Free-running Model Experiments Research Articles

A comprehensive propulsion system analysis: an integrated approach utilising engine simulation and free-running model experiments

Regulations concerning the environmental impact and energy efficiency of ships are becoming increasingly stringent. To meet these demanding criteria, new technologies are actively being developed to achieve emission reductions while maintaining efficient fuel energy conversion into propulsion power. However, the propulsion system behaviour in a dynamic environment of the actual sea is highly nonlinear and complex, and a profound understanding of the mutual interactions is still lacking. This paper proposes an integrated approach for comprehensive propulsion system analysis. The key feature of this technique is the utilisation of an intelligent propeller drive controlled by a marine diesel engine simulator (MDES) to drive the free-running model of a ship, providing real-like behaviour of the engine model in real-time in response to actual measured values. The core of the MDES consists of a continuous cycle-mean value dynamic engine model supplemented with a detailed combustion cycle evaluation. The developed fast calculation algorithm of crank-angle resolved thermodynamic equations of the gas state in the engine cylinder enables the complete engine model to be integrated into the real-time environment of a physical model ship. Thus, the propulsion system's performance can be analysed under conditions that closely mimic those of the actual sea environment.

Experimental study of the water on deck effects on the transverse stability of a fishing vessel running in stern quartering seas

The trapped water on the deck affects the transverse stability of purse seiners running in stern quartering seas. In this paper, we conducted free-running model experiments in regular and short-crested irregular following and quartering waves to clarify the state of trapped water on the deck and its effect on transverse stability. In the model experiments, pressure sensors, and cameras are installed on the weather deck of the ship model to measure the state of trapped water on the deck. The influence of trapped water on the deck is quantitatively evaluated by conducting a phase difference analysis of the measured trapped water on the deck motion and the time series of the ship model's rolling motion. The results confirm that trapped water on the deck acts as a hydrostatic heeling moment in the range around the design speed.

Probabilistic estimation of the large heel due to broaching associated with surf-riding for a ship in short-crested irregular waves and its experimental validation

The International Maritime Organization started using a numerical simulation to quantify the probability of a ship's stability failure as a direct stability assessment for broaching associated with surf-riding. Some of the authors proposed a critical wave method to estimate such probability, and its numerical verification and experimental validation in long-crested irregular stern-quartering waves were already published. The remaining issue is numerically verifying and experimentally validating the methodology applied to short-crested irregular stern-quartering waves. The work described in this paper is an attempt to resolve this remaining issue by executing numerical simulation and a free-running model experiment in both short-crested and long-crested irregular waves. The subject ship used in this work is an ocean research ship similar to the one lost in actual stern-quartering waves with higher speed, which was used in our previous study in regular waves. The experimental results indicate that the direct counting in the numerical simulation shows stochastically sufficient agreement with the model experiment, and the critical wave method provides reasonably conservative estimates even in short-crested irregular waves.

Ship-stopping algorithm utilizing VecTwin rudder system for automatic collision prevention

In this study, an algorithm for automatic ship stopping in an emergency is developed utilizing a VecTwin rudder system. Timing the emergency stop and determining the stopping capability of the ship equipped with the VecTwin rudder system are critical elements of the proposed stopping algorithm. The dangerous area of collision (DAC) between the ship and other ships is calculated to determine ship emergency stop. The algorithm is constructed such that the ship stops automatically after DAC, which is calculated based on the navigation situation between the own ship and the surrounding other ships, enters the emergency stopping region (ESR). The stopping capability of the subject ship equipped with the VecTwin rudder system was confirmed by a free-running model experiment with ship speeds determined by analyzing actual operational data. To validate the developed algorithm, two collision risk scenarios were selected—coastal navigation and in-port navigation—and simulations and free-running model tests were conducted for each scenario. It was demonstrated that the emergency stop algorithm developed for ships equipped with the VecTwin rudder system can reliably avoid collisions automatically. In addition, it was also confirmed that a shorter stopping distance was achieved than in the case of crash astern emergency stopping.

