Irregular Wave Motion Research Articles

A multiparameter simulation-driven analysis of ship response when turning concerning a required number of irregular wave realizations

The growing implementation of Decision Support Systems on modern ships, digital-twin technology, and the introduction of autonomous vessels cause the marine industry to seek accurate modeling of vessel response. Despite the contemporary 6DOF models can be used to predict ship motions in irregular waves, the impact of their stochastic realization is usually neglected and remains under-investigated. Especially in the case of turning, differences arising from the stochastic representation of the waves may result in excessive ship motions or even stability failure during maneuver execution. Therefore, in this study, statistical distributions of maximum amplitudes of roll, pitch, and lateral acceleration calculated in two representative locations on board a passenger vessel were analyzed concerning stochastic wave realization and existing extremes. The research utilized 6DOF simulation data and numerous realizations of the irregular wave with random phases of its components. Furthermore, the required number of wave realizations allowing for capturing the actual ranges of ship response at an assumed confidence level has been determined and analyzed. Ultimately, the results were compared in the safety-critical cases concerning various wave and operational conditions. The outcome of this study may be found useful by all parties involved in developing maritime autonomous systems and modeling ship motions.

An APSO-based TPA-BLSTM model for predicting ship motion in irregular waves using wave-series input

ABSTRACT Combining the bidirectional long short-term memory neural network and temporal pattern attention mechanism, a machine learning model based on multi-feature inputs is developed to predict ship motion in irregular waves. An adaptive particle swarm optimisation algorithm is proposed to adapt the parameters of neural network to different feature inputs. The study indicates that the improved particle swarm optimisation algorithm avoids the problem of poor adaptability of empirical parameters to different advance time. The combination of bidirectional long short-term memory neural network and temporary pattern attention mechanism further enhances the data mining ability. Compared with the prediction mode solely based on the ship motion input, the inclusion of wave elevation input can effectively improve the accuracy and advance time for ship’s motion prediction. Importantly, the prediction accuracy will improve with an increased number of inputs of wave probes, which are arranged along the wave propagation direction.

Experimental and CFD analysis of roll damping of a wind turbine installation vessel

The global transition to renewable energy has put pressure on wind turbine installation vessels (WTIV), thus creating an urgent demand for optimizing their operation. This includes more accurate predictions of the ship motions at sea, at which roll motion is of particular interest. For accurate prediction of the roll motion, roll damping is important to consider and is commonly found from empirical methods if experimental data are not available. Since the hull geometry and loading conditions of WTIVs are significantly different from conventional ships the validity of existing empirical methods has not been justified. This paper studies an alternative approach of determining the roll damping, by utilizing CFD to simulate free roll decay of a WTIV, from which the roll damping can be extracted. A WTIV is simulated under different loading conditions, varying the vertical center of gravity and adding a bilge keel to study the influence on the roll damping. Model tests are carried out to validate the CFD simulations. CFD simulations are performed in both model scale and full scale. The implied extreme roll motions in irregular waves are compared with empirical methods. It is found that CFD can predict roll damping accurately and that roll damping decreases for high centers of gravity. The viscous scaling effects for a 2m long model is significant, causing too large damping in model scale. The investigated bilge keel reduced roll motions significantly. The empirical methods consistently overestimated the roll damping, especially near the resonant frequency.

Motion Evaluation of Floating Point Absorber-type Wave Energy Converter by Kane's Equation

In this paper, we derive the equations of longitudinal motion for a floating point absorber type wave energy converter (FPAWEC) by Kane's equation. Since FPAWECs have a nonholonomic constraint, we selected Kane's equation which is based on the principle of virtual power. The time-domain motion equations of the FPAWEC with several control strategies were developed and its motion evaluations were conducted in regular and irregular waves. The motion evaluation results were reasonably consistent with the measured ones, both in regular and irregular waves. The dominant pitch motion in irregular waves was observed around pitch natural frequency. Using the derived equations, we investigated the pitch moment components and found that the pitch moment on the float had the largest contributions to pitching motion with the pitch natural frequency.

Online Prediction of Ship Coupled Heave-Pitch Motions in Irregular Waves Based on a Coarse-and-Fine Tuning Fixed-Grid Wavelet Network

A method based on a coarse- and fine-tuning fixed-grid wavelet networks is presented for online prediction of the coupled heave-pitch motions of a ship in irregular waves. The online modeling method contains two processes, i.e., coarse tuning and fine tuning. The coarse tuning is used to select the important wavelet terms, while the fine tuning is only used to compute the related coefficients of the selected wavelet terms. The Givens transformation algorithm is applied to realize the fine-tuning process. Due to the continuous fine-tuning process, the computational efficiency is improved significantly. Both simulation data and experimental data are used to verify the modeling method. The prediction results illustrate that the method has the ability to online predict the coupled heave-pitch motions of a ship in irregular waves.

