Experimental analysis and evaluation of a dynamic positioning thruster in heavy loading conditions

El-Lababidy, Said (2005) Experimental analysis and evaluation of a dynamic positioning thruster in heavy loading conditions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (22MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Over the past few years, Dynamic Positioning (DP) systems have seen increasing use in the offshore oil and gas industry. As operations move into more hostile and deeper waters, the need for DP systems has become almost mandatory. Thrusters working at low advance ratios are employed in a wide range of offshore and marine applications on Floating, Production, Storage and Offloading (FPSO) systems, shuttle tankers, tug boats, and mobile offshore units. Therefore, an understanding of the flow around a DP thruster is of great practical interest. -- Despite this interest, there is lack of knowledge on the description of the hydrodynamic characteristics of a DP thruster's wake under varied operating conditions and regarding a description of the effects of the current wash produced from DP thrusters when they are in operation. The large number of parameters involved in these particular problems does not allow an experimental and systematic coverage of the various phenomena of the DP thruster's wake. Therefore instead, recourse must be taken to a combination of experiments and calculations, which together eventually will yield a prediction tool that can be used to further improve the analysis of a DP thrusters wake. With the increasing importance and applications of DP thrusters in the offshore oil and gas industry, the designer as well as the operator must have more information about the operational propulsive characteristics. Future development and the success in such approaches depend greatly upon our understanding of the flow around a DP thruster. -- Different phenomena associated with the propeller wake can be measured by the introduction of laser techniques: Laser Doppler Velocimetry (LDV) and Particle Imagine Velocimetry (PIV), as propeller flow velocity-measuring tools. Moreover, with Computational Fluid Dynamics (CFD) codes finding their way into practical applications, a combined analysis is gradually becoming feasible. Experiments are then used to validate and complement computations. -- The aim of this study is to provide a comprehensive and systematic analysis of the flow produced from the operation of a DP thruster with and without a nozzle, under varied operating conditions. Furthermore, this study aims to quantify the interaction effects between the DP thruster wake and any structures placed in the working region of the DP thruster up to 15 diameters. This study is the first such approach to the particular problems associated with DP thruster operation. -- In this research, a DP thruster's performance characteristics and wake velocities were analyzed experimentally, and the DP thruster near and far wake profile was simulated computationally. The DP thruster performance characteristics were investigated at different propeller speeds, with and without a nozzle, and over a range of advance ratios including the bollard pull condition. The DP thruster wake velocities were measured using two different laser techniques in two different facilities: a LDV system at the National Research Council of Canada (NRC) Institute for Ocean Technology (IOT) cavitation tunnel; and a Stereoscopic PIV system at the Italian Ship Model Basin (INSEAN), Rome, Italy, large circulating tunnel. The numerical simulation of the DP thruster wake profile was performed using CFD software FLUENT 6.0. The experimental results of the DP thruster near wake velocity were used as input to the CFD numerical simulation. -- The results of the experimental work of this study show the advantages of using ducted thrusters rather than open thrusters in low speed and DP applications. Also, these results provide valuable data about the safest distance, which should be between the DP thrusters and any submerged structure in the slipstream. The evaluation of the DP thruster near and far wake characteristics under a variety of working conditions provide designers of structures in the slipstream of the DP thruster with reliable and accurate data of the DP thruster wake momentum, thrust and energy propagation. These data are fundamental in calculating the load increases on these structures due to the operation of the DP thruster. Therefore, reliable and sound design of these structures can be achieved. -- A number of DP thruster near and far wake simulations under a variety of working conditions were performed using FLUENT 6.0. The simulation of the DP thruster wake was simplified and was considered to be similar to a turbulent jet flow, in which the flow of the propeller was simulated without using the detailed propeller geometry. The results from the CFD simulations showed the capability of this method in simulating the DP thruster wake propagation and dissipation under a variety of working conditions. -- The results of this research provide detailed data about DP thrusters' performance characteristics, near and far wake hydrodynamic features, wake momentum propagation and far wake momentum effects under a variety of working conditions. Moreover, the results of this study provide reliable and accurate data about the DP thruster wake to be used for numerical and CFD simulation and validation work. These can be used as an important tool for evaluating the performance of DP thrusters and for predicting DP thrusters wake effect on structures in the slipstream.

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