P Krishnankutty

Evaluation of maneuverability of a ship at the early design stages is necessary for ensuring safety of its voyage. IMO recommends the test speed or approach speed for the maneuvering predictions as 90-100% of the service speed of the vessel. The confined model tests for ship maneuvering assessment are usually conducted at low speeds and the hydrodynamic derivatives obtained from these tests are used in the equation of motion even when vessel operates at much higher speeds. But the hydrodynamic derivatives and consequently the trajectory predicted using these derivatives differ substantially from the actual maneuvering conditions. Hence the dependency of the derivatives on vessel speed needs to be understood properly to get the correct estimate of the vessel trajectory prediction. This paper investigates the effect of vessel speed (Fn) on hydrodynamic characteristics of a container ship. Straightline test and horizontal planar motion mechanism (HPMM) tests are conducted for a container ship model for different speeds in a CFD environment.

The hydrodynamic performance of lunate tail fin propulsion inspired from Thunniform type of locomotion is analyzed numerically. The crescent-shaped foil is modeled using the NACA0012 section with a maximum cord length of 0.15m and an aspect ratio of 1.86. To mimic the exact foil motion of fishes, two actuators are used, each of them will give an oscillatory output to the foil so that the combined sway and yaw motion for the foil is obtained. The hydrodynamic performance of the flapping foil is investigated for different oscillation amplitudes and phase variations between the sway and yaw motions.

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.

Floating Production Storage and Offloading (FPSO) platforms are positioned in open sea using turret mooring systems and so Do Floating Liquefied Natural Gas (FLNG) platforms. The behavior of water inside the turret pit influences the platform operations considerably and hence a proper design and study of this aspect become essential for an efficient system. In the present work, the water column oscillation inside the turret of an FLNG during a tow condition at different speeds is studied. The natural periods of water column oscillations inside the turret moonpool are determined by performing decay tests in CFD environment. Free surface elevations at different locations inside the turret moonpool are analyzed both in time and frequency domains and the major natural modes are identified when the vessel is towed at constant speeds. It is found that the piston motion decays faster and sloshing motion remains for longer duration.

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.

The International Maritime Organization (IMO) has been keen in dealing with aspects of ship maneuverability, which are vital to its objectives of safer shipping and cleaner oceans for the past three decades. The standards specified by IMO on ship manoeuvring are for deep water condition, whereas it becomes more demanding on the designer to come up with a vessel which meets the more stringent requirements of controllability in shallow and confined waterways. The dynamic tests performed on a ship model using PMM in a towing tank yield these hydrodynamic coefficients, both linear and non-linear components in uncoupled and coupled modes of sway and yaw. The objective of the present work is to study shallow water effect on the manoeuvring coefficients of a container ship (S 175) model in a numerical towing tank. In dynamic simulations, the prescribed body motions have been imposed on the hull using user-defined field functions within the commercial CFD solver in pure sway and pure yaw mode. The hydrodynamic derivatives have been derived for the shallow water depths and are found to be in reasonably good agreement with the experimental data.

A CFD study to understand the hydrodynamics and fluid flow around a chordwise flexible hydrofoil with combined sway and yaw motion which imitates the caudal fin flapping in thunniforms, is presented. The dependency of motion parameters of the flexible flapping hydrofoil to its propulsive performance is studied by carrying out the analyses over a Strouhal number range of 0.1 to 0.4 in steps of 0.025 at three maximum angle of attacks viz. 10°,15°,20°. Qualitative observations of the wake field and trailing jet is presented using velocity magnitude contours and vorticity contours. The analyses carried out at 40,000 Reynolds number and sway amplitude of 0.75 chordlength, revealed that the average thrust coefficient increases with increase in Strouhal number and maximum angle of attacks. The highest efficiency is achieved when the maximum angle of attack is 15° and Strouhal number is 0.225.

A dynamic positioning (DP) system is a computer-controlled system which maintains the positioning and heading of ship by means of active thrust. A DP system consist of sensors, observer, controller and thrust allocation algorithm. The purpose of this paper is to investigate the performance of proportional derivative type fuzzy controller with Mamdani interface scheme for dynamic positioning of an oceanographic research vessel (ORV) by numerical simulation. Nonlinear passive observer is used to filter the noise from the position and orientation. A nonlinear mathematical model of the ORV is subjected to the wave disturbance ranging from calm to phenomenal sea. Robustness and efficiency of the fuzzy logic controller is analysed in comparison with the multivariable proportional integral derivative (PID) and the linear quadratic regulator (LQR) controller. A simplified constrained linear quadratic algorithm is used for thrust allocation. The frequency response of the closed loop system with different controllers is analysed using the bode plot. The stability of controller is established using the Lyapunov criteria.

Present study consists the effect of variation of the vehicle length on the hydrodynamic forces and moments of axisymmetric underwater vehicles due to angle of attack. Five different vehicle lengths are used for this study (L/D varying between 10 and 15) by experimental, numerical and empirical methods. Experimental investigation is done in a tow tank using a VPMM. Numerical analysis is carried out using commercial CFD code Fluent 6.2. The empirical estimation is based on method suggested by Allen & Perkins. Results show that the linear coefficients vary linearly with L/D, while the nonlinear coefficients vary nonlinearly with L/D for the range tested.