Design of an Underwater Towfish using Design by Rule and Design by Analysis

M. Muscat, M. Formosa, G. A. Salgado MartinDepartment of Mechanical Engineering

University of Malta

R.Sinatra, A. CammarataDepartment of Industrial Engineering

University of Catania

ERDF Italia – Malta 2007 – 2013

BIODIVALUE

SUMMARY OF PRESENTATION

• 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

• 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

• 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)

• Main cylinder thickness 3mm

• Cylindrical arms thickness 2mm

• Hemisphere thickness 2mm

• Conical end thickness 2mm

Design by Rule (Local loads)

• 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 pre-deformations 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

11X

Y

Z

CYLINDRICAL SHELL NOZZLE

-.725-.409333

-.093667.222

.537667.853333

1.1691.48467

1.800332.116

JAN 24 201412:12:25

ELEMENTS

PRES

• 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

MN

CYLINDRICAL SHELL NOZZLE

1.4527127.4024

53.352179.3018

105.252131.201

157.151183.101

209.05235

JAN 24 201412:31:34

ELEMENT SOLUTION

STEP=1SUB =20TIME=1SEQV (NOAVG)DMX =3.17683SMN =1.45271SMX =235

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

11X

Y

Z

AILERON NOZZLE & FIXED WING FLANGE LAODING

-.725-.409222

-.093444.222333

.538111.853889

1.169671.48544

1.801222.117

JAN 29 201410:07:01

ELEMENTS

FM

PRES

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

11 X

Y

Z

AILERON NOZZLE & FIXED WING FLANGE LAODING

JAN 29 201410:11:21

DISPLACEMENT

SUB =1TIME=1DMX =.673022

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

MX

AILERON NOZZLE & FIXED WING FLANGE LAODING

.01262626.1223

52.23278.3418

104.451130.561

156.671182.781

208.89235

JAN 29 201410:13:17

ELEMENT SOLUTION

STEP=1SUB =20TIME=1SEQV (NOAVG)DMX =.669009SMN =.012626SMX =235

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

MN

MX

X

Y

Z

CONE ELEVATOR NOZZLE

.45682326.5172

52.577578.6379

104.698130.759

156.819182.879

208.94235

JAN 30 201409:29:07

ELEMENT SOLUTION

STEP=1SUB =20TIME=1SEQV (NOAVG)DMX =.992815SMN =.456823SMX =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

X

Y

Z

TOWLUG QUARTER CYLINDER

JAN 30 201409:36:51

ELEMENTS

FCE

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

MN

MX

X

Y

Z

TOWLUG QUARTER CYLINDER

0.109E-03

.218E-03.327E-03

.437E-03.546E-03

.655E-03.764E-03

.873E-03.982E-03

JAN 30 201410:08:59

ELEMENT SOLUTION

STEP=1SUB =20TIME=1EPTO1 (NOAVG)DMX =.31252SMX =.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

Rear view assembly of the prototype

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

removible part while operating

Supports fixed on the towfish cylinder by welding

Ailerons

Ailerons motor support

WeldingView inside the main body

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