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1.
Muhittin Sylemez 《Ocean Engineering》1998,25(2-3)
The method of calculation of non-linear restoring forces presented in this paper is simple, concise and feasible to apply easily in the calculation of restoring forces of platforms in order to simulate motion responses of offshore platforms in the time-domain. In this method, hydrostatic restoring forces and moments are related to the translational and/or rotational displacements. Calculations of non-linear yaw, roll and pitch restoring moments are based on the catenary type of moorings. Although the method presented here is a simple one, it is capable of the calculation of restoring forces for use in the time-domain motion simulations of offshore platforms, with an acceptable degree of accuracy when the numerical simulation results were compared with the experimental measurements. 相似文献
2.
Muhittin Sylemez 《Ocean Engineering》1998,25(4-5)
The purpose of the study was to develop a prediction technique to simulate the motion response of a damaged platform under wave, wind and current forces. The equations of motion were obtained using Newton's second law and the numerical solution technique of non-linear equations of motion is explained for intact and damaged cases. The analysis technique employs large displacement non-linear equations of motion. Solutions were obtained in the time-domain to predict the motion characteristics. In this study, analysis procedures were developed to calculate: (a) wave loading on asymmetrical structural configurations; (b) hydrodynamic reaction forces (inertia or moment of inertia, damping and restoring forces) on asymmetrical shapes. During the damage simulation, change in the mass of the structure as well as wave and hydrodynamic reaction forces, were taken into account. The computer program developed for the time-domain simulation is introduced. In order to avoid slowly decaying transient motions of the structure due to wave excitation forces, an exponential ramp function is used. The application of a ramp function enables a quick convergence in the time-domain solution of equations of motion. Results of a numerical motion simulation program and the experimental studies are also presented in order to make comparisons. Comparison of the test results with the numerical simulations shows good agreement for heave, roll and pitch motions. The formulations and the computational procedures given in this paper provide useful tools for the investigation of the non-linear dynamic stability characteristics of floating structures in waves for intact, damaged and post-flooding conditions in six-degrees of freedom. 相似文献
3.
Osman Turan Zafer Ayaz Seref Aksu Jan Kanar Antoni Bednarek 《Ocean Engineering》2008,35(13):1339-1356
The study investigates the experimental and numerical analysis of the occurrence of auto-parametric rolling for large, high-speed pod-driven ships in waves. Considering unique design and performance targets, the aim here is to exploit susceptibility to auto-parametric rolling behaviour and to identify probable design and operational precautions. In order to achieve this aim, an existing non-linear time-domain software to simulate capsizing and other critical manoeuvring behaviours of slow- to medium-speed conventional and podded ships in waves is being enhanced for fast pod-driven vessels and then compared against the dedicated model test conducted in long-crested regular and random waves for a large, pod-driven containership model. This paper includes the presentation of current numerical modifications for pod-driven ships and the verification analysis. 相似文献
4.
A time-domain analysis is used to predict wave loading and motion responses for a ship traveling at a constant speed in regular oblique waves. Considered as a distribution of normal velocities on the wetted hull surface, the combined diffraction and radiation perturbations caused by the forward moving ship and her motions are determined simultaneously. This way, the ship-hull boundary condition is exactly fulfilled. The 3-D time domain Green's function is used to express the combined diffraction/radiation potential in terms of impulsive and memory potentials. Application of the Bernoulli equation yields the pressure distribution and accordingly, the necessary hydrodynamic forces. The equations of motion of the ship are then developed and solved in the time domain.Forces and motions at forward speed are predicted for a Wigley ship-hull in head waves and for a catamaran-ferry in oblique waves. Comparison is made with published theoretical and experimental results for the Wigley ship-hull, and the agreement is good. For the catamaran, a self-propelled model is built and tested both in a large towing tank and in a seakeeping basin in order to measure the six-degrees-of-freedom forces, moments and motions at forward speed in regular waves of different directions. For the longitudinal motions, the agreement between measurements and predictions is generally good. For the transverse motions, however, acceptable discrepancy exists. The discrepancy is thought to be mainly due to the exclusion from the analysis of the rudder forces and viscous damping. The inclusion of such nonlinear effects in the time domain simulation involves complex analysis and this problem is left to a future research. 相似文献
5.
