Safety Operation for Large Deck Cargo Barge at a U-Shaped Basin in Complex Port Areas

Abstract

It is challenging to manoeuvre large deck cargo barges within the confined, congested port waters, especially when berthing and unberthing at a U-shaped basin. To investigate the safety operation of those ships under these complex circumstances, the research employs an integrated methodology to enhance safety. Ship manoeuvring simulations were first conducted to determine the critical environmental limits (including wind, current, and wave thresholds) under which safe operations are feasible. Subsequently, for safe mooring, Computational Fluid Dynamics (CFD) simulations were applied to analyse the hydrodynamic forces acting on the barge while berthed. These CFD results were crucial for determining the optimal mooring configuration (number, type, and arrangement of lines) required to sustain the environmental loads. The combined insights from manoeuvring simulations and CFD analysis provide a comprehensive framework for port planners and mariners, which will substantially improve the operational safety of large deck cargo barges utilising U-shaped berths in busy and spatially constrained port areas.

1. Introduction

1.1. Background

The expansion of large offshore engineering (e.g., offshore wind power, offshore oil and gas) has substantially increased the demand for specialised transport vessels capable of handling oversized components and heavy equipment [1,2,3]. Among these, large deck cargo barges, normally with expansive deck cargo areas, have become critical carriers in offshore logistics due to their high cargo capacity and broad operational adaptability, and they are widely employed in the maritime transportation of modular installations, structural elements, and heavy machinery [4,5].

However, their inherent design characteristics introduce distinct operational challenges. Large deck cargo barges typically lack self-propulsion or are equipped with low-power propulsion, resulting in markedly reduced manoeuvrability compared to self-propelled ships. Their low rudder effectiveness, high inertia, and pronounced sensitivity to environmental disturbances further exacerbate the difficulty during berthing and unberthing operations [6,7], especially in complex port areas due to confined space and congested traffics. Consequently, ensuring the safe berthing and unberthing of large deck cargo barges has become an important research priority in port planning and maritime safety.

1.2. Literature Review

Berthing and unberthing operations in ports represent critical phases in maritime transportation, involving complex vessel manoeuvring under constrained conditions. First, vessels must be manoeuvred within limited under-keel and lateral clearance, where shallow water and bank effects substantially affect a vessel’s hydrodynamic response, thereby increasing the complexity and risk associated with manoeuvring control. As demonstrated in [8], numerical simulation reveals that, when manoeuvring in shallow waters, ship turning ability will degrade and turning circle will increase dramatically. When the lateral clearance of the vessel to banks is small, asymmetric flow leads to bank-induced lateral forces and yaw moments, which significantly alter the vessel’s lateral motion and heading stability [9,10]. Furthermore, vessels must reduce speed to improve manoeuvring precision and decrease inertia effects, which helps lower collision or ground risk. However, when vessels are advancing at low speed, rudder and propeller effectiveness diminish due to reduced inflow velocity, resulting in degraded yaw-rate response and turning capability [11,12]. Berthing large vessels is especially challenging because their smaller rudder-area ratio and weaker rudder-propeller interaction cause a more significant reduction in rudder and propeller effectiveness at low speeds [13].

Tugboat assistance has become a standard and often necessary measure to ensure safe berthing and unberthing of large vessels. The adverse hydrodynamic effects are more severe for the attending tugboats, as they are much smaller in size compared to the assisted vessels [14]. Computational Fluid Dynamics (CFD) is a powerful tool for analysing and optimising berthing operations through detailed hydrodynamic simulations. Mauro et al. [15] proposed a flexible method that combines a nonlinear thrust allocation algorithm with simplified hydrodynamic force estimates, allowing for the rapid assessment of maximum steering and braking forces under different tug configurations. Barrera et al. [16] used CFD to obtain hydrodynamic coefficients for the tug hull and implemented those coefficients in ship manoeuvring simulations to predict tug positioning and towing forces during docking operations. Jayarathne et al. [17] investigated the interaction effects induced on a tug during a ship-assist operation at different lateral and longitudinal distances from a larger ship and at different tug drift angles, revealing the optimal range of drift angles of tugboats and the optimal lateral distance between them. Abdelghafor et al. [18] demonstrated that the proper choice of orientation and speed of the tugboat might increase the pushing force to its capacity. These results support tug operational decision-making (e.g., positioning, drift-angle selection, speed regimes) during the reality of berthing operations.

