Roll Natural Frequency Research Articles

Research on parametric roll of the polar environmental transport ship based on time-domain Rankine panel method

In this study, the roll damping of a polar environmental transport ship (PETS) is estimated using viscous CFD method, and the parametric roll is simulated based on time-domain Rankine panel method. This method's reliability is validated by contrasting the simulation results with the experimental data of C11 container ship. The roll damping of C11 container ship is estimated using CFD method and improved Ikeda's method, respectively. Then, the time-domain Rankine panel method is adopted to simulate the roll motions of C11 container ship, and the simulation results are contrasted with experimental data. It confirms that the roll damping is calculated by CFD method and the roll motions of ships are simulated based on time-domain Rankine panel method has good applicability. Finally, the influence of wave steepness, wavelength, and the ratio of encounter frequency ωe to natural roll frequency ωroll on the parametric roll of PETS are studied. It is found that the maximum roll angle of PETS increases along with wave steepness. As the wavelength increases, the possibility of parametric roll for PETS is also increasing. When the parametric roll of PETS occurs, the ωe/ωroll tends to increase.

Estimation of ship roll damping and natural frequency using an extended Kalman filter applied to URANS output

Estimation of ship roll damping and natural frequency using an extended Kalman filter applied to URANS output

Experimental study on the motion reduction performance of a small suspension catamaran exploiting an active skyhook control strategy

A nine degrees of freedom suspension catamaran has been developed. The ship is so designed that the heave, pitch, and roll motions of the cabin are separable from those of the twin-hull. One brushed DC motor/generator (M/G) is employed at each of the four suspending locations to produce control force. An active skyhook control strategy utilizing proportional-only control is proposed to calculate the desired control input to reduce the cabin’s local vertical velocity and thus improve ride comfort. The mechanical responses of the suspension unit and the performance of the control system are verified through a group of bench tests. The ship motion responses in regular waves are investigated through a series of towing tank experiments. It is found that the proposed control strategy had significant effectiveness on motion reduction of heave and pitch of the cabin in head waves, but a small contribution to the roll motion reduction in beam waves except in the vicinity of the cabin’s natural roll frequency. In head waves, the average motion reduction ratios of the heave and pitch of the cabin both exceeded 50% at zero forward speed, which at 1.5m/s were 39% and 71%, respectively. In beam waves, the average motion reduction ratios of the heave and roll of the cabin were 62% and −57%, respectively.

Evaluation of onboard stability assessment techniques under real operational conditions

Stability-related accidents and crew lack of training are among the main causes of fatalities in the fishing fleet. Onboard stability guidance systems have been proposed by many authors as a possible solution. These guidance systems have been evolving from colour-coded posters to computer-based systems. However, their real-time operation without crew interaction is not solved yet.This work presents the validation of a methodology proposed by the authors in previous works for estimating the natural roll frequency of the vessel in real-time and thus the metacentric height. The performance of this method, based on the analysis of the roll motion, is tested during a fishing campaign of a mid-sized trawler, showing very promising results.

Lifting-induced stability loss of vessels in shallow water

By incorporating several series of results from both experimental scale-model tests and URANS-based simulations, the transverse stability of a model (1:20) under lifting condition in shallow water is investigated and quantified. According to the results, a non-dimensionalised mass of 0.66 (% Δ) on top of the lifting boom reduces GZ lever up to around 40% in calm water. In waves, the lifted object increases roll amplitude about 7–56% near roll natural frequency, and the smaller the waves, the larger the influence. Shallow water starts to have effects on roll, pitch, heave, and sway motions from a water depth/draught ratio of about 7 as opposed to 4 suggested by ITTC (2005). Maximum variations of those motion amplitudes are about 77%, 110%, 25%, and 500%, respectively. Double-peak-roll phenomenon appears to be both wave-frequency-dependant and water-depth-related, but at a depth/wavelength ratio of about 0.45 and above, motions are not affected.

