DESIGN OF AN UNDERWATER TOWFISH USING DESIGN BY RULE AND DESIGN BY ANALYSIS

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1 Proceedings of the ASME 204 Pressure Vessels & Piping Conference PVP204 July 20-24, 204, Anaheim, California, USA PVP DESIGN OF AN UNDERWATER TOWFISH USING DESIGN B RULE AND DESIGN B ANALSIS Martin Muscat Department of Mechanical Engineering University of Malta Msida, Malta ( martin.muscat@um.edu.mt) Mark Formosa German A. Salgado Martin Department of Mechanical Engineering University of Malta Msida, Malta Rosario Sinatra Alessandro Cammarata Department of Industrial Engineering University of Catania Catania, Italy ABSTRACT The European unfired pressure vessel code EN [] has been used to design a preliminary prototype of a towfish. The towfish is essentially an underwater vessel equipped with various sensors, cameras, hydroplanes and control systems that are used to capture data on the levels of pollutants in the sea and at the same time monitor plankton and jellyfish levels. The towfish is towed behind a surface ship and is designed to dive to a depth of 50m below sea level. The depth of dive can be controlled by means of hydroplanes. Data, signals and electrical power are transferred from the towfish to the surface ship and vice versa via the towing line. From a structural point of view the towfish is a vessel acted upon by external pressure and local loads. Design by rule (DBR) was first used to calculate some of the various dimensions and thicknesses of the towfish components. The various components were designed mainly to prevent failure due to buckling. Design by analysis (DBA) based on Annex B of the pressure vessel code EN [] was then used to carry out further buckling checks that were not possible to do using design by rule. At the end of the paper the results from the two design approaches are compared and any major differences are highlighted. KEWORDS Design by rule, design by analysis, buckling, towfish INTRODUCTION The monitoring of pollution is very important for maintaining a healthy environment in the ship congested Sicily- Malta channel in the Mediterranean sea. Pollution monitoring can be carried out using various methods. A method that is quite convenient to use when a large area of sea needs to be scanned is to use a towfish equipped with the necessary sensors that can detect various pollutants. The same towfish can also be used as a platform for cameras to capture images or video of jellyfish and plankton populations. The towfish is towed behind a surface ship and hydroplanes and elevators are used to control the depth of dive. In this way pollution level readings, images and video can be taken and recorded at various sea level depths. For this project the towfish was designed to dive to a maximum depth of 50m below sea level when under tow. The towfish is designed to be positively buoyant so that when it is not being towed it rises to the surface of the sea. This together with a radio beacon enables recovery in case of tow line failure. During a mission external pressure due to the depth of sea water acts on all the components of the towfish. In addition to this local loads act on the towing lug and on the nozzle/bearing attachment of the hydroplane and elevator shafts. The hydroplanes and elevators create both a moment load and shear force loads due to hydrodynamic drag and lift. Design by rule (DBR) was first used to calculate some of the various dimensions and thicknesses of the towfish components. The Copyright 204 by ASME

2 main components are a hemispherical head, a cylinder that carries the hydroplanes and towing lug and a conical end that carries the elevators and rudder. Two other cylinders (cylindrical arms) on each side of the main cylinder were designed so that they carry the jellyfish and plankton cameras as future add-ons to the towfish. Design by analysis (DBA) based on Annex B of the pressure vessel code EN [] was then used to carry out further buckling checks. The finite element analysis software ANSS Mechanical [2] was used for the design by analysis part of the structural analysis. The Computational fluid dynamics software ANSS FLUENT [3] and closed form analytical fluid dynamics equations were used in order to calculate the maximum loads acting on the hydroplanes, elevators and towing lug during a mission at sea. GENERAL STRUCTURE OF THE TOWFISH Figure shows the general assembly drawing of the prototype towfish. The main vessel is made up of a cylinder having an outside diameter of 300mm. The cylinder ends are closed by means of a hemisphere