Design of an Underwater Towfish using
Design by Rule and Design by Analysis
M. Muscat, M. Formosa, G. A. Salgado Martin
Department of Mechanical Engineering
University of Malta
R.Sinatra, A. Cammarata
Department of Industrial Engineering
University of Catania
ERDF Italia – Malta 2007 – 2013
BIODIVALUE
SUMMARY OF
PRESENTATION
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Scope for the project
Description of the towfish
Methodology of Design
Design by Rule
Design by Analysis
Conclusions and Discussion
Prototype manufacturing
Scope for the project
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Monitoring of sea water pollution
Monitor jellyfish population
Monitor plankton population
The towfish is very convenient to use when a
large area of the sea needs to be scanned
Description of the Towfish
• An underwater vessel towed behind a surface ship
• The towfish is equipped with various sensors &
cameras
• Positively buoyant
• Depth of dive controlled by hydroplanes
• Maximum depth of dive is 50m
• Data, signals, power transferred via tow line
Description of the Towfish
Methodology of Design
• The Design of the Towfish is not governed by
international legislation or code of standard
• The towfish can be considered to be a pressure vessel
acted upon by external pressure and various local loads
• DBR first used to calculate some of the various
dimensions and thicknesses of the towfish components
mainly to prevent buckling
• DBA Annex B of EN13445 part 3 was then used to carry
out other buckling checks that were not possible with
DBR
Design by Rule
• Section 8 & Section 16 of EN13445 Part 3
• Main cylindrical, hemispherical, conical shell &
cylindrical arms designed against buckling
(Section 8)
• Section 16 used to check for any reinforcement
required due to local loads acting at the nozzle
attachments and at the towlug area
Design by Rule
• Methods presented in EN13445 not always
applicable for the design of the towfish
components.
• Conical end bearing reinforcement
• Rectangular flange connection of the fixed part
of the hydroplane to the main cylinder
Design by Rule
Material used for the towfish
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Structural steel
Minimum yield stress ReH of 235 N/mm2
Maximum tensile stress Rm of 360 N/mm2
Resulted in a Design stress of f of 150 N/mm2
Corrosion allowance not required
Design by Rule (Buckling)
• The internal flanges connecting the main
cylinder to the hemispherical and to the conical
ends were assumed to act as heavy stiffeners
• No need to include shell junction
reinforcements
• Buckling expected to occur within each main
shell component
Design by Rule (Buckling)
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Main cylinder thickness 3mm
Cylindrical arms thickness 2mm
Hemisphere thickness 2mm
Conical end thickness 2mm
Design by Rule (Local loads)
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Total lift on both hydroplanes was -702N
Total drag on the towfish was 2519N
Lift on each rotating aileron was -254N
Drag on each rotating aileron was -87N
Lift on each fixed part of the hydroplane was -97N
Drag on each fixed part of the hydroplane was
100.6N
Design by Rule (Aileron nozzle)
• As a conservative assumption, for the DBR part it was
assumed that each aileron nozzle on the main cylinder
side is carrying the lift and drag acting on each aileron
• DBR section 16 of MSA EN 13445-3 resulted in a
reinforcement plate of thickness 3mm for the region of
the aileron nozzle
• The reinforcement was also extended so that it
reinforces the area around the rectangular flange
connection of the fixed part of the hydroplane
Design by Rule (Towlug)
• By considering the lift and drag on the
hydroplanes and towfish, the maximum value
and direction of the resulting force on the
towing lug could be calculated
• This resulted in a reinforcement plate of 3mm
thickness in the towing lug region.
Design by Rule (Towlug)
DBR (conical end nozzles)
• For the design of the nozzles at the conical end the
assumption that the local loads are acting on a
cylinder having the same diameter of the cone at
the point of the nozzle/bearing connection was
taken.
• No reinforcement required
Design by Analysis
• ANSYS Mechanical used within a DBA context in
order to ascertain the structural integrity of the
towfish especially in the parts that deviated away
from the scope of Sections 8 and 16 of EN13445-3
• GPD & I design checks were carried out to ensure
structural integrity of the pressure vessel
component
Finite element models
• Partial safety factors for the material different
for GPD & I checks
• For the GPD check the yield stress value used
in the finite element material model was 162.8
N/mm2
• For the I check the yield stress value was 235
N/mm2.
