Ship Motions In Waves Research Articles

Numerical analysis of coupling effect between sloshing and motion responses of a KCS in uni- and bi-directional waves

The seakeeping behaviour can be severely affected by the coupling between sloshing and ship motions in waves. The sloshing behaviour of liquid-loaded ships in bi-directional waves differs significantly from that in uni-directional waves. Given that ships operate in short-crested or multi-directional waves during their whole service period, it is significant to gain a deeper understanding of the effect of wave directionality on the coupling between sloshing and ship motions. In this study, a KRISO container ship (KCS) model with type-B liquefied natural gas (LNG) cargo tanks is adopted for coupling analysis. The 2D long-crested regular wave is adopted as an uni-directional wave, and three dimensional (3-D) cross wave is adopted as a bi-directional wave. The computational fluid dynamics (CFD) method is applied for the sloshing and seakeeping simulation of the ship model in different wave fields. The differences in wave elevation, sloshing forces, and coupling effects between ship motions and sloshing are studied in these two wave fields by adjusting control parameters such as ship heading angle and filling ratio. The comparative results indicate that sloshing behaviour and coupled motion responses in cross waves should be paid enough attention during ship design and evaluation.

Time-Domain Higher-Order Boundary Element Method for Simulating High Forward-Speed Ship Motions in Waves

Time-Domain Higher-Order Boundary Element Method for Simulating High Forward-Speed Ship Motions in Waves

First-principles algorithms for ship motion simulation based on ARMA and trochoidal wave models

An approach for the direct simulation of ship behavior in heavy sea conditions using modern computers methods is described. The approach includes a model for wind-driven waves, algorithms for hydrodynamic pressure calculation, a solution of the diffraction problem, and the integration of the approach into a problem solution environment. An ARMA (autoregressive-moving average) model is used for the simulation of a hydrodynamically adequate spatial-temporal wave surface and integrated with a calculation of hydrodynamic pressure at any point under the wave surface. The integration of the wave pressure over the ship hull and the solution of the diffraction problem provides the excitation forces and moments and permits direct numerical simulation of ship motions in waves. An effective numerical implementation of the algorithms with mapping to modern computer architecture is described. The combination of the numerical algorithms with a visual simulation environment is presented as a virtual testbed.

Data‐driven forecasting of ship motions in waves using machine learning and dynamic mode decomposition

SummaryData‐driven forecasting of ship motions in waves is investigated through feedforward and recurrent neural networks as well as dynamic mode decomposition. The goal is to predict future ship motion variables based on past data collected on the field, using equation‐free approaches. Numerical results in two case studies involving the course‐keeping of a naval destroyer in a high sea state using simulation data at model scale are presented. The proposed methods reveal successful in predicting ship motions both in short‐term and medium‐term perspectives with accuracy and reduced computational effort, thus enabling further advances in the identification, control, and optimization of ships operating in waves.

Prediction of ship future motions at different predictable time scales for shipboard helicopter landing in regular and irregular waves

ABSTRACT Differing from the traditional helicopter landing on land, due to ship motions induced by actual waves, the landing safety of shipboard helicopter on vessels in waves deserves to be studied. To provide a judgment basis for the timing of shipboard helicopter landing, a hydrodynamic analysis method based on the Long-Short-Term Memory (LSTM) neural network is proposed to achieve the real-time prediction of ship motions in this study. Firstly, the hydrodynamic formulations based on potential flow theory for nonlinear ship motions in waves are presented. The implementation details of the LSTM neural networks are described. Secondly, the numerical verification study is carried out in regular and irregular waves, respectively. Furthermore, a destroyer model is employed to illustrate the proposed framework, and the results corresponding to the different time scales in waves are presented. Finally, the work of this paper is summarized and the certain conclusions are drawn.

Comparative analysis of seakeeping methods: numerical, software simulation and experimental study

The current study case presents a comprehensive comparison of seakeeping analysis methods, utilizing the seakeeping numerical analysis based on Lewis’s form description of a hull, Ansys Aqwa, and experimental data obtained from the towing tank of the Faculty of Naval Architecture in Galati. The primary objective of this study is to assess the accuracy and reliability of two numerical approaches in predicting the seakeeping performance of a vessel under varying conditions. The experiment was carried out under two scenarios, one with the model at zero speed and another with the model towed at 1.28 m/s and 180o heading angle. The diverse set of conditions aimed to simulate real-world scenarios and capture the dynamic nature of ship motions in waves. The employing of Lewis forms numerical analysis, Ansys Aqwa simulation, and experimental data, the results were compared and evaluated in terms of their accuracy and agreement.

