Buoyancy-Engine Endurance Test by Use of Hydraulic Test Device
Article information
Abstract
Buoyancy engines used in underwater gliders require durability due to repeated operation over months. Using a hyperbaric chamber for durability testing is not only expensive but also challenging to operate, as the chamber require continuous monitoring for several days. In our laboratory, we develop a hydrostatic test device that allows us to test a buoyancy engine in open air to verify its durability. The hydrostatic test device allows us to observe problems associated with the buoyancy engine, which is repeatedly exposed to high-pressure environments in the laboratory, and to verify its durability via repeated operation under varying environmental pressures, which mimics the actual situation. The hydrostatic test achieves more than 1,000 cycles, which is equivalent to three months of operation of an underwater glider. As the power source used in this case is a battery, the amount of power consumed is measured to ensure that it is within the capacity of the battery used. The result shows that it is within the acceptable range.
1. Introduction
The Korea Research Institute of Ships and Ocean Engineering (KRISO) is developing a buoyancy engine for underwater gliders. The buoyancy engine is a core component for systems such as underwater gliders that propagate via buoyancy adjustment. In an underwater glider, the system can propagate by gliding as it descends or ascends underwater through buoyancy control (Jones et al., 2014). Although commonly referred to as an engine, the core of the buoyancy control system is in fact a high-pressure pump that aims to increase the volume of an artificial bladder against deep-sea pressure. This pump is powered by an electric motor that consumes energy while performing work under pressure.
As underwater gliders can operate for up to six months, the buoyancy engine must be sufficiently durable to withstand repeated operation during that time. Components subject to wear from repeated operation of the buoyancy engine include the pump, the motor, a bellows-type reservoir that stores and discharges hydraulic fluid, and an artificial bladder that expands and contracts due to the hydraulic fluid. The artificial bladder is composed of a rubber membrane and undergoes repeated expansion and contraction, which may result in cracks. The metal bellows used as a reservoir are fabricated by welding thin metal sheets, and since their overall volume increases and decreases continuously, their service life is limited. Meanwhile, the motor is a compact, high-performance brushless direct current motor, which implies limited durability.
The performance of the developed buoyancy engine was tested using a high-pressure chamber owned by the KRISO. Its operation was tested, and the amount of buoyancy generated was directly measured and verified. However, conducting durability tests for repeated operation using the high-pressure chamber is costly. In the buoyancy engine, the only component exposed to underwater pressure is the artificial bladder, whereas the other components are housed inside a pressure-resistant container to avoid exposure to water pressure. Therefore, we developed a device by fabricating a separate pressure tank that allows repeated operation of the buoyancy engine in an environment where only the bladder is subjected to pressure.
This type of hydraulic test device allows the buoyancy engine, which is repeatedly exposed to high-pressure environments, to operate under varying environmental pressures, thus mimicking actual conditions. This allows us to observe issues associated with the developed buoyancy engine in a laboratory setting and verify its durability through repeated operation. Since the buoyancy engine can be monitored while operating repeatedly for long durations, the cost of testing using a high-pressure chamber can be reduced.
2. Development of Buoyancy Engine and Hydraulic Test Device
2.1 Structure of Buoyancy Engine
The buoyancy engine was originally custom developed for underwater gliders and is now commercialized. It has been used extensively for decades, thus demonstrating its effectiveness. Recently, studies aiming to improve efficiency using thermally responsive materials in buoyancy engines for underwater gliders have been performed (Hou et al., 2023), as well as investigations into operating buoyancy engines for motion control in newly developed gliders (Wang et al., 2017).
The buoyancy engine developed in this study is designed such that only the bladder is exposed to seawater, whereas the other components are mounted inside the pressure hull of the underwater glider. In reference to Fig. 1, the left side of the bulkhead is the bladder, which is exposed to seawater and pressure, whereas the right side is installed inside the pressure hull of the underwater glider. The buoyancy engine was developed to be compatible with the Slocum glider currently in operation, as requested by the lead organization. Accordingly, we procured the same models of motor and pump used in the Slocum glider for development, as well as designed and fabricated the valves, valve sensors, bladder, and reservoir to similar specifications. All components of the buoyancy engine were arranged to fit within a 20 cm diameter to be housed inside the Slocum glider. The internal structure was developed based on estimation, as disassembly was not permitted. The project specifications required a maximum operating depth of 1,000 m (100 bar) and a maximum buoyancy capacity of 1 L. Fig. 2 shows a photograph of the buoyancy engine used in the endurance test. A linear sensor on the extreme left measures the reservoir displacement. Since the same model could not be sourced, a sensor with similar specifications was used. The flange of the bladder housing, which contains the bladder, was designed to connect with the opening of the hydraulic test device. The inside of the bladder housing was designed in a cup shape to accommodate a flexible membrane composed of 3-mm-thick Viton rubber. The outer edge of the membrane was secured with bolts in an annular structure.