• For the buckling check of the towfish no predeformations according to the critical
eigenvalue buckling shapes were considered.
• Local loads acted as force or moment
perturbations to induce buckling
• Partial safety factors for pressure action and
local load actions applied
• Linear elastic perfectly plastic material model
• The finite elements SHELL281 and BEAM181
were used in the software ANSYS Mechanical
• For all the FEA models (except for the towlug
model) the nozzle moments created a situation
of geometrical weakening so that both GPD and
I check used large deformation analysis.
• For the towlug model only the I check required
the use of large deformation analysis.
GPD / I check on aileron nozzle/main cylinder
• Same loading situation as
in the DBR method
• Pressure acting on the
cylindrical shell &
hydrodynamic lift and drag
acting on the aileron
• Fully fixed BC at the
flanges
• Symmetry BC along
length of cylinder
1
ELEMENTS
PRES
JAN 24 2014
12:12:25
Y
Z
-.725
-.409333
-.093667
CYLINDRICAL SHELL NOZZLE
.222
.537667
.853333
11
1.169
X
1.48467
1.80033
2.116
• The reinforcing plate in the region of the nozzle as
described in the DBR section of this paper had a
thickness of 3mm so that the total shell thickness in
this region was 6mm.
• The cylindrical shell thickness elsewhere in the model
was 3mm.
• The model deformation was as expected and confirmed
the applied boundary conditions and loadings.
GPD / I check on aileron nozzle/main cylinder
• The figure shows the von
Mises stress obtained from
the I check on the top
surface of the shells.
• Yield stress 235N/mm2
• In the region of the nozzle
the material remains
wholly elastic with
principal structural strains
much below 5%
1
ELEMENT SOLUTION
STEP=1
SUB =20
TIME=1
SEQV
(NOAVG)
DMX =3.17683
SMN =1.45271
SMX =235
JAN 24 2014
12:31:34
MN
1.45271
53.3521
105.252
157.151
209.05
27.4024
79.3018
131.201
183.101
235
CYLINDRICAL SHELL NOZZLE
GPD / I check on aileron nozzle/main cylinder
• The lift and drag on the
ailerons is counteracted by
both aileron nozzles that is
the one on the main
cylinder side and the one
on the cylindrical arm side.
• Different from what was
designed using DBR but
is more faithful to the
towfish prototype design
1
ELEMENTS
F
M
Y
PRES
Z
-.725
-.409222
-.093444
.222333
.538111
AILERON NOZZLE & FIXED WING FLANGE LAODING
11
.853889
JAN 29 2014
10:07:01
X
1.16967
1.48544
1.80122
2.117
GPD / I check on aileron nozzle/main cylinder
• The model includes the
loading at the end of the
fixed wings due to the
hydrodynamic drag and
buoyancy of the
cylindrical arms.
• The model deformation
was as expected and
confirmed the applied
boundary conditions and
loadings.
1
DISPLACEMENT
SUB =1
TIME=1
DMX =.673022
JAN 29 2014
10:11:21
Y
Z 11 X
AILERON NOZZLE & FIXED WING FLANGE LAODING
GPD / I check on aileron nozzle/main cylinder
• von Mises stress obtained
from the I check on the
top surface of the shells.
• In the region of the
aileron nozzleand fixed
wing connections the
material remains wholly
elastic with principal
structural strain much
lower than 5%.
1
ELEMENT SOLUTION
STEP=1
SUB =20
TIME=1
SEQV
(NOAVG)
DMX =.669009
SMN =.012626
SMX =235
JAN 29 2014
10:13:17
MX
.012626
52.232
104.451
156.671
208.89
26.1223
78.3418
130.561
182.781
235
AILERON NOZZLE & FIXED WING FLANGE LAODING
GPD / I check on elevator nozzle/conical end
• FE model used for the check
against buckling for the
region of the elevator nozzle
connection to the conical
end.