Reduced-order model for ship motions incorporating a volume-based calculation of body-nonlinear hydrostatic and Froude-Krylov forces

The theoretical background of a fast time-domain simulation tool for ship motion in waves, commonly referred to as SimpleCode, is reviewed. The tool implements a body-nonlinear formulation for the hydrostatic and Froude-Krylov forces with the instantaneous submerged volume and centroid of sections along the ship's length. Other forces are approximated by ODE (ordinary differential equation) like polynomials. SimpleCode implements a six degrees of freedom (DOF) solver as well two 3-DOF solvers: one for vertical plane motions (heave, roll, and pitch) and another for horizontal plane motions (surge, sway, and yaw). While the initial objective of the tool was limited to the statistical validation of extrapolation methods, it was later extended to other applications and is considered to be a general reduced-order numerical model (ROM) of ship motions in waves. The SimpleCode has been used for the development and statistical validation of the EPOT and split-time methods and for the identification of extreme response events in a multi-fidelity seakeeping analysis.

Asymmetry in sagging and hogging responses of a ship hull

In this paper asymmetry in sagging and hogging of a hull progressing in regular waves is investigated by applying a weakly nonlinear potential flow theory. At first instance ship motions in waves are evaluated using a linear strip-theory yielding transfer functions of ship motions and sectional loads. Then, by assuming time-histories of ship motions the nonlinear part of the Froude-Krylov and restoring forces and moments are evaluated using a panel representation. Linear and weakly nonlinear predictions are compared against available model tests for a roll-on/off (Ro-Pax) ship hull. The results confirm that importance of nonlinearities of restoring and Froude-Krylov part of wave loading on asymmetry of sagging and hogging.

A CFD-Based Data-Driven Reduced Order Modeling Method for Damaged Ship Motion in Waves

A simple CFD-based data-driven reduced order modeling method was proposed for the study of damaged ship motion in waves. It consists of low-order modeling of the whole concerned parameter range and high-order modeling for selected key scenarios identified with the help of low-order results. The difference between the low and high-order results for the whole parameter range, where the main trend of the physics behind the problem is expected to be captured, is then modeled by some commonly used machine learning or data regression methods based on the data from key scenarios which is chosen as Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) in this study. The final prediction is obtained by adding the results from the low-order model and the difference. The low and high-order modeling were conducted through computational fluid dynamics (CFD) simulations with coarse and refined meshes. Taking the roll Response Amplitude Operator (RAO) of a DTMB-5415 ship model with a damaged cabin as an example, the proposed physics-informed data-driven model was shown to have the same level of accuracy as pure high-order modeling, whilst the computational time can be reduced by 22~55% for the studied cases. This simple reduced order modeling approach is also expected to be applicable to other ship hydrodynamic problems.

Numerical Investigation on the Hydrodynamic Response of Pentamaran—Resistance Analysis of Different Outrigger Inclination Angles

Due to the interference between the main hull and the outrigger of the pentamaran, resistance is greatly affected. Therefore, research on the pentamaran front outrigger inclination angle has further practical significance for reducing resistance. In this study, the pentamaran front outrigger inclination angle was analyzed by CFD method, the ship motion in waves was simulated by overlapping grid technology, and the resistance of the pentamaran in static water and waves was predicted by using the unsteady RANS equation. First, a series of validation studies were carried out for the numerical methods used in the study. Then, the influence of the front outrigger inclination angle on the pentamaran resistance performance under different working conditions is calculated and discussed. In order to analyze the influence of the change of the front outrigger inclination angle on the resistance, free surface wave-making and hull pressure are further discussed. The results show that the influence of the front outrigger inclination angle change on the resistance of the pentamaran has a certain rule, and the resistance of the pentamaran can be reduced by adjusting the front outrigger inclination angle.

