Wave statistics for ship seakeeping in China offshore

Ocean waves are the core environmental elements affecting the movements and structure design of ships. Statistical analysis of wave parameters is the basis for the establishment of long-term ship environmental adaptability prediction model. The observations from coastal stations, buoys, altimeters and volunteer ships that cover from 1993 to 2011 were interpolated into miller Ion-lat grids by using bilinear method and the analytical fields of ocean waves were given. By using optimal interpolation, the analysis wave fields were assimilated into the WAVEWATCH III （WW3） simulation results. From the assimilated results, the wave rose statistics, the wave height of muitiyear return period and the extreme 2-D wave spectrum are related to the ship seakeeping were calculated. Finally, the wave statistics in China offshore were analyzed in detail.

Dynamic Analysis of High-Speed Boat Motion Simulator by a Novel 3-DoF Parallel Mechanism with Prismatic Actuators Based on Seakeeping Trial

In this study,we focused on a novel parallel mechanism for utilizing the motion simulator of a high-speed boat(HSB).First,we expressed the real behavior of the HSB based on a seakeeping trial.For this purpose,we recorded the motion parameters of the HSB by gyroscope and accelerometer sensors,while using a special data acquisition technique.Additionally,a Chebychev highpass filter was applied as a noise filter to the accelerometer sensor.Then,a novel 3 degrees of freedom(DoF)parallel mechanism(1T2R)with prismatic actuators is proposed and analyses were performed on its inverse kinematics,velocity,and acceleration.Finally,the inverse dynamic analysis is presented by the principle of virtual work,and the validation of the analytical equations was compared by the ADAMS simulation software package.Additionally,according to the recorded experimental data of the HSB,the feasibility of the proposed novel parallel mechanism motion simulator of the HSB,as well as the necessity of using of the washout filters,was explored.

Modeling the Seakeeping Performance of Luxury Cruise Ships

The seakeeping performance of a luxury cruise ship was evaluated during the concept design phase.By comparing numerical predictions based on 3-D linear potential flow theory in the frequency domain with the results of model tests, it was shown that the 3-D method predicted the seakeeping performance of the luxury cruise ship well.Based on the model, the seakeeping features of the luxury cruise ship were analyzed, and then the influence was seen of changes to the primary design parameters (center of gravity, inertial radius, etc.).Based on the results, suggestions were proposed to improve the choice of parameters for luxury cruise ships during the concept design phase.They should improve seakeeping performance.

Resistance and Seakeeping Numerical Performance Analyses of a Semi-Small Waterplane Area Twin Hull at Medium to High Speeds

The hydrodynamic analysis of a new semi-small waterplane area twin hull （SWATH） suitable for various applications such as small and medium size passenger ferries is presented. This may be an attractive crossover configuration resulting from the merging of two classical shapes： a conventional SWATH and a fast catamaran. The final hull design exhibits a wedge-like waterline shape with the maximum beam at the stem; the hull ends with a very narrow entrance angle, has a prominent bulbous bow typical of SWATH vessels, and features full stern to arrange waterjet propellers. Our analysis aims to perform a preliminary assessment of the hydrodynamic performance of a hull with such a complex shape both in terms of resistance of the hull in calm water and seakeeping capability in regular head waves and compare the performance with that of a conventional SWATH. The analysis is performed using a boundary element method that was preliminarily validated on a conventional SWATH vessel.

Optimization of Wigley Hull Form in order to Ensure the Objective Functions of the Seakeeping Performance

The research performed in this paper was carried out to investigate the computational procedure to design seakeeping optimized ship hull form. To reach the optimized hull form, four stages should be done, which consists of： generate alternative hull form, seakeeping calculations, objective functions and optimization techniques. There are many parameters that may be determined in ship hull form optimization. This paper deals with developed strip theory for determining the seakeeping performance, genetic algorithm （GA） as optimization method, high order equations for curve fitting of the hull form and finally reaching to the minimum bow vertical motion in regular head waves. The Wigley hull is selected as an initial hull and carried to be optimized. Two cases are considered. For the first case, the only form coefficients of the hull （CB, CM, Cw, Cp） are changed and main dimensions （L, B, 7） are fixed. In the second case both hull form and main dimensions are varied simultaneously. Finally, optimized hull form and its seakeeping performances are presented. The results of optimization procedure demonstrate that the optimized hull forms yield a reduction in vertical motion and acceleration.

