Ship Seakeeping Research Articles

CFD-FEM simulation of hydroelastic responses and slamming loads of a bow-flare ship advancing in head regular waves

ABSTRACT In this paper, the global motions and wave loads on a large bow-flare ship with the effect of hydro-elasticity, including whipping and springing, are numerically simulated by a CFD-FEM two-way coupled method. The simulation results of ship motions and wave loads by employing the numerical method are demonstrated by verification and validation studies including comparison with tank model experiment results. Additionally, the presented method is applied to study the motions, wave loads and slamming loads of the ship in different wave conditions. The vertical bending moment and vertical shearing force of different frequency components and their distribution along ship length are studied. The characteristics of slamming pressure and green water loads and their spatial distribution are studied. The simulation results indicate that the present numerical method well reproduces ship seakeeping and hydro-elasticity and has potential application values in the design and evaluation of ship wave loads.

Insights into the scale effects on ship motions and wave loads considering hydroelasticity

To investigate the scale effects on ship seakeeping and wave loads, ship motions and wave loads considering hydroelasticity responses of a 310-m-long ship are calculated in four different scales, i.e., model scale 1:100, 1:50, 1:25 and full-scale 1:1 by using a partitioned CFD-FEM two-way coupled method. The simulation results of the 1:50 scaled model are also compared with tank experimental data of a segmented model with a same scale and configuration for validation. Then the hydrodynamic coefficients, incident waves, heave and pitch motions, vertical bending moment, vertical shearing force and whipping loads obtained by different scaled simulations are compared and the associated scale effects are systematically analyzed. This paper also investigates the influence of segment scheme and backbone configuration on ship modal characteristics, which sheds some light on the design of segmented models with backbone for hydroelasticity experiments.

Vertical line time domain green function and its applications in numerical simulation of ship seakeeping performance

In present study, a vertical line source time domain Green function is proposed and the subsequent numerical evaluation methods are described in detail. For which, Taylor expansion and Chebyshev approximation algorithms are originally extended from point source to line source for small parameters, as well as asymptotic expansions are extended for large parameters. Meanwhile, polynomials are originally proposed to replace the asymptotic expansions to improve the computational efficiency. The accuracy and stability of the proposed method are validated through several cases and good agreement can be obtained.Furthermore, a hybrid Green function method based on the vertical line source Green function and Rankine source is developed for time domain simulation of seakeeping performances of ships. With respect to the hybrid method, the flow domain is divided into inner part and outer part by an artificial control surface. The first-order Taylor Expansion Boundary Element Method (TEBEM) based on the Rankine panel distribution is applied in the inner part, while three-dimensional panel method based on the vertical line source Green function distribution is adopted in the outer part. Under this circumstance, motions of the single-ship (Slender Wigley and Blunt Wigley) and two-ship (two side-by-side ships) in head waves are numerically investigated. The numerical results agree well with the experimental data, which proved the feasibility and the accuracy of present hybrid Green function method for motion prediction of both single-ship and two-ship.

Reducing CO2 Emissions through the Strategic Optimization of a Bulk Carrier Fleet for Loading and Transporting Polymetallic Nodules from the Clarion-Clipperton Zone

As global maritime cargo transportation intensifies, managing CO2 emissions from ships becomes increasingly crucial. This article explores optimizing bulk carrier fleets for transporting polymetallic nodules (PMNs) from the Clarion-Clipperton Zone (CCZ) to reduce CO2 emissions. Our analysis shows that larger bulk carriers, despite greater drifting forces from environmental conditions, emit less CO2 over the entire transport mission, including loading and transit. Deploying large ships in global maritime trade could significantly reduce CO2 emissions. This study also introduces a novel artificial neural network (ANN) model to estimate drifting forces during loading operations and proposes a new method for estimating CO2 emissions, considering environmental conditions and ship seakeeping properties. These findings highlight the importance of fleet size optimization and effective operational planning in achieving environmental sustainability in maritime transport.

Comparison and Evaluation of Learning Capabilities of Deep Learning Methods for Predicting Ship Motions

The development of intelligent ship control systems in real-world conditions relies heavily on the accurate identification and prediction of ship seakeeping and maneuvering trajectories. In this study, we comprehensively evaluate a selection of deep learning methods to assess their learning capabilities in terms of idealizing ship motion behavior in realistic operational environments. To recover real conditions, we utilize historical Automatic Identification System (AIS) data and a time domain 6 Degree of Freedom (6- DoF) grounding dynamics model to generate ship motion sequences for a Ro-Ro passenger ship operating in the Gulf of Finland. Via a rigorous evaluation process, we validate the performance of these methods using extensive data streams. The analysis includes the identification and estimation of uncertainties between two ports. The paper demonstrates the proficiency of the selected deep learning methods in capturing ship maneuvering features, their potential use in the design of ship control and intelligent decision support systems.

