Journal of Marine Science and Technology最新文献

This study presents a practical method for calculating the probability of exceedance (PoE) of the nonlinear failure function of the linear loads randomly fluctuating in irregular waves. In general, obtaining the exact PoE of such nonlinear quantities requires sophisticated computational method that are not well-suited for practical design. In contrast, the author in a previous study proposed a practical formula of the PoE distribution for von Mises stresses by asymptotic approximation, which can be applied when the criterion surface is ellipsoid in the stochastic variable space. Following this, the present study shows a method for calculating the PoE for a more general function of which the isosurface is expressed as a combination of ellipsoids. As a specific example of its application, this paper takes the limit state of the plates in ship structures specified in the common structural rules (CSR) and presents the calculation procedure of direct numerical integration as well as asymptotic approximation approaches. Calculation of the 1/1000 maximum expected value of the evaluation function of plates in short-term sea states using 700 actual plates in a ship structure confirms that the proposed method, which does not require integration, is in good agreement with rigorous methods using numerical integration.

A ship steering control law is designed for a nonlinear maneuvering model whose rudder manipulation is constrained in both magnitude and rate. In our method, the tracking problem of the target heading angle with input constraints is converted into the tracking problem for a strict-feedback system without any input constraints. To derive this system, hyperbolic tangent ((tanh)) function and auxiliary variables are introduced to deal with the input constraints. Furthermore, using the feature of the derivative of (tanh) function, auxiliary systems are successfully derived in the strict-feedback form. The backstepping method is utilized to construct the control input for the resulting cascade system. The proposed steering control law is verified in numerical experiments, and the result shows that the tracking of the target heading angle is successful using the proposed control law.

Recently, a considerable number of research and development projects have focused on automatic vessels. A highly realistic simulator is needed to validate control algorithms for autonomous vessels. For instance, when considering the automatic berthing/unberthing of a vessel, the effect of wind in such low-speed operations cannot be ignored because of the low rudder performance during slow harbor maneuvers. Therefore, a simulator used to validate an automatic berthing/unberthing control algorithm should be able to reproduce the time histories of wind speed and wind direction realistically. Therefore, in our first report on this topic, to obtain the wind speed distribution, we proposed a simple algorithm to generate the time series and distribution of wind speed only from the mean wind speed. However, for wind direction, the spectral distribution could not be determined based on our literature surveys, and hence, a simple method for estimating the coefficients of the stochastic differential equation (SDE) could not be proposed. In this study, we propose a new methodology for generating the time history of wind direction based on the results of Kuwajima et al.’s work. They proposed a regression equation of the standard deviation of wind direction variation for the mean wind speed. In this study, we assumed that the wind direction distribution can be represented by a linear filter as in our previous paper, and its coefficients are derived from Kuwajima’s proposed equation. Then, as in the previous report, the time series of wind speed and wind direction can be calculated easily by analytically solving the one-dimensional SDE. The joint probability density functions of wind speed and wind direction obtained by computing them independently agree well with the measurement results.

This article presents an automatic docking method suitable for fully actuated surface vessels for the purposes of assisting operators of maritime vessels when docking in time-varying environmental conditions. Docking of ships is a particularly stressful task for human operators, with high demands for both speed and precision, especially under influence from environmental disturbances such as wind, waves and ocean currents. The need for automatic docking systems is increasing as unmanned maritime vessels become more advanced and integrated into global maritime transportation. To address this task, a comprehensive automatic docking algorithm was developed, with path following and velocity control using a modified dynamic positioning control system, which makes the method applicable in existing industrial control systems. In addition, the method includes capability analysis of the docking procedure and evaluates strategies for counteracting disturbances. Specifically, this method utilizes a modified dynamic positioning control system using position sensor data only, to control position, heading and velocity in different stages when docking automatically. The methods are proven in simulations and field experiments.

