Experimental Analysis of Dynamic Response and Stability of Capsized Tugboat Under Varied Wave Conditions
Article information
Abstract
Tugboats are critical for maneuvering large vessels and conducting maritime operations, but their vulnerability to capsizing under severe conditions is not completely understood. This study experimentally analyzed the dynamic response of a capsized Buccaneer tugboat under various wave conditions to address this knowledge gap. The heave, pitch, and roll motions of a tugboat were examined under different sea states and wave encounter angles using a scaled-down model (1/23.35) in an ocean engineering basin. The results showed that heave motion maintains an almost constant significant period, with higher energy density at higher sea states. Pitch motion at the bow lengthened with increasing sea state, and an increased energy density was observed, particularly under stern waves, while the stern showed less pronounced pitch motion. Roll motion displayed two significant periods, with higher energy density in longer periods at the stern from sea state 4 onwards. These findings underscore the importance of considering heave, pitch, and roll motions in maritime accident response strategies, particularly under stern wave conditions, and provide valuable insights for improving rescue operations and safety measures for capsized vessels.
1. Introduction
Tugboats are specialized vessels that assist large ships with docking, departure, and navigation through narrow waterways. They are used primarily in harbors, rivers, and canals to aid the movement of larger vessels, and they play a crucial role in rescue operations and various marine construction tasks (Kristiansen et al., 2021). The robust structure and high maneuverability of tugboats allow them to perform reliably across diverse maritime environments (Wang and Zhu, 2021). On the other hand, these vessels are not immune to capsizing when faced with challenging sea conditions and adverse weather conditions (Yoon et al., 2011; Lee and Park, 2021). Capsizing is one of the most catastrophic occurrences in the maritime industry; tugboats are no exception. A capsizing incident can lead to significant loss of life, environmental pollution, and economic damage. Specifically, quick rescue and recovery operations are challenging after capsizing owing to the unique nature of tugboats, making understanding the dynamic stability of a capsized vessel crucial.
Although research on ship dynamics has often focused on the probability of capsizing, less attention has been given to the dynamic response of a vessel post-capsize. Previous studies concentrated on the likelihood of capsizing under conditions such as damaged ship roll dynamics, excessive roll motion in random waves, and pure loss of stability, often using computer simulations and numerical analysis to examine ship motion and the response of damaged ships in wave conditions (Gao et al., 2020; Chai et al., 2022; Liu et al., 2022; Gao and Shi, 2023). Consequently, there is a relative lack of understanding regarding vessel motion after capsizing. This represents a significant gap in knowledge, particularly for developing effective rescue operations and maritime accident response strategies. Understanding the dynamic behavior of a capsized vessel in various wave conditions is critical for improving the success rate of rescue efforts and preventing further incidences of capsizing or damage. The wave conditions are one of the most significant external forces in the marine environment, greatly influencing the stability of a capsized vessel. Therefore, experimentally analyzing the dynamic response of a capsized tugboat under different wave conditions is a crucial research topic. The safety of ships and offshore structures is a significant issue in the modern maritime industry. International organizations, such as the International Maritime Organization (IMO), have established various regulations to ensure safe navigation and prevent accidents (Chong, 2015; Baumler et al., 2021). Nevertheless, these regulations may prove insufficient without a thorough understanding of the behavior of ships post-capsize. Therefore, it is essential to understand the dynamic stability of capsized vessels and develop practical safety measures accordingly.
This study experimentally analyzed the dynamic response of a capsized tugboat under various wave conditions. The objective was to identify the dynamic response characteristics of a capsized vessel, determine the critical wave conditions that exacerbate the instability of a capsized vessel, and assess the potential for further capsizing or damage under these conditions. This study collected data on how a capsized tugboat moves and its characteristics at each degree of freedom by experimentally analyzing the six degrees of freedom (6-DOF) motion of a capsized tugboat under different wave conditions. These data are essential for understanding the impact of wave period, height, and direction on the dynamic response of a capsized vessel. The study examined how specific conditions influence the instability of the vessel by conducting repeated experiments across various wave conditions, thereby evaluating the increased risk of further capsizing or damage under certain conditions. This research provides a comprehensive understanding of the dynamic response and instability of capsized tugboats, offering practical insights for improving maritime accident response strategies and the success rate of rescue operations.