Prediction of Ship Motions in Irregular Waves Based on Response Amplitude Operators Evaluated Experimentally in Noise Waves

Abstract The most common methods for predicting ship roll motions in a specified sea state are direct measurements of motions in a representative irregular wave realisation (time domain) or calculations of motions from response amplitude operators (RAOs) in the frequency domain. The result of the first method is valid only for the tested sea state, whilst the second method is more flexible but less accurate. RAO-based predictions are calculated assuming a linear model of ship motions in waves. RAO functions are usually evaluated by means of tests in regular waves for a limited number of frequencies and a constant wave amplitude. This approach is time-consuming and the discrete form of the RAO functions obtained for a limited number of frequencies may lead to discrepancies in the prediction of seakeeping and often does not allow the actual amplitude of the response in resonant frequency to be determined. Another challenge is the appropriate selection of wave amplitude for tests due to the considerable influence of viscous damping on roll response in irregular sea waves. There are alternative methods for the experimental determination of RAO functions and one of them is presented in this study. The presented approach allows RAO functions to be evaluated in one run by the generation of irregular waves characterised by a white or coloured noise spectrum. This method reduces the experiment duration, with almost continuous RAO characteristics obtained. The flat (white noise) and linear (coloured noise) wave spectral energy characteristics are considered in the experiment and the obtained predictions are compared with the results of accurate measurements in irregular waves.

A unified seakeeping and maneuvering analysis of multiple linked towing system with triangular Bodies

Three-legged triangular tension leg platforms (TLPs), which are a kind of compliant-type floating structure, are drawing attention from industrial society. To reduce the transportation cost, a multiple linked ship towing system that the platform and anchor are linked and wet towed is studied. A unified seakeeping and maneuvering method which can take account of the hydrodynamic interaction between the bodies and utilize the experimental results for tuning the damping coefficients is proposed to model the towing system. In the method, a low-pass filter is applied to separate the slowly and fast motion components for the maneuvering and seakeeping calculations. The maneuvering and seakeeping problems can thus be solved simultaneously with a same time scale. SimMechanics™, which provides multi-body simulation environment for mechanical systems, is applied to implement the numerical model for studying the fully coupled six degrees-of-freedom (DOFs) motion of the towing system. Besides, experiments are carried out to demonstrate the feasibility of the system and the effectiveness of the numerical model. A circular motion in irregular waves is studied at the end and the issues of the system are discussed accordingly.

Fatigue evaluation of drill pipe by considering ship motions and its application in scientific drilling

The fatigue strength of a drill pipe is a critical factor governing the performance of drillings. Substantial research has been conducted related to fatigue evaluation and various factors that have an effect on fatigue. However, earlier studies have expressed concerns over the use of fatigue curves obtained from full scale tests and stress responses obtained via finite element analysis for evaluation: (a) fatigue curves were obtained by utilizing the stress measured at a point far away from the failure area rather than the stress exerted at the failure point, (b) the stress response is not equivalent to the stress expressing the fatigue curve. It is also important to obtain the stress-response amplitude by precisely considering the irregular wave and ship motion response. This study proposes a method to obtain stress and moment responses due to ship motion in irregular waves. Secondly, it proposes a method to obtain the fatigue curve from a full-scale fatigue test suitable for evaluating the cumulative fatigue with stress response and moment response results. Then, the cumulative fatigue is calculated by applying Miner’s rule. This paper also applies the proposed method to a scientific drilling program amid strong ocean currents and uses a special drill pipe. The results by the proposed method are compared with those obtained by previous methods.

Semi-submersible Offshore Coupled Motion in Irregular Waves

In order to predict the hydrodynamic performance of semi-submersible offshore platform accurately, based on CFD theory, continuous equation and N-S equation as the control equation, RNG type k-ε model as turbulence model, using the finite difference method to discretize the control equation，using the Semi-Implicit Method for Pressure Linked Equation (SIMPLE) algorithm to solve the control equation，using the VOF method to capture the free surface. The numerical wave tank of irregular wave is established, and the wave force and motion response of the semi-submersible platform under irregular wave are studied. Based on the Jonswap spectrum density function, for a certain area of two irregular waves (South China sea, a-ten-year return period, a-hundred-year return period) sea condition, five wave direction Angle (0 °, 30 °, 45 °, 60 °, 90 °), a total of 10 kinds of conditions of the motion response of semi-submersible platform are simulated, through analysis and comparison of simulation results, the influence law of wave angle, wave period and wave height on platform motion is obtained. Compared with the experimental values, the results of heave and pitch are close to the experimental data under the sea condition of 2, 0 degree wave angles. The research results in this paper can provide reference for the design and motion response prediction of practical semi-submersible offshore platforms.

