Mitigation of Ship Motion Using Passive and Active Anti-Roll Tanks

Roll motion is an undesirable feature of the behavior of a ship in rough seas, and so it is natural to consider ways of reducing it. The most common devices for increasing roll damping are bilge keels. However, the effectiveness of keels is limited, and anti-roll tanks and fins are used when more control is required. Moreover, unlike keels, anti-roll tanks can be used when the ship is not underway. Our objective is to develop design procedures for passive tanks for roll reduction in rough seas. To this end, we develop an improved model of the passive tank-liquid motion in this paper. This tank consists of U-shaped tubes placed side by side along the length of the ship. The equations of six-degrees-of-motion (6DOF) that govern the tank-liquid are coupled with those that govern the 6DOF motion of the ship, and all of the equations are integrated simultaneously in the time domain using the Large Amplitude Motion Program (“LAMP”). LAMP is a three-dimensional time-domain simulation of the motion of ships in waves. The unstabilized and stabilized roll motions of a S60-70 ship with forward speed and beam waves have been analyzed. For high-amplitude waves, the variation of the roll angle with the encounter-wave frequency exhibits typical nonlinear phenomena: a shift in the resonance frequency, multi-valued responses, and jumps. The performance of passive tanks on a S60-70 ship with forward speed is investigated in an irregular sea with different encounter-wave directions. It is found that passive anti-roll tanks tuned in the nonlinear range are very effective in reducing the roll motion. The effect of the tank mass and distribution of tank tubes on the performance of the tank system is studied. Also, it is found that passive anti-roll tanks are very effective in reducing the roll motion in sea state five of a ship whose pitch frequency is nearly twice its roll frequency.

Design of passive anti-roll tanks for roll stabilization in the nonlinear range

Wave propulsion and sea-keeping enhancement for ships in rough sea condition by flapping foils

Performing oceanographic measurements from the sea surface to the sea bottom is technically challenging under rough or icy sea conditions. As a measure to deal with rough seas, a new dynamic simulation program was developed and its validity was verified by comparison with the results of actual sea tests. This program can therefore be used as a design tool. To develop countermeasures against icing, we carried out basic tests in a large-scale snow and ice laboratory. Some of these countermeasures were then implemented in real sea tests and were found to be effective under icy conditions. These results indicate that deployment of moored surface buoys in regions of rough and icy sea conditions will soon be practical.

Comparative study of realistic ship motion simulation for optimal ship routing of a bulk carrier in rough seas

To ensure the safe navigation of ships in rough seas while reducing steering gear energy consumption and losses, a steering control system with small rudder output angles, low steering frequency, and high control performance was designed. A third-order closed-loop gain-shaping algorithm was employed in the development of the controller, with the ultimate control strategy derived by embedding a nonlinear compound function between the proportional derivative (PD) controller and the second-order oscillation link to enhance control effectiveness. A nonlinear Nomoto model of the “Yupeng” ship was employed for simulation validation. The simulation results illustrated a 14.5% improvement in overall control performance achieved by the proposed controller compared to a nonlinear feedback controller. The controller’s robustness was additionally validated through the application of the Norrbin ship model. The proposed controller enhances the stability of ships in rough seas, effectively limiting the maximum rudder angle during turns and reducing the average rudder angle and steering frequency during navigation. This design aligns with practical requirements for maritime operations in heavy weather, contributing significantly to the economic, safe, and efficient navigation of ships.

This paper analyzes the rough seas weather which may be encountered during the voyage and its influences on ship’s navigation safety. This paper draws lessons from related both domestic and abroad researches about ship’s navigation safety, according to the accident causes, analyzes the factors of ship’s navigation safety affected by rough seas, determines assessment index system of ship’s navigation safety in rough seas, uses Bayesian Network to realize the assessment, and eventually works out which index contributes the most to the marine accidents of ship in rough seas.

