Study on the Basic Design Method for Deployment of a Deep-Sea Buoyancy Engine Module
Article information
Abstract
A deep-sea test module was developed for the operation test of a buoyancy engine at deep-sea, which is one of the key elements of underwater gliders. When a deep-sea module is dropped in water, it is dropped with a weight with greater buoyancy than the deep-sea test module. The resistance coefficient of the test module must be determined to predict the dropping speed in water when the deep-sea test module is deployed and estimate a sufficient length between the weight and the test module (buoyancy engine). The test module may collide with the weight or the seabed if the speed of descent of the weight is too fast. In addition, the module may collide with the weight if the length of the rope connecting the weight and the test module is not long enough. Therefore, it is important to determine the resistance coefficient of the module to predict the descent velocity when dropping it. This study examined the resistance coefficient of the test module in relation to the flow velocity. The results confirmed that the length of the rope between the weight and the deep-sea test module was sufficient for the purpose.
1. Introduction
The International Maritime Organization (IMO) is implementing a range of marine SMART initiatives aimed at enhancing maritime safety and establishing a new marine Information and Communications Technology (ICT) ecosystem. Foundational technologies, such as Korea’s e-Navigation project and the development of submarine charts for digital navigation systems, are currently underway.
The Korea Research Institute of Ships and Ocean Engineering (KRISO) is currently developing core technologies for buoyancy engines designed for use in underwater gliders, as part of a project supported by the Ministry of Oceans and Fisheries (MOF). An independent test module was constructed to evaluate the actual performance of the developed buoyancy engine. This module, resembling an underwater glider, is capable of upward and downward vertical movements by integrating a control unit and battery within a pressure-resistant vessel. Performance assessments were conducted through tests in an ocean engineering basin, a deep-sea engineering basin, and a real-sea environment (Lee et al., 2023; 2024). To date, numerous studies have investigated the underwater drop characteristics of submarine observation equipment and related devices, such as submarine cables, as well as the structural design considerations related to the drop impact (Woo et al., 2017; Jang et al., 2018; Cho et al., 2012).
In this project, a dedicated deep-sea test module was designed to assess the capability of the developed buoyancy engine for long-duration operation on an underwater glider in a deep-sea environment. This module was used to conduct buoyancy engine performance tests over a period exceeding three months at a depth of 1,000 m, the maximum operational depth.
After deployment at a predetermined depth in the deep sea, the test module initiates operation and continuously records performance data. These records are later retrieved to evaluate the long-term operational performance of the buoyancy engine. The complete deep-sea test module system was assembled by separately attaching a battery to a pressure vessel housing the buoyancy engine, alongside another pressure vessel containing an acoustic release (AR) for long-term operation. A glass sphere buoyancy material was also incorporated to compensate for insufficient buoyancy.
After deployment, the deep-sea test module remained anchored by a weight attached via an AR mechanism. After the test period, the AR device received an acoustic command signal transmitted from the surface and severed the connection to the weight. As a result, the module, exhibiting positive buoyancy, ascended to the surface for recovery. At the initial stage of deployment, a weight exceeding the buoyant force of the deep-sea test module was attached to facilitate descent. The descending velocity is governed by the net negative buoyancy, or the difference between the combined weight and the buoyancy of the module, as well as the drag force acting on the module and attached weight. If the descending velocity is excessive, the rope connecting the weight and the deployment module may be insufficient in length, potentially causing the module to collide with the weight or the seabed. Such impacts could damage the AR mechanism or impair its proper functioning. Therefore, accurately determining the drag coefficient of the deployed module is essential for predicting its descending velocity during deployment.
This study examined the drag coefficients of a relatively complex deep-sea test module during descent and ascent to predict the drop velocity and ensure sufficient separation from the weight. The safety of the initial design distance was verified by experiments and calculations.
