Showing papers in "Journal of Marine Science and Technology in 2021"

Path planning and collision avoidance for autonomous surface vehicles I: a review

Abstract: Autonomous surface vehicles are gaining increasing attention worldwide due to the potential benefits of improving safety and efficiency. This has raised the interest in developing methods for path planning that can reduce the risk of collisions, groundings, and stranding accidents at sea, as well as costs and time expenditure. In this paper, we review guidance, and more specifically, path planning algorithms of autonomous surface vehicles and their classification. In particular, we highlight vessel autonomy, regulatory framework, guidance, navigation and control components, advances in the industry, and previous reviews in the field. In addition, we analyse the terminology used in the literature and attempt to clarify ambiguities in commonly used terms related to path planning. Finally, we summarise and discuss our findings and highlight the potential need for new regulations for autonomous surface vehicles.

Automatic ship collision avoidance using deep reinforcement learning with LSTM in continuous action spaces

Abstract: This paper presents an automatic collision avoidance algorithm for ships using a deep reinforcement learning (DRL) in continuous action spaces. Obstacle zone by target (OZT) is used to compute an area where a collision will happen in the future based on dynamic information of ships. Agents of DRL detects the approach of multiple ships using a virtual sensor called the grid sensor. Agents learned collision avoidance maneuvering through Imazu problem, which is a scenario set of ship encounter situations. In this study, we propose a new approach for collision avoidance with a longer safe passing distance using DRL. We develop a novel method named inside OZT that expands OZT to improve the consistency of learning. We redesign the network using the long short-term memory (LSTM) cell and carried out training in continuous action spaces to train a model with longer safe distance than the previous study. The bow cross range in collision detection proposed in this paper is effective to COLREGs-compliant collision avoidance. The trained model has passed all scenarios of Imazu problem. The model is also validated by a test scenario which includes more ships than each scenario of Imazu problem.

Path planning and collision avoidance for autonomous surface vehicles II: a comparative study of algorithms

Abstract: Artificial intelligence is an enabling technology for autonomous surface vehicles, with methods such as evolutionary algorithms, artificial potential fields, fast marching methods, and many others becoming increasingly popular for solving problems such as path planning and collision avoidance. However, there currently is no unified way to evaluate the performance of different algorithms, for example with regard to safety or risk. This paper is a step in that direction and offers a comparative study of current state-of-the art path planning and collision avoidance algorithms for autonomous surface vehicles. Across 45 selected papers, we compare important performance properties of the proposed algorithms related to the vessel and the environment it is operating in. We also analyse how safety is incorporated, and what components constitute the objective function in these algorithms. Finally, we focus on comparing advantages and limitations of the 45 analysed papers. A key finding is the need for a unified platform for evaluating and comparing the performance of algorithms under a large set of possible real-world scenarios.

Path following algorithm application to automatic berthing control

Abstract: This paper aims to verify a new automatic berthing system using a path following algorithm. Berthing operation is one of the most burdensome tasks for crews among several ship operations. The maneuverability of a ship at low speed during berthing operation deteriorates and becomes more vulnerable to disturbances such as wind. Therefore, it is necessary to support and automate operations that require advanced skills such as berthing operation. Previous studies on automatic berthing have investigated various methods to handle the nonlinearity of ship maneuvering motion and determine the optimal control variable. There is a trade-off between accuracy and real-time performance of berthing control from these studies. The algorithms must have sufficiently real-time performance while maintaining the accuracy of control. For these purposes, we propose the automatic berthing system applied a path following algorithm for a ship with one propeller and one rudder in this paper. We show the mathematical model for numerical simulation of berthing control and carried out system identification of the subject ship. In full-scale experiments, the proposed system performed automatic berthing control in both calm wind conditions around 2 m/s and strong wind conditions around 6 m/s.

Prediction of human–machine interface (HMI) operational errors for maritime autonomous surface ships (MASS)

Abstract: The human factor is a hot topic for the maritime industry since more than 80 percent of maritime accidents are due to human error. Minimizing human error contributions in maritime transportation is vital to enhance safety levels. At this point, the maritime autonomous surface ships (MASS) concept has become one of the most significant aspects to minimize human errors. The objective of this research is to predict the human–machine interface (HMI)-based operational errors in autonomous ships to improve safety control levels. At this point, the interaction between shore-based operator and controlling system (cockpits) can be monitored and potential HMI operational errors can be predicted. This research utilizes a Success Likelihood Index Method (SLIM) under an interval type-2 fuzzy sets (IT2FSs) approach. While the SLIM provides a prediction of the human–machine interface (HMI) operational errors, the IT2FSs tackles uncertainty and vagueness in the decision-making process. The findings of this paper are expected to highlight the importance of human–machine interface (HMI) operational errors in autonomous ships not only for designers but also for operational aspects.