An innovative EFD using FBG pressure sensors for ship seakeeping

AbstractThis paper examines the acquisition of the measurement data of the spatial distribution of unsteady pressure on a ship advancing in waves. The purpose of the study is to investigate the reliability of the pressure data obtained by the unprecedented experiment and to provide experimental data of unsteady pressure which can be used for validation of arbitrary computation methods. The optical fiber sensing technology, so-called FBG (fiber Bragg gratings) sensing, is employed as the pressure sensors. In the experiment, 379 FBG pressure sensors (version 7; the latest version) in total are attached to the port side surface of ship model and also ordinary strain-type pressure sensors are embedded in the starboard side for validating reliability of FBG pressure sensors by comparing both results. In addition to the pressure distribution, measurement is made for unsteady hydrodynamic forces such as added mass, damping coefficients, and wave exciting forces, and also for ship motions and added resistance. Most of the measurement is repeated at least five times for each condition, and results of measured pressures are evaluated with mean value and standard deviation. Regarding the added resistance, the distribution for estimating added resistance is also extracted from obtained pressure distribution and its pressure contour is illustrated to show visually which hull region affects the added resistance significantly. Validation of the measured pressure with FBG pressure sensors is carried out also by integrating measured pressures over the ship model surface and comparing it with forces directly measured with load cells. The measured pressure distribution is compared with typical calculation results with the Rankine panel method in frequency domain to demonstrate usefulness of the spatial pressure data for validating the calculation method. Through these studies, it is confirmed that the measured unsteady pressures with FBG pressure sensors are accurate enough and valuable as the validation data for any calculation method.

Numerical Analysis of Ship Motion of Crew Boat with Variations of Wave Period on Ship Operational Speed

Seakeeping is influenced by several factors such as speed, ship hull shape, and the direction of the waves (heading angle). In this research, the crew boat is analyzed using the 3D Diffraction method to obtain the response of the ship's motion at regular waves. The study focused on analyzing the sea motion of the crew boat at different wave periods (2.58 s, 4.12 s, 5.67 s, and 7.22 s) and variation of ship speeds (0 Knot, 3 knots, 6 knots, 9 knots, 12 knots, 15 knots, and 18 knots). It used input data including the heading angle of the wave (μ) which is 180° and at a wave height of 0.5 m. The numerical analysis was carried out to determine the ship's seakeeping and compare it with NORDFORSK 1987 criteria, and also to identify the Motion Sickness Incidence (MSI) of the crews on board. Based on the results obtained, the seakeeping value of the ship accepted the criteria of NORDFORSK 1987 up to a speed of 9 knots, starting at a speed of 12 knots it does not accept the criteria. As for the operability, the comfort level of the crew while on board has an MSI index of 0% for 0 Knot, and an MSI index for 18 knots is 4.46% of the total crew of the ship namely only 1 person from 25 persons will likely feel sea sickness.

Wave energy conversion using heaving oscillator inside ship: Conceptual design, mathematical model and parametric study

A concept of utilizing heaving oscillators inside a ship as a means of capturing wave energy is proposed, drawing inspiration from the two-body self-contained wave energy point absorber. This concept involves the ship acting as a small-amplitude-motion platform and the heaving oscillator as a large-stroke body, where the kinetic energy of their relative motion can be converted into electrical energy through a power take-off (PTO) system. The dynamic equations of the ship and the inboard heaving oscillator under regular waves are derived using the boundary element method and solved using a Python code with 4th-order Runge-Kutta integration. The numerical results exhibit favorable agreement with publicly available experimental data. Furthermore, the effects of forward speed, encounter angle, and PTO mechanical parameters on various aspects, including ship motion, oscillator response, wave energy capture performance, and inner loads, are investigated. The results show that the concept of heaving oscillator inside ship has no significantly adverse effects on the ship seakeeping performance. Both forward speed and encounter angle significantly influence the encounter period and thereby impact the sensitivity of capture width (CW) at a broader band of incident wave periods. Conversely, PTO mechanical parameters exerting a more localized impact primarily under long incident waves.