at the front end and a cone at its trailing end. Both hemisphere and cone are assembled on to the cylinder by means of internal flanges. Such flanges were used in order to reduce the hydrodynamic drag on the towfish and so reduce the stress on the towing line. The main cylinder carries the towing lug, the hydroplanes and the cylindrical arms attachments. The towing lug is designed so that it can swivel about an axis parallel to the longitudinal axis of the cylinder. The towing line is further attached to the lug in a way to allow the connection to act as a pivot or ball joint. The hydroplanes are made up of a fixed aerofoil part and a rotating part. The latter rotating part acts as the aileron. The aileron angle of attack can vary from to and is used to vary the depth of dive of the towfish when under tow. The fixed part of the hydroplanes is attached by means of rectangular flanges to the side of the main cylinder at a small anhedral angle to the horizontal. The anhedral angle acts as a passive system to control the rolling action of the towfish. The ailerons are mounted on two separate shafts that can rotate in two journal bearings one at each end of the shafts. The shafts are driven independently by two electric motors installed in the main cylinder of the towfish. In this way apart from controlling the depth of dive, the ailerons can be used for controlling the rolling action of the towfish in an active manner. The bearings of the aileron shafts on the cylinder side are made water tight by means of compression packing. The other bearings are carried by the outer end of the fixed part of the hydroplane as shown in Fig.. The outer end of the fixed part of the hydroplanes carry the cylindrical arms. These cylindrical arms whose external diameter is equal to 50mm will house the plankton and jellyfish cameras in a later add-on to the towfish. The conical and hemispherical ends are used in order to close the main cylinder and at the same time reduce the hydrodynamic drag acting on the towfish. The elevators are attached to the conical end and are used to stabilize the towfish and maintain it in a horizontal position while under tow. The elevators are mounted on one single shaft and are rotated and controlled by one electric motor fixed to a bracket inside the conical shell. Item No. Description Main body assembly 2 Fixed wing complete assembly 3 Cone assembly 4 Hemispherical head assembly 5 Aileron assembly 6 Elevator assembly 7 Rudder assembly 8 Cylindrical arm 9 Flat bar support Figure General assembly drawing of the towfish The shaft of the elevators rotates in bearings mounted in the nozzles as shown in Fig.. The bearings of the elevator shaft are made water tight by means of compression packing. The conical end also carries the rudder that can be used to steer the towfish away from the wake of the towing boat when taking sea surface or near sea surface measurements. The necessary sensors will be attached to the underside of the main cylindrical shell by means of welded brackets. The sensors need to make direct contact with sea water and so will be carried on the outside of the main cylinder. From market research it was concluded that all the required sensors roughly have a cylindrical shape. All sensors are to be mounted on three cylindrically shaped tubes of approximately 600mm in length. Since the sensors were not yet available at the design stage it was decided to include them in the computational fluid dynamic analysis but not in the structural analysis. The design of the 2 Copyright 204 by ASME

3 necessary connecting brackets and necessary reinforcement would then be made later on. DESIGN B RULE The design of the towfish is not governed by any European or any other international legislation and so no code of standard for its design exists. Notwithstanding this, the towfish can still be regarded to be a pressure vessel acted upon by external pressure and by other local loads acting on the towing lug and on the elevator and the aileron nozzle attachments. As a preliminary design for the prototype it was decided to follow Section 8 and Section 6 of the European unfired pressure vessel design code MSA EN 3445 Part 3 []. Section 8 gives methods and equations that can be used to design the thicknesses of the major towfish components against failure due to buckling when acted upon by external pressure. The major components designed using Section 8 were the hemispherical end, the main cylinder, conical end and the cylindrical arms. Section 6 was then used to check for any reinforcement required due to the