Once berthed, mooring safety becomes paramount for ensuring vessel security during cargo operations and port stays. The integrity of mooring arrangements directly determines whether vessels can safely withstand environmental loads during port stay, with failures potentially resulting in vessel drift, infrastructure damage, and breakdown of port operations [19,20]. The complexity of mooring system behaviour arises from the highly nonlinear load-displacement characteristics of mooring lines, the dynamic coupling between vessel motions and line tensions, and the time-varying nature of environmental loading [21,22]. Mooring design must rest on established principles and standards to ensure that a vessel remains safely positioned under environmental loads [23]. The primary principle of mooring design is to determine an appropriate mooring arrangement, line types (e.g., wires or synthetic ropes), and pretension based on the vessel size and type, the berthing characteristics (e.g., water depth, harbour geometry), and the anticipated environmental loads (e.g., wind, current, and waves) [24,25,26].

Mooring design requires rigorous analytical and numerical approaches to balance safety and operational efficiency. Common mooring analysis methods include static, quasi-static, and dynamic simulations, with the latter typically performed in the time or frequency domain to capture complex nonlinear and dynamic responses of the mooring system [27]. In [28], a time-domain model was used to compute the motions of a moored ship in fully nonlinear waves by integrating time-dependent wave loads and mooring forces. In [29], the time-domain model was adopted to analyse and compare the influences of the cable material, loading condition, cable arrangement position, and cable angle for a 16,000 TEU container ship. Moreover, some numerical tools, such as Advanced Quantitative Wave Analysis (AQWA) software version 2023, are used to numerically simulate the dynamic response of the mooring system under the coupling of wind, wave, and current in different sea states, facilitating assessment of potential line failure risks [30,31].

Despite extensive research on berthing operations and mooring safety for conventional vessels, limited attention has been directed toward the specific challenges faced by large deck cargo barges operating in semi-enclosed harbour configurations. U-shaped harbour basins, while providing shelter from offshore environmental conditions, introduce unique operational constraints to barges including intersecting with fairway at a large angle, restricted manoeuvring space, complex flow patterns near basin entrances, limitations on tug positioning and ballords for securing, and the need for shore-based assistance systems. This study aims to fill this research gap by presenting a comprehensive manoeuvring safety analysis framework specifically designed for large deck cargo barge operations in U-shaped harbour basins under complex waterway conditions. The research systematically identifies risk factors throughout the berthing, mooring, and unberthing processes and develops targeted control strategies. In this context, this study proposes an integration of ship manoeuvrability and mooring load responses within a framework for safety assessment. In this study, the proposed method transfers simulation outputs into measurable operational limits, including feasible environmental conditions, required tug assistance levels, and mooring-line load capacities. The result provides a link between model predictions and safety-related decisions, and addresses the interaction between manoeuvring performance, environmental limits, and mooring system behaviour in a constrained U-shaped berth. Therefore, this study contributes a systematic methodology that can be extended to other semi-enclosed harbour layouts with similar operational characteristics. By integrating CFD-based environmental force analysis, time-domain mooring dynamics simulation, and full-scale manoeuvring experiments, this study provides practical guidance for ensuring operational safety in similarly constrained harbour environments.

The remainder of this paper is organized as follows: Section 2 describes the research methodology, including the CFD simulation procedures and mooring analysis methods. Section 3 presents the case study, which involves full-mission simulator-based experiments and mooring calculations. Section 4 discusses the results. Section 5 concludes the paper.

2. Methodology

The operation of large deck cargo barges within U-shaped basins represent a critical practical requirement. However, during the processes of berthing, mooring, and unberthing, factors such as spatial constraints, complex flow fields, and limitations on tug operations pose significant challenges to manoeuvring precision and safety margins. Therefore, there is an urgent need for a comprehensive investigation of manoeuvring safety issues of large deck cargo barges within U-shaped harbour basins. Such research should identify the risk sources and key influencing factors throughout the entire berthing, mooring, and unberthing processes, conduct targeted analysis of manoeuvring strategies, and ultimately ensure operational safety.

To verify the feasibility and safety of the proposed manoeuvring strategies under complex waterway conditions, this study establishes an integrated analysis framework based on numerical simulations and experimental validations (see Figure 1). The framework assesses the entire berthing and unberthing processes of large deck cargo barges within U-shaped harbour basins, with a focus on the safety of tug deployment during berthing, mooring-line arrangements and tensions during the mooring phase, and the overall vessel attitude response.

2.1. Mathematical Model for Ship Motion Simulation

In this study, a full mission ship manoeuvring simulator, which incorporates tug subsystem, is employed to investigation the safety and operability of barge operations within the U-shaped basin by the mode of man-in-the-loop. By simulating different vessel types and reproducing manoeuvres that follow real navigational practices, the simulator enables a realistic evaluation of the proposed port layout and operating procedures. Through repeated simulation trials conducted under representative local environmental conditions, key data related to navigation, berthing, and unberthing processes can be obtained and analysed. These results allow for a systematic assessment of whether the design configuration supports safe operations, while also providing evidence-based recommendations for port authorities and operational decision-makers. As demonstrated in numerous domestic and international port development projects, manoeuvring simulation has become an essential tool in validating engineering designs and ensuring that navigational safety requirements are met.