This study considers the coupling effects of ship motion and sloshing. The linear ship motion is solved using an impulse-response-function (IRF) method, while the nonlinear sloshing flow is simulated using a finite-difference method. The IRF method requires the frequency-domain solution prior to conversion to time domain, but the computational effort is much less than that of direct time-domain approaches. The developed scheme is verified by comparing the motion RAOs between the frequency-domain solution and the solution obtained by the IRF method. Furthermore, a soft-spring concept and linear roll damping are implemented to predict more realistic motions of surge, sway, yaw, and roll. For the simulation of sloshing flow in liquid tanks, a physics-based numerical approach adopted by Kim [2001. Numerical simulation of sloshing flows with impact load. Applied Ocean Research 23, 53–62] and Kim et al. [2004. Numerical study on slosh-induced impact pressures on three-dimensional prismatic tanks. Applied Ocean Research 26, 213–226] is applied. In particular, the present method focuses on the simulation of the global motion of sloshing flow, ignoring some local phenomena. The sloshing-induced forces and moments are added to wave-excitation forces and moments, and then the corresponding body motion is obtained. The developed schemes are applied for two problems: the sway motion of a box-type barge with rectangular tanks and the roll motion of a modified S175 hull with rectangular anti-rolling tank. Motion RAOs are compared with existing results, showing fair agreement. It is found that the nonlinearity of sloshing flow is very important in coupling analysis. Due to the nonlinearity of sloshing flow, ship motion shows a strong sensitivity to wave slope. 相似文献
6.
The ability of a sailing yacht to re-right due to the effect of a breaking wave is investigated experimentally. Free and constrained physical models with varying mast height and centre of gravity were tested. To investigate the influence of retained mast height on sway force and roll moment, models were constrained by attachment to a force balance for sway motion tests in calm water and stationary tests in regular and breaking waves. Free model testing, with varying mast height and centre of gravity position, were carried out in breaking waves. For these tests, model motions in six degrees of freedom were measured using photogrammetry. The constrained tests showed that while the mast height had little effect on forces when stationary in waves it had a large effect when in sway motion. As models experience large sway motions when subject to a breaking wave the mast remnant plays a critical role in re-righting dynamics. This work demonstrates that re-righting probability is more dependent on mast height retention and wave characteristics than vertical centre of gravity. This conclusion has direct implications on re-righting safety assessment as the dominant design feature in most safety standards is the vertical centre of gravity. 相似文献
7.
A method to compute wave- and current-induced viscous drift forces and moments on floating platforms in regular and random waves is presented. The relative velocity drag term of Morison's equation is used in conjunction with frequency domain first-order motion transfer functions to compute the drift forces and moments. Mean viscous drift forces and moments in regular waves in all six degrees-of-freedom of a tension leg platform are computed. The relative importance of the free-surface force integration, steady current, wave-current interaction and platform motions on the computed drift forces and moments are discussed. The results from this method, in the frequency domain, are used to compute the drift forces and responses in irregular waves using existing methods developed for potential drift computations. Comparisons with results from time-domain computations are also presented and good agreement between the frequency-domain and time-domain results is found. Some comparisons with experimental data are also made. The frequency-domain method is found to be an efficient and useful tool for the analysis of semi-submersible and tension leg platforms during the preliminary design stage in which extensive parametric studies need to be undertaken. 相似文献
8.
Current paper presents a mathematical model based on 2D-asymmetric wedge water entry to model heave and pitch motions of planing hulls at non-zero heel angles. Vertical and horizontal forces as well as heeling moment due to asymmetric water entry are computed using momentum theory in conjunction with added mass of impact velocity in vertical and horizontal directions. The proposed model is able to compute sway and yaw forces, roll moment, as well as heave and pitch motions in calm water and regular waves. Validity of the proposed model is verified by comparing the results against existing experimental data in both symmetric and asymmetric conditions. Ultimately, different parametric studies are conducted to examine the effects of non-zero heel angle on dynamic vertical motions. The resulting sway and yaw forces due to asymmetric motion are also derived and effects of heel angle on these side forces are investigated. 相似文献
9.
