Heavy Sea States Research Articles

Course keeping control for very large ship using hyperbolic tangent function based on nonlinear decoration technique

To solve the problems of difficult course keeping, high energy output, and large wear of very large ships under different sea conditions, a robust controller combining the closed-loop gain shaping algorithm and nonlinear decoration is proposed in this paper. Firstly, a linear controller is designed by using the third-order closed-loop gain shaping algorithm and takes the hyperbolic tangent function as the nonlinear decoration of the controller output, and the robustness of the system is proved by H∞ robust control theory. Secondly, taking ‘Vale Brasil’, an ore carrier with a displacement of 400,000 tons, as the test plant. The nonlinear Nomoto model and nonlinear Norrbin model were established, which were compared with the control effect from the existing controller. Thirdly, simulation experiments were carried out under normal sea state and heavy sea state to prove the effectiveness. The results indicate that the proposed control strategy can achieve the required course faster, and enhances the smoothness of rudder angle actuation compared with the existing controller. It can maintain good control ability under different sea conditions. The proposed controller has the advantages of simple parameter adjustment, better robustness, less energy consumption, and reduced rudder angle. It is more in line with engineering practice and increases ship operation benefits.

Experimental and numerical investigation of lashing bridge and container stack dynamics using a scaled model test

Previously reported container losses were generally attributed to extremely violent motions of containerships due to adverse weather conditions. However, most existing specifications or standards adopted for containers and lashing equipment meet the requirement of static conditions. Hence, further researches on safer container shipping under heavy sea states are required. Consequently, an experimental study method is proposed to measure the dynamic response of 1/10 scaled lashing bridge and container stack. The scaled model of the lashing bridge is constructed based on the similarity theory. Based on two dimensionless numbers, Froude's number and Cauchy's number, eleven container scaled models are employed. A series of experiments with controlled parameters are performed using a three-degrees shaking table (roll, pitch, heave) to present sufficient data to verify the effectiveness of the numerical model. The results of experiments, numerical simulations and calculations of the VERISTAR procedure (developed according to the BV rule) are compared. This study aimed to explore the mechanical behavior of the lashing bridge and container stack under predetermined driving excitations (roll and pitch) which simulated heavy sea states. According to the results, the model can predict conditions similar to real situations of the lashing bridge and container stacks while storages on the weather deck.

Course-keeping control for ships with nonlinear feedback and zero-order holder component

Course keeping control is the core of automatic navigation for ships in marine transportation. To optimize the steering conditions of course-keeping controller and enhance its robustness in heavy sea states, a new nonlinear course-keeping control algorithm is designed with power function nonlinear feedback and ZOH model controller. Take the new generation of teaching-training vessel Yupeng from Dalian Maritime University as the research plant, and the nonlinear Nomoto ship model with a zero-order holder component is constructed to design the ZOH model controller. Then the deviation signal from the closed-loop system is modified by a power function nonlinear feedback component to reduce the control output. The stability of the closed-loop system is analyzed by the describing function method and Nyquist criterion. The simulation experiments in normal sea states and heavy sea states are respectively carried out to verify the control performance. The simulation experiments show that the performance of this nonlinear course-keeping control algorithm is satisfactory, which can still be effective in heavy sea states. The steering frequency is reduced significantly, and the steering condition of the controller meets the requirements of navigation practice. The nonlinear course-keeping control algorithm is more suitable for the navigation practice and engineering application.

Rudder Roll Stabilization Based on Arc Tangent Nonlinear Feedback for Ships

The rudder is used for damping the roll motion of a ship. Study of this motion is the essence of this paper. The responses of yaw and roll motions are different as the rudder angle varies. The low frequency motion of the rudder mainly affects the ship’s yaw motion, whereas the high frequency motion of the rudder mainly affects the roll motion. As long as the controller is well designed, the characteristic of the rudder can be used to reduce the roll motion. In order to save energy and reduce steer frequency, a nonlinear feedback controller is proposed based on the arc tangent function processing feedback error to save energy consumption. In addition, a ZOH component is applied in the system to reduce the steer frequency. In heavy sea state, simulation results illustrate that controllers based on pole assignment with and without nonlinear feedback can reduce roll motion by 32.1% and 30.3%, respectively, when the rudder turn rate is limited within 7°/s. Furthermore, the former reduces the amplitude of rudder angle by 23.3% compared with the latter, which means the nonlinear feedback control consumes less energy. Consequently, the ZOH can lower steer frequency to once every 1 s, which protects steering gear from abrasion.