Numerical study on the effect of sloshing on ship parametric roll

In this study, numerical simulations of the parametric roll of a ship coupled with liquid sloshing were performed. A model-scale ONR Tumblehome was used for the simulation, and the ship was equipped with a partially filled liquid tank. An in-house unsteady Reynolds-averaged Navier–Stokes solver was applied to the numerical simulation of the coupling effect between the sloshing and parametric roll of the ship in regular head waves. First, a validation study of the computational fluid dynamics (CFD) solver was conducted. Comparisons between the CFD and experimental results showed good agreement. Systematic grid and time-step convergence studies were performed to estimate the numerical uncertainty. The effect of liquid sloshing on the parametric roll of a ship was then investigated numerically with different liquid levels and different ship forward speeds. The results demonstrated that water sloshing could significantly decrease the natural roll frequency of the ship model, which led to a lower speed range where the parametric roll occurred compared with the model without sloshing. The phase difference between the total moment and sloshing-induced moment was close to 180°, resulting in a reduction of the parametric roll amplitude. Sloshing had slight effects on the amplitudes of the heave and pitch motions.

Improving the performance of a stability monitoring system by adding wave encounter frequency estimation

Onboard guidance systems have emerged to improve the safety of fishing vessels providing the skipper with simplified stability information. In the last years, the authors have developed a stability monitoring system that automatically estimates the natural roll frequency of the vessel. Although the performance was acceptable, there were some specific situations where the influence of external excitations reduced the accuracy of the stability estimations. In this work, a methodology to automatically estimate the wave encounter frequency from ship motions is proposed. Then, this methodology is included in the stability monitoring system. Towing tank tests of a mid-sized stern trawler in different conditions have been used to analyse the improvements obtained with this approach.

ALTERNATIVE ASSESSMENT OF WEATHER CRITERION FOR SHIPS WITH LARGE BREADTH AND DRAUGHT RATIOS BY A MODEL EXPERIMENT: A CASE STUDY ON AN INDONESIAN RO-RO FERRY

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient correponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

IDENTIFICATION OF INFLUENTIAL PARAMETERS IN A SHIP’S MOTION RESPONSES: A ROUTE TO MONITORING DYNAMIC STABILITY

Six degrees of freedom motion response tests of a Ro-Ro model have been carried out in irregular waves under intact conditions. A stationary model was tested in different sea states for following, astern quartering and beam seas. The investigation was limited to the effect of encountered frequency components and associated magnitude of energy of the ship’s motion responses. Analysis of heave, pitch and roll motions confirmed the vulnerability of the model to certain frequency ranges resulting in an adverse effect on the responses, and these were closely related to its natural frequencies. It was confirmed that the roll motion maintains its highest oscillation around the natural frequency in all sea conditions regardless of heading angles. However spectral analysis of the heave and pitch responses revealed the wave peak frequency. Roll is magnified when the peak frequency of wave approaches the natural roll frequency; therefore keeping them apart avoids a large motion response. It was concluded that peak frequency and associated magnitude are two important inherent characteristics of motion responses. Detection of influential parameters of encountered wave through heave and pitch responses could be utilised to limit a large ship’s motion at sea.

Controlling the parametric roll of a container ship model by changing the forward velocity

Parametric roll on ships is a resonance phenomenon whose onset causes heavy roll oscillations leading to dangerous situations for the ship, the cargo and the crew. It affects both small vessels with marginal stability and large container ships if four conditions are met simultaneously: the wave length and ship length are approximately equal, the frequency of encounter wave is twice the roll natural frequency, the ship’s roll damping is low enough and the wave height exceeds a limit value characteristic to each vessel. A good start to capture the basic features of the parametric roll is to cancel the couplings between roll and the other motions of the ship and to analyse the influence of parameters on the ship stability and the appearance of large roll amplitudes by using one-degree-of-freedom model. By doing this, one realizes that, besides increasing the damping, the roll motion can be stabilized by changing the wave encounter frequency in such a way that the resonance condition is no longer fulfilled. Two parameters mainly affect the encounter frequency: the ship forward velocity and the heading angle. In the paper, we have numerically investigated the effect of increasing or decreasing the forward speed on the roll amplitudes. We followed two strategies. In the first, we allowed the speed to change with a constant acceleration (positive or negative) so that the encounter frequency has left the dangerous region of the resonance. In the second strategy, we changed the forward speed up and down around a value that generates large roll amplitudes, by using different periodic profiles such as sinusoidal, saw tooth or trapezoidal. We noticed that both control techniques generally managed to significantly reduce the roll amplitudes (with a few exceptions). At the same time, we found that very important in how the amplitudes diminish are parameters like the moment of time when the control is activated, the speed rate of change or the period and amplitude of the speed change around the average value (in the second strategy). The data used in the paper have been obtained from experiments with a container ship model in a towing tank followed by expansion to a full scale ship.