The ship motion model is the core of the safety demonstration by using simulators. To describe the motion of the vessel, two coordinate systems are defined: the inertial system (earth-fixed) O-X₀Y₀Z₀ and the non-inertial system (body-fixed) G-xyz are adopted to describe the manoeuvring motion, as shown in Figure 2. In the body-fixed system, the ship motion equation can be written in the form of Equation (1).

$$\begin{array}{l}(m+m_x)\dot{u}-(m+m_y)vr=X\\ (m+m_y)\dot{v}+(m+m_x)ur=Y\\ (I_z+J_{zz})\dot{r}=N\end{array}$$

(1)

where m, m_x, m_y, I_z, and J_zz are mass of ship, added mass in x and y directions, moment of inertia, and added moment of inertia around z-axis, respectively. u and v denote surge and sway velocity, and r denotes yaw rate. X and Y are the components of external force vector F acting on ship in the reference system of G-xyz and N is the moment around z-axis. Following the MMG (Mathematical Manoeuvring Group) decomposition approach, the external forces consist of basic hydrodynamic forces due to hull and propulsion, the effects of environmental forces such as wind, wave, and current, and tug assistance.

2.2. Numerical Analysis on Tug Assistance Under Critical Condition

During tidal transition periods, vessels may simultaneously experience opposing tidal effects—particularly when the bow encounters flood/ebb currents while the stern is subjected to ebb/flood currents. In such scenarios, the vessel enters a rotational current field where the hull experiences a substantial yaw moment. To investigate the controllability of ship berthing and unberthing under such critical conditions, further numerical analysis is carried out for the large ship with tug assistance. This study employs CFD simulations using STAR-CCM+ with Version 2310 [6] to provide precise assess for the assistance. The Reynolds-Averaged Navier–Stokes (RANS) equations are solved, with the SST $k-\omega$ turbulence model applied for flow field simulation [7]. Numerical simulations can be conducted to calculate the yaw moment acting on the vessel under this most critical scenario. The governing equation is as follows:

Continuity Equation for the Control Volume:

$${\displaystyle \frac{{\partial }_{\rho }}{{\partial }_t}}=\nabla ·\left(\rho U\right)=0$$

(2)

In the equation, $\rho$ represents the fluid density, $t$ denotes time, and $U$ is the velocity vector of the continuous fluid.

RANS Equations for Incompressible Flow:

$${\displaystyle \frac{\rho {\partial }_{\left(\overline{u_i}\right)}}{{\partial }_{\left(t\right)}}}+{\displaystyle \frac{\rho {\partial }_{(\overline{u_i^{\prime }{u}_j^{\prime }})}}{{\partial }_{x_j}}}=-{\displaystyle \frac{{\partial }_{\overline{P}}}{{\partial }_{x_i}}}+{\displaystyle \frac{\partial }{{\partial }_{x_j}}}\left[\mu \left({\displaystyle \frac{{\partial }_{\overline{u_i}}}{{\partial }_{x_j}}}+{\displaystyle \frac{{\partial }_{\overline{u_j}}}{{\partial }_{x_i}}}\right)-\rho \overline{u_i^{\prime }{u}_j^{\prime }}\right]$$

(3)

In the equation, $u_i$ and $u_j$ are velocity components, with overlines indicating time-averaged values; P denotes pressure, μ is the dynamic viscosity of the fluid, $x_i$ and $x_j$ are spatial coordinates, and $\rho \overline{u_i^{\prime }{u}_j^{\prime }}$ represents the Reynolds turbulent stress tensor.

The above differential equations are numerically discretized and solved using the Finite Volume Method (FVM). The convective and diffusive terms are discretized with a second-order upwind scheme and second-order implicit scheme, respectively. Temporal discretization employs a first-order scheme. The velocity–pressure coupling is resolved using a segregated flow model with the SIMPLE algorithm for pressure–velocity coupling. The resulting linear systems from discretization are solved using an Algebraic Multigrid (AMG) linear solver. Relaxation for velocity U, pressure p, k, and ω is implemented via the Gauss-Seidel method. The free surface interface is captured using the Volume of Fluid (VOF) model.

The computational domain is discretized using an unstructured prismatic mesh (Figure 3). Overset mesh technology is employed to accurately capture the hydrodynamic forces acting on the vessel during motion, with additional mesh refinement applied within the rotational domains of the hull. Local mesh refinement is performed in the vicinity of the hull and free surface to resolve detailed flow structures and surface wave patterns. Regions with strong flow gradients, particularly near the bow and stern, are subject to targeted mesh refinement to improve solution accuracy. In addition, prismatic boundary-layer meshes are generated along the hull surface to accurately resolve near-wall flow behaviour.

