Skirt
The skirt is here only applied to the middle 6 sections of
the boom to save calculation time and to be able to simulate
more varieties of properties. The variable properties
are those of the lines and the bottom buoys. All combinations
of the following properties have been modelled.
The lines forming the skirt can have the following properties:
• Diameter: 5.0*10-3 m
• Mass: 0.0029 t/m
• Bending stiffness: 0.292 kNm²
• Axial stiffness: 3.5 kN
Or:
• Diameter: 50*10-3 m
• Mass: 0.019 t/m
• Bending stiffness: 2.920 kNm²
• Axial stiffness: 35 kN
While the bottom buoys can have the following properties:
• Mass: 0.01 t
• Mass moment of inertia: 0.020*10-3 tm²
• Volume: 1.3*10-3 m³
• Height: 0.108 m
Or:
• Mass: 0.05 t
• Mass moment of inertia: 0.29*10-3 tm²
• Volume: 6.4*10-3 m³
• Height: 0.185 m
Or:
• Mass: 0.1 t
• Mass moment of inertia: 0.91*10-3 tm²
• Volume: 13*10-3 m³
• Height: 0.234 m
The line diameter is based on the skirt thickness. One
skirt, which will be called the heavy skirt from here on,
has a thickness of 50*10-3 m. The other skirt, which will
be called the light skirt, has a thickness of 5.0*10-3 m
which is 10 times thinner in order to see clear differences
between the results.
The mass is based on the skirt thickness too. With a
thickness of 5.0*10-3 m the skirt should weigh about .
As may be noticed, a density of 1.16 t/m³ is used here instead
of the before used density of 1.1 t/m³ for the neoprene
connections between the steel sections.
It should probably be emphasized that the used materials
are not fully defined and will not be defined by this
model; therefore a lot of material properties may change
in the final design of the boom and these slight differences
that got into the model during creation should not
be important.
However, it is important to know which properties are
used within the model exactly in order to determine
whether using another material will have significant effects
on the results of the model.
Each meter of skirt is defined by 4 lines as explained in
chapter A4.3, section “Array of Lines”. So the mass of the
lines in the light skirt should be 0.0116/4 = 2.9 *10-3 t/m
while the mass of the lines in the heavy skirt should be
10 * 2.9 *10-3 =29*10-3 t/m. To enhance the visibility of the
effects of adding ballast to the model, this mass for the
lines in the heavy skirt is reduced to 19*10-3 t/m.
The bending stiffness is chosen in such a way that the
lines do not bend too much in the plane of the skirt, which
would create instabilities in the model.