• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 2
  • Tagged with
  • 520
  • 197
  • 178
  • 170
  • 149
  • 41
  • 31
  • 29
  • 27
  • 25
  • 24
  • 15
  • 14
  • 12
  • 12
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Dynamic modelling of SOFC marine power systems and shipboard applications

San, Baogang January 2014 (has links)
Sustainable and efficient provision of shipboard energy is an obvious challenge for the merchant marine industry. Various initiatives have been made to find alternatives to replace the currently used combustion engine for ship power and propulsion. In recent years, fuel cell technology, as has been widely advertised as a clean and efficient means of power generation, is drawing much attention from the marine industry. Among various types of fuel cells, the Solid Oxide Fuel Cell (SOFC) tops others in terms of energy conversion efficiency as it can be fed with both hydrogen rich fuels and traditional fossil fuels after being chemically reformed. These features make it most promising to meet the large power demands of seagoing ships. However, due to the comprehensive and hazardous working environment, shipboard installation of SOFC power systems is not available. Can the SOFC be a viable proposition for commercial shipping and how will it behave under severe seagoing circumstances? These questions need to be addressed before commercialising SOFC marine power systems. In the thesis, simulation methods are used to predict the performance of marine SOFC systems at both static and dynamic working loads. A mathematical model is developed for describing the thermodynamic nature of a tubular SOFC concerning the thermal equilibrium of the system. Electric-chemical reactions are reflected in the stack modelling. Reforming reactions of the fuel are included in the model. Auxiliary subcomponents within the SOFC power system are modelled based on their own mechanisms and working principles. The whole simulation system is composed by combining subcomponent models via reasonable control strategies to function the system's purpose. SOFC power system models are developed to represent different working scenarios which may possibly occur onboard ship. The dynamic responses of simulation systems are examined. Thermal flow transfer influence, manifold volume influence and controller 's influence are also taken into account in the dynamic modelling process. As concluded from the simulation outcomes, the sample SOFC system, while running alone, could satisfy the demand of dynamic load change for both propulsion and auxiliary power. However, the electric output of the SOFC system would be greatly smoothed if paralleled with a battery. In addition, risk and safety issues regarding SOFC onboard installation are examined from both design and operating perspectives. Relevant ship rules and regulations for verifications of system installation and maintenance are reviewed in detail. Conceptual design of marine SOFC application are also proposed at the last stage.

Dynamic analysis of raising sunken vessels using buoyant systems

Velayudhan, Arun Kumar Devaki Bhavan January 2014 (has links)
In this research, mathematical formulations for the dynamics of raising sunken vessels using buoyant systems are developed in a form which is suitable for integrating control techniques to ensure both hydrodynamic and structural stability for a safe and stable salvaging operation based on both rigid body modeling and flexible body modeling concepts. Due to the coupled nature of salvage dynamics and for integrating controller techniques, the mathematical modeling is carried out as two subsystems. In the primary model, the salvage dynamics is formulated in such a way that the variation in additional buoyancy due to flow rate of filling gas inside the lift bags is the controlling force with respect to hydrostatic force due to weight, buoyancy and suction break out, hydrodynamic forces and uncertainty arises due to any external disturbances. In the secondary model, the purging of gas through the valves is taken as the control parameter by accounting the excess buoyancy available after suction break out and to the variation in pressure difference between gas inside lift bag and surrounding sea water pressure for a stable ascent. According to the simplified two-degree-of-freedom equations of rigid-body vessel motion, a state space model is developed for integrating the primary controller. Initially a proportional derivative (PD) controller is selected as the primary controller for regulating the flow rate of filling gas inside the lift bags according to the buoyancy requirement and extended to other classic controllers like proportional integral and derivative (PID) controller and sliding mode controller (SMC) for improving the performance. Numerical simulations are carried out in MATLAB & SIMULINK by solving the standard State Dependent Ricatti Equation in a body-fixed coordinate reference frame. Preliminary results in terms of coordinate positions or trajectories, linear and angular velocity components of the raising body are evaluated based on an experimental pontoon model. A number of case studies are carried out for different target depths with the developed linear state space model including sensitivity analysis such as change in hydrodynamic coefficients, breakout lift force and the effect of external disturbances and uncertainty. SMC is found to be the optimum choice among these conventional controllers by satisfying the Lyapunov stability condition even for higher water depths with system robustness and capability to handle parameter variations, external disturbances and uncertainty. The tuning effort and chattering were found to be the two major draw backs of conventional sliding mode controller (CSMC), which is improved by integrating it with artificial intelligence such as fuzzy logic controller to bring together the advantages of both controllers to become fuzzy sliding mode controllers (FSMCs). In FSMCs, the performance of the CSMC is improved by dynamically computing the sliding surface slope by a FLC and adaptively computing the controller gain by another FLC. FLCs are designed using MATLAB's fuzzy logic interface based on Mamdani's implification method the combined models will be developed in SIMULINK. A two input fuzzy sliding mode controller (TIFSMC) is designed first and later simplified to single input fuzzy sliding mode controller (SIFSMC), for reducing the tuning effort and computational time. With the development of SIFSMC, the tuning process becomes standardized and hassle free and hence the well known chattering problem associated with SMCs is avoided. The comparative performance of the fuzzy sliding mode controllers over CSMC has been investigated by performing numerical simulations on the pontoon model. It is found that both FSMCs show 30% of improvement in the tracking performance when compared to the CSMC, while maintaining its robustness. It is also noted that FSMCs are less sensitive to external disturbances and uncertainties in comparison with CSMC. The responses obtained by the SIFSMC are the same as those obtained by the TIFSMC, with the former involving a much less tuning effort and computational time. Simulation studies reveals the fact that for complicated non linear underwater operations like marine salvage involving uncertainty and external disturbances, a closed loop control system is mandatory and an adaptive controller like SIFSMC is the optimum choice as the primary controller for regulating the gas flow rate. Purge valve modeling is carried out according to the excess buoyancy available after suction breakout and to the variation in pressure difference between gas inside the lift bags and surrounding sea water for a stable ascent.