2. Experiment Methodology
2.1 Description of Setup and Model
A series of experiments were conducted using the Buccaneer tugboat as the model vessel. The Buccaneer, known for its robust structure and high stability, is ideally suited for evaluating the performance across diverse marine environments. The fundamental physical characteristics and design of the vessel were examined through its key specifications and lines, as shown in Table 1 and Fig. 1. Furthermore, as the superstructure becomes submerged after capsizing, it experiences significant hydrodynamic effects; Fig. 2 presents a detailed diagram including the superstructure. The experimental setup included a scaled-down model of the actual Buccaneer tugboat. Table 2 and Fig. 3 show the specifications and photographs of this model. With a scale ratio of 1/23.35, the model maintains proportionality to the full-scale vessel, facilitating reliable data acquisition in a laboratory setting. This approach allows for a systematic analysis of the Buccaneer’s dynamic response under varying sea conditions.
Specifications of tugboat
Ship model (Buccaneer)
Multi angle view
Specifications of experimental model
Picture of the experiment model
2.2 Experimental Setup
The model experiments were conducted in an ocean engineering basin measuring 28 × 22 × 2 m at Research Institute of Medium & Small Shipbuilding . The wave generator system, located within the basin, consisted of 40 segments, with a total length, height, and width of 22.0 m, 1.0 m, and 0.4 m, respectively. A wave absorber was installed on the opposite side of the wave generator to absorb incoming waves and prevent wave reflection at the boundaries of the basin. This setup helped simulate the infinite fluid domain and facilitated the interaction between floating objects and waves. The tugboat model was positioned at the center of the basin, as shown in Fig. 4.
Model installation point and coordinates in the basin
For this experiment, the origin of the coordinate system was placed at the wave generator at a fixed water level. The x-axis was aligned along the horizontal direction of the wave generator, and the y-axis was aligned along the direction of wave propagation, with positive values in the wave propagation direction. A four-point mooring system prevented the model vessel from drifting due to wave action. The mooring system used springs with a natural period more than twice the maximum period of the model ship to ensure that the springs did not influence vessel movements. The springs were fixed at the vertical center positions of the bow and stern to minimize the impact of the mooring points.
The experiment used the following wave encounter angles (χ), as shown in Fig. 5: head waves, beam waves, following waves, and quartering waves. The wave encounter angle was 180º for head waves and 0º for following waves. The wave height was measured using a wire-resistance wave probe, which converted the wave height signals into voltage, transmitting them to a data acquisition board on a computer. All data from the wave probes were sampled at a frequency of 20 Hz. The motion characteristics of the vessel were measured using an AC electromagnetic motion measurement system, and the data were transmitted to a PC via radio frequency (RF) wireless communication. Although the motion measurement system could measure six degrees of freedom (6 DOF), this study focused on the heave, roll, and pitch motions. Fig. 6 presents the positions of the motion tracking equipment and gyro sensors installed on the model vessel.
Wave encounter angle
View of the sensor positioned on the model
Table 3 lists the irregular wave conditions used in the experiment to analyze the response characteristics. These conditions cover a range of marine environments from sea states 3 to 5. One hundred different wave types were used to generate the irregular waves. These conditions were designed to simulate the superposition of waves generated by the wave generator and the wave absorber. The experiment generated data over 300 seconds, with the analysis focusing on data collected after the initial 100 seconds to incorporate the superposition effects of waves generated by the wave generator and absorber. Two thousand and forty-eight data points from this period were used to evaluate the response characteristics of the vessel, using more than 40 times the wave period for the analysis. The average values from two identical experiments were used. The Texel–Marsen–Arsloe (TMA) spectrum was used for the wave spectrum. Fig. 7 shows the theoretical spectrum and the spectrum generated in the basin.