Resistance and Seakeeping Investigation for Optimization of the Floating Hull of Wave Glider

Wave glider is the first unmanned autonomous marine robot to use only the ocean’s endless supply of wave energy for propulsion. Wave glider comprises fin system, tether and float which harvest all of its energy from waves and sun to produce forward thrust. As a consequence of the lack of design information and data for the wave glider, the main aim of the study is using computational fluid dynamics (CFD) to present a method to predict calm water resistance for the floating hull through calculations of 3 different hull forms using the same mesh generation under the same conditions. Calculations are carried out using 3 different mesh sizes for Froude number in the range of 0.10 to 0.40 and compared for accuracy of the solution parameters. Wigley parabolic hull, high speed round bilge form (NPL) and Series 60 have been comparatively investigated in order to estimate the hydrodynamics performance of the hull. The linear seakeeping analysis, coupled heave and pitch motions, roll motion, in irregular waves, with one parameter Bretschneider and JONSWAP spectra. Numerical computations have been performed for motion response predictions of the three hulls which cover wave angles from 0? to 180? at 45? intervals for six different forward speeds from 0 to 4.304 knots using Maxsurf Motion software. The close agreement between the numerical predictions shows the importance of CFD applications in estimating the hydrodynamics performance to design the floating hull and the numerical method is useful in glider design. The fine grid is fit to the calculation and shows the most appropriate results because convergent results are obtained as the mesh size decrease so the fine grid is the one which will be applied for the other hulls. Also it can be observed that the added resistance and the RAOs for NPL hull are less than the other hulls. Therefore from the comparisons, the NPL hull is the optimum hull compared to the other hulls from the resistance and seakeeping point of view.

Smoothed-Particle Hydrodynamics Simulation of Ship Motion and Tank Sloshing under the Effect of Regular Waves

Predicting the response of liquefied natural gas(LNG)contained in vessels subjected to external waves is extremely important to ensure the safety of the transportation process.In this study,the coupled behavior due to ship motion and liquid tank sloshing has been simulated by the Smoothed-Particle Hydrodynamics(SPH)method.Firstly,the sloshing flow in a rectangular tank was simulated and the related loads were analyzed to verify and validate the accuracy of the present SPH solver.Then,a three-dimensional simplified LNG carrier model,including two prismatic liquid tanks and a wave tank,was introduced.Different conditions were examined corresponding to different wave lengths,wave heights,wave heading angles,and tank loading rates.Finally,the effects of liquid tank loading rate on LNG ship motions and sloshing loading were analyzed,thereby showing that the SPH method can effectively provide useful indications for the design of liquid cargo ships.

EXPERIMENTAL RESEARCHES AND COMPARATIVE STUDIES ON SEAKEEPING FOR A LARGE CONTAINER SHIP

Experiment was carried out to investigate the effects of wave direction and rolling motion on deck wetness and relative motion for a large fast container ship. The experimental results of the regular and irregular waves were presented. It is concluded that the wave directions and rolling motion have a considerable influence on the relative motion at mid-ship.

水面舰船耐波性指标及对Seakeeper软件的评价

针对目前国内军用舰船耐波性计算书中存在的问题,结合《舰船通用规范》中对适航性的规定,对水面舰船耐波性指标进行了较详细说明,指出军用舰船耐波性理论预报中值得注意的问题,并分析了目前较为流行的商用软件Seakeeper的适用船型和预报精度。

Hydrodynamic Coefficients for a 3-D Uniform Flexible Barge UsingWeakly Compressible Smoothed Particle Hydrodynamics