The evacuation of a modern passenger ship is a challenging task which might be hindered by a complex ship’s internal layout and/or the blocking of escape routes due to fire/flooding. In this work, the application of mobile technology to reduce travel time is investigated. A pilot system has been developed and tested on the RoPax ship GNV Bridge. It is composed of a server and a mobile application running on wearable smartbands. The guidance and localisation of devices have been carried out through Bluetooth beacons. A test area has been identified on GNV Bridge including 2 cabins corridors on deck 6 and the main lounge on deck 5. The corridors and the lounge are connected by three staircases, defining three alternative escape routes starting from cabins and arriving at the muster station in the main lounge. In the trials, the escape routes have been randomly blocked to assess the reduction of travel time achieved providing guidance through wearable devices to a sample population. It resulted in a 16.9% reduction in travel time. Besides, a strategy to simulate with a certified tool the effect of a guiding system has been defined. This is essential to make trials’ results transferable in different environments (e.g., other RoPax or cruise ships). In particular, experimental data coming from the trials have been used to assess agents’ speed reduction rate due to mobile device consultation. Although available experimental data were limited by the pandemic, the 2.5% agent’s speed reduction applicable to simulations has been assessed as most probable.

In this work, consideration is given to the kinetic energy recovery of a buoyancy engine and with particular reference to gliders. Utilizing a simplified physical model, an expression for the efficiency of kinetic energy recovery in terms of reduction of the required energy input from batteries was derived.

Ship response in short-crested irregular waves is calculated as the product of the frequency response function of ship response in regular waves and the directional spectrum of waves. However, the commonly used IACS and JONSWAP spectra may not accurately reproduce detailed spectra by wave hindcasting data and wave radar measurements, leading to over- or underestimation of ship responses. This study focuses on the reproducibility of a detailed directional spectrum by standard directional spectrum based on physical considerations concerning the mean spreading angle. This approach facilitates discussion on the limitations of reproducing a detailed directional spectrum with a standard directional spectrum. Furthermore, a procedure is proposed for the use of a standard directional spectrum based on the mean spreading angle as an indicator. Analysis using 6 example ships demonstrates that employing the procedure enables the extraction of highly reproducible and valuable data for short-term prediction of the added resistance in short-crested irregular waves.

For the ship stability estimation and ship design, it is helpful to know the probability of the roll amplitude exceeding a certain threshold. Therefore, it is necessary to obtain the probability density function of the roll amplitude. In this study, first, we derive the moment values of roll amplitude by combining the moment equations and the linearity of expectation. With this, we propose a method to estimate the probability density function of the roll amplitude by using the obtained moment values. The results of our proposed method are compared to those obtained from Monte Carlo simulations. When the higher-order cumulant neglect closure method is used, the moment values resulting from the moment equations are close to the results of corresponding Monte Carlo simulations. In addition, our proposed method for deriving the probability density function of the roll amplitude is validated by comparison with Monte Carlo simulation results. In conclusion, we can state that the proposed methods for deriving the moments and the probability density function of the roll amplitude are available for practical use cases.

To enhance the accuracy of hydrodynamic forces predictions for underwater vehicle within certain resource constraints, this study integrates high and low-fidelity samples using the Co-Kriging method, combined with an expected improvement (EI) sequential infill criterion to construct a hydrodynamic forces prediction model. Different grid densities of computational fluid dynamics (CFD) calculations are used to distinguish high and low-fidelity samples. Taking the Joubert BB2 underwater vehicle as the research object, hydrodynamic forces predictions are conducted for various angles of attack and speeds. The effectiveness of the EI infill criterion in improving model prediction accuracy is validated. Furthermore, compared to the traditional Kriging model under the same computational resources, the Co-Kriging method, which integrates high and low-fidelity samples, significantly outperforms the Kriging model constructed solely from high or low-fidelity samples in overall prediction accuracy for underwater vehicle hydrodynamic forces.

This article addresses the problem of speed and steering control of unmanned sea surface vehicles operating under unknown ocean environments affected by complex nonlinearities and uncertain hydrodynamic coefficients. An inertial delay control (IDC)-based sliding mode control (SMC) is proposed. The proposed controller is robust against the system’s nonlinearities, parametric uncertainties, external disturbances like a strong wind, complex disturbances due to wind-generated and ocean currents, etc. The proposed controller uses IDC to estimate these effects mentioned above, which makes the proposed SMC independent of the bound of uncertainties and disturbances, and provides chatter-free control. The effectiveness of the proposed controller is confirmed by considering the highly nonlinear model of Cypership-II using various performance indices.