Wave condition (real sea and experiment)
Validation of incident wave
3. Results and Discussion
3.1 Bow and Stern Sea
Figs. 8 and 9 show the a) heave, b) pitch, and c) roll motions for different sea states at the bow (χ= 135°) and stern (χ= 45°) of the vessel. For heave motion, the bow and stern show a relatively constant significant period, with increasing energy density as the sea state intensifies. This suggests that the heave motion is influenced more by the significant period of the incoming waves rather than by the inherent characteristics of the vessel. In the case of pitch motion, at the bow, the significant period lengthens as the sea state increases, indicating the effect of the increasing significant period of the incoming waves on the vessel characteristics. Pitch motion at the stern is influenced less by the incoming waves than at the bow.
Motion spectrum according to the sea state at the bow (χ= 135°)
Motion spectrum according to the sea state at the stern (χ= 45°)
The bow and stern tend to show two significant periods for roll motion. Based on the significant period observed in sea state 3, the significant period at the bow lengthens as the sea state increases, with the notable occurrence of a second significant period starting from sea state 4. On the other hand, the energy density of this second significant period is lower than the first. The trend at the stern was similar in that the significant period lengthened as the sea state increased. From sea state 4 onwards, the energy density of the second significant period was higher than that of the first.
At lower sea states, however, the motion characteristics are more influenced by the inherent properties of the vessel, while the characteristics of the vessel and the significant period of incoming waves play a role at higher sea states. This highlights the need for caution in maritime accident response, particularly as sea states increase because of the combined effects of the vessel characteristics and the significant period of incoming waves.
3.2 Directional Motion Characteristics by Sea State
Fig. 10 shows the heave motion for each sea state across different angles. The significant period for heave motion remains relatively constant regardless of the sea state or the angle of incoming waves. Fig. 11 presents the pitch motion by the sea state and angle. The energy density significantly increases when the vessel is overturned and exposed to stern waves. Therefore, caution is advised for pitch motion when encountering stern waves. Fig. 12 displays the roll motion by the sea state and angle. Roll motion is most pronounced when the vessel encounters the beam sea. Higher energy density is observed in the regions with longer significant periods at the stern as the sea state intensifies. Consequently, attention should be paid to roll motion when the vessel is exposed to stern waves. Roll motion is directly related to the vessel stability and is strongly influenced by waves. Compared to pitch motion under the same sea state, roll motion exhibits a higher energy density, indicating that roll motion absorbs more energy and results in greater movement. Hence, caution is needed when incoming waves are from the stern.
Heave motion according to the sea state and angle
Pitch motion by sea state and angle
Roll motion according to the sea state and angle
4. Conclusions
This paper reported a detailed analysis of the dynamic response of a capsized tugboat under varying sea states, focusing on heave, pitch, and roll motions. The experimental results showed that heave motion maintains a relatively constant significant period regardless of the sea state, with the energy density increasing as the sea state intensifies. This suggests that heave is predominantly influenced by the incoming wave conditions rather than the inherent properties of a vessel.
In contrast, pitch motion at the bow shows a significant period that lengthens as the sea state increases, reflecting the influence of the incoming waves. The stern exhibits less sensitivity to wave-induced significant periods, highlighting a discrepancy in the response of the vessel between the bow and stern. Roll motion is characterized by two significant periods at the bow and stern. Although the first significant period remains dominant, the second significant period, observed from sea state 4 onwards, showed increased energy density at the stern compared to the bow.
Overall, this study highlights the complex interplay between the inherent characteristics of a vessel and external wave conditions. At lower sea states, vessel-specific factors play a more substantial role, while the combined effects of the vessel properties and the significant periods of incoming waves become more pronounced at higher sea states. These findings emphasize the importance of considering inherent vessel characteristics and wave conditions in maritime accident response strategies, particularly as the sea states escalate. Future research will focus on the changes in the motion characteristics based on the approach to rescue vessels
Notes
No potential conflict of interest relevant to this article was reported.
This research was supported by a grant (20024457) of Ministry Cooperation and Safety Management Technology in National Disaster funded by Ministry of Interior and Safety (MOIS, Korea)