Fluid dynamics
The third niche described above, that of the immersed
vertical sides facing into the flow of currents, was cited
as most likely to impact the plastic collecting performance
of The Array.
There are two major factors to consider: first, whether the
booms are fouled with a biofilm or with macrofouling; and
second, the size distribution of the plastic particles being
gathered. In the case of only a biofilm being present, the
slimy polysaccharide matrix that houses the microbes
can be very sticky and could potentially make micro plastic
stick to the side of the booms. It is very possible that
even in the absence of macrofouling, sufficient small
particles of plastic will clump and stick to the biofilm,
subsequently becoming biofilmed themselves. A macrofouled
Array could have greater potential to entrap larger
plastic particles. In a hypothetical situation, the plastic
may become integrated with the fouling assemblage and
increase the surfaces available for colonization and affect
its structural integrity. Instead of plastic debris
smoothly progressing with the current along the smooth
surface of the boom in the direction of the capture apparatus,
it will accumulate on surface features created by
marine life and/or become ‘stuck’ to a biofilm. The shear
strength of flow across The Array will be related to the incidence
angle of the boom arms to the prevailing current
flow. However, considering the barriers will constantly
move laterally under influence of wave action, causing a
turbulent distance relation between the barrier and the
plastic, we deem this scenario unlikely. Future pilot studies
should further investigate this. Of course, all marine
fouling assemblages are different and exist in different
oceanographic conditions, but a general estimate is that
the critical current velocity for many species to reach
their maximum biomass is 0.2 to 0.5 meters per second.
If this critical limit is exceeded the biomass tends to decrease
rapidly. This is a generalization, as many fouling
species prefer shelter from flow while others require flow
to feed, for example.
5.3.3. Possible Combat Strateg ies
The two best strategies to combat fouling are antifouling
coatings and cleaning. These approaches work best
when used together and both have significant cost implications.
The author knows of no unmanned open ocean structures.
Manned structures in shallower waters, such as oil rigs,
are able to be cleaned by divers and mobile structures are
able to move to locations where they can more easily be
accessed for cleaning. Mobile structures are designed to
cut through the water with hydrodynamic efficiency. The
hydrodynamics of a vessel are very well understood and
are factored into the design of antifouling coatings. The
flow velocities experienced by a ship’s hull are likely to
be far greater than those passing across The Array. As
such, The Array will be an experiment to reveal how a
fouling assemblage might develop on a fixed position,
unmanned, open ocean structure. Limited nutrients may
constrain algal growth and too few larvae may encounter
the surface for a fouling community to grow. Animal life
may not be able to acquire sufficient appropriate food to
grow and reproduce. It is possible that growth will be slow
enough to be manageable, flow will be able to dislodge all
attached species, or that a sustainable cleaning regime
will allow The Array to function. However these are bestcase
scenarios and are improbable. A good design will
necessarily allow for a range of scenarios to ensure that
the project is not lost.
Antifo uling coat ing options
There are three solutions, each with pros and cons. Table
5.4 summarizes these solutions and estimates of average
prices. However, these prices are for marine vessels and
may differ depending on the structure being coated and
the customer (for instance, a shipping company is likely
to get lower prices with a fleet of vessels than for a single
project). The data also assumes that the structure would
be made from metal, as data is mostly available from the
shipping industry.