Numerical and experimental study on the parametric roll resonance for a fishing vessel with and without forward speed

The parametric resonance in roll motion (known shortly as parametric roll) was studied for a fishing vessel in regular waves. This is an instability and resonance phenomenon that can lead the roll to reach very high oscillation amplitudes at its natural frequency, depending on the damping level involved. In the worst cases it is responsible for vessel capsize. Here the problem was investigated numerically and experimentally, performing dedicated physical tests on a typical Norwegian fishing vessel with blunt hull and small length-to-beam ratio.Experimentally, a dedicated study was carried out on the SFH112 fishing vessel in scale 1:10 in regular head-sea waves with different wave frequencies and steepnesses. The tests were performed without and with forward speed, with corresponding Froude number Fn = 0.09 and 0.18. Numerically, a blended method was developed based on a 6-DOF 3D hybrid method where the radiation and diffraction potentials were computed for zero forward speed by WAMIT and used in the STF strip theory to obtain speed dependent loads. The convolution integrals were used to account for the effect of radiation free-surface memory effect. Nonlinearities in the Froude-Krylov and restoring loads were accounted for by integrating the corresponding pressure terms on the instantaneous wetted-hull surface defined by the incident waves and body motions. Use of the weak-scatterer hypothesis in radiation and diffraction loads has also been considered. The method was applied to reproduce and complement the experiments on the SFH112 fishing vessel. The numerical simulations showed good agreement with the experimental results. For the cases near the instability border of a 1-DOF Mathieu-type instability diagram, the physical and numerical predictions were different in terms of parametric roll occurrence. The instability borders for the experimental cases are also different from the instability borders of 1-DOF Mathieu-type instability diagram. The instability region for the experiments and 6DOF simulations cover a wider range of frequency ratio and the threshold value of metacentric height variation amplitude to have parametric roll seems to be lower than predicted by the 1-DOF Mathieu instability diagram. The results also show that the instability region for the cases with forward speed shifts to the lower frequency ratios(natural roll frequency to encounter frequency ratio) compared to the cases without forward speed. The effect of the weak-scatterer hypothesis on the results is also assessed and shown to be important for parametric roll. The results show that the simulations without this hypothesis tend to underestimate the occurrence and severity of parametric roll especially in longer and steeper waves.

Stability, performance and power flow of active U-tube anti-roll tank

Active control of ship roll using a U-tube anti-roll tank (ART) is considered. A linearized model of the ship and active ART with two degrees of freedom is formulated. The two degrees of freedom are the ART fluid free surface angle and the ship roll angle. The active control approach is based on the absolute roll-rate feedback assuming an idealized pump located in the connecting conduit of the U-tube. The pump generates a pressure difference proportional to the roll rate via a negative feedback gain. First, the stability of the feedback loop is analysed using the Nyquist criterion. The stability analysis indicates that the ART natural frequency should be set well below the ship roll natural frequency. The performance analysis is carried out in terms of the reductions of the frequency response function between the wave slope exciting the ship roll and the roll angle. It is found that also the active control performance improves with reducing the natural frequency of the ART. Finally, the analysis of the power flow through the system indicates that the pump power can be reduced with the reduction of the ART natural frequency. Also, it is shown that it is possible to reduce the wave power input into the ship plus ART system by engaging the active control.