M. Söylemez 《Ocean Engineering》1996,23(5):423-445
This paper presents the derivation of a general method for calculating wave forces on the cylindrical members of offshore structures. By means of the proposed method one can calculate the wave loading on cylindrical members of fixed or floating offshore structures orientated randomly in waves. This method of calculating wave forces is based on the linear Airy wave theory. Calculation procedure of wave force components is presented in great detail on the basis of wave particle kinematic properties obtained from the linear Airy wave theory. In the procedure of calculating wave forces presented, definitions of the wave reference system for propagating wave, the structure reference system for the platform and the member reference system for the tubular members of the structure are first established, and then the calculation of wave forces is given in terms of its components, which are pressure, acceleration and velocity forces, including current forces. At the end of the paper, expressions of total heave, sway and surge forces and total roll, pitch and yaw moments acting on the platform are given as a sum of these forces acting on each member of the platform. The calculation procedure derived in this paper provides a very efficient means of calculating wave forces and moments during the time-domain simulations of a floating platform experiencing large amplitude motion in intact, progressive flooding and damaged conditions. Comparisons of the predictions with the measurements which will be presented elsewhere reveal that the calculation procedure developed can predict large amplitude oscillatory and steady motion characteristics of an intact and damaged platform in waves with an acceptable degree of accuracy. 相似文献
10.
Real sea conditions are characterized by multidirectional sea waves. However, the prediction of hull load responses in oblique waves is a difficult problem due to numeral divergence. This paper focuses on the investigation of numerical and experimental methods of load responses of ultra-large vessels in oblique regular waves. A three dimensional nonlinear hydroelastic method is proposed. In order to numerically solve the divergence problem of time-domain motion equations in oblique waves, a proportional, integral and derivative (PID) autopilot model is applied. A tank model measurement methodology is used to conduct experiments for hydroelastic responses of a large container ship in oblique regular waves. To implement the tests, a segmented ship model and oblique wave testing system are designed and assembled. Then a series of tests corresponding to various wave headings are carried out to investigate the vibrational characteristics of the model. Finally, time-domain numerical simulations of the ship are carried out. The numerical analysis results by the presented method show good agreement with experimental results. 相似文献
11.
Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective. 相似文献
12.
This paper presents the results of an extensive parametric study to investigate various non-linear aspects of the prediction of the large-amplitude motion responses of a semi-submersible. The main objectives of the parametric studies were to investigate the following aspects, which can non-linearly influence the motion responses and which cannot be studied by linear frequency-domain prediction techniques. These aspects are the effects on the motion responses of flooding time and mass; non-linear wave-exciting and rigid-body induced motion (i.e. added mass and damping) forces; non-linear restoring forces; steady wind and current; variation of GM (transverse metacentric height); and the initial position of the semi-submersible. The investigations were carried out for a particular semi-submersible geometry using a numerical simulation technique in the time domain. The simulations were performed for the model during intact, progressive and post-flooding conditions under the combined loading of regular waves, steady wind and current for two different heading angles. This paper is thus intended to provide some insight into the physical effects of the non-linear terms in the equations of motion which are associated with the wave-excitation forces, rigid-body induced motion forces and restoring forces. Since the resulting motion responses could have a steady component as well as the oscillatory one, the force and motion phenomena were also highlighted through the computation of these components. 相似文献
13.
By extending the linear frequency domain theory, a quasi-non-linear time-domain technique has been developed to investigate the large amplitude motions of catamarans in regular waves. The non-linearity of hydrodynamic forces included in this practical method comes from variations of a ship's submerged portion. These forces are obtained from a database generated by the linear frequency domain method at each time step. The coupled equations, heave and pitch, are solved in the time domain by using the Runge-Kutta method with proper initial values. In order to investigate the non-linear effects of large amplitude motions of the V-1 catamaran in the head-sea condition, numerical results obtained from the linear and non-linear strip methods have been compared with those obtained from a series of experiments carried out in the towing tank of the Hydrodynamics Laboratory at the University of Glasgow. Based on the comparative studies, the numerical results obtained from the time-domain program can provide better predictions for the large amplitude motions of catamarans than the linear frequency domain method. It is concluded that the non-linear effects are significant when the model speeds and wave amplitudes increase. The peak values of large amplitude motions around the resonance frequencies, as obtained from the non-linear time-domain predictions as well as from measurements, are smaller than those obtained from the linear theory. 相似文献
14.
This paper considers a numerical analysis of ship maneuvering performance in the presence of incident waves and resultant ship motion responses. To this end, a time-domain ship motion program is developed to solve the wave–body interaction problem with the ship slip speed and rotation, and it is coupled with a modular-type 4-DOF maneuvering problem. In this coupled problem, the second-order mean drift force, which can play an important role in the ship maneuvering trajectory, is estimated by using a direct pressure integration method. The developed method is validated by observing the second-order mean drift force, and planar trajectories in maneuvering tests with and without the presence of incident waves. The comparisons are made for two ship models, Series 60 with block coefficient 0.7 and the S-175 containership, with existing experimental data. The maneuvering tests observed in this study include a zig-zag test in calm water, and turning tests in calm water and in regular waves. The present results show a fair agreement of overall tendency in maneuvering trajectories. 相似文献
15.