Absorption of wave reflections in way of an inlet boundary

In recent years, Computational Fluid Dynamics (CFD) is increasingly becoming more attractive for the analysis of ship seakeeping. This is because of the increasing efficiency of computers and the accuracy of solvers (e.g. Reynold Navier Stokes Averaged – RANS) that may be more accurate in terms of idealizing nonlinear phenomena in comparison to potential flow methods. Notwithstanding this, RANS methods remain sensitive to accurate wave modelling across the fluid domain as well as the reflection of domain boundaries. This paper studies the effective absorption of the reflected waves from a structure heading back towards the inlet boundary with a relaxation zone method. This is achieved by applying the RANS CFD solver OpenFOAM-v2206 and wave generation toolbox waves2Foam. The modelled case is a 2D box in waves with different wave lengths. The distance of the relaxation zone and the distance after the relaxation zone until the box varies and their effect to excitation forces is studied. It is demonstrated that longer distances reduce the problems caused by wave reflections. The effects are reduced with coarser meshes as the mesh itself dampens the waves.

Numerical prediction of seakeeping and slamming behaviors of a trimaran in short-crest cross waves compared with long-crest regular waves

The ship seakeeping and slamming behaviors in multi-directional waves differ significantly from those in uni-directional waves. Given that a ship operates in short-crest or bi-directional waves during its navigation, it is significant to have a better understanding of the effect of 3-D waves on ship motion and slamming behaviors. In this work, a trimaran is adopted for seakeeping and slamming analysis. Firstly, the bi-directional cross waves, which is selected as the 3-D short-crest waves, are generated by the numerical wave tank. Secondly, the trimaran motion and slamming responses in cross wave fields with different parameters such as relative position, wave length and forward speed are predicted and analyzed. The impact and green water characteristics of the trimaran are observed and analyzed. Finally, the motion responses and pressure distribution of trimaran in short-crest cross waves are compared with those in long-crest regular waves. The conclusions drawn in this work will provide certain guidance for the safety navigation of trimarans under the action of cross waves.

An experimental study of stabilizing ordinary fishing nets (SOFNets) on a stationary SWATH ship seakeeping behavior under irregular waves

The present research aims to experimentally demonstrate the effectiveness of an innovative method for reducing the ship motions using Stabilizing Ordinary Fishing Nets that we call it SOFNets from now on. To this end, the influence of a range of SOFNets on the seakeeping behavior of a Small-Waterplane-Area Twin Hull (SWATH) ship in irregular waves is investigated physically in a towing tank laboratory. Understanding, controlling, and improving the seakeeping behavior of ships has always been a major concern of ship designers. As a result, various techniques have been utilized to stabilize ships, including the use of anti-rolling tanks, gyrostabilizers, and stabilizer fins, all three of which are bulky, heavy, and costly. These disadvantages have prevented many ships from using conventional rolling reduction methods. In contrast, the present idea, SOFNets, does not suffer from these shortcomings and can be used in any type of ship with any tonnage. The prepared model is tested in the towing tank of the National Iranian Marine Laboratory (NIMALA) using three different net samples in order to compare the behavior of those nets. This research has introduced for the first time the concept of using a SOFNets to reduce motion amplitude in waves. The length of the SWATH model was 2.1 m, and the tests were carried out according to the International Towing Tank Conference (ITTC) guidelines. The research findings for the SWATH ship indicate that the heave and pitch amplitudes in the head sea state were reduced by up to 21%, and the roll amplitude in the beam sea state was reduced by up to 36%.

Comparative Study of Potential Flow and CFD in the Assessment of Seakeeping and Added Resistance of Ships

