1. Introduction
The demand for marine resource development and infrastructure construction has increased steadily worldwide. In particular, the proliferation of carbon neutrality and renewable energy 100% (RE100) policies has accelerated marine renewable energy development. This has resulted in the rapid growth of related industries including offshore wind, tidal, and wave energy. Accordingly, precise and safe underwater operations have been established as essential processes in areas such as marine structure construction, as well as the installation and maintenance of power generation facilities. Thus, it is becoming increasingly important to secure technological capabilities for successfully executing large-scale marine civil engineering constructions. The frequency of marine accidents is increasing owing to the increase in various maritime activities such as marine tourism, marine leisure, and maritime transportation. This further emphasizes the need for rapid and accurate responses.
Meanwhile, the rapid advances in technology and science have served as a foundation for dramatically enhancing the efficiency and safety of work in various industries. In particular, the advances in electronics and computer systems have accelerated the development of equipment capable of remote operations and automation. This has played a crucial role in minimizing direct human interventions and improving the reliability and sustainability of work even in high-risk environments. The marine industry is also experiencing this trend of technological innovation, and the introduction and utilization of cutting-edge equipment have been accelerated.
Accordingly, various studies have been conducted actively to secure the technical performance and operational stability of remotely operated vehicles (ROVs). Choi et al. (2008) developed a position control system using ultrasonic-based sonar sensors to overcome the limitations of the global positioning system (GPS). They significantly improved the positional accuracy compared with the conventional method through integration with proportional-integral-derivative controllers. Shim et al. (2011) improved the positioning accuracy in underwater environments by applying the extended Kalman filter. They applied this to the position control of a robotic arm in real time to implement precision work based on the World Coordinate System. Cui (2013) demonstrated the world’s highest deep-sea exploration performance by succeeding in a 7,062-m dive through stepwise dive tests performed over approximately 10 years. Ryu et al. (2020) contributed to an improvement in the precision of underwater structure inspections by developing an ROV for structural inspection by integrating stereo cameras and 3D sonar. Tanveer and Ahmad (2023) developed a proportional-integral controller that considered the effects of tidal currents. They ensured a high steering performance compared with the conventional method through optimization that utilized a genetic algorithm based on the integral of the time-weighted absolute error. Abdullah et al. (2024) proposed an ego-to-exo interface that provided a real-time third-person perspective by combining simultaneous localization and mapping (SLAM)-based posture estimation information. They significantly improved the location recognition and operational stability of ROVs even in dark underwater cave environments or turbid waters. Su et al. (2024) analyzed the dynamic interaction between ROVs and tether cables through smoothed particle hydrodynamics-discrete element method (SPH-DEM)-coupled modeling to optimize the underwater deployment of cable-controlled ROVs. Moreover, they quantified the effects of cable dynamics on the ROV control performance in strong tidal current environments. In addition, cable-less ROVs based on optical wireless communication and exploration technologies for deep-sea hydrothermal environments are under development.
Notwithstanding these technological advances, few studies have been conducted on ROV deployment strategies that reflect the various physical elements (e.g., tidal currents, waves, and low visibility) in real time in actual marine environments. In particular, there is no systematic methodology for predicting when, where, and how ROVs can be deployed at underwater rescue sites confronted with strong currents and establishing deployment plans based on such predictions. These problems were also evident during the underwater search and rescue operations at the Sewol ferry disaster site in 2014. At that time, ROVs were deployed for underwater photography. However, their performance was limited considerably owing to the strong currents and constraints such as the complex and narrow hull structure. This caused the rescue authorities to emphasize the need for a more meticulous approach for ROV deployment considering the technical limitations of the equipment and marine conditions (Yonhap News, 2014). Small ROVs and the multi-joint underwater exploration robot (Crabster) developed by the Korea Institute of Ocean Science and Technology were then deployed additionally. However, these failed in vessel entry and precision exploration owing to problems such as strong currents, limitations in equipment performance, and loss of control (Choi, 2014). According to certain reports, the disaster countermeasures headquarters announced the successful entry of the ROV into the vessel. However, the results of an analysis of the trunked radio system communication records revealed that the ROV was out of control because it was entangled in cables owing to tidal currents, and the search was interrupted (Cho, 2016). These series of cases appear to have resulted from the absence of systematic deployment plans and preparation that reflect actual marine environments and equipment performance, rather than simple technical failures.