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.

Wave induced motions of partially submerged bodies near a fixed structure

Wave driven impact of small icebergs with offshore structures is a design consideration in Arctic and sub-Arctic regions. The near field hydrodynamic interaction of these wave driven ice masses has been investigated through experimental and CFD studies by Sayeed et al. (2017b, 2018a, 2018b). These studies found that motions were significantly influenced by the standing wave generated due to the superposition of the incident and reflected waves in front of the fixed structure. In regular waves the small bodies may or may not impact the structure, depending on initial starting condition whereas in irregular waves the ice masses hit the structure in all cases. Spectrum analysis carried out over the entire distance travelled by the free floating ice mass in irregular waves revealed relatively small difference in surge and heave velocity due to the presence of the structure (Sayeed et al. (2018b)). The reason is attributed to insufficient data at each separation distances to capture the motion responses for all the frequencies in the spectrum. In this paper, the surge and heave motion time series data in irregular waves are reanalyzed in a similar manner as was previously done for regular waves (Sayeed et al. (2018a). The present analysis reveals correlation of significant wavelength with the motion history at variable separation distances in front of the fixed structure. A statistical approach is then adopted to both regular and irregular wave motion data sets to identify percentiles of velocity response vs position as the small floating bodies approach head on to the fixed structure. These findings enable the identification of the motions of small bodies at specific locations in front of the structure and provide understanding of how wave-induced surge and heave velocities change as small floating bodies approach a fixed structure in a realistic wave field.

Physically-based method for real-time modelling of ship motion in irregular waves

A physically-based computational model, which implements a variety of technologies to real-time simulation of the ocean surface dynamics and waves-body interaction, is proposed. The method uses data from experimental observation of the real ocean, stochastic description of irregular wind waves, inverse non-uniform fast Fourier transform mathematical tool, unstructured grid representation for changeable water surface and complex geometry of the floating object, reasonably full and accurate immersed rigid-body dynamics, fast and stable algorithms for solving the equations of motion. The integrated mathematical model combines the proven approaches, which are adapted and modified for stated goals. The developed approach does not impose restrictions on complexity of the immersed body configuration. The implemented algorithms allow to perform an interactive simulation on a desktop computer using high resolution surface meshes without loss of frame rate. Some examples are considered to demonstrate the abilities of the model to simulate the interaction of partially submerged body (rescue boat) with wind-generated irregular fluctuations of the water surface in the real-time mode.

Experimental and numerical comparisons of self-reacting point absorber wave energy converters in irregular waves

An experimental and numerical comparison of the performance of two self-reacting point absorber wave energy converter designs is undertaken for heave motions in irregular waves. The experiments consist of a series of 1:25 scale model tests that feature re-configurable WEC body shapes, a feedback controlled power take-off, and a heave motion constraint apparatus. While both designs have the same float, the first design features a streamlined reacting body and the second design features a damper plate reacting body. A time domain numerical model, validated by the experimental results, is used to extend the comparisons of the designs in terms of power matrices, capture width matrices, and mean annual power production. Results indicate that the wave energy converter design with damper plate produces 41% higher mean annual energy production than the wave energy converter design with streamlined reacting body at a representative location near the West Coast of Vancouver Island, British Columbia, Canada.

Evaluations of wave-induced steady forces and turning motion of a full hull ship in waves

In this paper, we provide new experimental data of the wave-induced steady forces (the added resistance, wave-induced steady lateral force, and yaw moment) acting on a full hull ship in regular waves, and we verify the validity of existing prediction methods for wave-induced steady forces by performing comparisons with results obtained experimentally. For the prediction methods, we consider the zero-speed three-dimensional panel method (3DPM) and the method based on formulas of the wave-induced steady forces that are expressed using the Kochin-function assuming a slender ship (strip theory-based Kochin-function method: SKFM). The results show that the calculation accuracy obtained using 3DPM and SKFM for added resistance and steady lateral force is acceptable for practical purposes although the accuracy is insufficient for the steady yaw moment. In order to confirm the applicability of both methods to the problem of maneuvering in waves, we predict the turning motions of the ship in irregular waves using the calculation results obtained by 3DPM and SKFM for the wave-induced steady forces, and we compare the turning motions with the free-running model test results. Both methods are useful for predicting turning motions in irregular waves.