local loads acting at the towing lug area and at the nozzle attachments. The Design by rule methods presented in [] were not always applicable to the design of the towfish components. MSA EN [] did not provide a method to design the conical end bearing nozzle reinforcement when the nozzle is acted upon by local loads. For this case of the conical end it was assumed that the local loads are acting on a cylinder having the same diameter of the cone at the point of the nozzle/bearing connection. This was done in order to use the equations presented in Section 6 of EN applicable to local loads in cylindrical shells. Another component not falling within the scope of the design by rule approach as presented in [] is the rectangular flange connection of the fixed part of the hydroplane to the main cylinder at one end and to the cylindrical arms at the other end as shown in Fig.. As a first trial the shell thickness of the fixed part of the hydroplane was taken to be 2mm. This value was used so as to be the same as for the shell thickness calculated for the cylindrical arms using the design by rule equations for cylindrical shells under external pressure. The shell reinforcement for the aileron nozzles/bearings was calculated using Section 6 of EN It was decided to extend this shell reinforcement so that it also reinforces the area around the rectangular flange connection of the fixed part of the hydroplane. Later on the finite element analysis software ANSS Mechanical [2] was 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 6 of EN The material selected for the towfish was structural steel having a minimum yield stress R eh of 235 N/mm 2 and a maximum tensile stress R m of 360 N/mm 2. This resulted in a design stress f of 50 N/mm 2. Corrosion allowance was not required for the shells making up the towfish since the welds and the shell material would be reliably protected by paint against the marine environment. It was also being assumed that all the main towfish shells would be fabricated within the shape tolerances as specified in Section 8 of EN []. SHELLS UNDER THE ACTION OF EXTERNAL PRESSURE The necessary thicknesses for the main shells to prevent buckling due to external pressure were calculated using the design by rule methods presented in Section 8 of MSA EN []. The internal flanges connecting the main cylinder to the hemispherical and to the conical ends were assumed to act as heavy stiffeners. Therefore buckling of the main shells could be expected to occur only within each main component of the shell. This way of design excluded both the use of introducing shell junction reinforcement and the need to check for buckling occurring across the shell junctions. The DBR equations for unstiffened cylinders under external pressure were used to confirm that thicknesses of 3mm and 2mm for the main cylindrical shell and for the cylindrical arms respectively were sufficient. The hemispherical end is only acted upon by external pressure and so the design of this end was quite straight forward using design by rule formulae. Using the method presented in Section 8 of MSA EN it was calculated that a hemispherical shell thickness of 2mm is able to withstand the external pressure at a sea water depth of 50m. Similarly the design by rule equations for unstiffened cones under external pressure were used to confirm that a thickness of 2mm for the conical end is sufficient. SHELLS UNDER THE ACTION OF LOCAL LOADS The Computational fluid dynamics (CFD) software ANSS FLUENT [3] and closed form analytical fluid dynamics equations were used in order to calculate the maximum loads acting on the hydroplanes, towing lug and elevators respectively. The geometry of the fluid surrounding the towfish was created using SolidWorks [4] and the mesh was created using the ANSS mesh tools [2]. A k-ε turbulence model was used for the fluid flow. The analysis was carried out at a fluid speed of 0 knots (5.4 m/s). CFD analyses at different angles of attack of the ailerons were carried out. The maximum values for towfish drag and lift occurred at an aileron angle of attack of Unsteady flow and aerofoil stall were detected at angles of attack near to the value of At the angle of attack of the total lift on both hydroplanes resulted in a value of -702 N while the total drag on the towfish resulted in a value of 259 N. At the same angle of attack the lift on each of the rotating ailerons resulted in a value of -254 N and a drag of 87 N while the lift and drag on each of the fixed part of the hydroplanes were calculated to be -97 N and 00.6 N respectively. It must be noted that the values of lift acting on each rotating aileron is not equal to half the total lift acting on the towfish since the fixed parts of the hydroplanes are also contributing to the lift. As a conservative assumption, for the DBR part using section 6 of MSA EN [], it was assumed that each aileron nozzle/bearing on the main cylinder 3 Copyright 204 by ASME