In particular, in challenging marine environments (e.g., strong currents), it is necessary to accurately understand the performance of ROVs in conjunction with strategic decisions and systematic operational measures on when, where, and how ROVs are deployed. This study aims to propose practical standards for strategically deploying ROVs in environments with strong currents based on an understanding of the limitations observed during the Sewol ferry disaster. Research was conducted on measures to effectively deploy ROVs with manned diving in underwater search and rescue operations. The results of this study can potentially be used as basic data to enhance the effectiveness and stability of ROVs in search and rescue operations in the event of large-scale marine accidents.
2. ROV Overview and Selection
2.1 Evolution of Underwater Vehicles
Humanity has long been willing to explore the deep sea. This has resulted in the development of various underwater exploration systems. The concept of submarines had emerged in sketches by the 16th century and the first underwater exploration equipment was produced in 1623. Submarines began to be used for military purposes in the 18th century. Furthermore, Jules Verne’s novel “Twenty Thousand Leagues Under the Sea” stimulated public imagination regarding the feasibility of deep-sea exploration. The virtual submarine “Nautilus” featured in the novel described the technology of descending to a depth of 20,000 m, which was considered a symbol of actual deep-sea exploration.
In reality, deep-sea exploration was realized for the first time when the submarine “Trieste” carrying U.S. Navy Lieutenant Don Walsh and Swiss oceanographer Jacques Piccard reached the Challenger Deep (approximately 10,800 m deep) in the Mariana Trench on January 23, 1960. Trieste was not equipped with precision equipment for scientific exploration and experienced limitations in conducting research owing to its large structure and low mobility. After 52 years, on March 26, 2012, film director and explorer James Cameron succeeded in diving solo into the Challenger Deep by directly steering the deep-sea submarine “Deepsea Challenger” that he designed. The submarine was equipped with hydraulic robotic arms as well as high-performance lighting and imaging equipment. This enabled deep-sea ecosystem exploration.
A significant turning point in the research on deep-sea biology and submarine geology occurred in 1977. The manned submarine “Alvin” operating in the United States identified a chemosynthesis-based ecosystem for the first time at a 2,500-m-deep hydrothermal vent in the Galapagos region. This discovery revealed that life may exist even in environments without sunlight. It served as a catalyst for various deep-sea explorations that used manned submarines over a few decades. However, the initiative in deep-sea exploration began to gradually move from manned submarines to ROVs. In particular, the ROV “Kaiko” developed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) successfully performed deep-sea sampling in the Mariana Trench in 1996. It succeeded in collecting biological specimens even at a maximum depth of 10,920 m. These technological advances enabled biological, chemical, and medical research even in extreme deep-sea environments where direct human access is difficult. ROVs continue to be used as key equipment for deep-sea exploration (Lutz and Falkowski, 2012).
2.2 Classification of ROVs
ROVs are classified in various ways depending on the purpose of operation, components, and operational conditions. Representative official classification systems include the standards of the International Marine Contractors Association (IMCA) and International Organization for Standardization (ISO).
IMCA (2021) classifies ROVs operated at industrial sites into six classes according to their applications and functions. The characteristics of each class are presented in Table 1. This classification is used to select appropriate equipment for the work environment and establish training systems and qualification standards for operational personnel.
Meanwhile, ISO (2002) classifies ROVs according to equipment configuration standards, which are based on the combination of equipment with different functions mounted on each ROV. The ISO classification defines necessary equipment configuration according to the characteristics of the mission and technical requirements. The details are shown in Table 2.
For actual marine structures and at underwater work sites, practical classifications based on operational methods and scales have been widely used in addition to the formal classifications such as IMCA and ISO. In particular, the classification of ROVs into the heavy work class and compact class reflects the differences in practical operational conditions including the configuration of equipment, operational personnel, power systems, and installation and recovery methods.