Fast Multigrid Algorithm for Non-Linear Simulation of Intact and Damaged Ship Motions in Waves

This paper proposes a fast multigrid algorithm to simulate the non-linear motion of ships in both intact and damaged conditions. The simulations of ship motions in waves are known to require much time to calculate due to the strong non-linear interactions between ship and waves. To improve the calculation efficiency while retaining the accuracy, a prediction-correction strategy was designed to accelerate the simulation through three sets of locally refined meshes. The flow field was first estimated in a coarse mesh and then mapped to a locally refine mesh for further higher-fidelity corrections. A partitioned radial basis function (PRBF) method is proposed to interpolate and reconstruct the flow field for the refined mesh. A new two-phase flow solver was developed with a fast multigrid algorithm based on the Reynolds-averaged Navier–Stokes equations (RANSE). The new solver was applied to study the non-linear behavior of a damaged ship in beam waves and the effect of damaged compartments on ship rolling motion. Validation against the solution with the original method of single set meshes and experimental data indicates that the proposed algorithm yields satisfactory results while saving 30–40% of the computational time.

CFD database method for roll response of damaged ship during quasi-steady flooding in beam waves

A database method is proposed to predict the roll response of a damaged ship upon quasi-steady flooding in regular beam waves. To construct the hydrodynamic database, the hydrodynamic loads acting on the damaged ship are decomposed into wave excitation moment and motion-induced moment, which are calculated based on computational fluid dynamics (CFD). The wave excitation moments for different wave frequencies are formulated using the high-order trigonometric function. The ship hydrodynamic coefficients (i.e., added moment of inertia and damping coefficient) for different roll frequencies and amplitudes are regressed from the motion-induced moment and then used as samples for database construction by cubic spline interpolation. The Runge-Kutta method is employed to solve the ship's roll motion equation, in which the hydrodynamic coefficients are updated according to the roll amplitude in real time. To test the performance of the present method, the roll motion responses of a damaged frigate DTMB-5415 are computed. The results obtained by the database method are in good agreement with the CFD results and experimental data. The characteristics of motion-induced moment, roll hydrodynamic coefficients and wave excitation moment are analyzed. In addition, the influence of various factors (e.g., sensitivity of mesh and time step, order of regression method, interval of sample points and type of load decomposition model) on the computation accuracy are also discussed. Compared to directly performing CFD simulation of damaged ship motion in waves, the proposed database method can predict the ship's roll response with high efficiency and acceptable accuracy.

Применение численных процедур в оценке волновых нагрузок танкера с учетом слошинга и скорости хода

Object and purpose of research. The investigation considers approaches for inclusion of sloshing and ship speed and discusses their influence on estimation of hull responses to external loads. Materials and methods. The 3D panel method is used to solve a hydrodynamic problem of potential flow around ship hull in regular waves. The sloshing is included by direct modeling of inner tanks also containing potential fluid. The ship speed is taken into account by additional potential of inflow. Main results. Analysis of response amplitude operators (RAO) and bending moments with and without additional factors introduced in the mechanical model. Conclusion. The problem of ship motions in regular waves taking into account sloshing and ship speed was solved in the work process. The influence on responses of the considered mechanical system for taking these factors into considerations is analysed. The applied panel method shows flexibility regarding additional factors of modeling and offers a prospect for further investigation of non-linear effects associated with ship motions in waves.

Data-Driven system identification of 6-DoF ship motion in waves with neural networks

Critical evaluation of ship responses in the ocean is important for not only the design and engineering of future platforms but also the operation and safety of those that are currently deployed. Short-term temporal predictions of ship responses given the current wave environment and ship state would enable enhanced decision-making onboard and reduce the overall risk for both manned and unmanned vessels, especially as the marine industry trends towards more autonomy. However, state-of-the-art numerical hydrodynamic simulation tools are too computationally expensive to be employed for real-time ship motion forecasting. Thus, a methodology is needed to provide fast predictions with levels of accuracy closer to the higher-fidelity tools. A methodology is developed with long short-term memory (LSTM) neural networks to represent the motions of a free running David Taylor Model Basin (DTMB) 5415 destroyer operating at 20 knots in Sea State 7 stern-quartering long-crested irregular seas. Case studies are performed for both course-keeping and turning circle scenarios. An estimate of the vessel’s encounter frame is made with the trajectories observed in the training dataset. Wave elevation time histories are given by artificial wave probes that travel with the estimated encounter frame and serve as input into the neural network, while the output is the 6-DOF temporal ship motion response. Overall, the neural network is able to predict the temporal response of the ship due to unseen wave sequences accurately. The methodology, the dependence of model accuracy on wave probe and training data quantity and the estimated encounter frame are all detailed.