Identification modeling and prediction of ship maneuvering motion based on LSTM deep neural network

Abstract: This paper proposes a novel system identification scheme to obtain a MIMO model of ship maneuvering motion, which can leverage the temporal correlation from the constructed training data to learn the underlying feasible model robust to extraneous noise. The scheme is based on long–short-term-memory (LSTM) deep neural network, which is more easily trained than traditional feedforward neural network with more complicated network structure. First, multiple datasets of simulated standard maneuvers (10°/10° and 20°/20° zigzag, 35° turning circle) of a KVLCC2 model are artificially polluted with white noise of various levels and used simultaneously to train the deep neural network model. Meanwhile, the data of 15°/15° zigzag maneuver are used to facilitate the training process to alleviate overfitting problem. Second, different datasets of modified zigzag tests are used to validate the generalization performance and robustness to noise of the trained neural network model. The training and validation results demonstrate that a mapping between the dynamics of ship motion and the computation in LSTM deep neural network is correctly identified. This mapping indicates that the complex nonlinear features of ship maneuvering motion can be learned from the measured temporal data, using standard training techniques for deep neural networks. An equivalent LSTM deep neural network model with better generalization performance and robustness is established, and its accuracy in predicting ship maneuvering motion is validated.

Autonomous ships for container shipping in the Arctic routes

Abstract: This study investigates the competitiveness of various autonomous ship categories for container shipping in the Arctic route. We propose a multi-criteria decision-making (MCDM) framework using four ship categories as alternatives and eight criteria for competitiveness evaluation. We analyse collected data using the Best–Worst Method (BWM), one of the recently developed MCDM methods. The findings reveal that operating expenses, navigation aspects, and environmental protection are the three most important criteria for deploying autonomous ships in the Arctic route. Among the three investigated autonomous ships alternatives, the semi-autonomous ship operated from a shore control centre (SCC) is prioritized for Arctic shipping in the foreseeable future, when benchmarked against the conventional ship. The SCC-controlled semi-autonomous ship alternative is competitive in the majority of the considered criteria including operating expenses, capital expenses, navigation, ship-shore and ship–ship communication, search and rescue, and environmental protection.

Mechanism of dynamic automatic collision avoidance and the optimal route in multi-ship encounter situations

Abstract: Autonomous navigation on the open sea involving automatic collision avoidance and route planning helps to ensure navigational safety. To judge whether all target ships (TSs) will pass safely and find the optimal route under multi-ship encounter situations, the relationship between the variations in the own ship (OS) velocity vector after nonlinear course altering motion and the collision avoidance result, which is defined as the collision avoidance mechanism, was analyzed. Methods producing the optimal route were also proposed. First, the static collision avoidance mechanism based on the ship domain and velocity obstacle (VO) was introduced. On that basis, the collision-free course alteration range of the OS, without consideration of the real manoeuvring process, was presented. Second, the ship motion equations and fuzzy adaptive proportion integral derivative (PID) control method were combined to develop a course control system. This system was then used to predict OS motions during the course-altering process. Based on this prediction, TS positions were calculated. Subsequently, the dynamic collision-free course altering ranges for the OS were obtained. Third, a model to compute the optimal route was introduced. Finally, simulations were performed under a situation including six TSs and two static objects, and the shortest collision-free route that satisfies both regulations for preventing collisions and good seamanship was found.

Path-following control of unmanned surface vehicles with unknown dynamics and unmeasured velocities

Abstract: The path-following control problem of an unmanned surface vehicle (USV) with unknown dynamics and unmeasured velocities is addressed in this paper. Main contributions are summarized as below: (1) considering that vehicle velocities are unmeasured directly, a finite-time velocity observer (FVO) is designed by utilizing available position information, which contributes to the path-following control and facilitates the implementation in practical engineering; (2) based on the traditional light-of-sight guidance, a heading-surge (HS) guidance scheme is proposed to guide USV surge velocity and heading angle, simultaneously; (3) in combination with wavelet neural network (WNN) and adaptive technique, unknown dynamics involving model uncertainties and environment interferences can be estimated accurately, thereby significantly enhancing system robustness. By the aid of the HS guidance based adaptive WNN controllers, all the error signals in the closed-loop system are uniformly ultimately bounded (UUB). Besides, simulation studies and comprehensive comparisons are conducted to demonstrate the satisfactory path-following performance and superiority of the proposed scheme.