Diagram illustrating concept of buoyancy engine
Photograph of developed buoyancy engine
2.2 Hydraulic Test Device
The hydraulic test device developed and used in this study connects only the bladder of the buoyancy engine to the pressure tank and applies pressure repeatedly, thus mimicking the actual operational environment of an underwater glider. This setup allows the buoyancy engine to operate repeatedly in a similar state to actual use and enables us to verify its durability.
The pressure tank of the hydraulic test device was fabricated using stainless steel 304, and its design safety was evaluated based on the “Standards for Pressure Vessel Design, Safety, and Inspection” issued by the Ministry of Employment and Labor. The thickness required to achieve the internal pressure stipulated in the ASME(American Society of Mechanical Engineers) Code was calculated using Eq. (1).
The thickness of a cylindrical body under internal pressure (minimum required thickness due to circumferential stress at the longitudinal seam) is expressed as follows:
P : Design pressure (Kg/Cm2)
R : Cylinder radius after corrosion (mm)
S : Maximum allowable tensile stress (Kg/Cm2)
E : Weld joint efficiency (Factor: 1.0–0.7 )
C : Corrosion allowance (mm, Corrosion allowance)
In the equation above, E and C are assumed to be 1.0 and 0, respectively. The thickness of the pressure tank under a pressure load of 100 bar (10 MPa) was calculated and compared with the actual thickness of the designed pressure tank to compute the safety factor. The thickness of the pressure-tank lid was calculated using Eq. (2), which was obtained from the “Japanese Mechanical Engineering Handbook.” Subsequently, the resulting stress value was compared with the allowable stress of stainless steel 304, which is 205 MPa, to compute the safety factor.
σ : Maximum stress (Kg/Cm2)
P : Design pressure (Kg/Cm2)
R : Radius of flat cover (mm)
t : Thickness of flat cover (mm)
In Fig. 3 shows the drawings of the pressure chamber of the hydraulic test device. The cylindrical-wall thickness was 12 mm, and the lid thickness was initially designed to be 60 mm but was fabricated at 40 mm as the original design was considered excessive. Based on the equations above, the safety factors were calculated to be approximately 1.77 and 1.79 under a pressure of 100 bar for the cylinder and lid, respectively. As classification societies generally recommend a safety factor of 1.25–1.5, the device was considered to be safely constructed.
Drawings illustrating pressure chamber of hydraulic test device
The pump used to pressurize the pressure tank (model AP A-250N-16/C-77, Doseuro) was a piston metering pump with a discharge capacity of 516 mL/min and a maximum pressure capacity of 196 bar. A pressure-reducing valve was connected to the hydraulic test device attached to the buoyancy engine. This system was controlled in coordination with the buoyancy engine’s operation, thus enabling monitored testing of the engine’s repetitive motion in a laboratory setting. Fig. 4 illustrates the configuration where the hydraulic test device is connected to the buoyancy engine. To monitor the engine during operation, additional instruments not used in actual underwater gliders, such as a tachometer for the pump motor and an ammeter to measure power consumption, were included. The control and logging computer at the top of the diagram controls the buoyancy engine, which is a passive device, and simultaneously manages the pump and valves of the hydraulic test device. Furthermore, it records sensor data and the operational status of the valves.
Concept diagram of hydraulic test device
2.3 Operating Flow of Hydraulic Test Device
Before starting the test, the control program allows users to specify the number of cycles for the buoyancy engine. Manual operation is possible when problems arise during automatic testing. When the test begins, the pressure-tank pump fills the tank with water and applies a pressure of approximately 5 bar.
At this pressure, the valve of the buoyancy engine is opened toward the reservoir such that the oil in the bladder flows into the reservoir. In an actual glider, air is removed from the hull to create a pressure lower than atmospheric pressure at the surface. When the valve is opened, oil in the bladder flows into the reservoir. The reservoir, which is fabricated using thin metal, cannot withstand high pressure. Therefore, once the reservoir is filled, the valve is closed to prevent the effects of deep-sea pressure. This state corresponds to the glider beginning to dive from the surface; therefore, the pressure in the tank is further increased until it reaches the specified depth pressure. When the specified pressure is reached, which signifies that the glider has arrived at the target depth, the pump in the buoyancy engine operates, and the engine valve opens toward the bladder, thereby allowing oil to flow back into the bladder. As the bladder volume increases, the pressure in the pressure tank increases as well. Consequently, the pressure-reducing valve is opened to reduce the tank pressure—this state corresponds to the glider ascending. Meanwhile, the pressure in the tank is reduced to approximately 5 bar. This entire process is repeated for the specified number of cycles.