4 side is carrying the lift and drag acting on each aileron. On the other hand when using finite element analysis (FEA) the lift and drag acting on each aileron was modeled as a distributed pressure on beam elements. For this case the FEA model was such that the lift is counteracted by both aileron nozzles/bearings i.e. the one on the main cylinder side and the one on the cylindrical arm side. It is therefore considered that DBA would result in a lighter design for the towfish. For the DBR part the moments M x and M y acting on the aileron nozzle were calculated to be 3800 Nmm and 3050 Nmm respectively (Fig. 2). Using the DBR section 6 of MSA EN [] it was found that a reinforcement plate of thickness 3mm is adequate for the region of the aileron nozzle. The local loads acting on the towfish rudder were smaller than those acting on the elevators so that the structural integrity check of the conical shell in the region of the rudder was not performed. Both the rudder and elevator bearing nozzles are the same and their point of attachment lies on the same section on the conical shell. Figure 3 Free body diagram of the major force vectors acting on the towfish Figure 2 Moment and force vectors acting on a nozzle 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 resultant force is shown in Fig. 3. Using section 6 of MSA EN [] and a reinforcement plate of 3mm thickness in the towing lug region it was found out that the cylindrical shell is able to withstand the resultant maximum towing force. For the design of the nozzle bearings 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. The drag and lift acting on the elevators at an angle of attack of 20 0 were calculated using analytical fluid dynamic equations. These equations resulted in higher drag and lift values than the CFD analysis and so in order to be conservative the values given by the analytical equations were used for the subsequent structural analysis. Thus the values of moments M x and M y acting on the elevator nozzles/bearing were calculated to be 590 Nmm and 53 Nmm respectively (Fig. 2). The aerofoil shape of the elevators is according to the NACA008 standard [5]. Using the DBR section 6 of MSA EN [] it was found that the conical shell made out of a 2mm plate thickness is able to withstand the applied external moments caused by the sea water flow over the elevators. DESIGN B ANALSIS Design by analysis can be used as an alternative or as a complement to design by formulae. MSA EN Annex B [] presents a number of design checks within a design by analysis context that can be used to check the structural integrity of pressure vessel components. The pressure vessel component must conform with the principle of each design check. Conformity with a principle can be proven by using what is called an application rule or else by using the principle directly. MSA EN 3445 suggests a number of application rules but the designer is free to use other application rules as long as they abide with the principle of the design check. Two of these design checks are the Gross plastic deformation (GPD) design check and the Instability (I) design check. The Instability design check complements the Gross plastic deformation check whenever compressive stresses occur in relatively thin components. The principle for the GPD check requires that the design value of an action, or of a combination of actions, shall be carried by the design model while limiting the largest principal structural strain to 5% []. The structural strains include membrane and bending strains caused by gross structural discontinuities but do not include peak strains caused by local stress concentration features. A shell finite element model gives structural strain directly because of the way such elements are formulated. The design model uses a linear elastic ideal plastic constitutive law, Tresca s yield condition and associated flow rule, small deflection first order theory and partial safety factors both for the actions (.2 for pressure and.5 for the unbounded and unfavourable variable actions due to the local loads) and 4 Copyright 204 by ASME