Energy Saving of Course Keeping for Ships Using CGSA and Nonlinear Decoration

In order to further reduce the rudder angle and the steering frequency on the premise of achieving good course-keeping control effect for ships, a concise robust control algorithm is proposed in this paper. The second order closed-loop gain shaping algorithm is employed to design the linear controller first. Then the final control law is achieved by using the nonlinear decoration technique. The stability of the closed-loop system is proved by the Nyquist criterion. Taking training vessel Yukun as a test plant, the simulation experiments under normal sea state and heavy sea state are carried out respectively to verify the effectiveness of the proposed scheme. The results indicate that compared with the existing methods, the proposed control algorithm not only has obvious advantages in energy-saving effect and smoothness, but also has stronger anti-interference ability under heavy sea state. The second-order closed-loop gain shaping controller decorated by bipolar sigmoid function has better robustness and a concise form. Meanwhile its remarkable energy-saving effect and smoothness make steering condition meet the requirements of navigation practice, which is of great significance for ships to realize safe and efficient navigation.

Nonlinear control algorithms for efficiency-improved course keeping of large tankers under heavy sea state conditions

In order to solve the problem of more energy-saving and safer course keeping control for large oil tanker under heavy sea state conditions, a hyperbolic tangent function was used as a new nonlinear feedback function to re-examine the nonlinear feedback in this paper. The new control algorithm was theoretically proved by describing function, and the stability analysis of the system was carried out through Nyquist curve. Taking "YUKUN" and "New Triumph" oil tanker as examples, systematic comparative simulations of hyperbolic tangent function and sine function were carried out. It was found that sine function was better than the hyperbolic tangent function for the course keeping control of general ship under normal sea state conditions. However, for large oil tanker under heavy sea state conditions, the hyperbolic tangent function could achieve almost the same control effect as sine function while the maximum rudder angle decreased by 39% and the average rudder angle decreased by 36%. Therefore, from the numerical investigations, the nonlinear algorithm modified by hyperbolic tangent has obvious advantages in the course keeping control of large oil tanker under heavy sea state conditions.

Course keeping Control Based on Integrated Nonlinear Feedback for a USV with Pod-like Propulsion

In this paper, a response model of an Unmanned Surface Vehicle (USV) with a pod-like propulsion device is established. To improve the robustness of motion control in heavy sea states, an integrated nonlinear feedback course-keeping controller is proposed. First, to establish a response model of a USV with pod-like propulsion, model parameters are obtained by the method of system identification, then an integrated nonlinear feedback control strategy is proposed. The essence of this method is to make the original error signal pass through a nonlinear function, and then the output of this function is used to replace the original error signal. Simulation results show that under ordinary sea states, nonlinear feedback can save up to 34.5% of energy used compared with standard feedback methods; under heavy sea states, this can rise to 40.8%. A set of field experiments were carried out with a USV with pod-like propulsion. Results show that under heavy sea states, the test USV can maintain the target course well, which proves the correctness of the model and the robustness of the proposed method.

Modelling and Control of ASV acting as communication node for deep-sea applications

Abstract: This paper describes the basic characteristics of a six degrees of freedom dynamic model of an innovative ocean-going unmanned surface vehicle. The model is used in an explicit and implicit way to ensure the operation of the autonomously acting vehicle, which serves as communication node between surface and underwater parts of a complex deep-sea monitoring system. In practice, it is a cumbersome task to identify the unknown parameters of such nonlinear models, due to strong couplings of the motion variables, measurement noise and unknown disturbances. In order to parameterize the models, special maneuvers have been carried out to decouple the motions and identify the corresponding parameters. Properties of the acoustic communication has been taken into account when designing the unmanned surface vehicle. Finally, it has been built as a shallow submerged vehicle with water surface-piercing towers to assure a reliable acoustic communication and positioning link up to a depth of 6.000 meters even in heavy sea states. As the vehicle motion has a decisive impact on its operation, the basic characteristics of the motion of the vehicle in waves have been investigated from the quasi-static case using potential theory to simpler dynamic models for the specific degree of freedom. Further, these models are used to design feed forward and feedback controller to ensure the autonomous vehicle operation. The paper concludes with a performance evaluation of the proposed controllers based on data recorded at field trials.

