Parameter identification of nonlinear roll motion equation for floating structures in irregular waves

In order to predict the roll motion of a floating structure in irregular waves accurately, it is crucial to estimate the unknown damping coefficients and restoring moment coefficients in the nonlinear roll motion equation. In this paper, a parameter identification method based on a combination of random decrement technique and support vector regression (SVR) is proposed to identify the coefficients in the roll motion equation of a floating structure by using the measured roll response in irregular waves. Case studies based on the simulation data and model test data respectively are designed to validate the applicability and validity of the identification method. Firstly, the roll motion of a vessel is simulated by using the known coefficients from literature, and the simulated data are used to identify the coefficients in the roll motion equation. The identified coefficients are compared with the known values to validate the applicability of the identification method. Then the roll motion is predicted by using the identified coefficients. The prediction results are compared with the simulated data, and good agreement is achieved. Secondly, the model test data of a FPSO are used to identify the coefficients in the roll motion equation. Then the random decrement signature of the roll motion is predicted by using the identified coefficients and compared with that obtained from the model test data, and satisfactory agreement is achieved. From this study, it is shown that the identification method can be effectively applied to identify the coefficients in the nonlinear roll motion equation in irregular waves.     

Wave pattern induced by a moving atmospheric pressure disturbance

A moving low atmospheric pressure is a main feature of tropical cyclones, which can induce a system of forced water waves and is an important factor that cause water level rise during a storm. A numerical model based on the nonlinear shallow water equations is applied to study the forced waves caused by an atmospheric pressure disturbance moving with a constant velocity over water surface. The effects of the moving speed, the spatial scale and the central pressure drop of the pressure disturbance are discussed. The results show that the wave pattern caused by a moving low-pressure is highly related with its moving speed. The wave pattern undergoes a great change as the moving speed approaches the wave velocity in shallow water. When the moving speed is less than the wave velocity, the distribution of water surface elevation is nearly the same as that of the pressure disturbance, and the maximum of the water surface elevation is located at the center of pressure. When the moving speed is larger than the wave velocity, a triangle shaped wave pattern is formed with a depression occurs in front of the pressure center, and the maximum of the water surface elevation lags behind the center of pressure. As the moving speed increases, the maximum of the water surface elevation firstly increases and then decreases, which reaches a peak when the moving speed is close to the wave velocity. The maximum of water surface elevation is approximately in proportion to the central pressure drop, and slightly affected by the spatial scale of pressure disturbance. Both the central pressure drop and the spatial scale of the pressure disturbance do not significantly affect the forced wave pattern. However, a clear difference can be noticed on the ratio of the maximum water surface elevation in moving pressure situation to that in static situation, when the moving speed is close to the wave velocity. A pressure disturbance with smaller spatial scale and smaller central pressure drop will give a larger ratio when the moving speed is close to the wave velocity.     

Wave loads on a coastal bridge deck and the role of entrapped air

Wave loading on a coastal bridge deck due to nonlinear waves during a storm, where air may be fully or partially trapped between the girders, is studied through an extensive set of laboratory experiments. Wave cases tested cover a range of shallow-water to intermediate-water depth waves. A range of model elevations is tested to include conditions where the bridge may be partially inundated, to where the model is fully elevated above the still-water level (SWL). The model is constructed to include different percentages of air-relief openings, to capture a range of cases where no air can escape between the girders, to where all the air can escape and the wave can freely interact with the bottom of the bridge deck. Effects of the compressibility of entrapped air as well as the effects of the model scale are investigated through numerical calculations solving the compressible and incompressible Euler's equations, at both the model and prototype scales, by use of the open source CFD software, OpenFOAM. Along with coastal bridges, this research is applicable to other coastal and offshore structures, such as piers, submerged breakwaters and offshore platforms, in which wave loading or entrapped air is of concern.     

A mixed continuous and discrete nonlinear constrained algorithm for optimizing ship hull structural design

A nonlinear search algorithm for optimizing constrained design of ship structures is presented. The decision variables can be continuous or discrete and the constraints can be homogeneous or inequality nonlinear functions of those variables. The algorithm does not use gradients; therefore, it can work with non-systematized functions such as tables or another class of design routine. It was tested in the structural design of a Patrol Boat and has proved to be a powerful tool decreasing the time expended in preliminary design when it is done by the conventional spiral approach.     