The need for maritime freight transport of various goods has never been greater. Consequently, ships are designed with ever-increasing dimensions, with the emphasis, of course, on length. One of the many challenges in the design of large ships is the prediction of their behavior in waves, i.e., motions, and consequently, added resistance. In this paper, a comparative study of two numerical tools for estimating ship motions and added resistance is presented. The first tool is the well-established DNV’s commercial seakeeping code Wasim, a weakly nonlinear potential flow (PF) solver based on a Rankine panel method. The other is the increasingly recognized open-source Computational Fluid Dynamic (CFD) toolkit OpenFOAM®, a viscous flow solver with a turbulence model; it is based on the finite volume method (FVM) combined with a volume-of-fluid (VOF) technique for sea-surface evolution. The study is carried out for two ship seakeeping cases in head-sea regular waves, respectively, without and with ship forward speed. The first case refers to a 6750 TEU containership scale model developed at the LHEEA laboratory in Nantes for a benchmark study, providing experimental data for all test cases. Pitch and heave response is calculated and compared with the experimental values. The second case refers to a KRISO container ship, an extensively researched hull model in ship hydrodynamics. In addition to the pitch and heave, added resistance is also calculated and compared with the experimental values. Hence, it provides a comprehensive basis for a comparative analysis between the selected solvers. The results are systematically analyzed and discussed in detail. For both cases, deterioration of the PF solution with increasing wave steepness is observed, thus suggesting limitations in the modeled nonlinear effects as a possible reason. The accuracy of the CFD solver greatly depends on the spatial discretization characteristics, thus suggesting the need for grid independence studies, as such tools are crucial for accurate results of the examined wave–body interaction scenarios.

Nonlinear seakeeping and hydroelasticity of ships using potential flow simulations

ABSTRACT Accurate predictions of the behavior of ships in steep regular waves or in a natural seaway must take into account nonlinear wave responses, especially for roll motions, section moments, and hull vibration excitation. In contrast to CFD methods, here fully nonlinear seakeeping simulations are presented based on potential flow with empirical corrections. This reduces the computing effort by several orders of magnitude. Comparisons with other calculations and model experiments for benchmark cases show that the accuracy of the present method is not worse than that of the best other computations and model experiments. After about ten years of development, the method appears mature for routine applications. The source code of the program is available from the author if an adequate gift is donated to the Mèdecins Sans Frontières.

Attitude prediction of ship coupled heave–pitch motions using nonlinear innovation via full-scale test data

—This paper predicts ship seakeeping based on nonlinear innovation. An improved extended Kalman filter (EKF) algorithm is proposed to predict ship pitch attitude based on Markov process model. The stochastic attenuation equation of coupled pitching motion is derived by using the Fokker-Planck equation. The parameters in these equations are then determined using the proposed algorithm. The hyperbolic tangent function is introduced and the nonlinear innovation method is used to process the error. By comparing the numerical simulation and full-scale test data, the effectiveness of the method is verified. The experimental data were obtained by using the “Yukun” ship. The method is more suitable for the case where the system is of small damping. Compared with traditional identification algorithms, the algorithm proposed in this paper has higher prediction accuracy. The real-time prediction results are compared with the full-scale data, which shows that the proposed ship prediction model has significant prediction accuracy and the algorithm is reliable. This parameter identification method can be used to establish ship maneuvering prediction model.

Comparative Study on Added Resistance and Seakeeping Performance of X-Bow and Wave-Piercing Monohull in Regular Head Waves

Bow shape has been recognized as an important factor influencing the seakeeping performance and added resistance of ships. This paper presents a numerical comparative study on added resistance and seakeeping of model ships with ‘X-bow’ and a wave-piercing monohull in regular head waves using a computational fluid dynamics (CFD) software. Different wave heights, wavelengths and forward speeds are considered in the systematic investigation in order to characterize the added resistance and wave-induced motions, and to explore the local wave patterns. The results show a considerably different hydrodynamic characteristic by different bow shapes.

Experimental parametric study on hydroelastic behaviour of a 15000-TEU container ship based on segmented ship model

ABSTRACT Large-scale ships with high elasticity and low natural frequency, behave high dynamic response amplitude under impulsive wave loads (whipping) and short period waves (springing), which could result in serious ultimate loads and fatigue damage. The segmented model test is a more powerful approach for the ship hydroelastic analysis compared with ship seakeeping test. However, the effect of some test parameters on the hydroelastic responses is still not a clear statement. Therefore, the number of segments, propulsion mode and backbone form are chosen for the parametric study on ship hydroelastic behaviour. A three-dimensional (3-D) nonlinear time-domain hydroelastic method is developed, in which the nonlinearity of instantaneous wetted surface and slamming force are considered. Through the analysis of natural frequencies and vertical bending moments (VBM), the impact on hydroelastic behaviour is discussed. Results show that self-propelling models with variable cross-section backbones are more suitable and reliable in studying nonlinear hydroelastic behaviour.