Fig. 5 shows the screen of the control program displayed on the computer used for the control and recording of the hydraulic test device. The diagram on the left side of the screen shows the configuration of the hydraulic test device, which displays the status of each component in the form of illustrations and numerical values. The middle section shows the pump motor rpm (revolutions per minute) of the buoyancy engine, current changes, oil-volume changes in the reservoir, and pressure changes in the hydraulic tank. The right side shows the status of each component in text.
Screen displaying hydraulic test device and buoyancy engine control program
3. Endurance Test
An endurance test using the hydraulic test device was conducted after the newly developed device was verified to operate as intended and issues in the control program were resolved. The test was performed to assess the durability of the buoyancy engine, with the goal of more than 1,000 continuous cycles. One thousand consecutive cycles correspond to the expected number of buoyancy engine operations during approximately three months of underwater-glider deployment. Any issues with the buoyancy engine occurring during testing were identified and resolved, and the test was restarted from the beginning. The test was conducted at a pace faster than that of actual underwater-glider operations. The endurance test was conducted in two main phases.
3.1 First Endurance Test
In the first test, problems occurred after 50 cycles of the buoyancy engine’s operation. Owing to the insufficient rigidity of the annular structure for securing the bladder in the housing, seawater entered through gaps in the bladder and mixed with the hydraulic oil. To prevent deformation, the annular structure was fabricated to be thicker. Additionally, residual air trapped inside the buoyancy engine prevented the intended operation; therefore, the air inside the engine was removed. In subsequent tests, the coupling structure connecting the buoyancy engine pump and motor became damaged due to repeated vibration. Thus, the structure was redesigned, and engineering plastic was inserted to solve the issue. Fig. 6 shows the values displayed by the linear sensor, which indicate the oil volume in the reservoir (top row) and the repeated pressure changes in the hydraulic tank (bottom row) during the first test. These results correspond to those of the 50-cycle test. Finally, abrupt abnormal pressure spikes and drops were observed due to air generated by pressure changes inside the buoyancy engine. Fig. 7 shows an enlarged view of the reservoir volume changes measured by the linear sensor during the normal operation of the buoyancy engine. ① is the pressure release section, and ② is the phase where the valve opens to allow oil to flow from the bladder to the reservoir for submersion. This process is not linear due to an increase in the external pressure. ③ is the 80-bar pressurization phase, during which there no change is observed in the reservoir. ④ is the phase where the buoyancy engine pump transfers oil from the reservoir to the bladder to generate buoyancy. The graph shows nonlinear displacement due to load variation as the bladder volume and the pressure in the hydraulic tank increase. ⑤ is the phase where pressure is released again. In the actual glider operation, phases ② and ④ occur over the same duration, although the ascent and descent of the glider requires a longer time. In the test, these phases were shortened to focus on evaluating the buoyancy engine's operation.
Endurance-test results of reservoir linear position and external pressure
Reservoir volume change
3.2 Second Endurance Test
In the second test, after addressing the previously encountered issues, 1,040 cycles were performed without any problems occurring during the test. Fig. 8 shows a photograph of the buoyancy engine connected to the hydraulic test device during testing. On the front desk, the control and recording computer and its screen are visible. Immediately behind it, a black box connects the components and supplies power. To the left of that unit at the back is the drive motor for the pump supplying pressure to the tank, and to the extreme left is the stainless steel pressure tank with the buoyancy engine connected on top.
Linear-sensor values from 1,040 cycles of endurance test
Fig. 9 shows the results recorded from the linear sensor of the reservoir during the endurance test, which was conducted over approximately 10 days. Since one operation cycle of the buoyancy engine corresponds to one complete fill-and-drain cycle of the reservoir, the number of cycles shown in the graph represents the engine operation. In actual underwater-glider operations, approximately 10 descent-and-ascent cycles are typically recorded daily. In this study, the test was conducted at 10 times that rate. After final improvements, the test was conducted continuously without long interruptions; however, because of limitations in monitoring personnel, it was performed intermittently. As shown in Fig. 9, the engine operated as intended for more than 1,000 cycles.
The linear sensor value results of 1,040 times endurance tests
3.3 Power-Consumption Analysis of Buoyancy Engine
Ensuring the durability of the buoyancy engine is essential because the engine must operate continuously over extended durations during underwater-glider missions. Since gliders are powered by batteries, the engine's power consumption must remain within the battery capacity. Measuring the current and voltage during the durability testing of the buoyancy engine allows us to estimate the total power consumption and assess the feasibility of the battery operation.