Application of particle filter combined with extended Kalman filter in model identification of an autonomous underwater vehicle based on experimental data

This study proposes a method for identification of the nonlinear dynamic model of an AUV while some states are unmeasured; hence, it concentrates on a nonlinear “state and parameter estimation” issue. In this method, a local linearization is used for solving the nonlinear dynamics based on the extended Kalman filter (EKF), and a particle filter (PF) is used to minimize errors and variances of the nonlinear system. In other words, the PF is combined with the EKF in the form of the extended Kalman particle filter (EKPF). The EKPF method is independent of the initial values and satisfies the limits of the parameters and also the assumption that the hydrodynamic coefficients are constant. Hence, it is shown when the ranges or signs of some parameters are known, the EKPF is a more accurate estimator than the EKF. Moreover, a new simulation is done using the model estimated by the EKPF and the results are compared and validated with the measured data of a new experimental test. It is shown that the obtained model can predict the trajectory path with the total normalized root-mean-square error (NRMSE) of 14% and the surge mean speed with the NRMSE of 5%; and it describes the 6DOF motion of the AUV more accurate than the EKF model.     

Burial and scour of short cylinders under combined random waves and currents including effects of second order wave asymmetry

This paper provides an approach by which the burial and scour of short cylinders under combined second order random waves and currents can be derived. Here the formulas for burial and scour for regular waves plus currents presented by Catano-Lopera and Garcia [Catano-Lopera, Y.A. and Garcia, M.H. (2006). Burial of short cylinders induced by scour under combined waves and currents. ASCE J. Waterway, Port, Coastal and Ocean Eng. 132(6), 439–449., Catano-Lopera, Y.A. and Garcia, M.H. (2007). Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows. Ocean Eng. 34(5, 6), 856–869.] are used together with Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. An example of calculation is also presented.     

Shock-capturing Boussinesq-type model for nearshore wave processes

This paper describes the formulation and validation of a nearshore wave model for tropical coastal environment. The governing Boussinesq-type equations include the conservative form of the nonlinear shallow-water equations for shock capturing. A Riemann solver supplies the inter-cell flux and bathymetry source term, while a Godunov-type scheme integrates the evolution variables in time. The model handles wave breaking through momentum conservation with energy dissipation based on an eddy viscosity concept. The computed results show very good agreement with laboratory data for wave propagation over a submerged bar, wave breaking and runup on plane beaches as well as wave transformation over fringing reefs. The model accurately describes transition between supercritical and subcritical flows as well as development of dispersive waves in the processes.     

Numerical simulation of nonlinear wave radiation by a moving vertical cylinder

In this paper the fully nonlinear potential model based on a finite element method is used to investigate the nonlinear wave motion around a moving circular cylinder. The results for the cylinder in transient motion are compared with the experimental data and a much better agreement than the linear theory is found. Further simulation for a circular cylinder in sinusoidal motion is made. It is found that when the ratio of the cylinder diameter D to the wavelength L is relatively small at a fixed motion amplitude the nonlinear components of the runup on the cylinder surface at the second- and third-harmonic frequencies become more important and this is confirmed by the experimental data. Results for the hydrodynamic force are also provided for a cylinder oscillating in a channel. It is noticed that when the frequency of the cylinder motion in a channel is between the first and the second natural frequencies of the symmetric mode, the time history has components not only at the frequency of the cylinder motion but also at the first natural frequency. The latter remains significant over the period that the simulation is made. This has important implications to model testing. If measurement is to be made at such a frequency it may take long time for the motion to become periodic at the frequency of the cylinder motion.     

基于中尺度模式MM5下的海洋蒸发波导预报研究

蒸发波导是发牛在海气边界层的一种异常折射现象,由于其分布范围广、发生概率大,被认为是对海上电子设备影响最为显著的波导类型,成为各国海军竞相研究的焦点.然而由于其形成机制复杂,且在近岸沿海地区由于海陆分布不均,以及海岸地形和海陆风等因素的作用,会造成蒸发波导的近岸效应,这种效应会影响蒸发波导高度诊断的准确率.目前国内外蒸发波导诊断模式有P-J模式、MGB模式、NPS模式、Babin模式等,但其基本原理都足依赖Monin-Obukhov相似理论,只是用于确定近地层通量和特征尺度的方法不同,且仅适用于定常和水平均匀的开阔海域,并没有考虑到蒸发波导的近岸效应.针对这一问题,文中在Babin模式的基础上引入张强、胡隐樵的通量廓线关系(非线性修正冈子αν)与阵性风速ωg,从而将蒸发波导诊断模式的适用范围扩展到近岸沿海地区和甚低风速条件下.此外在中尺度模式MM5的基础上,耦合改进的Babin模式,发展建立了一个海洋蒸发波导高度预报模式,并对预报模式进行数值模拟,利用2002年5月25-26日福建平潭岛的海上大气实测数据与雷达探测结果对预报模式输出结果进行了验证.验证结果表明:在0～48小时内模式输出值与实测值拟和较好且变化规律一致,预报蒸发波导高度平均误差为0.193;且蒸发波导高度预报结果与雷达实际探测结果一致.