Ming-Chung Fang

Hydrodynamic interactions between two ships advancing in waves

In this paper the hydrodynamic problems between two moving ships in waves are analyzed using a three-dimensional potential-flow theory based on the source distribution technique. The potential is presented by a distribution of source over the ship hull. The corresponding Green functions and their derivatives can be easily solved numerically by using the series expansions of Telste and Noblesses algorithm for the Cauchy principal value integral of unsteady flow. The numerical solution is evaluated by applying the present method to two pairs of models and compared with experimental data and strip theory. From the comparisons, it shows that the hydrodynamic interactions are generally important. In the resonance region, the hydrodynamic interaction calculated by the 3D method is more reasonable, which is not so significant as that by the 2D method. The technique developed here may serve as a more rigorous tool to analyze the related problems of two ships doing underway replenishment in waves.

SECOND-ORDER STEADY FORCES ON A SHIP ADVANCING IN WAVES

The nonlinear forces, including the mean drifting forces and added resistance, exerted on a ship advancing in regular waves are formulated in the paper. Neglecting the second-order potential, the first-order one is used to calculate the second-order forces which are expressed as products of the radiation potential, diffraction potential, the incident wave potential and the ship motion response. Based on the well developed strip theory, the first-order potentials of each ship section are derived by source distribution method and the related first-order ship motion responses are also obtained. Several wave headings and ship speeds are considered. Comparisons are made with other theoretical results and some experimental data and good agreements are generally obtained. It is also found that the second-order forces obtained based on the present strip theory are generally better than those obtained by other methods.

The simulation of the ROV motion with anti-pitch control in uniform current

In the paper, a hydrodynamic numerical model for predicting the ROV motion with anti-pitch control is presented. The six degrees of freedom of equations of motions are formulated, which includes the coupled effect due to the current and the umbilical cable. Because of the umbilical cable on the ROV, the two-point boundary value problem with respect to a set of first order ordinary differential equation system arises and a multi-step shooting method is suggested to solve this problem. The anti-pitch technique including the depth keeping, either by PD control or PID control, is proposed to maintain the attitude of ROV in the current to reduce the resistance and other disturbance. The results reveal that both controlled techniques can indeed reduce the pitch motion and keep the desired depth for the ROV even in strong current. The hydrodynamic model developed here can therefore serve as a useful tool to improve the performance of the ROV operated in uniform current.

SOLVING HYDRODYNAMIC PROBLEMS FOR TWIN-HULL BODY BY PHASE TRANSFER METHOD

Based on the strip theory, the authors use the phase transfer technique to treat the hydrodynamic problems for a twin-hull structure in the paper. Using the method, the total forces on the twin-hull structure can be obtained by calculating the corresponding hydrodynamic coefficients for a single body only. There is no need to calculate the force on each body twice and the hydrodynamic interactions are not included. Practically the present method is suitable for predicting the corresponding hydrodynamic properties of a semisubmersible and a SWATH ship with high speed because the hydrodynamic interactions are not significant for these two cases. The results obtained in the paper indeed show the present idea is reasonable and therefore the method developed here can be considered as a efficient tool to analyze the hydrodynamic problems for some twinxad hull body structures with less interaction effects in waves, either with or without speed.

On the stability and wave profile of A ship in oblique waves

In the paper, the analyses of the stability and resultant wave profile for a ship advancing in oblique waves are made. A time simulation technique based on the previously well developed strip theory is used to calculate the ship motion and the relative wave elevation along the ship. An Instant Grid-Generation (IGG) technique is also applied to reduce the laborious work for the input preparation. The resultant wave elevation includes not only the incident wave but also the diffraction and radiation waves. To analyze the ship stability in waves, the instantaneous wave profile along the ship is derived. The wave profile prediction is also improved by using a more accurate estimation between the incident wave and hull side. The related hydrodynamic coefficients are treated in the manner of large ship motion, hence it is a nonlinear problem. From the present analysis, it is found that the ship stability in waves indeed differs from that in calm water. The authors regard that the present technique can offer more general ship stability analysis and will be helpful to understand the ship capsizing behavior in waves.

The Lateral Drifting Forces and Moments on Two Ships in Proximity in Waves

An analytical procedure for evaluating the lateral drifting forces and moments between two ships in oblique waves by near-field method is presented in this paper. The velocity potential, including the hydrodynamic interactions are evaluated by a two-dimensional sink-source technique. Then the strip theory is applied to calculate the sectional force and the drifting forces and moments of the whole ships can be obtained by Simpson rules. Four components of the mean drifting force are obtained in which the relative wave term is dominant, whereas the Bernoulli quadratic component is secondary. The negative drifting force is observed at some frequency for the ship which is in the weather side of the wave. The lateral drifting force even occurs while the ships are in the head or following seas, which is consistent with the real physical phenomena at sea. The present technique offers the theoretical explanation for nonlinear phenomena between two ships in waves and will be helpful for the further practical study in random waves.