Numerical study of swell impact on seakeeping performance of surface combatant

The complex sea states with the co-occurrence of swell and wind-sea conditions represent threats to sailing vessels. This study presents the computational fluid dynamics (CFD) simulations for a surface combatant model advancing in mixed seas combined with the swell and wind-sea components, aimed at estimating the swell impact on ship seakeeping performance. An in-house unsteady Reynolds-averaged Navier–Stokes solver coupled with an overset grid approach is applied to the numerical predictions, along with the assessment study for the model advancing in calm water and harmonic wave group based on the existed model tests, indicating the CFD results agree well with the experimental data. Irregular wave simulations are then conducted for the model advancing in head seas combined with different swell and wind-sea components. Comparisons of ship seakeeping behaviors for wind-sea dominated, energy equivalent, and swell dominated mixed seas are carried out with the statistical assessment and frequency analysis. Overall, both the heave and pitch amplitudes are larger as the ship is advancing in swell dominated sea than those in the other two seas. The ship motions are also predicted in wind sea and it indicates that the ship encounters the larger heave motion in mixed seas compared with that in wind sea.

Magnus Antirolling System for Ships at Zero Speed

This study investigated the performance of an antirolling system based on the Magnus effect at zero speed. The test scheme and control method were designed based on a proportional integration differentiation controller. A scaled ship model equipped with an attitude monitoring and control implementation feedback system was developed. The effects of different swing speeds and angles, cylinder surface shapes, number of effective cylinders, and target ship model rolling angles on the performance of the antirolling system were studied using a shipborne gravity slider and normal transverse regular wave. The Magnus antirolling system exhibited a better swing speed range compared to conventional methods, and its performance improved as the swing angle increased. During the zero-speed antirolling test, the velocity loop control provided a better antiroll damping effect on the ship model than the position loop control, and the bionic cylinder improved the performance more than a smooth cylinder. Moreover, the system retained good antirolling capabilities even in partial failure cases (i.e., operating with fewer cylinders). This study can contribute to the research of ship seakeeping strategies.

COMPUTATIONAL ANALYSIS ON HEAVE AND PITCH MOTIONS PERFORMANCE OF A HYDROFOIL SHIP

Hydrofoil ship usually experiences high resistance and excessive heave and pitch that may lead to downgrade her seakeeping performance. Therefore, a reliable investigation on prediction of a seakeeping performance of a hydrofoil ship in head-seas is obviously required. To achieve this objective, an analysis of Computational Fluid Dynamic (CFD) approach on hydrofoil ship motion is then proposed. Several parameters such as angle of stern foil and Froude Number have been accordingly taken into account in the simulation, where the fore foil angle is constantly 5 . In general, the results revealed that the increase of the stern foil angle was proportional to the heave motion of the hydrofoil ship. As compared to the magnitude of the stern foil angle of 5 and 10 , the heave motion of the hydrofoil ship has sufficiently decreased at the stern foil angle of 7.5, which leads to have a better seakeeping performance. Furthermore, the subsequent increase of Froude number pointing towards reduction of heave motion, which was inversely proportional to the magnitude of her pitch motion. Inherently, these have led to degrade of the hydrofoil ship seakeeping performances presented in the form of high Response Amplitude Operators (RAO). In general, this CFD simulation is very beneficial to ensure an operational effectiveness of hydrofoil design in high sea states with respect to the aforementioned design parameter.

«Коэффициент мореходности» как обобщенный показатель уровня мореходных качеств судна

Object and purpose of research. This paper discusses the possibility of a ship design process that would consider seakeeping performance to the greatest extent possible. The purpose of this study was to work out a numerical indicator, an index, reflecting all the seakeeping properties relevant for suitability of given ship to its intended operational conditions. Materials and methods. The study was based on the data about various operational parameters of the ship under investigation. These data were further synthesized so as to obtain the most comprehensive picture of ship seakeeping behaviour in different operational conditions. Main results. The study yielded the method and the algorithm for the “seakeeping index” as an average annual probability that seakeeping performance of given ship will be adequate to the conditions of given water area. The method sug-gested in this paper for a generalized comparison of seakeeping properties can handle whatever variety of target parameters and whatever seakeeping standards for any kind of ship intended to operate in given water area, and the result of this comparison is given in form of a single number that can be further used to improve seakeeping parameters of given ship, as well as to estimate possible time of its fully-featured operation in given conditions, including cost efficiency analysis. Conclusion. For more accurate comparison, it is recommended to analyse target parameters as functions of both ship speed and wave heading angle keeping in mind that the assumption introduced, i.e. that these curves as functions of wave heading angle are cosines, is not necessarily true. In other words, it is recommended to rely on more accurate data, experimental or analytical, so as to take into account the effect of apparent frequencies upon these curves.