Alternative Assessment of Weather Criterion for Ships with Large Breadth and Draught Ratios By a Model Experiment: A Case Study on an Indonesian Ro-Ro Ferry

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient corresponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

Estimation of Effective Wave Slope Coefficient of Ships with Large Breadth and Draught Ratio

One of parameters to estimate heel angle of a ship in beam seas is effective wave slope coefficient. In the weather criterion of IMO, the effective wave slope coefficient is determined as function of ratio between distance of center of gravity from the sea surface and the ship draught. The others methods could be used to estimate the effective wave slope coefficient are simplified strip theory and model experiment. A ship with shallow draught and large vertical center of gravity can have an effective wave slope coefficient larger than 1.0 if the coefficient is calculated by using the formulae of weather criterion. Therefore, an alternative method to estimate the coefficient is necessary when it is applied to ships with geometry characteristics different with those used to develop the formulae. This research conducts to estimate the effective wave slope coefficient using three different methods, namely the formulae of weather criterion, the simplified strip theory and model experiment. Results of the three methods may provide enough evidence about suitable method to estimate the effective wave slope coefficient of ships with breadth and draught ratio larger than 3.5 like the Indonesian ro-ro ferries. Results and discussion show that the effective wave slope coefficient obtained by using the formulae of weather criterion is larger compared to that obtained by using the simplified strip theory and the model experiment. Here, the result of simplified strip theory for wave frequency the same as the roll natural frequency of subject ship is similar with the result of model experiment. This results show that the simplified strip theory can be used as an alternative method to determine the effective wave slope of a ship with breadth and draught ratio larger than 3.5 if the result of model experiment does not available.

A new identification method for non-linear roll resonance in irregular waves

In setting a safe route for a ship, one of the problems to face is represented by excessive roll motions deriving from resonance phenomena in adverse weather conditions. An effective prediction of the roll motion amplification is related to a proper mathematical modelling of the ship motions. Linear models may result to be inadequate in case of large roll motions, where non-linear effects dominate. This paper investigates the roll response resonance in irregular waves. The developed method aims at demonstrating the influence of the non-linear immersed hull geometry, as already acknowledged in literature for regular seas. A 6-Dof non-linear model is applied for generating the time histories of ship roll motions in irregular waves. These are fed into an optimization routine, able of finding the linear system (focusing at its natural roll frequency) that better approximates the non-linear one at a certain mean roll amplitude.The outcomes of this research are meant to support the estimation of roll resonance period accounting for non-linearities of immersed hull geometry, allowing for a more reliable solution of weather routing problems.

Effective wave slope coefficient of river-sea ships

The effective wave slope is a fundamental concept in simplified 1-DOF roll models, and predicted roll motion strongly depends on the assumed effective wave slope coefficient. This is relevant to regulatory frameworks based on 1-DOF roll models, such as IMO Weather Criterion or the under development Second Generation Intact Stability Criteria. The paper examines different methods for effective wave slope coefficient prediction, which could potentially be used for intact stability assessment of river-sea ships. In fact, there is a lack of studies specifically addressing this type of vessels. A database of existing inland and river-sea vessels is used for determining the effective wave slope coefficient by the considered methods, and results are compared and discussed. The used approaches comprise a direct calculation method based on linear seakeeping hydrodynamics, methods based on Froude-Krylov approximation, and semi-empirical methods. Obtained results suggest that the considered methods based on Froude-Krylov approximation provide similar predictions for river-sea ships, and tend to be conservative in the vast majority of cases. Results also suggest that the considered semi-empirical methods for effective wave slope coefficient prediction at natural roll frequency are unsuitable for river-sea ships, with a general tendency towards the overestimation.