Development of constant-force tank-testing techniques and associated instrumentation

Yemm, Richard William January 1995 (has links)
Over the last ten years there has been a resurgence of interest in the detailed experimental testing of sailing yacht designs. Part 1 of this thesis reports work carried out with the aim of improving test realism and data quality. Reduction of the cost of testing by increasing the rate of data generation is also reported. Constant-Force towing methods in oblique seas were developed. Both Constant-Force and Constant-Velocity data is presented along with a detailed comparison between the two modes. This comparison shows an apparent, frequency dependent, difference of up to +-2% between the two testing modes. During the work on yacht testing techniques a novel stiff DC loadcell was developed to meet the specific requirements of the tank testing apparatus. Part 2 of this thesis reports the work carried out on the development of this transducer. The final prototype achieved a full scale deflection of 10 microns with an overload factor of more than 20 times the rated load.

Decision support system for risk-based inspection and maintenance planning for ship hull structures

Hifi, Nabile January 2013 (has links)
This thesis aims to develop a decision support system for inspection and maintenance planning of ship hull structures considering the effects of repair activities performed during the different periodical inspection events through the service life of a ship. Because of the severe environmental conditions in which ships operate, their structure is continually subjected to fatigue and corrosion degradation and as a result of that their strength is reduced. Corrosion and fatigue cracking represent the most aggressive types of structural damage faced by ship structures, either of which, if not properly repaired or adjusted, can potentially lead to leakage, pollution, fire, critical failures or unanticipated out of service time and economic costs. For an economic design to be achieved, the ship structures need to be maintained during their life. Building a ship with enough safety margins so that repairs would not be required during its life would be uneconomical and not technically feasible. From the viewpoint of survey and inspection of ship hull structures, improvements in inspection planning, safety and reduction of maintenance costs are the most needed. These issues are addressed in the newly developed decision support system described in this thesis. Inspection planning may be based on experience (determined by Class Society guidelines), which generally treat all ships with the same inspection program or based on a risk-based maintenance planning program. In the first case, only some of the knowledge that could be used to predict structural problems, in the case of ship-to-ship variation (construction or use), is gained from the data gathered, while in the second case, risk based maintenance methods can deal with any individual structural component or with overall ship structural integrity. To bridge the gap between these two approaches, this thesis combines the knowledge gained from currently used practice in ship inspection and maintenance and from risk-based methods which have already been proven as a good practice in several industrial applications. The newly developed decision support system is employed to calibrate the results of prediction models based on the collected data. To assist in the prediction of structural degradation of ships, a new structural connections catalogue, an inspection oriented ship defects database and a calibration methodology for structural degradation prediction models are developed. The new system is designed to improve risk-based ship inspection and maintenance planning programs. Application of the newly developed system will benefit inspection companies, class surveyors, ship managers and ship designers by providing a mechanism for the calibration of risk based inspection planning activities. The decision support system developed in this thesis is inherently adaptable and can be applied to many other applications that require a cost effective maintenace, e.g. renewable energy devices, offshore platforms, machinery systems, large structures such as bridges and other transport systems.

The application of the finite element techniques to the transverse strength of ships

Al-Sani, M. A. January 1980 (has links)
No description available.

The control of ship motions using the rudder

Cowley, William Edward January 1973 (has links)
No description available.

Time domain simulation of ship motions in waves

Ballard, Edward John January 2002 (has links)
No description available.

Hydroelastic modelling for the prediction of wave induced loads on bulk carriers

Hirdaris, Spyridon Evgenios January 2002 (has links)
No description available.

Friction and wear of piston ring : cylinder liner contact in a large marine diesel engine

Zhou, Qing January 2002 (has links)
No description available.

Value-based maintenance using limited data

Pople, Andrew James January 2001 (has links)
No description available.

Page generated in 0.0264 seconds