Manoeuvrability of a Large Cruise Ship after Damage for Safe Return to Port

Free-running model tests were conducted using a scaled model of a large cruise ship with a damaged compartment, to investigate the effects of damage opening and floodwater on the manoeuvring performance in calm water and regular and irregular head waves. Drifting tests in regular beam waves were also performed. The experimental results indicated that the course-keeping ability in waves and turning ability became worse in the damaged condition. However, the target ship retained its manoeuvrability for safe return to the port, on its own, even in a damaged state. As it is time- and cost-consuming to conduct a free-running model experiment, a captive model test was also carried out to develop a system-based simulation model for evaluating the manoeuvrability of large cruise ships after damage.

Investigation on Maneuverability of a POD Propulsion Ship by Free-Running Model Experiments

Investigation on Maneuverability of a POD Propulsion Ship by Free-Running Model Experiments

Water-on-deck effects on roll motions of an offshore supply vessel in regular stern quartering waves

The applicability of the second generation intact stability criteria for preventing pure loss of stability in astern waves to offshore supply vessels (OSV) is investigated. OSVs show an inconsistency problem in the first and second level criteria. Thus, free-running model experiments of an OSV by changing its length of low weather deck in regular stern quartering waves were conducted and were compared with numerical simulation used for pure loss of stability so far. As a result, it was confirmed that the effects of water on deck (WOD) on the behaviour of the OSV could prevent the occurrence of typical pure loss of stability for the cases of long low weather deck. One of WOD's effects is the increase of roll damping and the other is the loss of static stability.

Effect of above-waterline hull shape on broaching-induced roll in irregular stern-quartering waves

In this paper, the effect of the above-waterline hull shape on broaching danger in irregular stern-quartering waves was numerically investigated using the US Office of Naval Research (ONR)-tumblehome and ONR-flare hulls. To indicate the danger of broaching, the probability of a broaching-induced large roll angle of the two vessels was examined along with the probability of broaching. The numerically estimated broaching-induced roll angles were compared with the time histories of the free-running model experiments. Then, the effects of above-waterline hull shape on broaching danger in the North Atlantic were simulated for various speeds, autopilot courses and rudder gains.

Design and Experimental Testing of a Free-Running Ship Motion Control Platform

In recent years, ship motion control has been used in unmanned merchant ships. It is a key issue in the sailing performance and navigation cost of unmanned vessels. The merchant ship trajectory and attitude are always generated from the response mechanism for ship motion, which uses one rudder and one propeller as the actuator. Therefore, it is a typical underactuated ship. However, there is not a specialized control test platform for unmanned merchant ships, in particular container ships. A precise merchant ship motion control platform is crucial; its computing power and motion attitude monitoring are difficult to solve by ship-borne computers. To solve this problem, a new free-running ship motion control platform is proposed, which realizes ship attitude monitoring and various motion control experiments. The proposed platform is novel as it embeds ship motion parameters and control parameters in the ship motion model, which successfully solves the precise control of the ship motion control experiment with linear or nonlinear dynamics. In this paper, the design scheme of the ship motion control platform is first presented. Then, the operating principle of the platform is shown, and the adaptive model of the ship plane motion is selected. A series of free-running model experiments and simulations is conducted to test performance of the new platform. To this end, the proposed platform can meet the requirements.

Estimation of broaching probability using wave-induced forces and moments measured in captive model tests

Broaching is a stability failure that involves many force components. Wave-induced forces, in particular, play an important role in the estimation of broaching, and it is necessary for these forces to be estimated accurately. Captive model experiments of wave forces in regular waves were, therefore, conducted for tumblehome and flare vessels (the vessels were based on ones developed by the US Office of Naval Research). Empirical correction factors of the wave forces for different wavelength-to-ship length ratios and heading angles were obtained; the difference between theoretical calculations and experimental values obtained in the present were used to develop these factors. These empirical correction factors were then added into a theoretical study and the results from this study were compared with that of a free-running model experiment not only in regular waves but also in irregular waves for both the tumblehome and flare vessels. The results indicate that if the empirical corrections of the wave forces depending on the wavelength are used, then the broaching probability in irregular waves can be well estimated for these two types of vessels.