The analytical method developed by Svendsen (1968) for a forced heave motion is extended to the general problem of wave induced heave, roll and sway motions of a long ship at a depth of water which is only slightly larger than the draught of the ship. This corresponds, for example, to the situation of a fully loaded ship in a harbour area.After linearization of the problem, the water motion is considered for each of the three individual motions and for the wave reflection-transmission problem for a fixed ship. The ensuing results for the forces on the ship are then synthesized to form the equations of motion, which are presented with all coefficients given, including mooring forces.Analytical and numerical results are given for the three components of motion, for the associated resonance frequencies, and for the hydrodynamic masses and moments of inertia. Finally, the assumptions used are analyzed and evaluated by comparison with measurements and with other results for a special case. 相似文献
16.
孤立波与带窄缝双箱相互作用模拟研究 总被引：1，自引：1，他引：0
针对孤立波与带窄缝双箱的作用问题,应用时域高阶边界元方法建立了二维数值水槽。其中,自由水面满足完全非线性运动学和动力学边界条件,对瞬时自由表面流体质点采用混合欧拉-拉格朗日法追踪,采用四阶龙格库塔法对下一时刻的自由水面的速度势和波面升高进行更新。采用加速度势法求解物体湿表面的瞬时波浪力。采用推板方法生成孤立波。通过模拟孤立波在直墙上的爬高以及施加在直墙上的波浪力,并与已发表的实验和数值结果对比,验证本数值模型的准确性。通过数值模拟计算研究了窄缝宽度、方箱尺寸对波浪在箱体迎浪侧爬高,窄缝内波面升高,箱体背浪侧透射波高及箱体受波浪荷载的影响。同时研究了有一定时间间隔的双孤立波与带窄缝双箱系统作用问题。 相似文献
17.
The paper studies the parametric stochastic roll motion in the random waves. The differential equation of the ship parametric roll under random wave is established with considering the nonlinear damping and ship speed. Random sea surface is treated as a narrow-band stochastic process, and the stochastic parametric excitation is studied based on the effective wave theory. The nonlinear restored arm function obtained from the numerical simulation is expressed as the approximate analytic function. By using the stochastic averaging method, the differential equation of motion is transformed into Ito's stochastic differential equation. The steady-state probability density function of roll motion is obtained, and the results are validated with the numerical simulation and model test. 相似文献
18.
《中国海洋工程》1998,(1)
A quasi-steady time domain method is developed for the prediction of dynamic behavior of amooring system under the environmental disturbances,such as regular or irregular waves,winds and cur-rents.The mooring forces are obtained in a static sense at each instant.The dynamic feature of the moor-ing cables can be obtained by incorporating the extended 3-D lumped-mass method with the known shipmotion history.Some nonlinear effects,such as the influence of the instantaneous change of the wettedhull surface on the hydrostatic restoring forces and Froude-Krylov forces,are included.Thecomputational results show a satisfactory agreement with the experimental ones. 相似文献
19.
为分析碟形越浪式波能发电装置初步设计方案的合理性和不足,应用三维势流理论和波浪的辐射-衍射理论,结合水动力分析软件AQWA,计算在规则波浪作用下装置浮体6个自由度上运动的响应幅值算子(RAOs),考虑不规则波浪、风、流载荷的共同作用,对装置在工作海况和恶劣海况下的运动响应进行数值模拟分析,得到其时间历程结果。结果表明:装置结构外形设计合理,装置在其锚链张力腿锚泊系统定位下,运动响应满足工作稳定性要求和安全性要求。 相似文献
20.
Parametric roll of a containership in head sea condition has been studied in the paper. A time domain routine for GZ righting arm calculation based on exact underwater hull geometry has been implemented into a two-degree-of-freedom procedure for roll response calculation. The speed variation due to e.g. added resistance has been accounted for in the model by the surge velocity. The ship roll motion due to a regular wave critical for parametric roll occurrence has been simulated, as well as the ship roll response in a severe stochastic sea. The present method has been compared with other existing methods for parametric roll prediction. 相似文献