2.2.1 Heavy work class
Heavy work-class ROVs are typically loaded onto ships or containers for operation and it is infeasible for personnel to move these. These operate based on the anchoring and positioning systems (e.g., dynamic positioning systems) of ships or barges rather than independent operation. These have limitations in mobility and rapid deployment because of the complex installation and recovery.
However, these exhibit remarkable performance for complex tasks because these are equipped with advanced equipment such as high-power thrusters, high-resolution cameras, multi-joint manipulators, and various sensor modules. The configuration varies depending on the manufacturer or purpose of operation. However, a typical heavy work-class ROV system consists of a control room, a launch and recovery system (LARS), a tether management system (TMS), and an ROV.
2.2.2 Compact class
Compact-class ROVs have high mobility and operational efficiency in that these can be directly moved and installed by personnel. Most of these are compatible with power supply systems based on rechargeable batteries or small generators. Additionally, flexible power operation is feasible according to the field conditions. Because these are designed in modular structures, various systems can be added conveniently depending on the mission purposes.
However, their work efficiency is limited compared with heavy work-class ROVs. These are mostly used for search and observation-oriented missions, particularly owing to the limited functions of the manipulator arm. A typical compact-class ROV system consists of an operator control console, a hand controller, a tether, and an ROV.
2.3 Selected ROV System
In this study, VideoRay’s Mission Specialist Defender (MSD) was selected as a compact-class ROV model applicable at actual sites based on underwater search and rescue activities (the main missions at the time of the Sewol ferry disaster). It has been practically applied by naval forces, maritime police, and disaster response agencies in various countries including the United States, Europe, and Japan. It has also been widely used at industrial sites in the private sector.
2.3.1 Deployment cases
MSD has been deployed in many cases in various public and private sectors. In a representative case, it was adopted as a core platform in the U.S. Navy’s explosive ordnance disposal (EOD) and deployed under the name of MK20 Defender (VideoRay LLC, 2025). The equipment was adopted as part of the Maritime Expeditionary Standoff Response (MESR) program. Herein, it was deployed for high-risk missions such as naval mine detection, identification, and dismantling, as well as the retrieval of underwater explosives. It reportedly operated even in a current velocity of up to 4 kn (approximately 2.06 m/s).
It has also been utilized in private industries. As a representative case, the Proceanic Group of Companies has introduced MSD for the inspection and non-destructive testing of large structures such as floating production storage and offloading, in Southeast Asia, West Africa, and the Gulf of Mexico. This equipment successfully performed over 200 high-level underwater operations based on high durability and precision control performance even in high-temperature, high-pressure, and high-velocity environments (VideoRay LLC, 2022; 2025).
2.3.2 Specifications and components
MSD is based on a pressure-tolerant modular design applicable at a depth of up to 2,000 m. It can combine various components depending on the mission characteristics. Its major specifications are listed below. Table 3 shows an example of the system configuration.
(1) Propulsion system: Equipped with seven vector array thrusters (four horizontal and three vertical units) to enable six-degree-of-freedom posture control (surge, sway, heave, yaw, pitch, and roll). Each thruster uses a 400-W brushless motor.
(2) Imaging system: Supports a 13.19 megapixel HD camera, a 16× digital zoom, a ±85° tilt, and an 80° viewing angle. The 5,760 lumen LED lighting facilitates image capturing even in low-visibility environments.
(3) Navigation system: Provides posture measurement precisions of ±0.2° for static posture and ±0.5° for dynamic posture based on the attitude and heading reference system (AHRS), a nine-degree-of-freedom IMU, an electronic magnetometer, and a depth sensor.
(4) Power and communication: The 400-VDC-based power system operates with a maximum power consumption of 3,000 W. It enables stable control and data transmission through a dual Ethernet and RS-485 communication structure.
(5) Tether system: The Kevlar-reinforced structure exhibits a maximum tensile strength of 450 kg. It is feasible to select neutral-buoyancy or negatively buoyant tethers. The standard length is approximately 550 m.
(6) Control system: It is controlled through the expeditionary console or workhorse console. An industrial hand controller or an Xbox Elite-based controller can be selected.