5 for the design material strength parameter (.2 for the GPD check). The design model is required to be initially stress free. In addition, von Mises yield condition may be used instead of Tresca s but then the design stress parameter must be additionally multiplied by 3 / 2. For cases were actions result in an unfavourable weakening effect geometrically nonlinear second order theory must be taken into account. Such actions are the nozzles in the shells of the towfish under transverse moments created by the hydrodynamic lift action of the aerofoils. The principle for the I check requires that For each load case, the design value of an action or of a combination of actions shall not be greater than the design value of the corresponding buckling strength, obtained, with a limitation on the maximum value of the principal structural strain of 5% []. The finite element model used for the instability check must contain pre-deformations according to the eigenvalue buckling modes and/or perturbation forces or moments, must use a linear elastic ideal plastic constitutive material law with a material safety factor of one, must use the von Mises yield criterion and associated flow rule, must use second order large deflection analysis and must be initially stress free. In addition to this, load actions are multiplied by a safety uncertainty factor (.2 for pressure and.5 for the unbounded and unfavourable variable actions due to the local loads) and the buckling load is also divided by a factor of safety once the analysis is performed. This latter factor of safety is equal to.25 when a pressure test as in EN3445 part 5 is carried out otherwise it is equal to.5. In this paper this latter factor of safety is not applied since the purpose of the work is to compare the design of the main towfish components using Design by rule and design by analysis. The shape deviations are required to be within the ones allowed by MSA EN 3445 Part 4 [6]. MSA EN3445 presents two application rules namely an experimental approach and the use of the design by rule Clause 8 of Part 3 of the same code. For the case of the Instability check the principle can also be used directly within the context of a finite element analysis. For both the GPD and the I checks EN 3445 Part 3 (Annex B) uses the value of R eh as the material strength parameter (yield stress) denoted by RM. For the material of the towfish (ferritic structural steel) the value of R eh or RM is equal to 235 N/mm 2. Table summarises the requirements of the principle for the GPD and I checks. THE FINITE ELEMENT MODELS The material selected for the towfish is structural steel having a minimum yield stress RM of 235MPa. Considering the partial safety factors for material properties given in Table, for the GPD check the yield stress value used in the finite element material model is 62.8 N/mm 2 while for the I check the yield stress value was 235 N/mm 2. Values of oung s modulus and Poisson s ratio were assumed to have values of 207,000N/mm 2 and 0.3 respectively. For the buckling check of the towfish no pre-deformations according to the critical eigenvalue buckling shapes are considered. These will be included later on in further analysis in future developments of the towfish design. Partial safety factor for pressure action Partial safety factor for local loads action RM d using von Mises yield condition Maximum principal structural strain limit Constitutive model GPD - check I - check R eh multiplied by R eh 3 / 2and divided by.25 5% 5% Linear elastic perfectly plastic Linear elastic perfectly plastic Type of deflection Small deflection Large deflection analysis analysis except for analysis cases were action causes a geometric weakening effect Buckling strength n/a Divided by.25 if pressure tested, by.5 otherwise (not used in the work presented in this paper) Table Requirements for the GPD and I checks The finite elements SHELL28 and BEAM8 were used in the software ANSS Mechanical [2]. SHELL28 is a quadratic element that can model curved shells. Each node has six degrees of freedom and the element has plasticity, stress stiffening, large deflection and large strain capabilities. BEAM88 is a linear 3D structural finite strain beam having 2 nodes and used in this work in order to apply the loads acting on the hydroplane, aileron and elevator. An elastic perfectly plastic material model was used in line with EN 3445 Part 3 Annex B. The loads for all the FEA models were all multiplied by the partial safety factors as shown in Table depending on the type of analysis (GPD or I check). 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. A mesh convergence study was carried out for all models. This showed that the stress and strain results shown in this paper are within 6% of the converged results and are conservative. Figure 4 shows the finite element model used for the check against excessive deformation or buckling for the region of the 5 Copyright 204 by ASME