Path following of under-actuated ships based on model predictive control with state observer

Abstract: A model predictive control (MPC) method with linear extended state observer (LESO) is presented to address the problems of the input constraints, uncertain parameters and external disturbances for path following of under-actuated surface ships without velocity measurements. The line of sight (LOS) guidance algorithm is employed to convert the path following into the heading control flexibly. The proper reference paths at waypoints are formulated by a variable acceptable radius approach. The matters of the heading control, input constraints including rudder amplitude and rate limit are addressed by virtue of MPC method employing the Norrbin model as the internal model. The LESO is introduced to estimate the uncertain parameters and external disturbances for the improvement of the internal model accuracy and the controller robustness. Moreover, the nonlinear observer and LESO are used to approximate the surge velocity, sway velocity and yaw rate, respectively. The effectiveness of the proposed control law is demonstrated via the compared simulations.

Wave motion compensation in dynamic positioning of small autonomous vessels

Abstract: Conventional dynamic positioning (DP) systems on larger ships compensate primarily for slowly time-varying environmental forces. In doing so, they use wave filtering to prevent the DP from compensating for the first-order wave motions. This reduces wear and tear of the thruster and machinery systems. In the case of smaller autonomous vessels, the oscillatory motion of the vessel in waves may be more significant, and the thrusters can be more dynamic. This motivates the use of DP to compensate for horizontal wave motions in certain operations. We study the design of DP control and filtering algorithms that employ acceleration feedback, roll damping, wave motion prediction, and optimal tuning. Six control strategies are compared in the case study, which is a small autonomous surface vessel where the critical mode of operation is launch and recovery of an ROV through the wave zone.

A New Fuzzy DEA Model for Solving the MCDM Problems in Supplier Selection

Abstract: Supplier selection is a crucial task in supply chain management, and good supply chain management is essential to a company's ability to develop sustainably and gain the trust and satisfaction of its customers. In modern supply chain management, supplier selection often takes the form of a multi-criteria decision-making (MCDM) problem. Thus, to solve this problem, this study formulated a novel fuzzy MCDM model, which is based on the fuzzy data envelopment analysis model. This novel approach can transform the linguistic variables of assessment criteria into well-founded quantitative data for calculation. Moreover, our approach can effectively discriminate between decision-making units (both efficient and inefficient) to aid managers in identifying the most suitable suppliers. We hope that our model can provide scholars and managers with new insights into solving the MCDM problem in supplier selection.

Systems analysis for deployment of internet of things (IoT) in the maritime industry

Abstract: Various industries are undergoing transformation given recently available pervasive sensors, low-cost and low-latency digital communication, and distributed control technologies. The objective of this paper is to support the introduction of Internet of things (IoT) technologies in the maritime industry. The maritime industry is analyzed as a system of systems to define performance criteria and functions to be modeled and analyzed through simulation. In this case, the simulation of a shipping system includes models of operation, cargo loading, fuel loading, and docking for maintenance. In the simulation, various kinds of IoT technologies are defined by several input parameters. By changing the parameters, the simulator evaluates the impact of those technologies quantitatively. As a case study, 11 IoT technologies are evaluated and compared. The result reveals several insights that weight of the ship is the most impactful for the profit, controlling damage of the ship’s hull by operation is the most important for safety, and improvement in efficiency at ports is the key to reducing delay time in operation. Moreover, this paper shows that the sensitivity analysis by changing the input parameters can support the decision making of how much investment will be effective in considering the technologies’ levels.