The numerical modelling of the interactions between water waves and floating structures is significant for different areas of the marine sector, especially seakeeping and prediction of wave-induced loads. Seakeeping analysis involving severe flow fluctuations is still quite challenging even for the conventional RANS method. Particle method has been viewed as alternative for such analysis especially those involving deformable boundary, wave breaking and fluid fragmentation around hull shapes. In this paper, the weakly compressible smoothed particle hydrodynamics(WCSPH), a fully Lagrangian particle method, is applied to simulate the symmetric radiation problem for a stationary barge treated as a flexible body. This is carried out by imposing prescribed forced simple harmonic oscillations in heave, pitch and the two-and three-node distortion modes. The resultant,radiation force predictions, namely added mass and fluid damping coefficients, are compared with results from 3-D potential flow boundary element method and 3-D RANS CFD predictions, in order to verify the adopted modelling techniques for WCSPH.WCSPH were found to be in agreement with most results and could predict the fluid actions equally well in most cases.

海洋信息环境下船舶耐波性指标研究

船舶在实际海洋环境中的性能代表航行性能的优劣,通过结合海洋环境信息数据来探究船舶的耐波性能以达到对船舶辅助航行的目的,首先使用以切片法为理论的seakeeper软件对船舶耐波性进行仿真研究,获得船舶运动的传递函数和船舶纵摇、升沉、横摇有义值曲线,采用莫尔公式对模拟结果进行验证,通过数值模拟找到对耐波性指标较大的因素,根据耐波性指标衡准对船舶耐波性能做出评价,并应用统计学中的正交试验方法对耐波性的影响因素(航速、风速、航向)对耐波性指标的影响程度进行研究,使海洋环境中船舶的耐波性达到最优,从而起到指导船舶航行的目的。

An Experimental Study on the Effect of the Side Hull Symmetry on the Resistance Performance of a Wave‑Piercing Trimaran

This paper presented the results of an experimental investigation into the resistance performance of a wave-piercing trimaran with three alternative side hull forms,including asymmetric inboard,asymmetric outboard,and symmetric at various stagger/separation positions.Model tests were carried out at the National Iranian Marine Laboratory(NIMALA)towing tank using a scale model of a trimaran at the Froude numbers from 0.225 to 0.60.Results showed that by moving the side hulls to the forward of the main hull transom,the total resistance coefficient of trimaran decreased.Findings,furthermore,demonstrated that the symmetry shape of the side hull had the best performance on total resistance among three side hull forms.Results of this study are useful for selecting the side hull configuration from the resistance viewpoint.

Sloshing-Coupled Ship Motion Algorithm for Estimation of Slosh-Induced Pressures

The effect of coupling between sloshing and ship motions in the evaluation of slosh-induced interior pressures is studied. The coupling between sloshing loads and ship motions is modelled through a hybrid algorithm which combines a potential flow solution based on transient Green function for the external ship hydrodynamics with a viscous flow solution based on a multiphase interface capturing volume of fluid(VOF) technique for the interior sloshing motion. The coupled algorithm accounts for full nonlinear slosh forces while the external forces on the hull are determined through a blended scheme of linear radiationdiffraction with nonlinear Froude-Krylov and restoring forces. Consideration of this level of nonlinearities in ship motions is found to have non-negligible effects on the slosh-coupled responses and slosh-induced loads. A scheme is devised to evaluate the statistical measure of the pressures through long-duration simulation studies in extreme irregular waves. It is found that coupling significantly influences the tank interior pressures, and the differences in the pressures between coupled and uncoupled cases can be as much as 100% or more. To determine the RAO over the frequency range needed for the simulation studies in irregular waves, two alternative schemes are proposed, both of which require far less computational time compared to the conventional method of finding RAO at each frequency, and the merits of these are discussed.

