working platform will lessen
the freeboard. Modelling this situation with Working
Model is important because deploying the net pen system
safely is half the battle in ocean aquaculture. The nets,
spars, and crane equipment are heavy, so safety is a
concern. And Loverich wants the freeboard to be as
minimal as possible in some applications, without ruining
the net pen stability, so that lifting other portions of
the net pen apparatus is not too difficult.
All of these Working Model simulations are critical
from a financial perspective. Deploying such a complex
system of spars and nets is difficult at best in open
seas, and the environment plays an important factor in
the execution. If equipment hasn't been tested properly,
it can break unexpectedly, and at inconvenient times. Bad
weather can also prevent repairs for days or weeks, time
that is money to any fishing operation. Loverich
estimates that it costs 40 times as much to repair
something at sea than in the shop. By using motion
analysis, he is decreasing his risk and increasing the
dependability of his product.
Maintaining a competitive edge
Ocean Spar Technologies will use Working Model not
just to perfect the net pen system but to continually
customise it for different fisheries around the globe.
Ocean Spar will keep various net pen models in the
simulation files, and when a customer supplies local
tidal and current information, or requests a different
net pen configuration, Ocean Spar will be able to input
the variable data and learn what adjustments will have to
be made to customise the product accordingly. "Our
engineers will call up Working Model and put in the local
parameters and then we'll simulate the motions and forces
that might impact our system."
Kinematics/dynamics simulation is saving Ocean Spar
Technologies money as well as time. The time to market
this complex and vital product has been slashed because
of the help Working Model has provided in simulating the
motions and forces that will affect this innovative
product. And Working Model helped explore a complex
floating system, not just single land-based masses - a
flexibility that was employed to great advantage by this
company. By continuing to employ Working Model
simulations, Ocean Spar will retain its competitive edge
in the ocean aquaculture market while improving the
ability of people worldwide to sustain their own fish
farming industries.