Robust adaptive asymptotic trajectory tracking control for underactuated surface vessels subject to unknown dynamics and input saturation

Abstract: In this paper, a robust adaptive control scheme is proposed for the trajectory tracking control of underactuated surface vessels (USVs) subject to unknown dynamics, external disturbances and input saturation. First, a coordinate transformation is introduced to deal with the underactuation problem of the USV. A Gaussian error function and an adaptive neural network (NN) are adopted to approximate the saturation function and the unknown dynamics, respectively. Then, an adaptive robust integral of the sign of the error (RISE) feedback term is introduced in feedback control design to compensate the NN and saturation approximation residual errors and unknown external disturbances. On the basis of the above, a robust adaptive trajectory tracking control law is proposed incorporating a coordinate transformation, Gaussian error function and NN into RISE method. In addition, the adjustable-online adaptive feedback gain reduces the conservativeness of the control design. The theoretical analysis indicates that the designed robust adaptive control law can force USVs to track the desired trajectory while guaranteeing the asymptotic tracking performance. Simulation results verify the effectiveness of the novel robust adaptive trajectory tracking control scheme.

Validation of 6-DOF motion simulations for ship turning in regular waves

Abstract: In this study, a six degrees of freedom (6-DOF) motion simulation method of a ship steering in regular waves is validated. The proposed simulation model is based on the two-time scale concept where the 6-DOF motions are expressed as the sum of the low-frequency maneuvering motions and high-frequency wave-induced motions. Turning simulations of a KCS container ship model with a rudder angle of $$\pm 35^\circ$$ in calm water and regular waves are performed and the obtained results are compared with the results of a free-running model test. The model tests were conducted using a ship model of length 3.057 m in a square tank at the National Research Institute of Fisheries Engineering, Japan. The wave conditions were as follows: the wave height was 3.6 m at full-scale, ratio of wavelength to ship length was 1.0, and the ship approached in the head wave direction before it was steered. The present method can simulate both the turning motion and wave-induced motions in regular waves with practical accuracy.

Underwater acoustic positioning based on the robust zero-difference Kalman filter

Abstract: The accuracy of underwater acoustic positioning is greatly influenced by both systematic error and gross error. Aiming at these problems, the paper proposes a robust zero-difference Kalman filter based on the random walk model and the equivalent gain matrix. The proposed algorithm takes systematic error as a random walk process, and estimates it together with the position parameters by using zero-difference Kalman filter. In addition, the equivalent gain matrix based on the robust estimation of Huber function is constructed to resist the influence of gross error. The proposed algorithm is verified by the simulation experiment and a real one for underwater acoustic positioning. The results demonstrate that the robust zero-difference Kalman filter can control both the effects of systematic error and gross error without amplifying the influence of the observation random noise, which is obviously superior to the zero-difference least squares (LS), the single-difference LS and zero-difference Kalman filter in underwater acoustic positioning.

Recent developments of titanium dioxide materials for aquatic antifouling application

Abstract: Biological fouling is undesirable accumulation of various organisms on submerged artificial surfaces. Combating biological fouling on watercrafts and immersed infrastructures is an eternal pursuit since ancient times. Traditionally, toxins, such as arsenic, mercury and tributyltin, were used as antifoulants on ship hulls but they are very harmful to both human and environment. Therefore, worldwide ban on these toxins has been implemented and other toxins with relatively lower environmental impact were used in antifouling coatings along with copper compounds. Unfortunately, these materials still pose a threat to various marine organisms. To completely eliminate this threat, coatings with complete absence of toxic materials have been developed. Nevertheless, these coatings have their own deficiencies, for instance, foul release coatings cannot protect stationary objects. In the past decade, many articles reported that titanium dioxide (TiO2) opens up new possibilities for antifouling materials. This article reports the current development of TiO2 containing materials for antifouling applications.

Analysis of hull, propeller and engine interactions in regular waves by a combination of experiment and simulation

Abstract: The goal of this study is to investigate ship propulsion system dynamics under sea wave conditions by including the interaction of hull, propeller, and engine. A mathematical ship propulsion system model was made and the related computer code was developed. To get the results as close as possible to real conditions, measured data for physical models, including the ship’s resistance in calm and sea waves and propeller performance, were implemented in the model. For a diesel engine, performances provided by the manufacturer were used. The wave force time series, as exciting force, changed the propulsion system state from steady to transient. It activated system variables including ship’s speed, advance number, propeller and engine torque, propeller and engine rotational speeds, effective and generated powers, and net thrust. The analysis was performed for a container ship for two regular waves. Using the developed computer code, the ship’s speed and system variables, as well as the consumed fuel and the voyage distance, were calculated and compared with the calm water condition. The voyage mode was set on constant rotational engine speed implementing a P-action governor with fuel rate and engine torque limiters. The outcomes of the research explain the influence of the governor and its limiters on fuel consumption, identify the nonlinear impact of sea waves on propeller characteristics, and underline the effect of voyage mode on system response and the consumed fuel. The results also show that the conventional method for calculating speed reduction based on the added resistance is not capable of justifying the system’s dynamic behaviour.