Monolithic Coupling of the Pressure and Rigid Body Motion Equations in Computational Marine Hydrodynamics

In Fluid Structure Interaction(FSI) problems encountered in marine hydrodynamics, the pressure field and the velocity of the rigid body are tightly coupled. This coupling is traditionally resolved in a partitioned manner by solving the rigid body motion equations once per nonlinear correction loop, updating the position of the body and solving the fluid flow equations in the new configuration. The partitioned approach requires a large number of nonlinear iteration loops per time–step. In order to enhance the coupling, a monolithic approach is proposed in Finite Volume(FV) framework,where the pressure equation and the rigid body motion equations are solved in a single linear system. The coupling is resolved by solving the rigid body motion equations once per linear solver iteration of the pressure equation, where updated pressure field is used to calculate new forces acting on the body, and by introducing the updated rigid body boundary velocity in to the pressure equation. In this paper the monolithic coupling is validated on a simple 2D heave decay case. Additionally, the method is compared to the traditional partitioned approach(i.e. "strongly coupled" approach) in terms of computational efficiency and accuracy. The comparison is performed on a seakeeping case in regular head waves, and it shows that the monolithic approach achieves similar accuracy with fewer nonlinear correctors per time–step. Hence, significant savings in computational time can be achieved while retaining the same level of accuracy.

Modeling of Roll-Heave-Pitch Motions of a Ram Wing Translating over Non-uniform Surface

Ground-effect vehicles flying close to water or ground often employ ram wings which generate aerodynamic lift primarily on their lower surfaces.The subject of this paper is the 3-DOF modeling of roll,heave,and pitch motions of such a wing in the presence of surface waves and other ground non-uniformities.The potential-flow extreme-ground-effect theory is applied for calculating unsteady pressure distribution under the wing which defines instantaneous lift force and moments.Dynamic simulations of a selected ramwing configuration are carried out in the presence of surface waves of various headings and wavelengths,as well as for transient flights over a ground obstacle.The largest amplitudes of the vehicle motions are observed in beam waves when the periods of the encounter are long.Nonlinear effects are more pronounced for pitch angles than for roll and heave.The present method can be adapted for modeling of air-supported lifting surfaces on fast marine vehicles.

Study on HOBEM Based on Analytical Panel Integrals Related to Translating-Pulsating Source for Hydrodynamic Responses of Vessels Sailing in Waves

A higher-order boundary element method(HOBEM)incorporated with analytical panel integrals related to translat-ing-pulsating source Green’s function is proposed for the hydrodynamic response prediction of ships advancing in waves.In this method,the 9-node bi-quadratic curvilinear elements employed to discretize the mixed-source/dipole boundary integral equation are mapped into the parametric plane through a coordinate transformation.Then in order to ease the numerical instability problem,a novel analytical quadrature is derived to calculate the influence coefficients by changing the integral order and using integration by parts.The singularity caused by infinite discontinuity is analyzed and eliminated by adopting some mathematical techniques.Through the calculations of panel integrals of Green’s function and its x-derivative,the analytical integral method is proved to be always accurate even for field points approaching the free surface,where numerical quadrature is impossible to give reasonable results.Based on this,a higher-order seakeeping program is developed and applied in the motion response prediction of two different types of ships(i.e.,a wall-sided ship Wigley III and a non-wall-sided ship S175).By comparing the computed results with the corresponding experimental data and numerical solutions of the translating-pulsating and higher-order Green’s function methods based on traditional Gauss quadrature,it is found that the HOBEM based on analytical quadrature is of better accuracy and stability.For the non-wall-sided ship,only the present method can produce reasonable pre-diction of motion responses,while obvious oscillatory phenomenon is observed in the results of the other two numerical methods based on Gauss quadrature.

Numerical Modeling of an Advanced Semi-SWATH Hull in Calm Water and Regular Head Wave

A small waterplane area twin hull(SWATH)has excellent seakeeping performance and low wave-making resistance,and it has been applied to small working craft,pleasure boats,and unmanned surface vehicles.However,with the increase in speed,the hydrodynamic resistance of SWATH will increase exponentially because of its large wet surface,followed by the uncomfortable situation of the hull underwater part relative to the water level and in terms of high trim by stern and high sinkage.A way to improve this situation is to reduce the depth of the draft at high speeds to ensure that all or a part of the volume of the submerged bodies is above the water level.Based on this idea,a new type of semi-SWATH hull form was analyzed in this paper.The two submerged bodies of the SWATH have a catamaran boat shape.This paper employed Sie-mens PLM Star-CCM+to study the hydrodynamic performance of an advanced semi-SWATH model.Bare-hull resistance was estimated for both SWATH and CAT(CATAMARAN)modes in calm water.Moreover,the efect of fxed stabilizing fns with diferent angles on the vertical motions of the vessel in regular head waves was investigated with an overset mesh approach.The vertical motion responses were estimated at diferent wave encounter frequencies,and the present numerical method results have been verifed by already published experimental data.

