It is shown that up to a boom length of approximately 1
km, the sea surface clearance stays approximately equal.
However, for longer booms, the sea surface clearance
value reaches a much lower minimum. At 2 km the force
is above the theoretical limit tension force.
This confirms the theory that the boom is less able to
follow the waves when high tension forces are reached
within it. By increasing the volume and mass of the boom,
the qG estimate in the calculations above will increase.
Higher tension is then allowed before the limit tension
force is reached where the boom is unable to follow the
shape of the waves.
For a design with equal properties as the models in this
study, it is advised not to use independently moored
boom lengths longer than 1,400 meters.
3.5.5 Optimizing Skirt and Ballast Weight
Besides obtaining loads acting on the system, simulations
were run with the purpose of achieving other results.
This section discusses simulations with different
skirts and ballast weights to investigate the influence of
these properties.
The skirt can move relative to the boom because the
boom is attached to the mooring at its ends and is restricted
in its movements while the skirt hangs underneath
it without any obstruction. This causes the skirt to
deviate from its vertical orientation as shown in Figure
3.65. If the angle is too big, captured plastics can easily
flow underneath it. This is both due to a reduced draft
and because fluid flow passes an inclined wall more easily
than a vertical wall.
Figure 3.65 shows the boom at a moment where the horizontal
wave forces in the boom are compensated by its
restriction at the ends due to mooring, Fh; Boom. The
ballast can still move in wave direction until Fh; Ballast
caused by the tension force Fballast is equal to the horizontal
wave forces.
Hanging ballast under the skirt can, in this way, help to
keep the skirt in a vertical position. Properties of the skirt
itself can have an influence on the movements too. The
purpose of this section is to give a rough estimation of the
combination of skirt and ballast that work best.
In the refined concept described in Sub-chapter 3.6, a
different skirt attachment is proposed. This type is not included
in this section due to parallel engineering activities.