Averaging methods for estimating parametric roll in longitudinal and oblique waves

The International Maritime Organization is still developing second generation intact stability criteria for the roll motion of ships. For level 2 vulnerability criteria for parametric roll that require no expert knowledge about nonlinear dynamics, various analytical approaches are proposed in this study based on averaging methods. Firstly, an averaging method is applied to an uncoupled roll equation without fitting a polynomial to the calm-water GZ curve, thereby taking into account more accurate effect of the restoring arm. Secondly, the parametric roll amplitude is estimated more accurately using an averaging method that takes into account the effect of superharmonic roll motion. Thirdly, the critical speed for parametric roll is estimated, which is expected to reduce calculation processes in the design stage. Fourthly, the averaging method is extended to parametric roll in oblique waves, thereby taking into account the effect of both the ship’s speed and course, which are helpful for an operational decision. Finally, the proposed analytical approaches are verified against an existing averaging method, numerical simulations, and free-running model experiments.

Model experiments on the early detection and rudder stabilization of KCS parametric roll in head waves

Free running model experiments on the early detection and rudder stabilization of parametric roll in regular and irregular head waves are conducted in the towing tank of Yokohama National University. The KCS (KRISO Container Ship) containership model is used in the experiment. Parametric roll motions are observed in various ship speeds and wave lengths. The parametric roll early detection algorithm based on incremental real-time Hilbert–Huang transform (IR-HHT) technique is validated by the model experiments. Results show that the occurrence of parametric roll under different cases can be successfully detected through the detection algorithm when the roll amplitudes are still small. The rudder anti-roll control is applied for the stabilization of parametric roll in regular and irregular head waves. The rudder anti-rolling is activated when parametric roll is detected by the detection algorithm. It is found that the rudder anti-rolling can successfully stabilize parametric roll in regular and irregular head waves especially when the ship speed is high. However, the effectiveness of rudder anti-rolling is limited in severe parametric roll cases with roll amplitude larger than 25°.

A Study on the Relationship between Ship Stability and Maneuverability Using Free Running Model Experiments

A Study on the Relationship between Ship Stability and Maneuverability Using Free Running Model Experiments

Experimental and numerical study on several crucial elements for predicting parametric roll in regular head seas

The methods for direct stability assessment of parametric roll are currently under development at the International Maritime Organization for the second generation intact stability criteria. To establish guidelines for developing accurate, but sufficiently simple prediction method, several elements required for modeling parametric roll in head waves are investigated by executing free running model experiments, partially restrained model experiments with and without surge motion and numerical simulations of uncoupled roll model, coupled surge–roll model and coupled heave–pitch–roll model for a C11 class containership. As a result, the following outcomes are obtained. First, the surge effect on parametric roll is generally small. Second, heave and pitch motions can have subharmonic components when parametric roll occurs. Third, the inclusion of the radiation and diffraction effects on restoring variation can result in more conservative prediction for parametric roll in head seas. Finally, the use of the uncoupled roll model could provide more conservative, but less time-consuming prediction for parametric roll than that of coupled heave–pitch–roll model.

Numerical prediction of parametric roll in oblique waves

Numerical prediction of parametric roll in head and following waves has been intensively investigated so that the requirements for a reasonably good prediction are almost entirely known. However, prediction of parametric roll in oblique waves has not yet been sufficiently investigated owing to the unavoidable coupling with sway and yaw motions. Since parametric roll for actual ships occurs with almost zero forward velocity, accurate prediction of the sideways drift of the ship in waves is difficult. Therefore, in this study, the authors present a numerical model of parametric roll in oblique waves by considering low-speed manoeuvring forces. The numerical predictions are compared with newly performed free-running model experiments of a hypothetical ship, in which the maximum roll amplitude was about 20°. The obtained results show that the present model captures some qualitative tendencies in the experiment but the estimated amplitudes are larger than the experiment by about 7°. This information can be used to identify the minimum requirements for good prediction of parametric roll in oblique waves.

Broaching probability for a ship in irregular stern-quartering waves: theoretical prediction and experimental validation

To avoid stability failure due to the broaching associated with surf riding, the International Maritime Organization (IMO) has begun to develop multilayered intact stability criteria. A theoretical model using deterministic ship dynamics and stochastic wave theory is a candidate for the highest layer of this scheme. To complete the project, experimental validation of the theoretical method for estimating broaching probability in irregular waves is indispensable. We therefore conducted free-running model experiments using a typical twin-propeller and twin-rudder ship in irregular waves. A simulation model of coupled surge–sway–yaw–roll motion was simultaneously refined. The broaching probability calculated by the theoretical method was within the 95 % confidence interval of that obtained from the experimental data. This could be an example of experimental validation of the theoretical method for estimating the broaching probability when a ship meets a wave.