パワースペクトル密度関数を用いた不規則波中船体横揺れ最大角の評価 : 三種類の推定法を用いた比較・検証(奨学褒賞)

パワースペクトル密度関数を用いた不規則波中船体横揺れ最大角の評価 : 三種類の推定法を用いた比較・検証(奨学褒賞)

Analysis of electromagnetic transponders tracking data to quantify intrafraction prostate motion during radiotherapy treatments

The Calypso tracking system (Varian, Palo Alto, CA, USA) is used to track the prostate isocenter on patients undergoing prostate radiotherapy after implantation of electromagnetic transponders. Aim of this study was to assign 226 recorded prostate tracks to different patterns of prostate intrafraction motion (i.e. stable target (ST), continuous target drift (CTD) and irregular wave motion (IWM)) and excursion (i.e. transient excursion (TE), persistent excursion (PE) and high-frequency excursion (HFE)). Relative frequencies of STs, CTDs and IWMs were 51.8%, 44.6% and 3.6%, respectively. TEs, PEs and HFEs were revealed in 9.4%, 5.4% and 14.3% cases, respectively, with maximum values of 8.0 mm, 8.7 mm and 15.5 mm, respectively. The equation D(t) = 8.0*10-3 mm/s * t + 0.93 mm was established to calculate the average prostate drift D with time t. Intrafraction prostate motion and excursions can be significant and should be in particular taken into account with treatment deliveries that require a prolonged treatment time, as for instance stereotactic body radiotherapy (SBRT) or hadrontherapy.

Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves

Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process.

A study on evaluation of ship motion in irregular waves

In this paper, the results of evaluating the passenger comfort due to the standard deviation of acceleration in vertical and lateral direction regarding the ship response in irregular wave by ordinary strip method in regular wave and energy spectrum using linear superposition theory in order to evaluate the motion of experimental ship are as follows. According to the results of ship response, it was possible to find that, in order to reduce the motion of ship, a ship operating in bow sea was more stable than in quartering sea. In the results of analyzing the standard deviation of acceleration in vertical direction according to each component wave pattern, when there was a wave length of 56m and an average wave period of 6 sec, most of cases showed the peak value. And among them, the standard deviation was 0.35 which was the highest in head sea. And in case of lateral direction, the maximum value was shown in a wave length of 100m and an average wave period of 8 sec. And it was 0.16 in beam sea and   . In the evaluation of passenger comfort due to standard acceleration in vertical and lateral direction, it was 80% in head and bow sea. On the other hand, it was shown to be 15% in follow sea. Accordingly, when the expected wave height in a sea area where a training ship was intended to operate was known, it was possible to predict the routing of ship. And altering her course could reduce the passenger comfort by approximately 50%.

The Simulation of Ship Oscillatory Motions in Irregular Waves

Based on the theory of nonlinear random dynamic and ship oscillatory motions, by analyzing the spectrum of the random ocean wave and the force of the ship, the roll and pitch motion equations were erected respectively, then the irregular waves were created according to the superposition theory. Meanwhile, the coupling motion between rolling and heaving was studied, then the time-domain responses were the driving data of the motions simulation. Using VisualBasic, the interaction interface was established. Using MATLAB, the motions were solved. Using OpenGL the 3-D model of ship was described. We just need input the ship and environment parameters then the results will be displayed with the 2-D figures and the 3-D model quickly, then the visualization of ship motions simulation is achieved. The program developed can simulate 3-D ships and can be used for designers to analyze the properties of ship intuitively.

Wave actions on the side caissons of the Venice gates

The present study originates from a construction problem found in the planned deployment of the side caissons of the Venice gates barrier. Each of these caissons is made to float, then sunk into a lateral trench and jointed to the soil at its bottom. As a result, a C-shaped channel forms between the vertical caisson surfaces and the surrounding trench. Incoming storm waves propagating from the sea can then induce forced oscillations in this C-channel. An analytical model based on the method of matched asymptotics is developed in order to obtain the free surface oscillations in the channel, caused by a grazing incident wave. The resonant response of the basin and the amplification factors are then determined. From the free surface elevation, the pressure field and hence the total forces are also found. The analysis moves on to irregular wave motion. The analytical solution enables us to compute the dynamic actions, showing that their magnitude can be significant, due to the excitation of different resonant modes.