Experimental Investigation into the Effect of Roll Amplitude on Roll Damping

An in-house large eddy simulation (LES) code has been developed in the previous study. In order to validate the code for simulation of roll motion, effect of roll amplitude on forced roll damping is experimentally investigated in a circulating water channel at Shanghai Jiao Tong University (SJTU). KRISO very large crude carrier 2 (KVLCC2) is taken as a target ship, and a model with a scale of 1: 128.77 is manufactured. Tests are carried out for the ship model at shallow draft with frequencies around its natural roll frequency, where natural roll frequency is attained via free roll decay. Six forced roll tests with roll amplitudes ranging from 1° to 4° are performed experimentally, and three forced roll motions are simulated with the in-house LES code for middle section of the ship model. Discrepancies between the computed roll damping coefficients and the ones from forced roll tests are quite small. By means of particle image velocimetry (PIV), velocity fields in the vicinity of the ship model, and generation and evolution of vortices near the ship bilge are measured in detail. It is shown that roll amplitude has a significant effect on the vortex behavior, and therefore on the magnitude of roll damping coefficient. Further experimental and numerical investigation into roll motions with large amplitudes is planned.

Real-time detection of transverse stability changes in fishing vessels

Fishing is a very hazardous activity. Stability failures, caused many times by the crew lacking information related to the level of stability of the vessel, are among the most dangerous accidents affecting this fleet. A possible solution is offered by stability assessment systems that can help the skipper to identify potential risks and support his decision making process.The metacentric height is a key parameter for vessel stability and its real-time monitoring may be beneficial for alerting the crew about changes in stability. The paper proposes the design of a novel stability monitoring system that automatically detects changes in metacentric height based on estimates of the roll natural frequency solely using the measured roll angle. The core of the monitoring system is a combined estimation-detection system that exploits methods in advanced signal processing and statistical change detection to properly address issues of robustness.To analyze the monitoring performance, a nonlinear mathematical model of a stern trawler is used to generate roll motion time series in beam irregular waves of different peak period and significant wave height. Estimation and detection results obtained on the simulated data show a convincing ability of the monitoring system in discerning between safe and unsafe sailing conditions for most of the investigated cases.

Numerical study of a container ship model for the uncoupled parametric rolling

Parametric roll on ships is a resonance phenomenon whose onset causes heavy roll oscillations leading to dangerous situations for the ship, the cargo and the crew. It affects both small vessels with marginal stability and large container ships if four conditions are met simultaneously: the wave length and ship length are approximately equal, the frequency of encounter wave is twice the roll natural frequency, the ship’s roll damping is low enough and the wave height exceeds a limit value characteristic to each vessel. For a container ship, the first two conditions result in a time-varying geometry of the submerged hull and thus to a periodic variation of the transverse stability. In the last two decades, several mathematical models for parametric rolling have been proposed by scientists. In the paper, we cancelled the couplings terms between heave, pitch and roll in a 3 DOF model proposed by Neves and Rodriguez to obtain an uncoupled 1 DOF model for the parametric roll. Even if it seems too simplistic, such a model based on Mathieu type equation is a good start to capture the basic features of the analysed phenomenon. There are no less than seven parameters that can influence the ship stability and the appearance of large roll amplitudes. By modifying two of these parameters one by one and keeping the others constant, a series of colour plots have been drown that capture the role played by the ship’s parameters in obtaining dangerous roll amplitudes or too short times to take countermeasures. Analysing the instability regions suggested by these colour plots, one can imagine control measures to reduce the roll amplitudes, such as changing the forward velocity or the heading angle. The data used in the paper have been obtained from experiments with a container ship model in a towing tank followed by expansion to a full scale ship.

Experimental Validation of Transverse Stability Monitoring System for Fishing Vessels

Active monitoring of transverse stability in fishing vessels is paramount due to its significant incidence in operational accidents. Access to systems for automatic detection of changes in vessel’s stability related parameters would better support the crew during fishing and navigation operations. The paper presents an initial experimental validation of a signal-based transverse stability monitoring system, which consists of an estimator-detector kernel that solely uses measurements of roll motion to identify changes in vessel’s metacentric height by estimating the roll natural frequency. Its performance is evaluated based on experimental data from a towing tank scale model test campaign. The proposed transverse stability monitoring system well performs by identifying the potential risks and changes in loading condition.