Prediction methods for the surf-riding threshold and the wave-blocking threshold based on Melnikov’s method

A ship operating in following and/or quartering seas may be susceptible to broaching-to preceded by the surf-riding phenomenon. Therefore, for the safety assessment of fast vessels such as destroyers and patrol craft, the estimation of the surf-riding condition is important. As shown by previous research, there are two boundaries for ship motion in following and quartering seas. These are the surf-riding threshold and wave-blocking threshold. In this study, the theoretical methods to estimate both boundaries are obtained by making use of Melnikov’s method. In order to validate the formulae, free-running model experiments are conducted in the towing tank. Comparisons between the results obtained from calculations and experiments show good agreement. It is concluded that the formulae based on Melnikov’s method could be applicable to the safety assessment of surface ships.

Broaching prediction of a wave-piercing tumblehome vessel with twin screws and twin rudders

The new intact stability criteria which are under development at the International Maritime Organization (IMO) are required to cover a broaching phenomenon, well known as a great threat to high-speed vessels which can lead to capsizing. Some reports exist which demonstrate that their numerical models can predict a highly nonlinear phenomenon of broaching. However, additional validation studies are needed for unconventional vessels, in addition to conventional ones, to develop direct stability assessment methods for the new intact stability criteria. In this research, we selected as the subject ship a wave-piercing tumblehome vessel with twin screws and twin rudders, a design expected to be one of a new generation of high-speed monohull ships. Firstly, a series of captive model tests were conducted to measure the resistance, the manoeuvring forces, the wave-exciting forces, the heel-induced hydrodynamic forces, and the roll restoring variation for the unconventional tumblehome vessel. Secondly, the existing mathematical model which had been developed for broaching prediction of conventional vessels with a single propeller and a single rudder was extended to unconventional vessels with twin propellers and twin rudders. Finally, comparisons between numerical simulations and the existing free running model experiments were conducted. As a result, it was demonstrated that fair quantitative prediction of broaching is realised when the rudder force variation, the roll restoring variation and the heel-induced hydrodynamic force for large heel angles are taken into account.

An Improvement of Broaching Prediction with a Nonlinear 6 Degrees of Freedom Model

This paper reports on the improvements of a simulation model for ship motions in following and quartering waves. The authors use a wave-piercing tumblehome vessel as a subject ship. The existing 4-DoF (degrees of freedom) model shows some limitation in its capability for predicting broaching of this vessel. In a free-running model experiment for the vessel, we observed rudder and propeller emersion from the water leading to broaching. This rudder and propeller emersions can significantly reduce steering performance. To express these rudder and propeller emersions, we should take account of not only roll but also pitch and heave.Therefore the authors develop a 6-DoF model. With the heave and pitch motion, we can calculate the local draft variation for calculating wave exiting forces and moments. Roll restoring variations are one of the important factors to estimate roll motions.Therefore we theoretically estimate restoring variations in wave then we found the tendency of variation dramatically changes due to the roll angle. Submergence of superstructure can be important for that variation. We also consider the variation of maneuvering coefficients and rudder forces in waves which are relevant to broaching. To estimate these variations, we conduct PMM tests, straight-line tests and rudder angle tests in waves using a scale model of subject ship. With some of these improvements, we obtain better agreements with experimental results than existing 4-DoF model.

Mathematical model of single-propeller twin-rudder ship

A mathematical model of a single-propeller twin-rudder ship has been developed from captive and free running model experiments. An open water rudder experiment was carried out to figure out the characteristics of the rudder. Captive experiments in a towing tank were carried out to figure out the performance of a single-propeller twin-rudder system on a large vessel. Interactions between the hull, propeller and twin rudders, including mutual interactions between the twin rudders, were expressed with several coefficients that were calculated from the experimental results at various ship speeds. In the analysis, the unique characteristics of a single-propeller twin-rudder ship, which affects rudder forces, were explained and formulated in the mathematical model. The captive model tests were conducted with zero ship’s yaw rate, so the interaction coefficients, which are influenced by the yaw rate, are determined from free running model experiments. Validation of the mathematical model of a single-propeller twin-rudder system for a blunt body ship is carried out with an independent set of free running experiments, which were not used for determining the interaction coefficients. The validated numerical model is used for carrying out simulations. Based on simulation results, some recommendations have been proposed for installing a single-propeller twin-rudder system.