Simulation of marine operations for launch and recovery of bluff bodies such as autonomous underwater vehicles (AUV), remotely operated vehicles (ROV) or subsea templates is traditionally performed in calm to moderate sea conditions. The reason for doing so is partly due to the interaction between the complex dynamic response of an installation vessel, a moving bluff body and the wave kinematics of the rough sea condition. This is in addition to the need for accurate hydrodynamic coefficients that would enable proper simulation and modelling of the launch and recovery process. The key objective of the current methodology is to minimize risks of damage to the vessel and total loss of assets during the deployment and recovery process for marine operations in rough sea conditions. The aim of this paper is to present the results of experimental and numerical investigation on the prediction of dynamic response of a bluff body during launch and recovery from a surface vessel in rough sea condition. Experimental measurements of hydrodynamic coefficients and responses of a large scale bluff body using a scaled model were completed. Further studies using a time-domain numerical tool have been undertaken to measure the response characteristic of bluff bodies in rough seas. The study also predicted the contributions of vessel motion in rough seas to the dynamic response of the bluff bodies. The results obtained have shown that simulation of launch and recovery operations in rough seas can be carried out efficiently if their hydrodynamic coefficients through the wave active regions of the rough seas are predicted and then adequately implemented in the simulations.

Control of ship roll using passive and active anti-roll tanks

For some ships in special use or the warships as sea platforms, the out range rolling will produce bad effects on the operation of the ships or the equipments on the warships. The current researches show that the synthetic stabilization scheme combined several stabilization sets is the main trends in the future. Considering the relationship between fin stabilizer and passive ling tank and the stabilization effectiveness in all kinds of sea condition, a configuration scheme of fin stabilizer and passive anti-rolling tank is proposed. An index of function-effectiveness is defined according to the economic factors. The simulation shows that the scheme employing low-capacity fin stabilizer to match anti-rolling tank is more rational.

It has recently been recognized that structural failures of large ocean-going vessels are essentially related to the dynamic stresses induced by bottom and bow-flare slamming in rough seas. Bottom slamming may cause an excessive vertical bending moment in a hull girder which may lead buckling of the upper deck, while bow-flare slamming may cause an excessive torsional moment in the fore-body of high speed ships in oblique waves. Local dents and cracking failures due to slamming are also observed in the fore body of large ships, they might be accelerated by severe corrosion.In high speed ocean-going ships, cracking failures are sometimes observed in middle and aft parts of the hull girders after passing through a rough sea. Since this type of failure is not necessarily exceptional to newly build ships, it may be caused by low cylce fatigue due to repeated whipping stresses induced by bow-flare slams. In the present paper, fatigue strength is investigated for structural members with high stress concentration factors in the neighbourhood of the front end of the superstructure of a container ship, where the applied stress is composed of a wave induced bending stress including non-linear effects, whipping components of stresses, and possibly coupled vibratory stresses of the superstructure.Having calculated the dynamic stress components among regular waves in head seas, the fatigue strength of highly stressed portions are examined by using appropriate S-N curves, in which effects of ship speeds and wave heights are taken into considerations. When a high speed vessel passes through very rough seas, the fatigue strength may be reduced due to the significant elevation of the repeated stress range caused by the whipping components of stresses superposed on the wave induced bending stress.

Z-shaped navigation for surface ships in rough seas based on constraint MPC

This paper presents a practical method of predicting the longitudinal stresses of ships in rough seas by means of Neumann-Pierson's wave spectrum theory and St. Denis-Pierson's theory. Given the principal dimensions of ships, the wind velocities, the durations and the fetches, we can statistically find the logitudinal stresses. In an experiment the predicted stress was in good agreement with the observation.

This paper presents a computational model which describes motion of an object of six degrees of freedom (DoF), intended for simulation of spatial motion of one- or two- rope-sling lifeboat or rescue boat during its launching from ship in rough sea. This is a complex model which accounts for sea conditions as well as elasticity and damping properties of davit’s elements and mechanisms, rope and boat hull. Also, are presented results of example calculations for an assumed set of technical parameters of davit and boat as well as sea conditions.

Model experiments were carried out for a container ship in long crested irregular waves to investigate the non-linear effect on ship responses such as ship motions, wave induced pressure and wave load varying with wave height. The wave condition was changed from moderate to rough sea state in head waves. Numerical computations, by means of the strip method, the slender ship theory and the non-linear time domain strip method, were also carried out. The calculation results were compared with the experimental data to confirm the accuracy and to check on the range of the applicability.As a result, non-linear effect on vertical bending moment and wave induced pressure were remarkably observed in large waves : with increase of wave height, the bending moment acting on fore part is accelerated due to the influence of slamming and the induced pressure at the position near water surface is repressed due to the influence of exposure to the air. A time domain strip method can predict well the significant/maximum value of non-linear vertical bending moment in large waves. Toki's correction method is useful for improving the calculation accuracy of the induced pressure in regular and irregular waves.