NUMERICAL SIMULATION OF ICE MILLING LOADS ON PROPELLER BLADE WITH COHESIVE ELEMENT METHOD

In ice-infested waters, propellers of a polar ship are likely to be exposed to ice loads in different scenarios. Propeller milling with ice is one of the most dangerous cases for ice-propeller interaction. In this study, we try to simulate dynamic milling process of ice-propeller and reproduce resulting physical phenomena. Cohesive element method is used to model ice in the simulation. To simulate material properties of ice, an elastoplastic softening constitutive law is developed. Both crushing and fracture failures are included in the ice-propeller milling process. The ice loads in 6 Dofs acting on blades of a propeller are calculated in time domain. The average and standard deviations of simulated dominant ice loads are compared with those from model test. A good agreement is achieved. By varying propeller rotation speed, advance velocity and cutting depth on ice block, the sensitivity study has been carried out. The results show that dominant ice loads are affected much by the three parameters. It is shown that decreasing rotation speed, or increasing advance velocity and cutting depth may lead to higher ice loads. Care should be taken to avoid over-loading on propeller when operating in ice for polar ship

Nonparametric Modeling and Control of Ship Steering Motion Based on Local Gaussian Process Regression

This paper aims to study the nonparametric modeling and control of ship steering motion. Firstly, the black box response model is derived based on the Nomoto model. Then, the establishment of a nonparametric response model and prediction of ship steering motion are realized by applying the local Gaussian process regression (LGPR) algorithm. To assess the performance of LGPR, two cases are studied, including a Mariner class vessel by using simulation data and a KVLCC2 tanker model by using experimental data. The results reveal that the response model identified by LGPR presents good prediction accuracy and low computational burden. Finally, the identified response model is used as the basis for developing the ship heading controller, and the results demonstrate that the proposed controller is able to achieve good dynamic performance

Path following of a surface ship sailing in restricted waters under wind effect using robust H∞ guaranteed cost control

The path following problem of a ship sailing in restricted waters under wind effect is investigated based on Robust H∞ Guaranteed Cost Control (RHGCC). To design the controller, the ship maneuvering motion is modeled as a linear uncertain system with norm-bounded time-varying parametric uncertainty. To counteract the bank and wind effects, the integral of path error is augmented to the original system. Based on the extended linear uncertain system, sufficient conditions for existence of the RHGCC are given. To obtain an optimal robust H∞ guaranteed cost control law, a convex optimization problem with Linear Matrix Inequality (LMI) constraints is formulated, which minimizes the guaranteed cost of the close-loop system and mitigates the effect of external disturbance on the performance output. Numerical simulations have confirmed the effectiveness and robustness of the proposed control strategy for the path following goal of a ship sailing in restricted waters under wind effect. Keywords: Restricted waters, Path following, Wind effect, Guaranteed cost control, Robust H

Online Prediction of Ship Roll Motion in Waves Based on Auto-Moving Gird Search-Least Square Support Vector Machine

A novel method based on auto-moving grid search-least square support vector machine (AGS-LSSVM) is proposed for online predicting ship roll motion in waves. To verify the method, simulation data are used, which are obtained by solving the second-order nonlinear differential equation of ship roll motion using the fourth-order Runge–Kutta method, while the Pierson–Moskowitz spectrum (P–M spectrum) is used to simulate the irregular waves. Combining the sliding time window with the least square support vector machine (LS-SVM), the samples in the time window are used to train the LS-SVM model, and the model hyperparameters are optimized online by the auto-moving grid search (AGS) method. The trained model is used to predict the roll motion in the next 30 seconds, and the prediction results are compared with the simulation data. It is shown that the AGS-LSSVM is an effective method for online predicting ship roll motion in waves

Wavelet denoising method with a novel wavelet threshold function applied in denoising ship maneuvering test data

1780-1787Based on the classical wavelet threshold functions, a wavelet denoising method with a novel wavelet threshold function is proposed for denoising the data from ship maneuvering tests. The wavelet denoising methods combined with the hard-threshold function, the soft-threshold function and the proposed wavelet threshold function are applied respectively to denoise the polluted 20°/20° zigzag test data which are obtained by adding some random noise into the simulated 20°/20° zigzag test data. Comparison of the denoising errors shows that the proposed wavelet threshold function is superior to the hard-threshold function and the soft-threshold function, and the wavelet denoising method combined with the proposed wavelet threshold function is a promising method to denoise the data of ship maneuvering tests

STUDY OF CONTINUOUS ICEBREAKING PROCESS WITH COHESIVE ELEMENT METHOD

Accurate simulation of the continuous icebreaking process in level ice is crucial for the design of icebreakers. The crushing and bending failures of ice sheet, as well as the rotating, sliding and accumulating of ice cusps broken from the ice sheet constitute a complex system for the icebreaking process. In this paper, cohesive element method is combined with an elastoplastic softening constitutive model to simulate continuous icebreaking process in level ice. Firstly, the elastoplastic softening constitutive model in modelling ice local crushing is calibrated by simulating the ice cone crushing tests. Three different softening laws are proposed and their effects on simulation results are evaluated by comparing with the experimental data. Then, the continuous icebreaking process in level ice is simulated by cohesive element method. The regular tri-prism mesh is applied to ice bulk elements to realize the random propagation of crack. The mesh dependency study is carried out, and the simulation results are validated by comparing with model test results in both of time domain and frequency domain. The ice failure patterns during continuous icebreaking process are also compared between the simulated and experimental results. Finally, the influences of ship velocity on ice resistance and ice failure patterns are investigated by numerical methods and semi-empirical formulas

Numerical study on hydrodynamic interaction between two tankers in shallow water based on high-order panel method

A three-dimensional high-order panel method based on Non-Uniform Rational B-Spline (NURBS) for predicting the ship-ship hydrodynamic interaction during meeting and overtaking in shallow water is developed. The NURBS surface is used to precisely represent the hull geometry. The velocity potential on the body surface is described by B-spline after the source density distribution on the boundary surface is determined. A collocation approach is applied to the boundary integral equation discretization, and the velocity potential is being solved at each time step. Under the low-speed assumption, the effect of free surface elevation is neglected in the numerical calculation, and the infinite image method is used to deal with the finite water depth effect. Two tankers in model tests are investigated, and the predicted hydrodynamic interaction forces and moments are compared with experimental measurements to clarify the validity of the proposed numerical method. Calculations are then conducted for different water depths, lateral ship-ship distances and ship speeds, and the detailed results are discussed to demonstrate the effects of these factors