Evaluation of bow foils on ship delivered power in waves using model tests

Bow foils have the potential to significantly reduce engine load and CO2 emissions of ships operating in waves. This paper presents the methodology and results of an improved free running experiment for the evaluation of bow foils. The model tests directly measured the change in ship delivered power in waves with and without a bow foil. The results show that the bow foil reduces the delivered power in regular head waves by up to 50% and 12% in irregular waves. In addition to quantifying the comparative contribution due to foil thrust and reduced ship motions in waves, the experiment measured the wave phasing and feathering parameters. The presented dataset, analysis and experimental procedures, which can be used for numerical validation and performance predictions in a range of conditions, demonstrates the potential of energy saving bow foils to reduce engine load and ship emissions.

船体運動の応答関数における支配パラメータの抽出と簡易推定式の提案

船体運動の応答関数における支配パラメータの抽出と簡易推定式の提案

Simulation Modeling of a Ship Propulsion System in Waves for Control Purposes

The article deals with a simulation approach to the representation of the ship motions in waves, interacting with the propulsion system behavior (diesel engine and propeller). The final goal is the development of a simulator, as complete as possible, that allows the analysis of the main engine thermodynamics in different sea conditions, also in the unfavorable event of dynamic instability of the hull, and the correct management of the other propulsion components. This latter aspect is particularly interesting in some of the last new energy solutions for decarbonization of ships, concerning, for example, auxiliary electric motors, powered by batteries, to support the traditional diesel-mechanical propulsion (especially in heavy weather conditions). From this point of view, a proper analysis of the engine dynamic performance, affected by particular sea states, is fundamental for a smart management and control of shaft generators/auxiliary electric motors, batteries, etc. To this end, the work presents and highlights the main features of a ship simulator, suitable for the study of the new propulsion solutions that are emerging in maritime transport. Some representative results will point out the complex non-linear behavior of the propulsion plant in waves. Moreover, a parametric roll scenario will be investigated, in order to highlight the capability of the conceived simulator in modeling the effects of the dynamic instability of the hull on the propulsion plant.

Prediction of Flooded Compartment Damage Locations in Ships by Using Spectrum Analysis of Ship Motions in Waves

Considering the nature of marine accidents, even a single accident can result in significant damage to the environment and property, as well as loss of life. Therefore, the initial response should be rapid and accurate, and various decision support systems have been developed to achieve this. Research on simulating progressive flooding on board immediately after an accident is being actively conducted, but this requires high levels of computing power. In this study, a methodology for converting simulated ship motion data into a ship motion database is presented. The model of a training ship from the Korea Institute of Maritime and Fisheries Technology and KRISO in-house code SMTP was used for ship motion computations. The short-time Fourier transform was used to convert time-series motion data into a spectrogram motion database. A methodology for deriving a predicted location of the damage center is presented. The candidate locations of the damage centers were obtained by comparing the root mean square error values of the ship motion database from the simulation and real-time ship motion data. Finally, a probability function was suggested to confirm the predicted location of the damage center. Using 100 randomly selected test cases, our method showed 95% accuracy.

VISCOSITY IN SEAKEEPING

Application of strip theory for the prediction of ship motions in waves relies on sectional hydrodynamic coefficients; i.e. the added mass and damping coefficients. These coefficients apply to linearised problems and are normally computed for inviscid fluids. It is possible to account for viscosity but this cannot be done by the RANS equations, as in linear problems there is no room for turbulence. The hydrodynamic coefficients can include the effect of viscosity but this can be done rightly through the classic Navier–Stokes equations for laminar (non-turbulent) flows. For solving these equations commercial RANS software can be used, assuming no Reynolds stresses.

CFD Prediction of Ship Seakeeping and Slamming Behaviors of a Trimaran in Oblique Regular Waves

The main hull encounters waves at first and causes waves to break, when trimarans are subject to the slamming in head waves. At this moment, emergence phenomena of side hulls will not occur. Thus, the slamming study of trimarans in oblique waves presents further practical significance. In this study, a CFD method is used for trimaran seakeeping and slamming analysis. An overset grid technique is adopted to simulate ship motions in waves. Firstly, to further verify the present method, a series of verification and validation studies is conducted. Then, the motion responses and slamming pressure with different control parameters, such as forward speed and ship heading angle, are calculated and discussed. The comparative results indicate that the seakeeping and slamming behaviors of trimarans differ significantly from those of conventional monohull ships. Finally, severe bow slamming and green water in oblique waves are also observed and investigated, which should be given enough attention during ship design and evaluation.