3.5.3.1 Boom
The Orcaflex boom model was built from three main
structures. The required buoyancy was achieved by the
floating boom elements. The two boom concepts included
a flexible boom and a stiff boom with flexible connecting
elements, to enhance the capability of the boom to
follow the waves. The selection of these concepts was
based on a concept study explained in Sub-chapter 3.2.
Rigid boom
The stiff boom model has a hinge every eight meters,
which can be seen in Figure 3.56. The left side of the figure
shows the cross section and the right shows the side
view of part of the boom. The stiff sections are the 7.5 m
long sections in the figure.
For the hinges, links were used (The 0.5 m long sections
which are drawn a bit wider than the other parts in Figure
3.56). The skirt was hanging beneath the boom, shown in
white with a black outline. Compared with the CFD simulations
presented in Sub-chapters 3.3 and 3.4, the boom
buoyancy element and skirt geometry is slightly different.
This is due to parallel engineering activities.
As for materials, steel was used for the rigid parts, and for
the links, neoprene is used. Steel was chosen because it
is one of the main materials used in the offshore industry
and neoprene has good environmental resistance, yield
strength and other properties to use as flexible material
in the boom. Please note that this is a first selection of
materials for this design phase. During the detailed design
phase, a thorough material selection will be included.
Figure 3.56 Physical interpretation rigid boom
The length of a steel section (7.5 m) selected was based
on 1/10 of the mean wavelength of the most occurring sea
state. Longer steel sections tend to bridge waves. When a
steel section bridges a wave, this results in wave crests
going over the boom and reduced draft from the skirt,
preventing the possibility to capture plastics.
Flexible boom
The flexible boom outer geometry is equal to the steel
boom without the stiff sections. A flexible tube can follow
the waves without the need for hinges. The tube now has
the properties that have been given to the links between
the steel sections in the stiff boom. The initial material is
chosen as neoprene with an outer diameter of 1.55 m and
an inner diameter of 1.35 m.
During actual simulations, an adjusted type of flexible
boom was introduced. For this type the axial stiffness is
equal to a neoprene material boom with a Dyneema cable
running through the boom. In the model, the geometry is
equal for both materials.
3.5.3.2 Skirt and ballast
A skirt was attached to the boom and oriented vertically.
To reduce the loss of captured plastics, a ballast weight
is likely to be attached to the bottom of the skirt. This ballast
was part of the computation as well. The skirt was
the same in both models, as well as the ballast weight
underneath the skirt. In Section 3.5.5, the skirt and ballast
weights are varied for design optimization purposes.
More details on modeling of the skirt and ballast can be