2.3.3 Optional modules
MSD has module systems that can be mounted selectively depending on the mission purposes. Representative components are as follows:
(1) Multibeam sonar: As an example, BluePrint’s Oculus M750d can perform detection at a distance of up to 120 m and provide a horizontal viewing angle of 130° using ultrasonic waves in the 750 kHz–3 MHz range. Furthermore, it enables precision detection even in turbid environments.
(2) Manipulator arm: It can respond to various missions (e.g., underwater object retrieval, tether management, and sample collection). It has conveniently replaceable forms (e.g., parallel, V, trident, and cutting types).
(3) Cyclone CP system: It is an electronic sensor that quantitatively measures the cathodic protection. It supports contact, non-contact, and cell-to-cell methods. Additionally, it enables real-time voltage data collection and analysis.
As described, MSD is designed for flexible installation of various sensors and accessories to provide high operational flexibility and effectiveness in various fields such as underwater structures, marine structure inspections, and explorations.
3. Operation Strategy
3.1 Environmental and Operational Conditions
3.1.1 Wreck status of the Sewol Ferry
This study set the marine environment and field conditions at the time of the Sewol ferry disaster (which occurred approximately 3.1 nautical miles northeast of Byeongpung Island in Jindo-gun, Jeollanam-do, in April 2014) as the foundation of the analysis. According to the report on the Sewol ferry sinking accident (Korea Coast Guard, 2014) submitted by the Korea Coast Guard to the Special Committee for the national investigation of the Sewol ferry disaster in 2014, the maximum depth of the accident area was approximately 48 m, and the depth on the starboard side was approximately 26 m considering the width of the hull.
According to a report by the Korea Institute of Ocean Science and Technology (KIOST, 2015), the Sewol ferry was stranded with its port in contact with the seabed while a relatively constant depth of water was maintained on the starboard side (Fig. 1). This depth distribution serves as a key factor in establishing manned diving plans. It can be used as essential basic data for setting hull access routes and determining underwater search areas.
3.1.2 Tidal level and current environment
Fig. 2 shows the tidal level and current velocity over time at the Sewol ferry sinking site for 31 days from April 16 to May 16 in 2014. The data is based on the ocean current simulation results of Kim et al. (2023). The typical characteristics of a semi-diurnal tide that repeat the ascent and descent of the tidal level with a period of approximately 12 h and 25 min are reflected well. During the period, the amplitude of the tidal level fluctuates significantly over time. Moreover, the spring and neap tides can be distinguished clearly.
The current velocity also exhibits fluctuation characteristics similar to those of the tidal level. The current velocity is high and varies significantly at spring tides, whereas it is relatively gentle at neap tides. On May 16 (which was a spring tide), strong currents occur with the current velocity determined to be 0.923–1.128 m/s. The current velocity on May 7 (which is a neap tide with weak tidal currents) range from 0.123 to 0.323 m/s.
Such tidal level and current environmental information can serve as key basic data in establishing strategies for underwater search and rescue operations using ROVs. In particular, the strength of tidal currents and its variations over time are directly related to the mobility, position control, and search accuracy of ROVs. Thus, these are essential considerations in determining the timing and location of search.
3.2 ROV Operation and Limitations
3.2.1 Standard operating procedures for ROVs
ROV operation should follow a periodic inspection procedure to ensure system reliability and operational safety. This study proposes the standard operational procedure summarized in Table 4 based on IMCA (2020, 2021, 2025). The main steps are as follows:
(1) Pre-operation: The appearance of the equipment, LARS/TMS, and power and communication systems should be inspected. This includes an examination of the operation of the insulation and emergency stop systems for high-voltage equipment. The related safety documents should also be secured.
(2) Wet test: The operation of thrusters, lighting, and sensors should be inspected in a confined water tank or harbor. The integrated operation of systems should also be examined.
(3) Operation: Camera images and sensor data should be examined in real time through the control console at the work site for potential tidal currents, failures, and malfunctions.
