SHIP-SHIP HYDRODYNAMIC INTERACTION IN CONFINED WATERS WITH COMPLEX BOUNDARIES BY A PANELLED MOVING PATCH METHOD

This paper presents a potential flow solution for online estimation of hydrodynamic interaction between ships moving in restricted waters with complex boundaries. Each ship in concern is linked with a moving patch representing the arbitrary bathymetry beneath it. The wetted surfaces of ship hulls are meshed and loaded prior to the simulation, while the moving patches are dynamically discretized by a fast and robust mesh generator. The proposed method is validated for the ship- ship interaction case in the shallow water case with a flat and horizontal seabed where the mirror image technique is applicable, and satisfactory agreement is obtained. The method is further applied to simulate two interaction scenarios involving arbitrary seabed topography, and the numerical results are obtained and discussed.

Identification of Ship Hydrodynamic Derivatives Based on LS-SVM with Wavelet Threshold Denoising

Nowadays, system-based simulation is one of the main methods for ship manoeuvring prediction. Great efforts are usually devoted to the determination of hydrodynamic derivatives as required for the mathematical models used for such methods. System identification methods can be applied to determine hydrodynamic derivatives. The purpose of this work is to present a parameter identification study based on least-squares support-vector machines (LS-SVMs) to obtain hydrodynamic derivatives for an Abkowitz-type model. An approach for constructing training data is used to reduce parameter drift. In addition, wavelet threshold denoising is applied to filter out the noise from the sample data during data pre-processing. Most of the resulting derivatives are very close to the original ones—especially for linear derivatives. Although the errors of high-order derivatives seem large, the final predicted results of the turning circle and zigzag manoeuvres agree pretty well with the reference ones. This indicates that the used methods are effective in obtaining manoeuvring hydrodynamic derivatives.

Hardware in the Loop Simulation and Control Design for Autonomous Free Running Ship Models

This paper presents an hardware-in-the-loop (HIL) simulation system tool to test and validate an autonomous free running model system for ship hydrodynamic studies with a view to verification of the code, the control logic and system peripherals. The computer simulation of the plant model in real-time computer does not require the actual physical system and reduces the development cost and time for control design and testing purposes. The HIL system includes: the actual programmable embedded controller along with peripherals and a plant model virtually simulated in a real-time computer. With regard to ship controller design for ship model testing, this study describes a plant model for surge and a Nomoto first order steering dynamics, both implemented using Simulink software suit. The surge model captures a quasi-steady state relationship between surge speed and the propeller rpms, obtained from simple forward speed towing tank tests or derived analytically. The Nomoto first order steering dynamics is obtained by performing the standard turning circle test at model scale. The control logic obtained is embedded in a NI-cRIO based controller. The surge and steering dynamics models are used to design a proportional-derivative controller and an LQR controller. The controller runs a Linux based real-time operating system programmed using LabVIEW software. The HIL simulation tool allows for the emulation of standard ship hydrodynamic tests consisting of straight line, turning circle and zigzag to validate the combined system performance, prior to actual for use in the autonomous free-running tests.

A Numerical Method for Calculation of Ship–Ship Hydrodynamics Interaction in Shallow Water Accounting for Sinkage and Trim

Sinkage and trim, which often occur to ships moving in shallow water, do not only have an effect on the ship–ship hydrodynamic interaction forces but also increase the risk of grounding. Potential flow-based online calculation of ship–ship hydrodynamic interaction forces without accounting for dynamic sinkage and trim is able to capture the hydrodynamic interaction effects with fair accuracy; however, there are still discrepancies in many cases, especially in the case of shallow water. An algorithm based on the potential theory has been devised for real-time simulation of the hydrodynamic interaction between two ships in shallow water accounting for sinkage and trim. The shallow water condition is modeled using the mirror image method. The sinkage and trim are solved iteratively based on the principle of hydrodynamic balance, where a mesh trimming procedure is carried out when the waterline is changed. Simulations are performed with and without accounting for the sinkage and trim, and comparison with experimental results shows a fair agreement.

A Stereolithographic Model-Based Dense Body Plan Generation Method to Construct a Ship Hydrodynamic Coefficients Database

A ship’s body plan is a vital data resource of ship hydrodynamics analysis, especially for time-domain simulations. Motivated by 3D printing technology, a novel dense body plan generation method is developed in this study. The slicing algorithm is adopted to generate dense 2D body plans from ship stereolithographic models. The dense body plan can be produced automatically under arbitrary rotational angles and slices. Moreover, a section redistribution algorithm is integrated to eliminate the non-uniform distribution features in sliced data inherited from the stereolithographic models. The benchmark ship models are selected to validate the accuracy of the method. The hull volumes of three ship models are calculated based on the produced data. The calculated results show satisfactory agreement with the published values. Furthermore, the estimation formulas of wetted surface area (WSA) are reviewed and utilized for validation. The calculated WSAs by slice integration turn out to be adaptive and accurate. The time costs of different slices are provided to illustrate the computational efficiency. A ship hydrodynamic coefficients database is constructed based on a 2D strip method and the produced data. The proposed method aims to improve the generation process of the body plan, which could meet the accuracy requirements of the strip method. As a result, hydrodynamic coefficients utilized in time-domain simulations could be obtained smoothly from the database.

Numerical Simulation of Damaged Ship’s Motion in Beam Waves

The dangerous situation caused by the breakage of the ship will pose a serious threat to crew and ship safety. If the ship’s liquid cargo or fuel leaks, it will cause serious damage to the marine environment. If damage occurs accompanied by roll and other motions, it may cause more dangerous consequences. It is an important issue to study the damaged ship in time-domain. In this paper, the motions of the damaged DTMB 5512 in calm water and regular beam waves are studied numerically. The ship motions are analyzed through CFD methods, which are acknowledged as a reliable approach to simulate and analyze these complex physical phenomena. An in-house CFD (computational fluid dynamics) code HUST-Ship (Hydrodynamic Unsteady Simulation Technology for Ship) is used for solving RANS equations coupled with six degrees of freedom (6DOF) solid body motion equations. RANS equations discretized by finite difference method and solved by PISO algorithm. Level set was used for free surface simulation. The dynamic behavior of model was observed in both intact and damaged condition. The heave, roll and pitch amplitudes of the damaged ship were studied in calm water and beam wave of three wavelengths.