6 hydroplane nozzle connection to the main cylinder. In order to compare directly with the DBR results, Fig. 4 shows the same loading situation as in the DBR method. This means that in Fig. 4 each aileron nozzle/bearing on the main cylinder side is carrying the lift and drag acting on each aileron. Later on, in a further FE analysis in the model shown in Fig. 7, the lift is counteracted by both aileron nozzles/bearings i.e. the one on the main cylinder side and the one on the cylindrical arm side. STEP= TIME= SEQV (NOAVG) DMX = SMN =.4527 SMX =235 JAN :3:34 ELEMENTS PRES MN Z X JAN :2: CLINDRICAL SHELL NOZZLE Figure 5 The von Mises stress distribution near the nozzle attachment on the top shell surface for the I-check CLINDRICAL SHELL NOZZLE Figure 4 Finite element model for the hydroplane nozzle connection to the main cylinder The half symmetry model shown in Fig. 4 includes the external pressure acting on the cylindrical shell and the lift and drag acting on the aileron. Part of the flanges that connect the main cylindrical shell and the hemispherical shell at one end and the conical shell at the other end are also modeled. The nodes at the conical end of the cylindrical shell were fully constrained. Symmetry boundary conditions were applied to nodes lying on the symmetrical longitudinal plane of the cylindrical shell. The effect of the external pressure acting on the hemisphere was modeled as a pressure acting on the edge of the cylindrical shell at the hemispherical end. The end pressure acting on the nozzle/bearing due to the external pressure was not modeled. Beam elements were used in order to be able to apply the aileron loading. The beam elements were connected to the nozzle by creating a rigid region on the line of nodes lying on the edge of the nozzle. The nozzle, cylindrical shell and end flanges were modeled with shell elements of different thicknesses according to the design drawings. 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. 2.6 Figure 5 shows the von Mises stress obtained from the I check on the top surface of the shells. This plot is shown since the yield stress used in the analysis is 235N/mm 2. The latter is the minimum yield stress for the specified structural steel used. The analysis indicates that at the main cylinder internal flanges some plasticity has occurred. In case that this plasticity affects the service conditions of the flanges then a design modification would be required for these components. These flanges were outside the scope of DBR and their design can therefore only be carried out using FEA. Figure 5 indicates that in the region of the nozzle/bearing the material remains wholly elastic. The maximum structural strain for both the GPD check and the I check occurred in the flange regions and was equal to 0.28% and 0.07 % respectively. The maximum principal structural strain in the nozzle region was 0.09% in the GPD check and % in the I check. Figure 6 shows the st (maximum) principal structural strain distribution near the nozzle attachment for the I-check. Figure 7 shows the main cylinder model in which the lift and drag on the ailerons is counteracted by both aileron nozzles/bearings i.e. the one on the main cylinder side and the one on the cylindrical arm side. This model is different from what was designed using the DBR method but is more faithful to the towfish design shown in Fig.. The same boundary conditions on the main cylinder and flanges as for the model shown in Fig. 4 were used. Similarly the effect of the external pressure acting on the hemisphere was modeled as a pressure acting on the edge of the cylindrical shell at the hemispherical end. The aileron nozzle/bearing was modeled as in the model shown in Fig. 4. Again beam elements were used to model the aileron and apply the loads due to the hydrodynamic lift and drag. As shown in Fig. the fixed part of 6 Copyright 204 by ASME