The solution of the differential equations requires boundary condition specification. The computational setup employs the following boundary conditions: No-slip wall boundary conditions are imposed on the hull surface and seabed. Pressure outlet boundary condition is applied at the top boundary. Velocity inlet boundary conditions are assigned to all remaining boundaries. The entire flow field is initialised with zero velocity, with rotational motion exclusively imparted to the overset mesh region to simulate hull rotation.

Due to the complexity of physically implementing reverse water flow, this study employs a simplified approach based on Newton’s third law. According to the law, action and reaction forces are equal in magnitude and opposite in direction. Consequently, by prescribing a rotational angular velocity to the vessel, the hydrodynamic forces acting on the hull under reverse flow conditions can be effectively simulated. In this configuration, the damping moment experienced by the vessel equates to the effective value of the actual yaw moment exerted on the hull by rotational water flow. This methodology not only simplifies the computational model but also accurately captures essential flow characteristics.

2.3. Numerical Simulation Method for Mooring

The basic concept of arrangement of mooring lines is multi-point mooring configuration, that is to multiply breast and spring lines in addition to the main bow and stern lines. Looking into actual General Arrangement Plan of typical large deck cargo barge and a limited berth dike length, the relative positions of the probable mooring lines and the vessel are shown in Figure 4. The mooring-line materials are selected in accordance with the ‘Port Engineering Load Code’.

The mooring of a vessel is a complex process involving the coupled interactions between hull motions and mooring-line constraints. It is closely related to factors such as the configuration of quay mooring facilities, environmental conditions near the berth, vessel tonnage and loading condition during mooring, and mooring-line arrangements and selection. The time-domain solution is performed using the convolution integral as follows:

$$\begin{array}{c}\left\{m+A_{\infty }\right\}\ddot{X}\left(t\right)+c\ddot{X}\left(t\right)+KX\left(t\right)+{\int }_0^{t}h\left(t-\tau \right)\ddot{X}\left(\tau \right)d\tau =F\left(t\right)+TM\left(t\right)+FF\left(t\right)+FW+FC\end{array}$$

(4)

In the equation, $m$ is the vessel mass matrix, $A_{\mathrm{\infty }}$ is the added mass matrix, $c$ is the added damping matrix, $K$ is the total stiffness matrix of the vessel, $h$ is the acceleration impulse response function matrix, and $t$ denotes time. $X$ and $\ddot{X}$ represent the vessel motions and accelerations in six degrees of freedom. $F\left(t\right)$ includes wave excitation forces, while $T_M$, $F_F$, $F_W$, and $F_C$ represent external forces due to mooring lines, fenders, wind, and current, respectively.

By using AQWA, the vessel geometry, mooring-line characteristics, and environmental parameters for each operating condition are input into the model. The software then computes the time histories of vessel motions and the corresponding mooring-line tensions. By analysing the resulting tension responses, the risk of mooring-line breakage under the specified environmental conditions can be assessed. The specific input and output parameters for each condition are listed in

Table 1. Input and output parameters in calculation.

Category	Parameter Type	Description
Input	Vessel	Three-dimensional geometry of underwater hull;
		vessel draft, weight, centre of gravity, local coordinate system; projected areas under different wind directions
	Mooring Posts	Coordinates of mooring posts; mooring stiffness
	Mooring Lines	Coordinates of both ends of each mooring line; breaking strength; material stiffness coefficients K1~K5 (depending on breaking load); elongation ratio
	Fenders	Coordinates of fenders
	Environmental Parameters	Water depth; wave height spectrum and peak period; current speed and direction; wind speed and direction
Output	Vessel	Time histories of six-degree-of-freedom motions
	Mooring Lines	Time histories of mooring-line tensions

.

3. Case Study

3.1. Water Area Overview

Figure 5 is solely used to illustrate the appearance of a real deck cargo barge. These barges serve as crucial carriers for offshore engineering, transporting ultra-large components with their expansive decks and high load capacity. For the convenience of loading, stern-loading or roll-on/roll-off operation is frequently experienced, which requires a relatively calm operational environment and sufficient space to ensure precise alignment of handling equipment and safe cargo work. While conventional open berths or alongside quays offer spatial advantages, they are directly exposed to wind and waves, resulting in reduced operational stability. Furthermore, due to their specialised loading and unloading methods, these barges often need to perform operations in a ‘stern-in’ configuration, and operations at open berths can be significantly affected by nearby vessels. Consequently, fully or partially enclosed waters can mitigate environmental disturbances to some extent, thereby better meeting the safety requirements for berthing and cargo handling of such vessels.