• The conical shell has a
thickness of 2mm while
the thickness of the shell
for the nozzle is 5mm.
1
ELEMENT SOLUTION
Y
STEP=1
SUB =20
TIME=1
SEQV
(NOAVG)
DMX =.992815
SMN =.456823
SMX =235
Z
X
JAN 30 2014
09:29:07
MX
MN
.456823
26.5172
CONE ELEVATOR NOZZLE
52.5775
78.6379
104.698
130.759
156.819
182.879
208.94
235
• The model deformation was as expected and confirmed
the applied boundary conditions and loadings.
• The maximum principal structural strain occurred in
the elevator nozzle/conical shell region and had values
of 0.667% in the GPD check and 0.293% in the I
check.
GPD / I check on the towlug/main cylinder
• The thickness of the towlug
shell is 4 mm.
• The thickness of the
reinforcing plate region is
6mm.
• The cylindrical shell thickness
elsewhere in the model is
3mm.
• The reinforcing plate in the
region of the towing lug is of
rectangular shape and has
dimensions 70mm by 200mm.
1
ELEMENTS
F
CE
JAN 30 2014
09:36:51
Y
Z
TOWLUG QUARTER CYLINDER
X
GPD / I check on the towlug/main cylinder
• In the region of the towlug
area the material remains
wholly elastic with some
plasticity occurring in the
internal flange that connects
the main cylinder to the
hemisphere.
• The maximum structural strain
for both the GPD check and
the I check occurred in the
flange regions and was much
less than 5%.
1
ELEMENT SOLUTION
STEP=1
SUB =20
TIME=1
EPTO1
(NOAVG)
DMX =.31252
SMX =.982E-03
MN
JAN 30 2014
10:08:59
MX
Y
Z
0
.109E-03
.218E-03
TOWLUG QUARTER CYLINDER
.327E-03
.437E-03
.546E-03
.655E-03
.764E-03
X
.873E-03
.982E-03
Discussions and Conclusions
• The DBR approach and assumptions taken were quite
suited for the preliminary design of the towfish.
• DBA is required in order to get more insight into the
kind of failure mechanisms especially for the
components that were outside the scope of the DBR
method.
• At the main cylinder internal flanges some plasticity has
occurred. In case that this plasticity may effect the
service conditions of the flanges and so their thickness
was increased to reduce the size of the plastic region.
Discussions and Conclusions
• In all components the maximum structural strain in the
FEA models when subjected to the maximum loads was
less than 5%.
• Therefore the principles of the GPD check and I check
were satisfied and the design of each component
acceptable according to Annex B of EN13445 Part 3
• DBA can be further used to reduce the weight of the
towfish while at the same time maintaining its structural
integrity and fitness for purpose as regards to allowable
deformations.
Prototype Manufacturing
Main changes to the original design:
• Flange tickness equal to 5 mm for all collars.
• Thickness reduced from 2 mm to 1,5 mm for ailerons, stabilizers
and rudder.
• Cutting of the central screw to allow screwing of the main body to
the ailerons.
• Skid supports moved to the central body’s external surface.
• Modified flange system for the camera housing (pod’s cap)
• conical surface approximated through planar surfaces
Assembly of the Prototype
Front view
assembly of the
prototype
Assembly of the Prototype
Rear view assembly
of the prototype
Coupling Flange
Flange used for the coupling of the
rudder motor to the shaft.
Connection flange-shaft is made
using hole with steel plug
Rudder Motor
View mouting of the
motor and rudder’s
shaft.
Motor Support
Motor support and
anchorages to the
towfish frame
Holes: 2 dof
Cone Assembly
Inside view of the
cone assembly.
Particular view
assembly of the
stabilizers and rudder
Fixed Wing Support
view of the fixed
wing support
The component has
been modified to
remove the problems
associated with the
screwing
Cone support
Supports fixed on the
towfish cylinder by
welding
removible part while
operating
Ailerons
Ailerons motor support
View inside the main body
Welding
Open Pod
Pod without cover to
position a GoPro Camera
Close Pod
Pod with cover
O-Ring Stabilizers
O-Ring housing
Thank you for attending