Parametric design and optimization of SWATH for reduced resistance based on evolutionary algorithm

Abstract: This study develops a numerical simulation-based optimization procedure of Small Waterplane Area Twin Hull (SWATH) design. The procedure integrates the parametric geometry model, numerical analysis of total resistance, response surface methodology (RSM) model, and evolutionary optimization (EVOL) algorithm. The parametric hull form model is constructed based on general characteristic curves and several parameters to achieve model fully automated geometry variation quickly in the process. The resistance numerical simulation is performed and the results of resistance forces are taken as the optimization objective. The RSM model is built by stepwise selection method to accelerate the optimization process. The EVOL algorithm is applied to solve the optimization problem. A 32 m SWATH has been taken as an example to present the efficiency of the optimization. The optimization result shows that the optimized hull form has lower resistance than the original hull. The effect of the design parameters on the total resistance is discussed, which offers a few suggestions for the improvement of SWATH.

Unburned hydrocarbon formation in a natural gas engine under sea wave load conditions

Abstract: Although natural gas is documented as a low-emission fuel compared to the other traditional fossil fuels in internal combustion engines, recent research indicates large amounts of methane emission released by lean burn gas engines and highlights the importance of this emission on global warming. This paper aims at illustrating the main sources of unburned fuel in internal combustion engines with an emphasis on spark ignited natural gas engines. In addition, two unburned hydrocarbon modeling patterns, empirical and thermodynamic, are proposed. Moreover, a verified engine model including all components with an implemented dynamic load based on harmonic sea waves has been set up and coupled to the unburned hydrocarbon formation models. Results show that load variation may contribute to further methane slip and this increment rises sharply when the load amplitude enlarges. The maximum amount of methane slip occurs at reduced loads when the time lag of the control system of the turbocharger causes additional fresh air to flow towards the combustion chamber and brings the flame into the quenching area. As well, inspecting unburned hydrocarbon emission in diverse air–fuel ratios but with the same wave frequency and amplitude uncovers the sensitivity of lean burn gas engines to the dynamic load.

Influence of heave plates on the dynamics of a floating offshore wind turbine in waves

Abstract: This paper presents an analysis of the effects of heave plates with large skirts on the dynamics of a semi-submersible FOWT in waves. The analysis was based on a preliminary hull configuration envisaged for Brazilian waters. A numerical model based on linear frequency-domain hydrodynamics evaluated with panel methods was used for predicting the effects of the heave plates on the natural periods of motions. The predictions were compared to the results of model tests conducted in a wave basin involving ten different configurations of plate diameters and skirt heights. The experimental results confirmed the validity of the numerical predictions and provided quantitative results regarding the variations in viscous damping associated with the dimensions of the heave plates. The data will be used for future calibration of a parametric model for hull geometry optimization.

Sea state estimation using monitoring data by convolutional neural network (CNN)

Abstract: In recent years, the size of container ships has become larger, thus requiring a more evident assurance of the hull structural safety. In order to evaluate the structural safety in operation, it is necessary to grasp the encountered sea state. The aim of this study is to estimate the encountered sea state using machine learning from measurement data of ocean-going 14,000TEU container ships. In this paper, as a first step in the study, considerable amounts of virtual sea state data and corresponding ship motion and structural response data are prepared. A convolutional neural network (CNN) is developed using these data to estimate the directional wave spectrum of encountered sea based on the hull responses. The input parameters of the formulated CNN include the spectral values of ship motion and structural response spectrum. The output of the CNN includes the sea state parameters of the Ochi-Hubble spectrum, specifically, significant wave height, modal wave frequency, mean wave direction, kurtosis, and concentration of wave energy directional distribution. It is found from the performance examination that the developed CNN is capable of accurately estimating the sea state parameters, although the level of accuracy decreases when the hull response is low. However, the decrease in accuracy when the hull response is low has a weak influence on the evaluation of the structural response to the estimated sea state.

