Platforms developed for oceanographic purposes
are rarely adapted to the deployment of radiometers at
sea. Indeed, recording the light field within the ocean
interior is difficult because the instruments themselves
and, more dramatically, the platform onto which they
are installed, inevitably introduce perturbations (shadowing
in particular). Other difficulties originate from the
need to keep the instruments as much horizontal as
possible, either because a plane irradiance is aimed
at (cosine sensors), or if a given direction (generally
nadir) is aimed at. The actual measurement depth is
also difficult to accurately assess, because rapid
vertical displacements of the instruments sometimes
occur, which prevent any precise estimation of pressure,
thence of depth. Considering the above observations
(among others), a new type of platform has been developed,
dedicated to radiometry measurements. This platform
is able to minimize shadowing effects, to minimize
perturbation of the sub-marine light field, and to
warrant the stability of the instruments.
The constraints were:
- The data to be measured
are Eu, Ed, and Lu(nadir)
at two depths, plus at the surface;
- Minimizing shading of the instruments;
- Maximizing
the stability of the instruments; and
- Deployment
at a site with a depth of 2,440m, and swells
up to 8m (but low currents).
The principle is that of a reversed pendulum, with
Archimedes thrust replacing gravity. A large sphere (with
a diameter of about 1.8m) is stabilized at a depth out
of the effect of most swells, at the end of a cable that
goes down to the sea floor. This sphere creates the main
buoyancy of the system. A rigid, tubular, structure is
fixed above the sphere, which hosts the instrumentation
onto horizontal arms (at 4 and 9m). The resulting approximately
three tons of thrust ensures the stability of the system,
which is subject to very limited forces from the so-called
transparent-to-swell superstructure. This is a taught
mooring, definitely different from what is usually referred
to as spar buoys.
With such a design, there is no large body at the surface
generating shade, the stability of the instruments is
warranted even for quite large swells, and the possibility
exists to accurately measuring the water level above
the instruments.
Theoretical calculations were performed, by specifying
an initial and preliminary design (Fig 6.) and material
for the construction, a swell of height 5m and period
7s, typical of the deployment site. The inherent periods
of the whole mooring were determined, as well as the
period and amplitude of the oscillations and displacements
due to swell and currents. These calculations were extremely
encouraging in terms of tilt and oscillation, so that
it was decided to first build a reduced-scale model in
order to perform tests in an engineering pool.
Schematic
drawing of the BOUSSOLE buoy
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