Design and analysis of low-cost underwater glider for shallow water

This paper aims to design a low-cost underwater glider to operate in shallow water. The proposed design was developed by manufacturing engineering software. Analysis of the hull using manufacturing engineering software and 3D computer-aided design (CAD). The analysis of hydrodynamics using computational fluid dynamics (CFD). This glider was designed to operate in shallow water, coastal, lake and river for a maximum depth of 10 m and a maximum speed of current 12,96 km/h, or 3,6 m/s. To reduce and minimize the cost to manufacture this underwater glider, the mechanics, electrical, electronics, and power source were using common tools on the market, not on demand. Based on numerical model, the hull pressure had 30,127 psi or 0,2077162 MPa for maximum depth 10 m and max speed 12,96 km/h. Maximum pressure occurs on the nose and behind the wings. This unmanned vehicle was designed to be in 9 compartments. The first compartment and 8th compartment are used for ballast tanks. The others for: mechanics of ballast system, altimeter and attitude controller, payload, battery pack, main controller part, propulsion system, and propulsor.


Introduction
Underwater gliders are used in oceanographic research applications such as underwater topography or mapping, environment monitoring and inspection, marine observation, and any other operation including military [1][2][3][4]. With this vehicle, a lot of missions or studies of scientists in exploring the sea or underwater become easier, for example, research on marine life, studies of mineral potential, biology and chemistry, and environmental rescue missions. In addition, missions in the military field such as underwater observation and monitoring of the threat of mines, chemical contamination, and radioactivity can also use this vehicle. The capacity of sea gliders to collect data for long durations and travel long distances. Sea gliders are based on buoyancy engines, that change the buoyancy relative to the underwater environment by changing their relative buoyancy to sequentially descend and ascend in saw-tooth gliding patterns [1].
The first underwater glider project was published by Stommel [5] in 1989. This article discusses the imagination and possibilities of using a floating fleet in the sea. Recently, a lot of research projects regarding underwater gliders such as ALBAC glider [6], SLOCUM glider [7], Spray glider [8], Sea glider [9], Libredae XRAY glider [10], Deep glider [11], Folaga glider [12], etc. Underwater glider projects are not limited to a laboratory-scale [5][6][7][8][9] but also on a large scale [10][11][12] and even commercial [13]. In general, underwater gliders have two kinds of mechanisms to glide downward and upward. The first mechanism uses a payload system [6] and the second way utilizes a buoyancy

Material and Method
In this study, the underwater glider was designed with a total length of 2000 mm, a diameter of 225 mm and a total width including the wingspan of 1220 mm. High Density Polyethylene (HDPE) is used as the main material for the hull of this underwater glider. The underwater glider is equipped with two wings that are installed in the middle of the right and left sides of the body with have the angle of 61.5 degrees relative to the main body of the glider. This prototype has the elliptical front and rear construction to reduce the resistance received when the underwater glider dives. The elliptical front is 315 mm long and the elliptical rear is 350 mm long. While the length of the main body without the elliptical front and rear covers is 1335 mm. Finally, at the rear section of this prototype has a rudder fin with a heigh of 227 mm and is designed to maintain the stability of the underwater glider when operating. Figure 1 shows the hull design of our underwater glider. To support its operational activities, this underwater glider is divided into 9 compartments. The first and 8th compartment are used as ballast tanks. While the other compartments are used for other components such as: mechanics of ballast system, altimeter and attitude controller, payload, battery pack, main controller part, propulsion system, and propulsor. The detail information can be seen in section five.  There are three main criteria used in this glider simulation, namely: a. floating condition, with a glider water-laden (T or D) of 175 mm, b. Submerged floating condition or in equilibrium condition, at a depth of 5 m, c. fully submerged conditions, at a depth of 10 m. This hydrostatic analysis was carried out to obtain the values of the hydrostatic characteristics of the underwater glider which was developed using the Maxsurf software. Hydrostatic analysis in this study uses 2 criteria, floating conditions and submerged conditions, taking into account the hydrostatic value of a fully submerged glider at any depth, will produce the same hydrostatic value. Figure 2 shows the result of hydrostatic analysis in submerged condition and the result of hydrostatic analysis in floating environment as shown in Figure 3.   The hydrostatic curve at submerged condition can be shown in Figure 4 and while Figure 5 shows the hydrostatic curve analysis at floating condition. Table 1 presented the parameters of hydrostatic analysis of the glider at floating and submerged conditions. By using the same criteria as mentioned above, namely a. floating condition, with a glider waterladen (T or D) of 175 mm, b. Submerged floating condition or in equilibrium condition, at a depth of 5 m, c. fully submerged conditions, at a depth of 10 m. With one assumption, the Bonjean Curve can be calculated using Maxsurf Software with the results in Table 2 and Figure 6.