7 the wing is connected to the main cylinder by means of a rectangular flange. This flange is modeled in FEA by using shell elements to form a rectangular shaped nozzle as shown in Fig. 7. STEP= TIME= EPTO (NOAVG) DMX = SMN =.264E-05 SMX = JAN :33:47 applied on these beam elements. The model shown in Fig. 7 also includes the loading at the end of the fixed wings due to the hydrodynamic drag and buoyancy of the cylindrical arms. This loading consists of the drag of 509N acting on the arm in the z direction, an upward force of 20.6N due to buoyancy acting in the y direction and a moment of 3502Nmm about the x-axis. Figure 8 shows a view of the deformed model when the GPD check was carried out. The model deformation was as expected and confirmed the applied boundary conditions and loadings. DISPLACEMENT SUB = TIME= DMX = JAN ::2 MN Z X.264E-05.24E-03.22E E E E-03.77E E E CLINDRICAL SHELL NOZZLE Figure 6 The maximum principal structural strain distribution near the nozzle attachment for the I-check ELEMENTS F M PRES Z X JAN :07:0 AILERON NOZZLE & FIXED WING FLANGE LAODING Figure 8 Deformed model for the GPD check of the model that incorporates the aileron nozzle/bearing and a representation of the fixed part of the wing STEP= TIME= SEQV (NOAVG) DMX = SMN = SMX =235 JAN :3: AILERON NOZZLE & FIXED WING FLANGE LAODING Figure 7 - Finite element model for the hydroplane nozzle connections to the main cylinder MX The end pressure acting on the nozzle/bearing due to the external pressure was not modeled. The beam elements representing the fixed part of the wing were given approximately the same cross section as the real component. These beam elements were connected to the rectangular nozzle of the fixed wing by creating a rigid region on the line of nodes lying on the face of the nozzle. Loading to represent the hydrodynamic lift and drag on the fixed part of the wing was AILERON NOZZLE & FIXED WING FLANGE LAODING Figure 9 - The von Mises stress distribution near the nozzle attachments on the top shell surface for the I-check 7 Copyright 204 by ASME

8 Figure 9 shows the von Mises stress obtained from the I check on the top surface of the shells. The analysis again indicates that at the main cylinder internal flanges some plasticity has occurred. On the other hand in the region of the aileron nozzle/bearing and fixed wing connections the material remains wholly elastic. Figure 2 shows the von Mises stress obtained from the I check on the top surface of the shells. Figure 2 indicates the presence of a small plastic region in the junction between the conical shell and the elevator nozzle/bearing. ELEMENTS F STEP= TIME= EPTO (NOAVG) DMX = SMN =.624E-07 SMX =.985E-03 JAN :26:40 Z X JAN :23:22 MX CONE ELEVATOR NOZZLE.624E-07.29E E E E-03.0E E E E E-03 AILERON NOZZLE & FIXED WING FLANGE LAODING Figure 0 - The maximum principal structural strain distribution near the nozzle attachments for the I-check Figure - Finite element model for the region of the elevator nozzle/bearing in the conical shell The maximum structural strain for both the GPD check and the I check occurred in the main cylinder internal flange regions and was equal to 0.6% and % respectively. The maximum principal structural strain in the nozzle region was % in the GPD check and 0.039% in the I check. Figure 0 shows the st (maximum) principal structural strain distribution near the nozzle attachment for the I-check. Figure shows the finite element model used for the check against buckling or excessive deformation 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. The shaft of the elevator was modeled using BEAM88 [2] elements in order to be able to apply the drag and lift acting on the elevator aerofoil. These drag and lift distributed loads amounted to 8.6 N and 245 N respectively. The beam elements were connected to the bearing nozzle by creating a rigid region on the line of nodes lying on the edge of the nozzle. In the analysis, all the degree of freedom of the nodes lying on the circumferential edge of the main flange that connects the conical end to the main cylinder were fully constrained. An external pressure of 0.5 N/mm 2 was applied on the outer surface of the cone. The model deformation was as expected and confirmed the applied boundary conditions and loadings. STEP= TIME= SEQV (NOAVG) DMX = SMN = SMX = CONE ELEVATOR NOZZLE MX Z MN X JAN :29:07 Figure 2 - The von Mises stress distribution near the elevator nozzle attachment on the top shell surface of the conical shell (I-check) Figure 3 shows the st (maximum) principal structural strain distribution near the nozzle attachment for the I-check. 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. 8 Copyright 204 by ASME