Numerical simulations of a fully submerged propeller subject to ventilation

Numerical simulations aimed at modeling the phenomenon of ventilation on a fully submerged propeller were performed. Ventilation occurs on thruster propellers operating at high loadings and heavy sea states, experiencing continuous cycles in and out-of water. This leads to sudden thrust losses and violent impact loads, which can damage shaft bearings and gears of azimuth and tunnel thrusters. Damages in rough seas were reported also during transit operations. In the simulated configuration the propeller is fully submerged ( h/ R=1.4) and working at high loading ( J=0.1), where the blade becomes surface-piercing only after ventilation occurs. The dynamic loads computed with the numerical model are in satisfactory agreement with the experimental data at the upright position where the blade is piercing the free-surface, whereas thrust is over-estimated at all the other angular positions. A thorough analysis of the causes of this deviation was performed, identifying the inability of the numerical simulation to properly resolve the tip vortex at some distance from the propeller blades as the most likely responsible factor. Unlike ventilation of surface-piercing propellers with super-cavitating profile, it was found that the tip vortex plays an important role in ventilation of conventional propellers, which is the object of the present study.

Identification of ventilation regimes of a marine propeller by means of dynamic-loads analysis

In the present work a relation is established between the mechanisms leading to ventilation and the corresponding dynamic loads a marine propeller's blade is subject to during its rotation. Ventilation occurs on thruster propellers operating at high loadings and heavy sea states, experiencing continuous cycles in and out-of water. This leads to sudden thrust losses and violent impact loads, which can damage shaft bearings and gears of azimuth and tunnel thrusters. Open-water tests were performed in order to better understand the dynamic forces due to ventilation, and ultimately predict the corresponding losses. They were carried out in calm water with varying propeller submergence and loading. Two main ventilation mechanisms, depending on the propeller submergence, loading and advance ratio, were identified: (i) at deeper submergence through a free-surface vortex and (ii) at moderate submergences through the tip-vortex and ultimately the blade itself piercing the free surface. These two mechanisms can exist separately or at the same time, identifying three distinctive ventilation regimes for the dynamic loads. Unlike ventilation of surface-piercing propellers with super-cavitating profile, it was found that the tip vortex plays an important role in ventilation of conventional propellers, which is the object of the present study.

Rogue Waves: Results of the MaxWave Project

Safety of shipping is an ever growing concern. In a summary, Faulkner investigated the causes of shipping casualties (2002, “Shipping Safety: A Matter of Concern,” Ingenia, The Royal Academy of Engineering, Marine Matters, pp. 13–20) and concluded that the numbers of unexplained accidents are far too high in comparison to other means of transport. From various sources, including insurers data over 30% of the casualties are due to bad weather (a fact that ships should be able to cope with) and a further 25% remain completely unexplained. The European project MaxWave aimed at investigating ship and platform accidents due to severe weather conditions using different radars and in situ sensors and at suggesting improved design and new safety measures. Heavy sea states and severe weather conditions have caused the loss of more than 200 large cargo vessels within the 20years between 1981 and 2000 (Table 1 in Faulkner). In many cases, single “rogue waves” of abnormal height as well as groups of extreme waves have been reported by crew members of such ships. The European Project MaxWave deals with both theoretical aspects of extreme waves and new techniques to observe these waves using different remote sensing techniques. The final goal is to improve the understanding of the physical processes responsible for the generation of extreme waves and to identify geophysical conditions in which such waves are most likely to occur. Two-dimensional sea surface elevation fields are derived from marine radar and space borne synthetic aperture radar data. Individual wave parameters such as maximum to significant wave height ratios and wave steepness, are derived from the sea surface topography. Several ship and offshore platform accidents are analyzed and the impact on ship and offshore design is discussed. Tank experiments are performed to test the impact of designed extreme waves on ships and offshore structures. This article gives an overview of the different work packages on observation of rogue waves, explanations, and consequences for design.

Transverse Horizontal Coherence of Explosive Signals in Shallow Water

Transverse horizontal cross-coherence lengths as a function of signal frequency were measured by a hydrophone array 80 m long under homogeneous winter conditions with heavy sea state. The decrease of the normalized coherence length with the acoustic wavenumber is shown as an example of sound propagation parallel to the wave crests and is compared with published reverberation measurements. The cross-coherence length is interpreted in a simple model computation as the reciprocal of a medium-dependent angular uncertainty.

津軽海峡の波浪(日本航海学会第45回講演会)

Many ships encounter heavy severe sea states in the northern Japan, especially near Hokkaido. When the sea state can be predicted, ship operation and maneuvering become safer and sea casualties will be decreased. For this purpose, this paper presents the numerical sea state calculation method on the Strait of Tsugaru, between Honshu and Hokkaido, and discuss the difference between the observed wave heights obtained by the suspended type wavemeter and the calculated values previously obtained. Moreover, the methods to obtain the wind direction and force in the strait are shown by using the track and the pressure of the center of a cyclone.