随机激励下非线性海洋结构物响应分析方法研究进展

对在随机载荷作用下非线性海洋结构物的运动响应分析的各种方法进行了综述,对有些方法的基本原理做了阐述,并指出一些最新的研究进展和今后进一步研究的方向。     

Prediction of short-term distributions of load extremes of offshore wind turbines

This paper proposes a new methodology to select an optimal threshold level to be used in the peak over threshold (POT) method for the prediction of short-term distributions of load extremes of offshore wind turbines. Such an optimal threshold level is found based on the estimation of the variance-to-mean ratio for the occurrence of peak values, which characterizes the Poisson assumption. A generalized Pareto distribution is then fitted to the extracted peaks over the optimal threshold level and the distribution parameters are estimated by the method of the maximum spacing estimation. This methodology is applied to estimate the short-term distributions of load extremes of the blade bending moment and the tower base bending moment at the mudline of a monopile-supported 5MW offshore wind turbine as an example. The accuracy of the POT method using the optimal threshold level is shown to be better, in terms of the distribution fitting, than that of the POT methods using empirical threshold levels. The comparisons among the short-term extreme response values predicted by using the POT method with the optimal threshold levels and with the empirical threshold levels and by using direct simulation results further substantiate the validity of the proposed new methodology.     

浮式风机极限载荷与疲劳载荷对比分析

根据IEC61400-3设定工况,采用NREL开发的5 MW风机基础模型,应用FAST,以Aero-Hydro-Servo-Elastic耦合仿真技术对风机进行研究。对时域仿真得到的短期载荷,应用分块极大值法联合Gumbel分布外推计算风机极限载荷;以雨流计数法、线性累积损伤理论和S-N曲线为理论基础应用MLife软件,计算风机疲劳载荷。对比分析不同工况下浮式风机、近海单桩风机和陆上风机的极限载荷与疲劳载荷,进而探讨影响浮式风机动态响应的因素。结果表明,对于陆上风机和近海单桩风机,风是其主要载荷来源;而波浪是浮式风机主要载荷来源。对风机进行设计要根据特定海域统计的海洋气候条件,避免风机及其支撑结构的固有频率与波浪频率近似而产生共振;风机制造装配在一定误差范围内,质量不平衡对风机载荷几乎没有影响。     

On the Exceedance Probabilities of Extreme Drift Motions of An Offshore Structure

The response statistics of a compliant offshore structure excited by slowly varying wave drift forces is calculated by use of a numerical path integral solution method,rlhe path integral solution is based on the Gauss-Legendre interpolation scheme,and the values of the response probability density are obtained at the Gauss quadrature points in sub-intervals.It is demonstrated that a distinct advantage of the path integral solution is that the joint probabihty density of the response displacement and velocity is one of the by products of the calculations.This makes it possible to calculate the mean level up-crossing rates,which provides estimates of the exceedance probabilities of specified response levels for given time periods.     

Global Geometric Analysis of Ship Rolling and Capsizing in Random Waves

The nonlinear biased ship rolling motion and capsizing in random waves are studied by utilizing a global geometric method.Thompson's α-parameterized family of restoring functions is adopted in the vessel equation of motion for the representation of bias.To take into account the presence of randomness in the excitation and the response,a stochastic Melnikov method is developed and a mean-square criterion is obtained to provide an upper bound on the domain of the potential chaotic rolling motion.This criterion can be used to predict the qualitative nature of the invariant manifolds which represent the boundary between safe and unsafe initial conditions,and how these depend on system parameters of the specific ship model.Phase space transport theory and lobe dynamics are used to demonstrate how motions starting from initial conditions inside the regions bounded by the intersected manifolds will evolve and how unexpected capsizing can occur.     

Wave period distributions in non-Gaussian mixed sea states

The wave period probability densities in non-Gaussian mixed sea states are calculated by utilizing a transformed Gaussian process method. The transformation relating the non-Gaussian process and the original Gaussian process is obtained based on the equivalence of the level up-crossing rates of the two processes. A saddle point approximation procedure is applied for calculating the level up-crossing rates in this study. The accuracy and efficiency of the transformed Gaussian process method are validated by comparing the results predicted by using the method with those predicted by the Monte Carlo simulation method.     

Modified Rayleigh distribution of wave heights in transitional water depths

This paper concerns the calculation of wave height exceedance probabilities for nonlinear irregular waves in transitional water depths, and a Transformed Rayleigh method is first proposed for carrying out the calculation. In the proposed Transformed Rayleigh method, the transformation model is chosen to be a monotonic exponential function, calibrated such that the first three moments of the transformed model match the moments of the true process. The proposed new method has been applied for calculating the wave height exceedance probabilities of a sea state with the surface elevation data measured at the Poseidon platform. It is demonstrated in this case that the proposed new method can offer better predictions than those by using the conventional Rayleigh wave height distribution model. The proposed new method has been further applied for calculating the total horizontal loads on a generic jacket, and its accuracy has once again been substantiated. The research findings gained from this study demonstrate that the proposed Transformed Rayleigh model can be utilized as a promising alternative to the well-established nonlinear wave height distribution models.     

Modified Rayleigh Distribution of Wave Heights in Transitional Water Depths

This paper concerns the calculation of wave height exceedance probabilities for nonlinear irregular waves in transitional water depths, and a Transformed Rayleigh method is first proposed for carrying out the calculation. In the proposed Transformed Rayleigh method, the transformation model is chosen to be a monotonic exponential function, calibrated such that the first three moments of the transformed model match the moments of the true process. The proposed new method has been applied for calculating the wave height exceedance probabilities of a sea state with the surface elevation data measured at the Poseidon platform. It is demonstrated in this case that the proposed new method can offer better predictions than those by using the conventional Rayleigh wave height distribution model. The proposed new method has been further applied for calculating the total horizontal loads on a generic jacket, and its accuracy has once again been substantiated. The research findings gained from this study demonstrate that the proposed Transformed Rayleigh model can be utilized as a promising alternative to the well-established nonlinear wave height distribution models.     

瘫船稳性第二层薄弱性衡准研究

基于IMO第二代完整稳性草案中瘫船稳性第二层薄弱性衡准的有关研究,分析草案提出的简化计算方法,采用一个单自由度船舶横摇运动微分方程,利用例船CEHIPAR2792数据,编译相应程序,求得不规则横风横浪作用下瘫船横摇运动相应特征,进而计算特定海况下的倾覆概率及特定海区的平均倾覆指数,并与草案值进行对比。分析不同拟合扶正力臂曲线方法和剩余稳性高对计算结果的影响,并对瘫船稳性第二层薄弱性衡准方法进行修改。     

用迈尔尼可夫法分析海洋结构物动态稳性

介绍用迈尔尼可夫法分析海洋结构物动态稳性的原理和公式,并用迈尔尼可夫法预报了为防止一驳船倾覆而需要的阻尼,指出了在分析动态稳性时迈尔尼可夫法比传统的数值仿真在效率方面的优越性,同时也指出了在应用迈尔尼可夫法时应注意的问题.