Due to the facts that each meter of skirt is modelled by
4 lines, and each of those lines has a drag diameter of
0.6 m, the resulting surface area of the skirt for drag purposes
will be 2.4 m2/m length.
Connect ion buoys
Underneath each steel section, there are 8 top 6D Buoys
and 8 bottom 6D Buoys at the intersections of the top
respectively bottom lines. They connect the skirt lines to
each other and are not visible in Figure A4.4. A detailed
view is shown however, in Figure A4.3.
The top buoys have the following properties:
• Mass: 1.0*10-3 t
• Height: 1 m
The bottom buoys have the following properties (as determined
from tests done to optimize ballast and skirt thickness
for further simulations). This is done in subchapter
3.5:
• Mass: 0.05 t
• Mass moment of inertia: 0.29*10-3 t*m²
• Volume: 6.4*10-3 m³
• Height: 0.185 m
4.4 Modeling of the neoprene boom
This model is build up out of 2 parts: neoprene sections
and the connections between those sections.
Neoprene sect ions
The neoprene boom consists of a total of 96 neoprene
sections of 1 m long. Each section is connected to the
next with infinite bending stiffness. In this way a continuous
neoprene tube is modelled. Based on the links in the
steel boom model the properties of the neoprene sections
are given by:
• Outer diameter: 1.55 m
• Inner diameter: 1.35 m
• Mass per unit length: 0.750 t/m
• Bending stiffness: 170 kNm²
• Axial stiffness: 825 kN
• Poisson ratio: 0.50
• Torsional stiffness: 114 kNm²
• Drag Coefficient: 1.2 (Axial: 0.008)
To create more stability in the model, the stiffness is
based upon neoprene sections with 0.3 m wall thickness
(instead of 0.1 m).
The mass is here defined so that the waterline is at nearly
the same height as the centerline of the neoprene sections
in a static situation. This is because OrcaFlex gives
the best estimate for the draft of floating tubular sections
when their centreline is near the waterline.
Connect ion between neoprene sect ions
The neoprene sections are connected to each other
through 6D Buoys. There are a total of 95 connections;
in addition, both boom ends have an end buoy. The used
connection buoys have the following properties:
• Mass: 1.0*10-3 t
• Mass moment of inertia: 0.01 t*m²
• Volume: 1.0*10-3 m³
• Height: 1 m
• Center of gravity: -0.1 m
The mass of the connection buoys is reduced to a negligible
value in this model because the connections do not
have any physical meaning in the flexible boom. They are
present however, to be able to connect the skirt to the
neoprene sections.
Modeling of the skirt in the neoprene boom
model
Array of lines
In the flexible boom model the skirt is defined in the
same way and has the same properties as for the steel
boom, with one exception.
Each vertical top line now has its own buoy between the
boom sections to connect to, as can be seen in the Figure
A4.5 below. This is instead of the offsets to a buoy
as described in chapter 4.2, section “Array of Lines” (see
above).