1. Introduction
Unmanned Underwater Vehicles (UUVs) are crucial in ocean exploration and development due to their autonomous capabilities [
1]. They are used in rescue missions, pipeline inspections, marine science studies, the exploration of seabed energy, and fishing. However, their limited energy supplies restrict their operations, necessitating periodic retrieval by a mother ship, which is time-consuming and labor-intensive. Advances in seafloor observatory networks have introduced docking stations [
2,
3] where UUVs can recharge, upload data, and download missions, reducing the need for frequent retrievals. This innovation extends the operational range of UUVs and improves their autonomous potential [
4]. As underwater tasks become more complex, the incorporation of UUVs into seafloor networks is increasing, making docking stations as essential as refueling or charging stations on land.
The variety of UUVs has resulted in the following three main docking configurations [
5]: a funnel-shaped or cone-shaped entrance [
6], a vertical design with a V-shaped capture mechanism [
7], and a docking platform [
8]. The cone-shaped entrance is used most frequently because it requires minimal modifications to the UUV and does not require additional mechanical parts [
9]. This design is especially effective for retrieving torpedo-shaped UUVs and usually provides a large cross-sectional docking area.
Docking stations shaped like funnels or cones can be categorized into the following three main types [
10]: fixed, mobile, and floating stations. Fixed docks are securely mounted to the seabed or underwater stations, providing high stability but necessitating flat seabeds. They may include self-adjusting mechanisms to adapt to uneven terrain. Examples are the Dorado AUV station [
6] and the Dolphin II AUV station, which features a two-axis swing [
11]. Innovations such as motor-driven rotation are used to counteract cross-flow effects during docking, as seen with the Hybrid Underwater Glider (HUG) single-axis rotating station [
12] and the hydraulically adjustable station [
13]. Although highly stable, fixed docks are complex, susceptible to marine biofouling, and difficult to deploy in the long term [
14]. Mobile docks are attached to larger UUVs [
15,
16] or unmanned boats [
17], using towing speed to ensure stable positioning. These docks are mainly designed for short-term recovery operations. Lastly, floating docks can either hang freely or be suspended at a specific height. They can be incorporated into buoy systems [
18], surface ships, or seabed mooring systems [
19], maintaining stability under ocean currents. Although simple in structure and adaptive to ocean currents, they can sway due to surface waves [
20]. Their stability can be improved by anchoring them to the seabed.
A suspended seabed docking station is easy to deploy and retrieve, is not affected by seabed flatness, and is effective for UUVs to dock in diverse marine environments. As shown in
Figure 1, the underwater local resident observation network around offshore platforms includes a suspended seabed dock, UUVs, and underwater wireless nodes for ocean monitoring and identification of equipment problems and safety risks. This paper focuses mainly on a suspended seabed dock (marked within the red box in
Figure 1). It adopts a mooring type that links the dock to the counterweight through a mooring chain or a load-bearing cable. The design includes a suspended body structure that offers buoyancy. In addition, it can be connected to the seafloor junction box with a cable or remain unconnected (orange dashed line in
Figure 1). Furthermore, the docking function can be extended to wireless nodes to form dock nodes, allowing UUVs to power them, prolonging the operating time of the monitoring network [
21]. The design allows UUVs and Remotely Operated Vehicles (ROVs) to operate independently of established subsea resources [
22]. The flexible design of suspended stations uses ocean currents for passive posture adjustments, enhancing stability and adaptability in dynamic environments. As researchers seek efficiency, stability, and reliability, there is a strong need to implement adaptive docking stations and explore their potential for performance.
As a novel structural form, research on underwater suspended docking stations is currently limited. Ensuring the success and safety of UUV docking operations requires attention to the station’s static stability and dynamic contact reliability. First, static stability is essential for accurate positioning and safe docking of UUVs. R. Zheng et al. examined the impact of wing size, tow cable length, and hinge position on the motion state of a suspended dock with mooring [
23]. X. Wen et al. used a numerical method and a modified P controller with an added pre-condition and limiter to assess the dynamic response of a floating dock in accidents, enhancing its safety and stability [
24]. Stability analysis often uses Computational Fluid Dynamics (CFD) for tow-mobile docks, examining hydrodynamic coefficients, tow cable characteristics, and tow motion parameters [
25]. Edoardo I. et al. simulated the automated launch and recovery process using OrcaFlex and performed a parametric study [
17]. Furthermore, studies on wave energy converters [
26] and floating offshore wind turbines [
27,
28] provide information on the numerical modeling, dynamic performance, and survivability of moored floating platforms.