The Analysis of Hull Performance
The hull resistance is analyzed to determine the velocity in the body and the area traversed by this underwater glider. The resistance analysis is carried out using the Computational Fluid Dynamics (CFD) method. In accordance with the purpose of this design, which is intended for shallow water, with a wavelength less than 19 m and a current strenght of about 5 knots. The speed of the underwater glider used for hull analysis is 7 knots or 12.96 km/h. Figure 7 shows the distribution of the velocity flow rate around the sea glider, simulated by Solidwork Software as Computational Flud Dynamics method, to determine hydrodinamics characteristic around sea glider while having velocity speed. Based on this analysis, the highest flow rate of 7.8 knots occurred in the front area of the hull. While at the stern, precisely in the propeller area, the flow rate decreased in speed. Figure 7(a) shows the velocity analysis on the side view. Figure  7(b) shows the top view. While the isometric view or 3D view of this analysis can be shown in Figure  7(c).

The Analysis of Hydrodinamics
Hydrodynamic analysis was developed using the CFD method to determine the pressure that occurs on the body of the glider when it is under the water level. The analysis was carried out at a depth of 10 m below of the water level and to get the value of hydrostatic pressure we used the calculations as follows: Ph total = Ph 10 m below the water level + Ph above the water level Ph above water/sea level = 101325 pa / 1 atm Ph 10 m below water/sea level = . g . h Based on the calculations above, we conducted the CFD analysis at a depth of 10 meters with a pressure of 2 atm or 0,201775 MPa or 29,26 psi. According to the results of the pressure analysis, it was found that the greatest pressure was occured at the bow end of the sea glider with a maximum pressure of 2.05 atm or 30,127 psi or 0,2077162 MPa. While the lowest pressure was in the area between the bow to midship and aft to the midship, with the lowest pressure experienced at 1.9 atm or 27,92 psi or 0,192518 MPa as shown in Figure 8. Which are figure 8(a) is an isometric view, figure  8(b) is a side view, and figure 8(c) is a top view. These results will be used for the calculation to meet Rules and Regulation on Biro Klasifikasi Indonesia [20].

Plans for Materials, Mechanical and Control Parts
The plans for materials, mechanical and control parts on this glider are based on compartment division. The first and eighth compartments will be functioned as tanks with a volume in each of 0.006 m3 or about 6.15 kg and followed by compartments 2-7 : altimeter and attitude controller, mechanic of ballast system, payload, main controller, and propulsion system. The last compartment will be functioned as a propulsor compartment and it can be seen in Figure 9. Others, this underwater glider will be equipped with wing on each side, and a rudder fin on the stern, to maintain the stability of this sea glider. The design of the ballast tank on the bow and stern sides is intended as ballast to control the sea glider's operating depth.
The Hull Material will be build by three optional materials. Firstly: HT Pipe, that fulfills JIS K6776, by diameter above 65 mm and working temperatur between 5 through 40 degrees, could withstands pressure up to 1,0 Mpa. Secondly, PVC Pipe, that fulfills JIS, with nominal diameter 8", could withstands 250 psi or 1,7237 Mpa. Thirdly, HDPE Pipe, with the larger amount with the same diameter. All of these optional material had nominal thickness 4 mm at lowest strength and 9 mm at highest strength. The front and rear cones will be build by casting and forging HT, PVC or HDPE materials, the thickness of hull including front and rear cones will be the same with the nominal thickness of the pipe. The bulkheads that separated compartment, wings and rudder fin will be build from mica (acrylic) materials or materials with similar strenght, and the thickness will be the same thickness with the hull. The bulkhead will have others purpose as internal construction members that support the hull. Figure 9 shows general arrangement of sea glider in isometric view.    Figure 9. Plans for the material, mechanical and control parts Among the two calculated power results, 0.2178 kW while floating and 0.245 kW while submerged, the largest value is selected, 0,245 kW. That can be used for the glider when floating operation and submerged operation. Table 3 shows the main engine selection in this study.  Table 4 shows the calculation result of volume and tonnage by water lines. Interpolation to determine middle of displacement:

Calculation of Weight of Materials, Mechanics of Ballast, Propulsor, and Control Parts:
Below the calculation of light weight (LWT) of underwater glider. Table 7 shows the hull  construction, table 8 shows the propulsor specifications, table 9 shows the buoyancy engine  specifications, table 10 shows the propeller specifications, table 11 shows the control parts  specifications, table 12 and table 13 show the total light weight of underwater glider, including propulsor and propulsor not included. As we can see, LWT maximum is below displacement, it's mean the use of ballast tank when underwater glider submerged is needed and the remaining is used for payload. Thus ballast 6,15 kg each in total 12,3 kg, could submerge this glider.        Table 14 shows the Principal Dimension and Technical Specification of the glider :

The Simulation Result
Simulations are obtained based on 3-dimensional CAD modelling, as shown in Figure 1 with a diameter of 225 mm and a total length of 2000 mm and then followed by analysis using the CFD method with external flow simulation at a depth of 10 meters under the sea which has a pressure of 201775 pa or 2 atm, a speed of 7 knots or 12.96 km/h, and 318 iterations. Then the simulation results are obtained as below. Table 15 shows the simulation result of flow analysis for this glider. Based on the results of this analysis, the largest pressure received by the hull of the sea glider is 2.0517 atm, the maximum velocity is 7.8492 kn, and the maximum force is 668.178 newtons. Meanwhile, the maximum iteration for each main value can be seen in the graphs in Figure 10, Figure  11, and Figure 12. Figure 10 shows the highest static pressure value of 2.098 atm occurs in the third iteration and will have a stable value in the 9th iteration and so on with a static pressure of 2,051 atm. The peak velocity value of 8.79 kn is experienced during the 6th iteration and will become a steady state condition in the 33rd iteration with a velocity value of 7.84 kn as shown in Figure 11. While in Figure 12, the maximum force value of 685.61 N appears in the third iteration and will fluctuate downward in the next iteration.  Figure 12. Iterations for force

Conclusions
Based on the CAD design and analysis of the low-cost underwater glider for shallow water that costs about Rp. 30.000.000, -, the main dimensions of the sea glider are 225 mm in diameter, with a total length of 2000 mm, and wings of 1220 mm. The front shape of the sea glider is designed in an elliptical shape to reduce the resistance received. While the wings and rudder fins are designed to maintain the stability of this glider.
CFD analysis is carried out to determine the coefficients of velocity, pressure, and force that occur in the sea glider area. Using an external flow simulation with a speed parameter of 7 knots and at a depth of 10 meters in the sea, which is then calculated in such a way as to obtain valid values for data input. The simulation results show that the maximum velocity of 7.8 knots and a maximum pressure of 2.05 atm occurs at the end of the bow of the underwater glider.
This glider compartment is designed for 9 compartments, where the first and eighth compartments are used as ballast tanks with a volume of each tank of 0.006 m3 or 6.15 kg, 0, 01230 M3 or 12,3 kg in total. While compartment numbers 2 to 7 have the following functions: altimeter and attitude controller, mechanic of ballast system, payload, main controller, and propulsion system, while the last compartment functions as a propulsor.
In order to complete this research in the future, we will consider the following items including analysis of the influence of the angle of the wings and rudder fin on the sea glider, experiments or sea glider model tests, and the design of the control sea glider based on the design that has been carried out in this study, buoyancy engine research instead.