ENVIRONMENT
in 2016, used this technology. But what
soon became apparent that the long-term
application, and the working conditions of
the middle of an ocean, where waves can
reach 20m high, were too harsh for the
spill boom, which is intended to be used
short-term in calm waters near shore.
The test rubber boom, which featured
separate air chambers, was so buffeted
by the North Sea waves that individual
sections started to tear apart.
What keeps an oil boom in place is
air pressure acting on the inner surfaces
of the coated fabric tube; it’s a bit like
a bouncy castle. Likewise, the Ocean
Cleanup system needs structural rigidity,
because it can only collect plastic when
the net is held open against currents,
waves and wind. In the xed system,
each corner of the net would have
been anchored to the sea bed. Lacking
anchors, the mobile system had to be
built to do the same thing.
Like an oil boom, the latest version of
the oater is also an air- lled rigid tube.
But neither its rigidity nor its buoyancy
high-density polyethylene. “We ght evil
using evil,” quips Tjallema. There is a
perverse logic to this; those material
characteristics that enable plastic to
persist in the environment for so long
are the same that safeguard a oating
structure from the ravages of sun, salt
and spray.
The oater’s bending stiffness, which
acts to resist the action of the waves to
keep the net open, is determined by the
relationship between its diameter (60cm)
and wall thickness (35.3mm); the magic
number found for this design is the ratio
between them, 17.
Tjallema points out that it would be
impossible to make this large structure
out of the offshore engineer’s material of
choice, steel. First, it would be impossibly
heavy. And to get the required bending
stiffness in steel, the diameter of the pipe
would be relatively small. But because
steel is a stiffer material, that pro le
might buckle under operational loads.
8 www.ied.org.uk
Early Paci c tests consisted of
large 100m or 200m--span net/boom
combination, supporting a collection
net. And it (sort of) worked; it captured
oating plastic, but then it let it go again.
The speed of movement of the plastic
changes with sea conditions, mainly the
current, but also due to wave forces. As
these types of plastics tend to oat just
below the surface, wind speed has only
an indirect effect. To ef ciently collect
oating objects, the system needs to
do two things. It needs to travel slightly
faster than they do, to physically intercept
the material. And it needs to be able
to turn to reorient itself toward shifting
currents, to make sure debris remains
in the back of the net. Performance
on neither aspect was ideal. Explains
Tjallema: “The exact forces were hard
to predict. Indeed, the speed difference
between the system and the plastic was
not consistent.” He estimates that the
system needs to travel 10-20 cm/sec
(0.19-0.39 knots) faster than the debris.
Also, despite all of the calculations, in
late 2018 the oater still broke, and had to
be towed out of the water at the end of the
year. The problem was discovered to be
the connection between oater and screen,
come from the solid
material, not from
pressurised air (in fact
it is assembled and
shipped at atmospheric
pressure, as pictured
above). In an ironic
twist, it’s made from the
same stuff that it is designed to collect:
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