Ananthakrishnan P

Analytical algorithms developed and optimized for quantifying the wake behind in-stream hydrokinetic turbines are presented. These algorithms are based on wake expressions originally developed for wind turbines. Unlike previous related studies, the optimization of empirical coefficients contained in these algorithms is conducted using centerline velocity data from multiple published experimental studies of the wake velocities behind in-stream hydrokinetic turbine models or porous disks and not using computational fluid dynamics. Empirical coefficients are first individually optimized based on each set of experimental data, and then empirically based coefficient expressions are created using all of the data sets collectively, such that they are functions of ambient turbulence intensity. This expands the applicability of the created algorithms to cover the expected range of operating conditions for in-stream hydrokinetic turbines. Wind turbine wake model expressions are also modified to characterize the dependence of wake velocities on radial location from the centerline of in-stream hydrokinetic turbines. Thus, expressions with empirically optimized coefficients for calculating wake velocities behind in-stream hydrokinetic turbines are described in terms of both centerline and radial positions. Wake predictions made using the Larsen model for radial dependence are shown to diverge from experimental measurements near the wake radius defined by the Jensen model, suggesting that this is a good indication of the cutoff point beyond which numerical estimations no longer apply. Results suggest that using a combined Larsen/Ainslie approach or combined Jensen/Ainslie approach for characterizing wake have similar mean errors to using only a Larsen approach.

The Floating Drilling Production Storage and Offloading (FDPSO) Platform is functionally a non-propelled vessel and belongs to a subset of a wider group of vessels called Floating Production Systems (FPS). The FDPSO, besides its unique attribute of drilling capability, has all the functionalities of FPSOs. Given the remote offshore locations of oil fields, FDPSOs provide viable economic solution for drilling, production, storage and offloading to smaller vessels for transportation of oil. This paper addresses the designer’s concern in the critical role of positioning of the moonpool since it has a direct bearing on the dynamic effects experienced by the platform. The investigation provides valuable quantitative insights into the important effects on drag during towing, sloshing effect within the moonpool and motion dynamics due to the liquid oscillation. The results of this investigation using numerical and experimental tools, is expected to give key inputs for the designer towards the design of better performing FDPSO platform and benefit the industry. This paper investigates the vessel dynamics including the liquid oscillation behaviour inside the moonpool as a function of the moonpool location in the vessel. The results of the study are important inputs to a designer for consideration of new designs. The FDPSO is a dedicated design of a large capacity platform, different from the conventional FPSO which is usually a retrofitted ship for the purpose. The investigation is based on computational simulation using a commercial RANSE solver namely, STAR-CCM+. Comparison with results from towing tank tests serves to initially validate the numerical simulation-based results. The study performs hydrodynamic diffraction analysis using a potential flow-based solver, namely ANSYS–AQWA. A simplified hull form represents the FDPSO, considering that it is a platform predominantly stationary in operation. However, drag and interactive effects with the moonpool as well as a liquid oscillation in the moonpool are important dynamic conditions for investigation in stationary as well as transit conditions during the tow. The focus of the investigation is on moonpool dynamics in calm water and regular sea conditions and ship motion in waves. The moonpool is vertical circular cylindrical shaped and the investigation considers three moonpool locations namely, at the forward, the midship and the aft, respectively. The water column in the moonpool experiences large oscillatory motions in the piston mode or the sloshing mode. The analysis captures the flow physics, amplitude of oscillations, hydrodynamics resistance and the vessel motion response. The results obtained establish that the location of the moonpool contributes to the augmentation of the cavity drag in the FDPSO. For the selected parameters, the study indicates that the moonpool located in the forward region gives better performance as compared to responses in the other locations.

This paper presents a numerical approach for the prediction of thrust generated by a fish-type biomimetic propulsion system. We model the fish as a non-uniform Timoshenko Beam (TB) with varying Young’s modulus and moment of inertia along its length from head to tail. The non-uniform Timoshenko beam is assumed to be a cantilever with the fixed end at the head and free end at the tail and the corresponding mode shapes are determined. Based on the mode shapes, the thrust is calculated with respect to the input parameters such as wave velocity, propulsive velocity and wavelength. The method developed here is shown to be useful for the design of bio-inspired propulsive systems for marine vehicles and also for gaining a better understanding propulsive mechanics of a robotic fish. In results, the thrust delivered by the simple flapping beam is identified and the relationship between the non-dimensional frequencies is determined.