Effects of nonlinear wave loads on large monopile offshore wind turbines with and without ice-breaking cone configuration

Abstract: In the present paper, the computational fluid dynamics (CFD) method is used to investigate the variation of linear and nonlinear wave loads on a 10-MW large-scale monopile offshore wind turbine under typical sea conditions in the eastern seas of China. The effect of adding a structural ice-breaking cone configuration close to the mean water level on the monopile’s hydrodynamic response is studied further. Results are derived with the use of the CFD model and are compared with the relevant results that are calculated using the Morison equation and the potential flow theory based on the high-order boundary element method. The fifth-order Stokes’ theorem is used to model the incoming wave kinematics, and the volume of fluid (VOF) method is used to capture the free surface of waves and to accurately calculate the wave run-up on the monopile and cone configuration. The influence of different water depths and wave heights on the wave maximum vertical extent of wave uprush on the structure, pressure and horizontal wave forces on the monopile is investigated for both with and without the use of the cone configuration. Up–downward cone configuration results in better performance compared to the inverted cone configuration in terms of reduction of hydrodynamic nonlinear excitation loads and wave maximum vertical extent of wave uprush on the structure.

Performance studies of MIMO based DCO-OFDM in underwater wireless optical communication systems

Abstract: The research in underwater communication system is focused towards wireless networks in view of recent developments in wireless technology and increasing need for deep sea data mining. The underwater wireless optical communication (UWOC) have more advantages for short-range wireless links due to very high bandwidth and data rate than acoustic communication. Although UWOC offers high capacity links at low latencies, they suffer from limited communication range due to various distinctive characteristics of underwater channel. A major degrading effect associated with the underwater channel is turbulence-induced fading. To mitigate this fading and to extend the viable communication range, spatial multipath diversity techniques can be adopted over UWOC links. The proposed work evaluates the performance of a multicarrier modulation scheme, i.e., MIMO based DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) for underwater wireless optical channel. The simulation of MIMO-DC biased optical OFDM technique employing spatial diversity in underwater optical communication is a novel approach towards the determination of an optimal modulation technique for underwater optical wireless channels. The performance is evaluated based on the various parameters such as Signal to Interference plus Noise Ratio (SINR), Throughput and Bit Error Rate (BER). A simple experiment is conducted to demonstrate the BER performance assessment for different link distance of 0.45 m and 1.5 m with different transmitter and receiver configurations (SIMO and MIMO) for both clear water and coastal water using MIMO DCO OFDM technique.

Theoretical estimation of roll acceleration in beam seas using PDF line integral method

Abstract: The prevention of excessive roll acceleration is one of the fundamental requirements of an oceangoing vessel at sea. In this paper, aiming at this requirement, we propose a new theoretical method for calculating roll angular acceleration, called “PDF line integral method.”. This paper presents the derivation of this method and a numerical comparison with Monte Carlo simulation (MCS) results; also, its validity is shown. Although the utilized GZ curve (restoring curve) exhibits strong asymmetricity, the proposed method can provide for such a condition. With only the information of roll and roll rate joint probability density function (PDF), this method can approximately calculate the roll angular acceleration, which is a high-order differential property. In addition, a relatively good agreement is achieved between the theory and MCS results.

Collision avoidance path planning in multi-ship encounter situations

Abstract: Collision avoidance path planning is still one of the essential problems in the design and application of an intelligent maritime navigation system. Its main obstacle is how to determine effective and cooperative collision avoidance maneuvers within a multi-ship encounter situation. By deconstructing a multi-ship encounter, this study adopted ship domain around target ships to assess the collision danger that own ship should avoid. Subsequently, the fitness function that has multiple dynamic obstacle constraints was designed in a two-dimensional map. Based on DE algorithm, a path-planning method was developed to compute collision-free and optimal navigation paths for ships. Simulation results show that the algorithm can generate a safe and suitable path from each perspective in a multi-ship encounter. The results also validate the practicality of the generated paths, consistency of the algorithm outputs and performance of the algorithm. It would be expected to provide a reference for collision avoidance decision making as well as contribute to the development of autonomous navigation systems.