Based on the operational requirements of large deck cargo barges, some ports adopt a U-shaped harbour basin design for dedicated berths (Figure 6). A U-shaped basin is enclosed on three sides by quay walls or breakwaters, with only one end open to the main navigation channel, effectively reducing the impact of offshore wind and waves and creating a relatively calm operational area. This structural layout facilitates the concentrated arrangement of multiple berths and large lifting equipment, enhances shoreline utilisation, and provides the sheltered environment and operational space required for stern-in berthing of large deck cargo barges, thereby improving both operational efficiency and safety.

However, the structural characteristics of U-shaped harbour basins also impose certain manoeuvring limitations. The internal water area is narrow and the opening is limited, providing restricted turning space for vessels during berthing and unberthing operations. Flow velocities are concentrated near the entrance, easily generating cross currents and backflows. Under conditions of dense berthing, hydrodynamic interactions between vessels become significant. Furthermore, the confined internal space limits tug positioning and pushing angles, and in some cases prevents tug operations altogether, making conventional tug-assisted manoeuvring modes less effective. While these features enhance operational stability, they also introduce new challenges to the manoeuvring safety of berthing and unberthing processes.

A typical U-shaped harbour basin is selected as the case study (see Figure 7). The basin entrance is oriented approximately 12° west of north and is enclosed by permeable breakwaters on the east and west sides and a quay wall on the south, forming a semi-enclosed water area with only the northern end open to the main navigation channel. The basin has a depth of approximately 250 m and a width of only 97 m, indicating significant spatial constraints. An elliptical turning area is located outside the entrance, connecting with the main channel and adjacent berths to form a shared operational zone with complex environmental disturbances.

The hydrodynamics of the harbour area belongs to an irregular semi-diurnal shallow-water tidal regime, with pronounced tidal currents. During the ebb of spring tides, surface current velocities in the main navigation channel can reach 0.91 m/s, with directions nearly perpendicular to the basin entrance, generating strong cross currents. The velocity difference between the quay front and inside/outside the basin is approximately 0.37 m/s. Vessels passing through the entrance experience a marked hydrodynamic transition zone, while the internal backflow within the basin is about 0.15 m/s. The prevailing wind direction is N–NE, accounting for 48.4% of observations, producing a continuous oblique onshore wind along the basin.

3.2. The Design Ship and Tug Deployment

In China, port design is generally based on the class of Dead Weigh Tonnage (DWT) to the design ship. For a standard class of 50,000 DWT the upper limit will be approximately 65,000 DWT. For the sake of safety, the following ship is selected as the representative ship for the analysis. The principal particulars of the ship are summarised in

Table 2. Principal Particulars of the Representative Vessel.

Experimental Vessel Model	DWT/(t)	L	B	H	Full-Load Draft/T	Restricted Draft/T
65,000-ton deck cargo barge	65,000	231.1	46	14.5	10.9	9.0

.

Based on the parameters of the experimental vessel and the harbour basin conditions, tugs cannot operate inside the basin and are only responsible for heading control during the turning phase and for providing bow thrust assistance outside the entrance and turning areas. Stern entry traction and mooring attitude control are entirely managed by the shore-based pilot car system. Regarding the tug optimisation, normally 2 tug assistance is required by port regulations for the berthing of a ship with Dead Weigh Tonnage (DWT) more than 50,000 tons. If the complexity and limitation of the sea are concerned, the port authority highly recommends one more tug in case of breakdown of any one tug assistance. In line with the strategy of ‘optimising tug arrangement by pre-planning tug deployment and specific tasks, with reserved standby positions,’ three 4000 HP tugs are pre-assigned according to the experimental vessel and the actual berth conditions. Figure 8 illustrates the tug deployment during the critical berthing phase, corresponding to the third step of the berthing procedure.

3.3. Simulation of Berthing and Unberthing

The ship motion model is integrated into the ship-handling simulator to conduct berthing operation tests under various environmental conditions. By configuring multiple navigation scenarios within the simulator, the vessel’s motion parameters and trajectories can be obtained for each test condition. These simulation results are then combined with the potential hazards associated with ship manoeuvring and berthing to evaluate the safety of the navigational environment. In this way, the suitability of the wharf layout and surrounding conditions for safe ship handling and berthing operations can be systematically assessed.

Based on the environmental characteristics of the study area, a series of representative ship-handling simulation conditions were designed to evaluate the safety of berthing and unberthing operations. These conditions incorporate variations in wind, current, and wave parameters, as well as differences in approach direction and manoeuvring strategy. To clearly present the full range of scenarios considered, a summary table of ship-handling simulation conditions was developed. Table 3 provides a structured overview of all environmental settings and operational configurations used in the simulator, ensuring that each berthing and unberthing test reflects realistic and operationally relevant conditions. This systematic design allows for a comprehensive assessment of ship manoeuvrability and safety under varying environmental influences.