Autonomous underwater vehicle (AUV) is an uncrewed underwater robotic vehicle that operates independently of humans. The present work illustrates an effective design of the biomimetic autonomous underwater vehicle (BAUV) model and its trajectory control for the predefined path. The vehicle possesses a definitive biomimetic structure with specific lengths and widths. Three trajectory tracking control methods are used here: conventional proportional–integral–derivative (PID) control, H∞ control, and feedforward (along with feedback) control. The advantages and drawbacks of these would be governed by examining the characteristics of the methods. Finally, the future development of AUV trajectory tracking scenarios is discussed based on a comparison of control systems.

A brushless alternator with damper windings in the main alternator and with combined ac and thyristor fed dc loads has been handled ab initio as a total modelling and simulation problem for which a complete steady state performance prediction algorithm has been developed through proper application of Park's equivalent circuit approach individually to the main and exciter alternator units of the brushless alternator. Details of the problems faced during implementation of this algorithm through PSPICE for the case of a specific 125 kVA brushless alternator as well as methods adopted for successfully overcoming the same have then been presented. Finally a comparison of the predicted performance with those obtained experimentally for this 125 kVA unit has also been provided for the cases of both thyristor fed dc load alone as well as combined ac and thyristor fed dc loads. To enable proper calculation of derating factors to be used in the design of such brushless alternators, the simulation results then include harmonic analysis of the alternator output voltage and current waveforms at the point of common connection of the ac and thyristor fed dc load, damper winding currents, main alternator field winding current, exciter alternator armature voltage and the alternator developed torque and torque angle pulsations.

The hydrodynamic performance of a rigid foil inspired from the thunniform kind of propulsion is analyzed numerically. The hydrofoil is allowed to oscillate in pure pitch, pure heave and combined motion by varying the Strouhal number from 0.1 to 0.4. Both 2D and 3D analyzed are done for the combined motion of oscillation. The NACA0012 foil of 0.7m span and 0.141m chord is selected for the study. Qualitative observations of the wake field and trailing water jet is studied and presented using average vorticity and velocity contours. The parameters such as thrust coefficient, power coefficient, and efficiency are plotted against the Strouhal number. It is found that the maximum efficiency is achieved in Strouhal number range of 0.15–0.3.

As oil and gas exploration moves toward deeper, harsher waters, and marginal fields, the issues concerning the associated floating production storage and offloading ships (FPSOs) get more complex. Traditionally, at the early stage of the design of FPSOs, subsystem design is individually taken up and subsystem interactions are accounted for through an uncoupled approach, as against a more rigorous fully coupled approach. In fully coupled approach, the system components and mutual interactions are coupled concurrently. Issues related to mooring system of FPSOs are some of the key aspects needing close attention. Approaches for determining the peak behavior belonging to turret-moored FPSOs to random wind, seawater flow, and wave forces for a specified life are still evolving. In the case of weathervaning FPSOs, special consideration is essential for the representation of realistic non-collinear environments. Also, choice of location of turret inside hull is an important design decision, as turret position influences motions. Similarly, another aspect requiring attention is the development of resonant sloshing due to excitation owing to external waves. Also combined piston modes happen within the turret and in the volume between turret and moon pool walls, which is an important hydrodynamic phenomenon. The flow parting via chain table openings heavily damps the piston mode. The paper reviews the literature on some of these hydrodynamic aspects concerning motion response of turret-moored FPSOs.

Analysis of the fault performance of a brushless alternator with damper windings in the main alternator has been handled ab initio as a total modelling and simulation problem through proper application of Park's equivalent circuit approach individually to the main and exciter alternator units of the brushless alternator and the same has been implemented through PSPICE. The accuracy of the parameters used in the modelling and results obtained through PSPICE implementation are then evaluated for a specific 125 kVA brushless alternator in two stages as follows : First, by comparison of the predicted fault performance obtained from simulation of the 125 kVA main alternator alone treated as a conventional alternator with the results obtained through the use of closed form analytical expressions available in the literature for fault currents and torques in such conventional alternators. Secondly, by comparison of some of the simulation results with those obtained experimentally on the brushless alternator itself. To enable proper calculation of derating factors to be used in the design of such brushless alternators, simulation results then include harmonic analysis of the steady state fault currents and torques. Throughout these studies, the brushless alternator is treated to be on no load at the instant of occurrence of fault.