P Krishnankutty

Traditionally, ship maneuvering performances are predicted in calm water condition to evaluate the directional stability and turning ability of the ship in the early design stages. Evaluation of maneuvering performance of a ship including wave effect is more realistic and important for the safety of ships at sea. Determination of hydrodynamic derivatives is the basic step in solving maneuvering equations of motion. Accurate estimation of hydrodynamic derivatives is necessary for the prediction of vessel trajectories. As the ship maneuvers through seaway, the effect of wave load will alter the maneuvering derivatives, consequently the vessel trajectory will be affected significantly. Hence, the influence of wave on hydrodynamic derivatives needs to be determined. For the present study, horizontal planar motion mechanism tests are numerically simulated for a container ship in head sea condition using RANS-based CFD solver. Obtained force/moment time series include both wave excitation forces/moment and hydrodynamic forces on the hull due to PMM motions. Fast Fourier transform (FFT) algorithm is used to filter the hydrodynamic forces/moment from the estimated total force/moment time series. The Fourier series expansion method is used to derive the hydrodynamic derivatives from the estimated force/moment time series. A comparison study is done with the wave-effected hydrodynamic derivatives and derivatives in still water condition.

Captive ship model tests are conducted to determine the hydrodynamic derivatives appearing in the maneuvering equations of motions of a ship. These hydrodynamic derivatives have an important role in maneuvering prediction of a ship at early design stages. Practically, surface ships operate at different speed conditions. Variation in vessel speed will affect the hydrodynamic derivatives and subsequently maneuvering characteristics of the ship. This paper investigates the effect of vessel speed on the derivatives and maneuvering characteristics of a ship. Captive model tests are numerically simulated in a CFD environment for a container ship at different Froude numbers to estimate the influence of Froude number on hydrodynamic derivatives and on the turning characteristics of the ship.

Ships operate in seaways, but ship manoeuvrability is usually studied in calm water conditions, thus ignoring the effects of wave on the ship hydrodynamic behaviour. So, the assessment of surface ship manoeuvrability in wave environment is more realistic and accurate than its estimation in calm water condition. In the present study, captive dynamic ship model tests are numerically simulated in a computational fluid dynamics environment in regular head sea waves and the hydrodynamic derivatives are derived from the estimated force/moment time series using the Fourier series expansion method. These derivatives and wave excitation forces are fitted in the manoeuvring equations of motion and are solved to simulate the ship standard manoeuvres in head sea waves. Parameters of these definitive manoeuvres in wave condition are compared with those in still-water condition.

Accurate determination of hydrodynamic derivatives, appearing in the equations of motion of a marine vehicle, is essential for the correct prediction of its manoeuvring performance. Solution of these equations leads to the simulation of the ship motions in the horizontal plane for a surface ship and thus helps in understanding its course stability, turning ability, rudder effectiveness and ship responsiveness. This paper presents the numerical simulation of the Planar Motion Mechanism test using RANS-based equation to obtain the hydrodynamic derivatives appearing in the equations of motion and to assess the manoeuvrability of a container ship using standard definitive manoeuvers. Experimental validation of the numerical test is carried out by conducting free running model tests in a manoeuvring basin.