Rigid‑Body Analysis of a Beveled Shape Structure in Regular Waves Using the Weakly Compressible Smoothed Particle Hydrodynamics(WCSPH)Method

In many cases of wave structure interactions,three-dimensional models are used to demonstrate real-life complex environ-ments in large domain scales.In the seakeeping context,predicting the motion responses in the interaction of a long body resembling a ship structure with regular waves is crucial and can be challenging.In this work,regular waves interacting with a rigid foating structure were simulated using the open-source code based on the weakly compressible smoothed par-ticle hydrodynamics(WCSPH)method,and optimal parameters were suggested for diferent wave environments.Vertical displacements were computed,and their response amplitude operators(RAOs)were found to be in good agreement with experimental,numerical,and analytical results.Discrepancies of numerical and experimental RAOs tended to increase at low wave frequencies,particularly at amidships and near the bow.In addition,the instantaneous wave contours of the sur-rounding model were examined to reveal the efects of localized waves along the structure and wave dissipation.The results indicated that the motion response from the WCSPH responds well at the highest frequency range(ω>5.235 rad/s).

Calculation of ship motions in steep waves with restoring and Froude-Krylov forces on an adaptive panel mesh with Gauss and analytic integration methods

The impulse response method is a frequently used method to calculate ship seakeeping behavior.In this paper,the restoring and Froude-Krylov calculation is conducted with constant evaluation of panel pressures as well as Gauss quadrature and an analytical integration.The applied panel grid is coarsened by an adaptive algorithm which is based on a normal vector condition.The comparison of methods is based on grid convergence studies which are followed by a verification of forces with computational fluid dynamics(CFD)results on the fixed duisburg test case in waves.Validations with experimental results in head,oblique and following waves show that all integration methods are accurate.The exact integration is numerically sensitive in some cases.Gauss quadrature is highly accurate;however,the additional effort is not beneficial since the geometrical accuracy has-stronger influence on the force amplitudes than the integration method.Adaptive grid coarsening reduces the simulation time and is accurate up to a level,where the panel length comes close the wavelength.The added resistance at the investigated Froude number of 0.05 shows higher uncertainty levels,this applies to the results of both the numerical methods and model tests.

Nonlinear air dynamics of a surface effect ship in small-amplitude waves

In many existing works,the seakeeping motions and air dynamics of a surface effect ship(SES)were assumed to be linear under small-amplitude waves(wave amplitude to wave length ratio≤5%)to en-hance the computational efficiency.However,according to SES model test results,it was found that even in small-amplitude waves,the fluctuating air cushion pressure shows significantly nonlinear effects.To precisely reveal this distinctive feature,the origin of nonlinearity was carefully investigated and the air leakage was considered as the main source of nonlinearity based on mathematical analysis in this paper.The reason is that the variance of clearance height under seals is comparable to the clearance height at equilibrium state in small-amplitude waves,which makes the air leakage area intermittently equal to zero without any harmonic variance.Therefore,an efficient partial nonlinear numerical model for the SES dynamics was proposed by combining a linear frequency-domain hydrodynamic model based on the ef-ficient 2.5D methods with a nonlinear time-domain air dynamic model.The nonlinear parts of numerical results from the partial nonlinear model,including the fluctuating air pressure and midship accelerations,agree well with experimental results.The results demonstrate the effectiveness of the partial nonlinear model on the SES seakeeping performance prediction,and confirm that its nonlinearity mainly originates from the air leakage.