(4) Post-operation: The damage to equipment should be examined, and the error logs are to be analyzed. Consumable replacement and preventive maintenance are to be performed if necessary. Additionally, the results of operation are to be documented.
This procedure serves as a key framework to standardize the operation and ensure safety beyond simple inspections. Each step should be performed based on the documented guidelines.
3.2.2 Operational staffing requirements
The operational staff of ROVs consists of pilots, technicians, system engineers, and supervisors in accordance with the IMCA standards (2021). The minimum staffing requirements vary depending on the ROV class. For example, at least two personnel are required for Classes I–II ROVs and at least three for Class III or higher.
(1) Main roles: Pilots (direct control of ROVs), technicians (equipment maintenance), supervisors (overall operation supervision), and personnel dedicated for LARS/TMS (for large systems).
(2) Work schedule: A 12-h two-shift system in general (shift planning is essential to prevent fatigue accumulation during long-term operations).
(3) Operational history management: All the personnel should maintain personal logbooks. These are used as data for maintaining qualifications and ensuring quality.
This study proposes that the personnel for underwater search and rescue operations be organized in the form of pilots working in pairs on 12-h shifts for small ROVs based on IMCA (2021).
3.2.3 Current-induced limitations on ROV operations
In the underwater operations of ROVs, tidal currents are key environmental factors that significantly affect the mobility and operational feasibility of equipment. Tidal current conditions impose direct physical constraints on the precise positioning in the work area, accessibility to the target structure, and accuracy of exploration using images and sensor data. In particular, in strong tidal current environments, problems such as tether tension, ROV posture instability, and control delays may occur. These may determine the success of the entire mission.
According to a report by Ritter (2019), the deep-sea exploration operation performed by the U.S. NOAA ship Okeanos Explorer in the deep-sea waters off the southeastern United States in 2019 is a representative case of the operational limits imposed by tidal currents. At that time, several ROV deployments were cancelled or postponed owing to strong tidal currents. The major constraints were indicated as follows:
(1) Underwater positioning capability degradation
(2) Risk of damage owing to excessive tension in tether cables
(3) Increased risk of collision with underwater terrain and structures.
Tidal currents have complex effects on stratification distribution according to the water depth, temporal variations in directionality, and ROV mobility according to the turbulence configuration and horizontal flow velocity. In such environments, tethers are bent or twisted rapidly. This increases the likelihood of the ROV deviating from its planned path or losing control. Although the operational guidelines of IMCA and ISO do not present specific limits on tidal currents, each ROV manufacturer describes the tidal current range for the operation of their equipment. For example, it is specified that Oceaneering’s Isurus ROV can operate even at a maximum flow velocity of 5 kn (2.57 m/s) (Oceaneering International, Inc., 2019). However, the levels of 3–4 kn (1.54–2.06 m/s) are the limits for most commercial ROVs. Meanwhile, numerous reports mention that stable operation is difficult in tidal current environments that exceed a flow velocity of 2 kn (1.03 m/s) in actual operation cases.
VideoRay’s MSD is the target of this study. It can operate at a maximum flow velocity of 4 kn (2.06 m/s) based on its specifications because it is equipped with seven high-performance thrusters. However, in marine environments that exceed 3 kn (1.54 m/s), problems such as thruster overload, control delays, and image quality degradation occur frequently. Therefore, a current velocity of at most 2 kn was set as the ROV operational limit by applying practical and conservative standards. Based on this, operational strategies for underwater search and rescue were established. This operational flow velocity limit can be adjusted based on future advances in equipment performance. In this study, an analysis was conducted based on the scenario set according to the current technological level of commercial equipment.
3.3 Operational Limitations of Manned Diving
Kim et al. (2025) and Lee et al. (2025a, 2025b) analyzed the daily operational time of SCUBA and SSDS based on the tidal current prediction results of Kim et al. (2025). The daily operational time during representative spring and neap tides is presented in Table 5. The operational limit flow velocities of SCUBA and SSDS were set to 1 kn (0.514 m/s) and 1.5 kn (0.77 m/s), respectively (refer to the limit lines in Fig. 2).