2. Development of Buoyancy Engine Deep-Sea Test Module
2.1 Overview of the Deep-Sea Test Module
A test module was developed for deep-sea operation of buoyancy engines. As explained previously, the module was powered by a battery designed to operate the buoyancy engine continuously for three months in the deep sea. The module also featured an acoustic telemetry modem for underwater communication, enabling remote command transmission to facilitate recovery of the test module after the test was complete. A satellite communication system was installed to provide surface location tracking and enable long-distance data transmission. Fig. 1 presents the structure and identifies the components of the buoyancy engine deep-sea test module.
Buoyancy engine deep sea test module
2.2 Concept of Buoyancy Engine Deep-Sea Test
The deep-sea test module of the buoyancy engine was designed to possess a positive buoyancy of approximately 40 kgf (392 N). During deployment, a weight of approximately 100 kgf (980 N) was attached to facilitate its descent (initial separation distance between the weight and the deep-sea test module: 15 m). The deep-sea test module was anchored to the seabed by a weight attached to it. When the buoyancy engine was activated, the seabed location of the module was transmitted via an acoustic communicator for monitoring and recovery purposes. The EvoLogics S2C 18/34 model, which features acoustic positioning capabilities, was used as the underwater acoustic communicator. Fig. 2 presents a simplified overview of the deployment and recovery process for the deep-sea test module.
Procedure of the buoyancy engine deep-sea test
2.3 Post-Impact Motion Equations for the Deep-Sea Module Weight
Upon the deployment of the deep-sea test module, equations predicting the drop velocity, as well as the post-impact speed and displacement following seabed collision, are derived, as shown in Fig. 3.
Definitions of the motion equations for the deep-sea module and weight
The following equation was used to calculate the additional descent distance of a weighted object after the weight impacts the seabed, based on its descending velocity. First, at the time of deployment, the weight is denoted as Mt = M + MW, and the object (deep-sea module, M–A) connected to the weight (MW; Weight: W) has positive buoyancy. The buoyant force B0 exceeding its self-weight is expressed as Eqs. (1) and (2):
At the terminal velocity, acceleration drops to zero, making the velocity immediately prior to impact equal to the terminal velocity vt, as expressed in Eq. (3):
By defining the motion model depicted on the right side of Fig. 3, the upper deep-sea module (M–A), which has overall positive buoyancy, descends under the influence of the attached weight (MW; Weight: W). Upon impact of the weight (W) with the seabed, the module continued to descend briefly because of its terminal velocity vt, after which its speed decreased because of the positive buoyancy, eventually coming to a stop before ascending. The time when the velocity decreased to zero after impact is expressed as Eq. (4), allowing for calculations of the time and distance at the moment of impact:
3. Numerical (CFD) Analysis Model
3.1 Overview of the Numerical (CFD) Analysis Model
The drag coefficient of the independent test module of the buoyancy engine was analyzed in terms of the flow velocity to predict the deployment velocity and establish an adequate separation distance between the weight and the deep-sea test module during deployment. Accordingly, the drag coefficient of the battery pack configuration shown in Fig. 4 was determined by calculating the hydrodynamic resistance at various ascent and descent flow velocities. Their three-dimensional geometries were modeled to analyze the flow characteristics of alkaline and lithium-ion battery packs under longitudinal flow conditions, as shown in Fig. 4. Based on these models, corresponding shape and mesh structures were generated for numerical simulation using CFD analysis. The drag coefficients for ascent and descent were derived through flow simulations conducted using the generated mesh model, with ANSYS CFX 2023 (ANSYS, Inc., 2023).
Buoyancy engine and AR assemble fit
3.2 Numerical (CFD) Analysis Model and Analytical Method
For flow analysis based on the longitudinal flow velocity of the alkaline and lithium-ion battery packs, their three-dimensional geometries were developed as analytical models (Fig. 5), with the corresponding mesh model shown in Fig. 6. A cylindrical analysis domain was established, extending approximately 12.5 m upstream and downstream of the buoyancy test module, with a 15 m diameter. This setup ensured sufficient clearance around the device to minimize the impact of boundary conditions on the accuracy of the simulation results.