Contact reliability is crucial for docking efficiency and success, with the aim of minimizing contact forces and using them to guide the UUV smoothly into the dock [
29]. Water flow disturbances often cause UUVs to collide at the dock entrance, increasing difficulty and the risk of damage. Frequent problems involve the UUV becoming lodged at the entrance [
30], damage to the guide cone, or deformation of the UUV’s front. B. Fletcher et al. suggested using water-filled bags to increase and stabilize suspended dock mass. They highlighted the need for the dock to offer adequate reaction forces and torque (from inertia and/or thrust) to ensure that the UUV’s docking efficiency is not affected by the dock’s movements [
31,
32]. M. Lin et al. developed an ADAMS contact model to study floating docks, analyzing factors such as the cable length and mass ratio [
30]. In the context of fixed docks, Zhang et al. created a contact model to examine the impact of the guide cone material, the thrust of the Autonomous Underwater Vehicle (AUV), and the initial position [
33]. Our previous work evaluated the effects of different guide-cone shapes (convex, conic, and concave) on the adjustment of UUV motion [
34]. Other researchers have explored optimizing the shape of contact bodies and using multi-objective optimization to reduce contact forces. Furthermore, Wu Lihong was the first to use CFD techniques to analyze the hydrodynamic characteristics of AUV docking, taking into account factors such as speed, acceleration, entrance geometry, sliding behaviors, and rudder angles [
35,
36]. Meng Lingshuai used Star-CCM+ to gather contact force data from captured rod docking simulations [
29], and Xu Yunxin investigated the hydrodynamic feasibility of dynamically docking a remora-inspired AUV on a benchmark submarine [
37]. A novel “soft dock” design featuring flexible appendages and active gripping mechanisms can significantly reduce collision impacts [
38]. Despite the advances in the field, research on the hydrodynamics of UUV docking with suspended docks is still lacking, particularly in terms of fully understanding the post-contact motion response crucial for successful docking. Hence, further studies are essential to utilize underwater fluid dynamics and contact interactions more effectively to enhance UUV docking, focusing specifically on post-contact motion.
This paper presents a Seabed Moored Suspended Dock (SMSD) model and a UUV docking model with contact coupling to simulate SMSD stabilization under various currents and the UUV’s dynamic contact response during docking. The goal is to deepen the understanding of the SMSD’s motion response to water flow and contact forces to optimize UUV docking, providing design insights for floating docks. The SMSD is composed of a guiding funnel, a suspended body, and a catenary. To realize six-degree-of-freedom (6-DOF) motion for both the UUV and SMSD, techniques such as Dynamic Fluid Body Interaction (DFBI), overlapping grids, and adaptive mesh refinement are employed. The accuracy of the UUV docking model is validated using experimental data. Then, by systematically analyzing the effects and sensitivities of the design parameters on the attitude of the SMSD and the contact response during docking, we provide suggestions for optimizing floating dock designs. Finally, based on the proposal for optimal parameters, we construct the SMSD and perform sea tests to confirm the stability and contact reliability of its suspended moored design.
The main contributions of this work are summarized as follows.
This study proposes an SMSD model using a hybrid dynamic overlapping grid technique through DFBI, featuring a guiding funnel, a suspended body, and a catenary. Moreover, a UUV dock within the SMSD model is developed with contact coupling. These models successfully simulate the SMSD’s equilibrium stability and dynamic contact response under ocean flow conditions.
We analyze the effects of mass, catenary stiffness, and flow velocity on SMSD stability. In addition, an investigation is conducted regarding how volume, moment of inertia, mass, and catenary stiffness impact UUV docking. This improves understanding of flexible mooring methods and contact interactions, guiding the optimization of SMSD design.
Through sea trials and numerical simulation results, we verify the stability and contact reliability of the suspended moored structure of the SMSD. The well-considered design enhances the SMSD’s adaptability to environmental conditions and stability during UUV docking.
This paper is structured as follows.
Section 2 details the numerical methods used for simulation of the attitude of the SMSD and the UUV docking within the SMSD, including the catenary equations, contact coupling and forces, and the DFBI motion.
Section 3 outlines the calculation model, grid division, and validation of the numerical model.
Section 4 analyzes SMSD equilibrium stability and contact interaction during UUV docking, as well as the impacts of volume, moment of inertia, mass, and catenary stiffness.
Section 5 presents the sea experiments and results of the optimized SMSD. Finally,
Section 6 summarizes the findings of this study and identifies deficiencies and directions for future work.