Table 3. Summary of Ship-Handling Simulation Conditions.

Simulation Tests for Representative Ship Berthing Operations
No.	Wind Direction	Wind Speed (m/s)	Tidal Current (Full Flow Field)	Figure 9
1	ENE	13.8	One hour before low tide	a
2			One hour after low tide	b
3	WSW	13.8	One hour before low tide	c
4			One hour after low tide	d
5	N	13.8	One hour before low tide	e
6			One hour after low tide	f
7	ENE	13.8	Reversing current	g
Simulation Tests for Representative Ship Unberthing Operations
No.	Wind Direction	Wind Speed (m/s)	Tidal Current (Full Flow Field)	Figure 10
8	ENE	13.8	Flood tide	a
9			Ebb tide	b
10	WSW	13.8	Flood tide	c
11			Ebb tide	d
12	N	13.8	Flood tide	e
13			Ebb tide	f
14	WSW	13.8	Ebb tide, 0.7 m/s	g

The numbers 1–7 in the table represent the ship berthing simulation under seven prevailing or unfavoured conditions, while the numbers 8–14 represent the ship unberthing simulations correspondingly. According to the above 14 working conditions, the simulation results are obtained as shown in Figure 9 and Figure 10.

Table 3. Summary of Ship-Handling Simulation Conditions.

Simulation Tests for Representative Ship Berthing Operations
No.	Wind Direction	Wind Speed (m/s)	Tidal Current (Full Flow Field)	Figure 9
1	ENE	13.8	One hour before low tide	a
2			One hour after low tide	b
3	WSW	13.8	One hour before low tide	c
4			One hour after low tide	d
5	N	13.8	One hour before low tide	e
6			One hour after low tide	f
7	ENE	13.8	Reversing current	g
Simulation Tests for Representative Ship Unberthing Operations
No.	Wind Direction	Wind Speed (m/s)	Tidal Current (Full Flow Field)	Figure 10
8	ENE	13.8	Flood tide	a
9			Ebb tide	b
10	WSW	13.8	Flood tide	c
11			Ebb tide	d
12	N	13.8	Flood tide	e
13			Ebb tide	f
14	WSW	13.8	Ebb tide, 0.7 m/s	g

The numbers 1–7 in the table represent the ship berthing simulation under seven prevailing or unfavoured conditions, while the numbers 8–14 represent the ship unberthing simulations correspondingly. According to the above 14 working conditions, the simulation results are obtained as shown in Figure 9 and Figure 10.

Figure 9. Berthing trajectory (in brown).

Figure 9. Berthing trajectory (in brown).

Figure 10. Unberthing trajectory.

Figure 10. Unberthing trajectory.

The trajectory plots indicate that, under all test conditions, the vessel remained within the designed waterway boundaries throughout the entire process from turning to berthing and unberthing, without grounding or hard contact with quay structures. Only under extreme conditions did the minimum safety distance to the eastern and western breakwaters approach critical limits, imposing very high requirements on manoeuvring precision. Under crosswind conditions, the vessel generally exhibited lateral drift leeward; however, with coordinated control by the tugboats and ship-handling tractors, it was able to return to the planned trajectory.

During current-turning scenarios, the vessel’s trajectory displayed certain irregularities, reflecting a momentary increase in difficulty for maintaining attitude control. In flood- and ebb-peak conditions, it was observed that the bow tug needed to sustain high-power thrust for an extended period to counter strong lateral currents, placing higher demands on both tug performance and crew coordination. The CFD-based numerical results show the quantitative results (Figure 11). The yaw moment of the model ship under the maximus rotational tide is 0.49 N·m and, correspondingly, to the full-scale ship, it will be up to 501,796,000 N·m = 5120 t·m.

3.4. Mooring Simulation

Currently, high-performance synthetic fibre ropes are widely used on board 50,000 DWT vessels. The primary mooring lines are with diameters ranging from 50 mm to 78 mm. For a line of 68 mm diameter, the safe working load (SWL) is about 995 kN (101.5 tonnes). The overall elongation rate is within 5%. As the environmental conditions concerned, above all, these conditions were selected based on local hydrometeorological characteristics, where wave conditions play the dominant roles and can be modelled using the Pierson–Moskowitz (PM) spectrum. Two groups of mooring simulation tests were conducted under different wind and current conditions based on AQWA software. One is applied to investigate the safety of routine moorings, and the other for the ultimate of safety. Each simulation scenario encompassed a full tidal cycle.

During routine moorings, the conditions such as wave with height of 1 m, period of 4 s, direction of 90~180°, and approximate wind speed of 7.4 m/s will be more frequently experienced; therefore, after a simulation span of 10,000 s, the results were shown as

Table 4. Maximum work load of line during routine moorings (t).