The maximum buoyancy output of the engine is 1 kgf, which corresponds to a buoyant volume of 1,000 mL (0.001 m3). The operating pressure corresponds to the pressure at a depth of 1,000 m, which is 100 bar or 10 MPa (10 × 106 N/m2). Therefore, the energy required to fully inflate the bladder at 100 bar is calculated as follows: 0.001 m3 (pressure) × 107 N/m2 (volume) = 1 × 104 Nm = 10,000 J. The pump used to deliver oil to the bladder has a capacity of 0.1 ml/rev, which implies that it discharges 0.1 mL per revolution (Bieri hydraulik AG, n.d.). The motor driving this pump operates at 15 V with a motor constant of 206 rpm/V; thus, it rotates at 15 V × 206 = 3,090 rpm = 51.5 rps (AXI model motors, n.d.). The pump–motor combination has a discharge rate of 5.15 mL/s. Hence, the time required to deliver 1,000 mL is 1,000 mL/5.15 mL/s = 194.17 s. Theoretically, the amount of power to transfer 1,000 mL of oil to the bladder at a depth of 1,000 m is 10,000 J / 194.17 s = 51.5 W. This value represents the pure power for one operation, excluding mechanical losses in the motor and pump.
Fig. 6 shows the actual operation record of a Slocum underwater glider, which was deployed to a depth of 800 m and performed 10.5 dive and ascent cycles over 24 h. Therefore, the number of buoyancy engine operations per month is 10.5 (cycles/day) × 30 (days) = 315, which requires a net energy of 315 × 51.5 W × 0.8 = 12.978 kW. The factor 0.8 reflects the reduced depth of 800 m. Fewer operations would be required for missions reaching 1,000 m.
Based on the actual measurements presented in Fig. 6, if 315 cycles per month are performed at 800 m, then the total net energy required is 8,000 J × 315 = 2,520,000 J, which is the pure energy required to inflate the bladder. Therefore, the actual energy calculated from the measured values during the endurance test would exceed 2.52 MJ. In Fig. 10, the spacing difference between the first and second halves reflects different diving depths. The first and second halves dive to 400 and 800 m, respectively.
Diving record of Slocum glider
During the endurance test, voltage and current were measured at 1-s intervals. By integrating the power over time, all power values from 315 operations were added and then multiplied by the 1-s interval, thus resulting in the total energy. Table 1 below presents the cumulative energy values for the 315 operations. The total energy consumed for the 315 operations was 2.9 MJ, which exceeded the net work required. At the rated motor voltage of 15 V, this corresponds to 53.7 Ah.
Measured data set and total energy
4. Conclusion
To verify the durability of the developed buoyancy engine, a hydraulic test device was custom developed for the first time. A test was performed to evaluate whether the buoyancy engine can operate for more than 1,000 cycles without failure under conditions equivalent to those of actual deployment. One advantage of the developed hydraulic test device is that the buoyancy engine is exposed, thus allowing direct observation of its operation during testing. By contrast, high-pressure chamber tests require the buoyancy engine to be enclosed in a pressure-resistant vessel, which renders it difficult to identify and resolve issues without opening both the chamber and vessel. Various problems observed during testing using the hydraulic device were addressed, thereby improving the reliability of the buoyancy engine.
The buoyancy engine developed in this study was designed to withstand pressure at the bulkhead and at the discharge ends of the valves and pump. The bulkhead was sufficiently thick to form internal flow paths and was designed to withstand pressures exceeding 100 bar. The pump was rated for discharge pressures exceeding 250 bar, and the valves were selected to satisfy pressure resistances exceeding 250 bar. Ultimately, a continuous operation test was performed up to 1,040 cycles. In future studies, the performance of the developed buoyancy engine shall be validated via open-sea trials.
Durability is critical for underwater gliders operating for approximately three months, and since the buoyancy engine is powered by the glider’s battery, its power consumption must remain within the battery's available capacity. Through testing, the current and voltage of the buoyancy engine were measured during its operation. Moreover, its total power consumption was calculated to be 53.7 Ah at 15 V, thus confirming its compatibility with underwater-glider battery systems.
Notes
The authors declare that they have no conflict of interests.
This research was supported by the “Development of Core Technology for Buoyancy Engine (Project No: 20200482)” funded by the Ministry of Oceans and Fisheries (MOF) through the Korea Institute of Marine Science and Technology (KIMST) Promotion in 2023, as well as under KRISO's basic research project, “Development of Underwater Acoustic Amplification and Bio-inspired Flow Measurement Technology Using Metastructures (4/5) (PES5551)” funded by the MOF in 2025.