Modeling for numerical (CFD) analysis
Mesh generation and performance test
Numerical (CFD) analysis was conducted using seawater as the working fluid. A steady-state approach was used, and the shear stress transport (SST) turbulence model was applied. The differences in drag coefficient results from turbulence models under steady-state conditions at constant velocity were minimal (Kim, 1993). Table 1 provides details of the numerical analysis. A Tetra/Prism mesh was generated for the numerical analysis domain, and a mesh independence test was conducted. Based on the results, a mesh consisting of approximately 1.31 million nodes and 3.6 million elements was selected for the analysis. The mesh was selected for which the variation in with respect to the mesh density was minimal, as shown in Table 2, confirming that the analysis results were independent of further mesh refinement.
Conditions for the numerical (CFD) analysis
Result of the mesh generation and performance test
4. Numerical (CFD) Analysis Results and Discussion
4.1 Numerical (CFD) Analysis Method
Flow simulations were conducted to assess water movement around the battery pack. The bidirectional longitudinal resistance was evaluated by conducting numerical analyses at flow velocities of 0.5, 1.0, 2.0, 3.0, and 4.0 m/s in the upward and downward directions. The drag coefficients (CD) were calculated using the resulting resistance force (F):
where F represents the force in the direction of flow; ρ denotes the density of the working fluid; v is the flow velocity; A is the measured longitudinal projection area, 0.45 m2. The longitudinal projection area (A) was determined by superimposing the relevant components in the CAD design to generate a projection surface, as shown in Fig. 7.
Projection Area for the longitudinal direction
4.2 Numerical (CFD) Analysis Results
Figs. 8 and 9 present the velocity and pressure distributions for the ascending and descending directions at the maximum flow velocity of 4.0 m/s. The velocity and pressure distributions indicate greater flow deceleration and higher pressure in the descending direction, a trend consistently observed across all flow velocities. In the descending direction, the geometry results in a higher pressure at the leading edge than in the ascending direction, and a more pronounced reduction in velocity can be observed.
CFD analysis result (1) - upward (velocity = 4.0 m/s)
CFD analysis result (2) - downward (velocity = 4.0 m/s)
Table 3 lists the flow analysis results of the deep-sea buoyancy test module in the ascending and descending directions, yielding the drag coefficients (CD). The results indicate the corresponding resistance forces acting in each direction.
Numerical (CFD ) analysis result
4.3 Estimation of Post-Impact Travel Distance of a Deep-Sea Module
The additional sinking depth of the deep-sea test module after the attached weight impacts the seabed was calculated using Eq. (4), derived from the previously established motion equations, as shown in Fig. 10. The module continued to descend approximately 1.25 m when M = 135 kg, MW = 100 kg, and B0 = 30 kgf (294 N). In contrast, the sinking depth increases to approximately 1.7 m when M = 135 kg, MW = 100 kg, and B0 = 10 kgf (98 N) under the same mass conditions.
Predicted moving distance for the deep-sea module after weight impacts with the seabed
5. Conclusion
This study examined the drag coefficient of the independent buoyancy engine test module as a function of the flow velocity during the deployment of a deep-sea test module for ocean exploration. Although the calculated drag coefficients were slightly higher in the descending direction, the values were generally comparable in both directions. The derived drag coefficient was applied to estimate the deployment velocity of the deep-sea test module and determine an appropriate separation distance between the module and the attached weight. According to the derived equation of motion, the deep-sea module sank an additional approximate depth of 1.25 m when M = 135 kg, MW = 100 kg, and B0 = 30 kgf (294 N). This additional sinking increased to approximately 1.7 m when M = 135 kg, MW = 100 kg, and B0 = 10 kgf (98 N) with the other parameters unchanged. Consequently, the originally designed separation distance of 15 m was deemed sufficiently safe.
Notes
The authors declare no potential conflict of interest.
This study was supported by the Korea Research Institute of Ships and Ocean engineering, grant from Endowment Project of “Research on Development of Agenda Based on Global Cooperation and Technology for leading International Standardizations and Regulations (2520000690)” funded by the Ministry of Oceans and Fisheries, and “Development of Core Technology for Buoyancy Engine (Project No: 20200482)”.