P Krishnankutty

Oceanographic research vessels (ORVs) are used to collect ocean data and analyse them to understand the physical, chemical and biological characteristics of seawater, seabed and other ocean-related factors affecting climate changes. These task require vessel to track a predefined trajectory at different speeds. It is crucial to understand the effect of different speed on manoeuvring characteristics of the ship so as to assign appropriate control parameter and techniques to make its position/track keeping ability to an acceptable level. This paper attempts to estimate hydrodynamic derivatives of ORV by performing planar motion mechanism experiments at different speeds to understand the effect of Froude number on the hydrodynamic forces and subsequently on the different coefficient appearing in equations of motion and then on its turning performance. Sensitivity analysis is carried out as part of current work to understand the effect of different hydrodynamic derivative value variation on different turning parameter of vessel.

Correct prediction of the hydrodynamic derivatives is essential for the accurate determination of ships manoeuvring performance. Numerical and experimental methods are widely used for the determination of these derivatives. Even though experimental methods are more reliable, these facilities are rare and often prohibitively expensive. More viable option, primarily during the early stages of the ship design, is to determine these derivatives numerically. And also most of the ship manoeuvring studies and regulations are on deep water conditions, whereas the ship manoeuvring performance is much worse in shallow waters, and its controllability is difficult. An attempt is made in this paper to study the shallow water effects on the sway velocity-dependent derivatives and rudder derivatives numerically. KRISO container ship (KCS), a benchmark example used by different research groups, is taken for the present study. Straight line or static drift tests are performed in a numerical environment at different drift and different rudder angles using a commercial CFD package. These tests are conducted in both deep and shallow water conditions. Effects of water depth on the sway velocity-dependent hydrodynamic derivatives and rudder derivatives are evaluated, and the results are presented and analysed.

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.

Manoeuvring behaviour of a vessel changes drastically when it enters from deep water region to a shallow water region. Flow characteristics, around the hull changes and the vessel, respond poorly to the use of control surfaces. Aim of this paper is to study the manoeuvring behavioural changes in a container ship for different water depth conditions. Computational fluid dynamic (CFD) methods are used for simulating static and dynamic captive model tests. Variation in hydrodynamic reaction forces and moments caused by the reduction in water depth and the subsequent effect in hydrodynamic derivatives appearing in the equation of motion are explained in detail. Standard turning circle and zigzag manoeuvring tests are simulated using the CFD generated hydrodynamic derivatives to assess the manoeuvring characteristics of the vessel.