5. Sea Experiment
A trade-off is required to choose the optimal combination of design parameters. A design principle can be established by initially determining the volume, then examining the effects of mass and moment of inertia on stability and reliability; choosing a suitable catenary stiffness based on structural strength; and, finally, refining the design optimization. As summarized in
Table 8, we balance the parameters to minimize the docking time while allowing for a moderate increase in maximum contact force within an acceptable range. The angle between the UUV’s central axis and the guiding funnel’s central axis on the horizontal plane, the number of contacts, the UUV yaw angle, and the SMSD yaw angle are kept at moderate levels to ensure comprehensive system performance.
In August 2022, we tested SMSD performance in a UUV docking experiment at sea in Dalian. The depth was 9–10 m, with a current of 1–2 knots. The SMSD deployment process is illustrated in
Figure 19. The guide funnel is made up of steel rods. The suspended body is made of carbon steel, and the syntactic foam. The mooring chain is made of stainless-steel material. Given the test conditions, the length of the mooring chain was set at 2 m, and the counterweight was 1.5 tons. Initially, the guide funnel, the suspended body, the mooring chain, and the counterweight were assembled as a single unit (
Figure 19a). The SMSD was then deployed with the sling taut under load (
Figure 19b). Finally, the counterweight led to the complete submersion of the system. This resulted in the slacking of the sling and a gradual rotation of the SMSD in the current until it reached equilibrium (
Figure 19c). The attitude sensor on SMSD showed a stable pitch of 7.5° and a stable roll of 8°. The SMSD takes advantage of the hydrodynamics of the marine environment to maintain stability, thereby enhancing adaptability.
The yaw angle of the SMSD can be calculated on the basis of acoustic measurements [
47]. As illustrated in
Figure 20, the SMSD maintains a stable orientation of 225°, demonstrating its ability to remain suspended in the presence of current. Furthermore, between 100 s and 160 s, the relative angle between the UUV and the SMSD approaches 0°, indicating that the UUV heads toward the funnel-shaped entrance for docking. The cross-track error decreases to 0 m at 160 s, indicating successful UUV docking. However, after 160 s, there is a fluctuation in the relative angle and the cross-track error. This is due to a contact between the UUV and the guide funnel during docking, causing the SMSD to rotate. Moreover, at the sea test site, it was observed that the proper rotation of the SMSD can facilitate the successful docking of the UUV. The SMSD then returns to its original direction due to a restoring moment of force generated by the current and mooring chain, demonstrating the dynamic stability of the SMSD.
6. Conclusions
This paper uses CFD and DFBI to simulate the stability of an SMSD under ocean currents and to analyze its dynamic response during UUV docking. Furthermore, it offers a detailed evaluation of how critical design parameters such as volume, moment of inertia, mass, and catenary stiffness influence the behavior of the UUV and SMSD during docking. The simulation results improve our understanding of flexible mooring methods and contact dynamics, providing insights into the design of suspended docks, which are listed as follows.
The pitch of the SMSD should be kept within 10°. Increasing the net buoyancy force and incorporating stabilizer fins can improve SMSD stability and reduce the pitch angle of the SMSD.
Low stiffness in the mooring chain facilitates the SMSD in quickly reaching equilibrium, resulting in a stable state.
The SMSD’s mooring connection allows for flexibility, absorbing the contact impacts and resulting in low UUV angular velocity, high UUV velocity, and reduced contact force, lowering damage risk.
The key parameters affecting UUV docking, in order of impact, are volume, moment of inertia, and mass. Catenary stiffness is negligible.
Optimal design parameters involve a careful trade-off. Moderate volume balances docking efficiency and UUV navigation. Increased mass enhances both docking efficiency and SMSD stability. A suitable moment of inertia maintains SMSD flexibility, aiding docking accuracy and safety. High catenary stiffness increases contact force.
Finally, after developing the SMSD with a design optimized for stability, sea experiments were conducted. The results revealed that before docking, the SMSD held a stable attitude with a pitch of 7.5°, a roll of 8°, and an azimuth of 225°. During docking, the SMSD rotated but was restored to its initial azimuth after UUV docking. This confirms the stability and contact reliability of the moored suspended structure.
The experimental results show the effectiveness of the proposed SMSD design. However, in weak currents, it shows yaw-angle oscillations that prevent it from stabilizing. Additionally, there is a reduction in docking accuracy, highlighting the need to improve the capability of SMSDs to adjust the UUVs’ steering. Future work will design a “self-guiding wing plate” for the suspended body that adjusts its size based on water flow, optimizing the flow field around the docking station and, thereby, increasing SMSD stability in various currents. Additionally, we consider adding an active control mechanism for the SMSD. Furthermore, the incorporation of a two-way fluid–structure interaction (FSI) approach in the numerical model should be considered, along with a penalty function [
48] to refine contact force calculations. This approach will improve the analysis of UUV docking strategies and further improve system performance.