O.	L1	L2	L3	L4	L5	L6	L7	L8	L9	L10
Maximum Tension/t	21	41.3	19.5	29.7	16.4	16.8	34.6	34.7	26.8	26.6

.

In contrast, the ultimate conditions, e.g., wave with height of 2.23 m, period of 8 s, and direction of 150°, approximately wind force 7, which was hardly met at the inner basin within the island waters. The result is shown in

Table 5. Example of Maximum Mooring Line Tensions under Selected Conditions.

	Scenario 37			Scenario 38
No.	Maximum Tension/t	Whether Dangerous	Whether Line Breaks	Maximum Tension/t	Whether Dangerous	Whether Line Breaks
L1	34.3	No	No	78	No	No
L2	99.4	No	No	52.1	No	No
L3	50.6	No	No	77.4	No	No
L4	42.8	No	No	65.7	No	No
L5	24.2	No	No	47.2	No	No
L6	24.8	No	No	47	No	No
L7	71.8	No	No	100	No	No
L8	75.2	No	No	95.5	No	No
L9	53.3	No	No	45.3	No	No
L10	55.1	No	No	44.7	No	No

.

4. Analysis and Discussion

4.1. Risk Analysis

Based on the simulation results, the maximum trajectory width within the channel is approximately 121.3 m. The turning basin area occupied is about 415 m × 362 m, which is significantly smaller than the designed turning circle. The minimum clearance observed between the vessel’s stern and the breakwaters ranges from 8.4 m to 22.3 m. Even under the most critical rotational tide conditions, the deployment of three 4000 HP tugs (each providing 40 t of thrust) with a lever arm of 100 m will generate a total yaw checking moment of 12,000 t·m. This moment is more than double the barge’s yaw moment of 5120 t·m. Therefore, safe operation is highly feasible. The validation of the simulation is currently short of data, because the port is not yet built. However, same models were applied to analyse the mooring of the LNG ship in the same water area, where the results meet well with the measured data by the mooring monitor system on shore and also were examined by local pilots and shore masters [32].

Considering relevant regulations and studies, and the specific conditions of U-shaped harbour basins, risk awareness should be further enhanced in the private and public sector. In such semi-enclosed and spatially constrained operational waters, the primary sources of risk during berthing and unberthing operations are the limited manoeuvring space and the complex hydrodynamic conditions.

(1)

Berthing Phase

For large deck cargo barges performing stern-in berthing within U-shaped harbour basins, the risks are higher than during unberthing operations. First, the angle between the basin entrance and the main navigation channel is relatively large, and the flow direction and velocity vary significantly as the tide passes through the entrance, creating a complex hydrodynamic field. During the approach, the bow and stern sequentially enter the main current zone, making the vessel susceptible to uneven lateral forces, generating considerable yaw moments and potentially causing vessel instability. Winds from the entrance direction, combined with cross currents, further increase the risks of lateral drift and deviation. Second, the internal space of the basin is limited, the turning radius is small, and tug positioning and pushing angles are constrained; in some cases, certain tugs cannot enter the basin, requiring the vessel to rely on pilot cars or shore-based equipment for stern traction. The bank effect is particularly pronounced in the final berthing stage: as the vessel approaches close to the quay, water pressure generates suction, which, if the approach speed is too high or attitude corrections are delayed, may lead to sudden contact or scraping against the berth. Furthermore, dense berthing and limited spacing between quays, along with disturbances from other vessels’ wakes or ongoing lifting operations, can compromise the stability of stern-in berthing, making the berthing phase the highest-risk stage of the entire operation.

(2)

Unberthing Phase

The risks during the unberthing phase are relatively lower, but uncertainties still exist. During unberthing, vessels typically depart bow-first, and the bank effect combined with the prevailing wind may push the hull toward the quay. If mooring-line release is not well-coordinated or thrust is insufficient, a ‘re-berthing’ phenomenon may occur. In addition, the outflow direction of the basin is strongly influenced by tidal currents; when the offshore current direction is not aligned with the unberthing course, uneven forces act on the bow and stern, potentially causing minor deviations. As tugs are often positioned outside the basin, the thrust transmission path is relatively long, making precise control of the vessel’s heading difficult in confined waters. Poor communication or delayed responses may also result in heading deviations or instability during the unberthing process.

4.2. Countermeasures for Berthing and Unberthing Phases

To address the risks, strategies can be proposed from the perspectives of operational organisation and manoeuvring control. The following measures aim to enhance controllability and safety during the berthing and unberthing phases under varying environmental conditions, thereby mitigating risks arising from environmental disturbances and manoeuvring errors.

(1)

Strict compliance with safe operational limitations. Based on the results, the following limitations are suggested: Under wind force 7, current speed 0.7 m/s (approx. 70% of the maximum speed in the channel, at least 2 tug assistance and one reserve for emergency response in critical situation.