9 Figure 4 shows the shell finite element model used for the check against buckling or excessive deformation for the region of the towlug connection to the main cylinder. The thickness of the towlug shell is 4 mm while 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. 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 equal to 0.% and 0.098% respectively. Figure 6 shows the st (maximum) principal structural strain distribution near the towlug attachment for the I-check. The maximum principal structural strain occurring in the towlug region was 0.038% in the GPD check and 0.032% in the I check. STEP= TIME= EPTO (NOAVG) DMX = SMN =.929E-06 SMX = JAN :32:0 ELEMENTS F CE JAN :36:5 MX Z X MN.929E E-03.65E E CONE ELEVATOR NOZZLE TOWLUG QUARTER CLINDER Figure 3 - The maximum principal structural strain distribution near the nozzle attachment in the conical shell (I-check) Figure 4 Finite element model for the towlug attachment to the main cylinder Part of the flanges that connect the main cylindrical shell and the hemispherical shell at one end and the conical shell at the other end are also modeled. The same boundary conditions on the main cylinder and flanges as for the model shown in Fig. 4 were used. Similarly the effect of the external pressure acting on the hemisphere was modeled as a pressure acting on the edge of the cylindrical shell at the hemispherical end. The half symmetry model shown in Fig. 4 includes the external pressure acting on the cylindrical shell and the force acting on the towlug. The horizontal and vertical components of the towing force were applied on the circumference of the through hole in the towlug. For this purpose a rigid region was created on the hole circumference and the loading applied on a central master node created within the hole. This was done in order to distribute the horizontal and vertical component of the towing force over a number of nodes. The model deformation was as expected and confirmed the applied boundary conditions and loadings. Figure 5 shows the von Mises stress obtained from the I check on the top surface of the shells. Figure 5 indicates that in the region of the towlug area the material remains wholly elastic STEP= TIME= SEQV (NOAVG) DMX =.3252 SMN = SMX =235 JAN :00: TOWLUG QUARTER CLINDER Figure 5 - The von Mises stress distribution in the towlug area (top shell surface) for the I-check 9 Copyright 204 by ASME

10 STEP= TIME= EPTO (NOAVG) DMX =.3252 SMX =.982E E-03.28E E E E E E E E-03 TOWLUG QUARTER CLINDER MN MX Z X JAN :08:59 Figure 6 - The maximum principal structural strain distribution near the towlug attachment for the I-check DISCUSSION AND CONCLUSIONS The DBR and DBA methods presented in the Unfired pressure vessel code EN3445 part 3 [] have been used to perform a preliminary design of an underwater towfish. The towfish is essentially a vessel acted upon by external pressure and local loads. The internal flanges that connect the conical and hemispherical ends to the main cylinder and the aileron/fixed wing and elevator connections/nozzles were outside the scope of the DBR procedure []. Some assumptions were therefore taken in order to use DBR to come out with preliminary values for component thicknesses and reinforcement pads. The DBA method presented in Annex B of [] was then utilized to check the structural integrity each component. It was concluded that 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. The DBA analysis indicated that at the main cylinder internal flanges some plasticity has occurred. In case that this plasticity effects the service conditions of the flanges then a design modification would be required for these components. 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 EN3445 Part 3 []. In view of this 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. the use of the ANSS software academic research licenses and the education edition software SolidWorks provided to the University of Malta and the University of Catania by ANSS Inc. and Dassault Systemes respectively. NOMENCLATURE f Design stress R eh, RM Minimum yield stress R m Minimum tensile strength RM d Design material strength parameter CFD Computational fluid dynamics DBR Design by rule DBA Design by analysis FEA Finite element analysis GPD Gross plastic deformation I Instability REFERENCES [] Malta Standards Authority EN : Unfired Pressure Vessels. [2] ANSS Academic Research Mechanical, Release 4, Ansys Inc., Canonsburg, USA [3] ANSS Academic Research FLUENT, Ansys Inc., Canonsburg, USA [4] SolidWorks Education Edition Software, Dassault Systemes, USA [5] NACA airfoil sections, NASA advisory council (NAC), USA ACKNOWLEDGMENTS This project is being funded through ERDF (European Regional Development Funds) Italia-Malta within the project BIODIVALUE. The authors wish to acknowledge 0 Copyright 204 by ASME