As shown in Table 5, SCUBA with an operational limit flow velocity of 1 kn may operate in a stable manner for up to 1,000 min/day on May 7 (neap tide) and 270 min on May 16 (spring tide). Meanwhile, SSDS with a higher limit flow velocity (1.5 kn) may operate for up to 420 min/day during the spring tide. This is approximately 1.56 times longer than that for SCUBA. SSDS can operate continuously for up to 1,310 min (approximately 22 h) during the neap tide with weak tidal currents. A comparison of the one-time operational time shows a large difference of 1.95 times (specifically, 590 min for SCUBA and 1,150 min for SSDS) during the neap tide. During the spring tide, the operational time of SSDS is 40 min longer, which is approximately 1.57 times longer than that for SCUBA.
These results show that SSDS has higher operational sustainability and efficiency than SCUBA under tidal current conditions. Conversely, the limited operational time of SCUBA indicates that the effect of ROVs can be enhanced when SCUBA-type manned diving is combined with ROVs.
3.4 Operational Duration of ROVs
Based on the ocean current simulation results of Kim et al. (2023), the variations in tidal level and current velocity at the Sewol ferry sinking site are shown in Fig. 2. In the figure, the operational limit flow velocities of SCUBA, SSDS, and ROV are also expressed. The operational time of the manned diving methods was calculated based on the limit flow velocities (Table 5). By applying this concept, the operational time of ROV can also be analyzed quantitatively.
During the neap tide with low tidal level amplitudes and weak tidal currents (May 6–10), the ROV can operate for 24 h (1,440 min). This is 1.44 times longer than the daily operational time of SCUBA on May 7 and 1.1 times longer than that for SSDS. Even on April 28 (which is a spring tide with strong tidal currents), the ROV can operate for 640 min. This is 2.37 and 1.52 times longer than that for SCUBA and SSDS, respectively.
As described, the ROV is more favorable than the manned diving methods in terms of operational sustainability under tidal current conditions. Therefore, the strategic role of ROV during integrated operation with manned diving can be set precisely by quantitatively evaluating the operational time of ROV. This would enable safer and more efficient underwater search and rescue operations.
3.5 Pre- and Post-Operational Tasks of ROVs
The analysis described above verifies that an ROV can ensure a longer operational time than manned diving methods. Accordingly, measures to strategically utilize the ROV at underwater search and rescue sites were examined. The results show that pre-operation (before manned diving) and post-operation (after manned diving) are the most effective methods for an integrated operation.
Based on this concept, the integrated operation system of ROV and manned diving is shown in Fig. 3. The figure presents the integrated operation method with SCUBA based on the current velocity distribution on May 16, 2014 estimated by Kim et al. (2023).
The integrated operation is divided into the following four phases:
(1) Phase I (Standby): Standby condition in which neither SCUBA nor the ROV can be deployed owing to significantly strong tidal currents.
(2) Phase II (Pre-dive ROV tasks): A pre-dive phase in which only the ROV can operate owing to marginally reduced tidal currents.
(3) Phase III (Manned diving): A main search and rescue operation phase in which manned diving is feasible owing to the weakest tidal currents.
(4) Phase IV (Post-dive ROV tasks): A post-dive phase in which only the ROV can operate before tidal currents regain strength.
From Fig. 3, the time for each phase is verified as follows: 220 min for Phase I, 50 min for Phase II, 60 min for Phase III, and 30 min for Phase IV. The efficiency and safety of search and rescue operations can be maximized through an integrated operational strategy that assigns clear missions and roles to each phase based on these time frames.
The strategic operational measures for ROV summarize the operational objectives, responsible personnel, and tasks for each phase. These are presented below, with the main content summarized in Table 6.
(1) Phase I
The time frame when it is difficult to perform tasks owing to strong tidal currents can be redefined as a strategic preparatory period to enhance the efficiency and success rate of the entire mission, rather than a simple idle period. During this period, the safety and efficiency of the subsequent phases can be enhanced by focusing on equipment inspection, manpower rest, operational plan review, data analysis, and documentation. However, if the real-time flow velocity information is insufficient and tidal current predictions are not reflected in the planning stage, inefficient standby time may occur without assessment in the field. This results in schedule delays and increased costs. Therefore, an accurate flow velocity prediction-based operational decision system is required.