Time-domain TEBEM method for wave added resistance of ships with forward speed

Abstract: A numerical method for solving 3D unsteady potential flow problem of ship advancing in waves was put forward. The flow field was divided into an inner and an outer domain by introducing an artificial matching surface. The inner domain was surrounded by a ship-wetted surface and a matching surface as well as part of the free surface. The free-surface condition for the inner domain was formulated by perturbation about the double-body (DB) flow assumption. The outer domain was surrounded by a matching surface and the rest had a free surface as well as an infinite far-field radiation boundary. The free-surface condition for the outer domain was formulated by perturbation of the uniform incoming flow. The simple Green function and transient free-surface Green function were used to form the boundary integral equation (BIE) for the inner and outer domains, respectively. The Taylor expansion boundary element method (TEBEM) was adopted to solve the DB flow and inner-domain and outer-domain unsteady flow BIE. Matching conditions for the inner-domain flow and outer-domain flow were enforced by the continuity of velocity potential and normal velocity on the matching surface. Direct pressure integration on the ship-wetted surface was applied to obtain the first- and second-order wave forces. The numerical prediction on the displacement, acceleration and added resistance of the 14000-TEU container ship at different forward speeds were investigated by the proposed TEBEM method. Reynolds-Averaged Navier–Stokes (RANS) equations based on Computational Fluid Dynamics (CFD) method were adopted to compare with TEBEM method. The physical tank experiment results also validated the accuracy of the numerical tank results. Compared with the experimental solutions, TEBEM obtained good agreement with the RANS CFD method. TEBEM, however, was much more efficient and robust.

Statistics of thickness and strength of first-year ice along the Northern Sea Route

Abstract: In this paper, statistics of ice thickness and ice strength of first-year sea ice along the Northern Sea Route (NSR) is studied to provide useful information for the design and operation of Arctic ships. Specifically, ice thickness, ice strength and other physical parameters of the sea ice are estimated. Four representative sites are selected to study the ice environment and ice strength in different sea areas along the NSR during the ice growth season. Besides that, a good knowledge of the co-variation relationships between these ice parameters in a particular region would promote the estimation of ice loads acting on structures located in this region. In this work, a novel probabilistic model is introduced to describe the probability distribution of the ice flexural strength. The co-variation relationships between ice thickness and ice strength are quantified in terms of correlation coefficients. The influence of air temperature on ice properties is also investigated and discussed.

Error-driven-based adaptive nonlinear feedback control of course-keeping for ships

Abstract: This paper presents a robust adaptive nonlinear feedback algorithm based on error-driven function for ships course-keeping which are subjected to the unknown time-varying disturbances, uncertain ship model parameters and control saturation. Nonlinear feedback and adaptive techniques are used to design the control law. The error-driven function is designed to avoid the input saturation and adjust the control gain. The designed adaptive law not only achieves the self-regulation of course-keeping control system but also adjusts parameters adaptively. In addition, the Lyapunov direct method is utilized to analyze the stability of course-keeping system. Theoretical analysis indicates that the designed control law can achieve the ship course-keeping while ensuring that all signals are bound. Finally, the effectiveness of the developed control strategy is demonstrated by simulations and comparative results.

Application of Physically Based Semi-Distributed Hec-Hms Model for Flow Simulation in Tributary Catchments of Kaohsiung Area Taiwan

Abstract: Hydrologic modeling is a commonly used tool to understand the rainfallerunoff processes of gauged and ungauged catchments for proper quantitative estimation of water resources availability. In the present study, an attempt is made to simulate surface runoff using a physically based semi-distributed hydrological (HEC-HMS) model for four local-scale tributary catchments of Kaohsiung area Taiwan. The input physical parameters of the model were calculated and preprocessed in HEC-GeoHMS based on digital elevation model (DEM), land use, soil and hydro-meteorological data. The model performance was satisfactory with Nash Sutcliffe Efficiency (NSE) 1⁄4 0.51 to 0.86 and the coefficient of determination (R) 1⁄4 0.63 to 0.86 during calibration (2016e2017) and validation (2018) period based on the selected loss, transform and flow routing, Soil Conservation Service Curve Number (SCSeCN), Soil Conservation Service Unit Hydrograph (SCS-UH) and Muskingum methods, respectively. The comparison of the observed and simulated hydrographs showed that the model is appropriate for hydrological simulations in Kaohsiung area. Therefore, the model was applied using calibrated parameters (CN and Ia) in ungauged Meinong creek catchment. The comparative results between the maximum design flow and the average simulated monsoonal flows verified that HEC-HMS model can synthesize hydrologic processes and phenomena for both wet and dry seasons. It is concluded that the developed methodology can be applied in ungauged catchments for water resources management and planning purposes under future climate scenarios that will help hydrologists to understand the efficiency and application of HEC-HMS model in rainfall-runoff modeling.