(2)

Selection of Appropriate Operational Windows: Berthing and unberthing operations should be conducted during periods of weak currents and minimal crosswinds. When necessary, tidal window control can be employed to avoid operations under strong winds or during rapid ebb or flood currents.

(3)

Optimisation of Tug Arrangement: Tug deployment plans and specific operational tasks should be prearranged in advance, with designated standby positions reserved for tugs.

(4)

Control of Speed and Attitude: During berthing, maintain low speeds and make small-angle adjustments to avoid sudden changes in bank suction caused by rapid acceleration or deceleration. During unberthing, gradually release mooring lines and depart slowly. Coordinated control between tugs and pilot cars should be employed to ensure the vessel moves along the central axis, thereby minimising lateral drift.

(5)

Real-Time Monitoring of Key Parameters: Parameters to be monitored include bow-stern offset angles, lateral velocity, minimum distance to the quay, and variations in mooring-line tension.

(6)

Communication and Coordination: A unified command system should be maintained during berthing and unberthing operations to ensure consistent communication among the vessel, tugs, and shore-based control systems.

4.3. Mooring Phase

Based on the calculated results of all operating conditions, the following finding can be noted:

(1)

Normal operating conditions

Relatively small tensions in the mooring lines and fenders will be experienced, but as the wave direction gradually deviates from the ship’s heading, the maximum line tension increases, reaching up to 40 t under beam-wave conditions. When waves approach from the bow and the stern is supported by only one breast line, this single line bears most of the surge-induced load, resulting in significantly higher tension.

(2)

Severe wind–wave scenarios

The maximum mooring-line tensions are much higher than those in normal conditions. In a head-sea case with a wave height of 2.23 m and a wind with Beaufort scale 7, the maximum line tension reaches 100 t, while during oblique-wave conditions with the same wave height (2.23 m) and a wave period of 5.3 s, the maximum tension reaches 99 t, approaching the critical threshold and indicating conditions near the dangerous limit.

Therefore, the proposed mooring configuration can maintain vessel stability under harbour recirculation and disturbances from adjacent vessels, ensuring that no mooring-line failure occurs. However, in the U-shaped harbour basins, the confined space and dense quay walls restrict water movement, creating suction between the hull and the quay that continuously draws the vessel toward the berth. Additionally, internal backflows, waves, and operations of adjacent vessels may induce slow lateral movement and oscillations in the bow and stern angles. In case extreme conditions arise, such as wind and wave levels exceeding the above thresholds, emergency measures should be promptly implemented. These may include temporarily adding breast lines or spring lines and increasing pretension to prevent excessive load concentration on a single line, thereby avoiding overload or line breakage and ensuring sufficient safety margins during the mooring stage. Improper mooring-line arrangement or uneven tension distribution can result in overloading, slackening, or snapping of certain lines, thereby compromising mooring stability. Moreover, changes in weather and tidal conditions over extended periods of mooring can alter the force distribution and further increase risks.

5. Conclusions

The study focused on the safety of manoeuvring and mooring operations for large deck cargo barges within U-shaped port basins, a matter of significant importance to both mariners and port designers. Due to special features of the basin and barge, the operation is challenging. To investigate the safety operation of those ships under these complex circumstances, an integrated methodology is presented, including ship manoeuvring simulations and CFD-based analysis. There are two core technical components that directly affect safe operation in a U-shape inner basin, which are ship manoeuvring capability to access the berth and mooring safety to maintain the vessel securely alongside the berth under environmental loads.

Based on the simulation results, the following conclusion can be safely drawn:

(1)

The safety operation can be reached for a barge with DWT class of 50,000 tons under normal situation.

(2)

These operations are subject to the following conditions: under wind force 7, current speed 0.7 m/s, wave under 2 m, and period within 8 s.

(3)

At least 2 tug assistance with capacity of 4000 HP and one reserved for emergency response in critical situation.

(4)

Ten mooring line configurations is enough to secure the vessel ashore, accommodating non-standard bollard arrangements.

(5)

The manoeuvring simulation indicate the trajectories and the ship clearance to channel and the breakwaters.

(6)

The numerical result for the barge yaw moment, calculated under critical situation of rotation tide, indicates that the arrangement of tug is reasonable.

(7)

Although the arrangement of mooring lines is enough to counter the environmental effects; under the extreme condition, the tension of some lines will reach its safe work loading. Therefore, good management and deliberation for mooring line is still urged.

Generally, the finding of this study could provide mariners and port personnel with a practical guide for the manoeuvring of a large ship at the entrance of and within a narrow basin, including tugboat assistance, berthing, unberthing, and mooring operations, under various sea and wind conditions.