(2) Phase II
The time when tidal currents are weakened moderately is not suitable for manned diving. However, it provides appropriate conditions for advanced reconnaissance and risk factor identification through small ROV deployment. In this phase, terrain analysis in the search zone, obstacle identification, visibility identification, and initial detection of major targets are performed. This contributes to the acquisition of spatial information and path optimization before manned diving. In addition, ROVs enable precise exploration even in waters with low visibility using high-resolution cameras and sonar. They can also remove risk elements such as ropes and waste nets through manipulators or cutting equipment. These functions play a key role in ensuring the safety of divers and preventing mission failures.
(3) Phase III
Manned diving is a key phase in which complex missions such as search, rescue, and tasks are performed. It requires flexibility to respond immediately to various variables such as the flow velocity, visibility, and water depth. Diving is performed based on the limited gas capacity. Therefore, a reduced awareness of the gas consumption while focusing on complex missions involves an inherent risk of accidents. In particular, omitting gas inspection may result in severe accidents such as decompression disorders and arterial gas embolism. This is considered a structural operational risk. To reduce this risk, a role allocation-type operational strategy in which search and obstacle removal are completed by deploying ROVs in advance and divers perform only assigned tasks is required.
(4) Phase IV
At the time when tidal currents become strong again after manned diving, re-exploration and omission identification are performed by re-deploying ROVs. Rather than a repetitive task, this phase is a procedure to supplement omissions that may occur owing to variations in the flexible underwater environment. ROVs can quantitatively and visually record the work history of the search zone based on high-resolution image, sonar, and path tracking data. Therefore, these contribute significantly to preventing duplication and ensuring efficiency in establishing future deployment strategies. In particular, for large accidents that require wide-area search (e.g., the Sewol ferry disaster), the systematic records of the searched and unsearched areas and the distinctions between these are key to the success of the mission.
4. Conclusions
In this study, the operational time of manned diving methods (SCUBA and SSDS) was calculated quantitatively based on tidal current velocity prediction results. Accordingly, the feasibility of strategically deploying ROVs was analyzed. The analysis results are summarized below:
(1) At underwater search and rescue sites, the daily operational time of manned diving (SCUBA and SSDS) is limited significantly by the tidal current environment. In particular, during spring tides, the operational efficiency reduces significantly owing to the short operational time.
(2) ROV ensured a significantly long operational time compared with the manned diving methods in an identical tidal current environment. This enabled stable operations even in waters under the significant influence of tidal currents.
(3) Accordingly, the integrated operational strategy for ROV and manned diving was divided into four phases (standby–pre-dive ROV tasks–manned diving–post-dive ROV tasks). Furthermore, measures to enhance the search and rescue efficiency, as well as safety, were presented by specifying the missions, responsible personnel, and roles of equipment for each phase.
The analysis in this study conducted with a focus on tidal current prediction needs to be supplemented. This is because it could not fully reflect the various marine environmental elements (e.g., waves, water temperature, and turbidity) that may occur at actual rescue sites. Integrated operational strategies were presented for small ROVs with demonstrated field applicability. However, because functions differ depending on the ROV size, it is necessary to quantitatively analyze the operational limits and performance differences by grade and establish detailed strategies based on these. Therefore, future tasks include the systemization of operational conditions by equipment, construction of support systems based on real-time environmental information, establishment of transition procedures between systems, and development of integrated simulation tools. These strategies can support the scientific decision-making by rescue commanders and significantly contribute to ensuring the efficiency of search and rescue operations and the safety of workers. In particular, the applicability of ROVs as auxiliary means to mitigate the physiological load of underwater workers has attracted attention. In this regard, strategic operational measures to reduce physiological risks (such as nitrogen absorption, oxygen toxicity, and decompression stress) would also be examined through convergence with existing diving simulation technology (Lee et al., 2019a, 2019b; Lee, 2020; Lee et al., 2021). This approach is likely to contribute to the development of safer and more precise underwater rescue operation systems.
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