Driscoll, Frederick R.

This work details the development of tools and controllers for station keeping
control of twin screw vessels. A fundamental analysis is conducted of the dynamics of
twin screw displacement hull vessels and their actuator systems, where the response
characteristics and maneuverability are quantified through a series of full scale trials
conducted in different environmental conditions while recording the environmental
conditions, actuator states, and geodetic and inertial measurements. The data from these
maneuvers were repeatable from run to run and thus provide valuable benchmarks for
several maneuvers and the measured actuator response provides valuable set points of
performance characteristics/limitations for control development. A comprehensive
general simulation of small twin screw displacement hull boats is developed as a tool to
estimate ship and actuator responses in support of developing and tuning of control
systems. The model and computer simulation is capable of modeling a wide range of the
surface vessels, including their actuators and environmental conditions. This model
proved to be accurate, when compared to the sea trial data, and model estimates have rms velocity errors for the various steady maneuvers of 1.2-4.6% for surge, 12.6-17.9% for
sway, and 7.6-20.2% for yaw.
A path following station keeping controller is developed that uses Lyapunov
stability analysis to determine the path the vessel should follow to effectively eliminate
position error. This controller showed good performance for several different
environmental conditions. Encouraged by these finding, three additional station keeping
control methodologies are developed for twin screw surface ships. All four of these
controllers are examined for their robustness to environmental conditions, as well as their
sensitivity to sensor precision, sensor update rates, and actuator limitations. All
controllers are evaluated in sea state 4 yielding rms position errors from 3.3 to 16.2 m,
the rms surge and sway accelerations are under 0.62 m/s , and the engine shifting
frequencies are between 0.011 and 0.145 Hz. These four controllers are then tested over a
wide range of environmental conditions, sensor precisions and update rates, and actuator
response rates. The results from these tests give quantitative data that will aid in selecting
the appropriate controller for a specific application, and will assist in selecting
appropriate sensors.

At sea cargo transfer has historically been a logistical challenge for both the military and the offshore industry. Even in moderate seas, three to five foot wave heights, extreme pendulations of cargo and large relative motions between vessels can occur that halts cargo transfer activities. This work develops a six-degree-of-freedom rigid crane dynamics model that is used to investigate the feasibility of crane target tracking which could extend and enhance offshore crane operations. A double girder crane system is developed that easily adapts to different configurations and efficiently supports long reach heavy lift applications. Target tracking is feasible in sea states up to 5 when using the double girder crane. When compared to a present crane system, the target tracking crane requires, on average, only 3.65% more absolute total system power and 13.4% less continuous power, indicating that the proposed system should be realizable.

Underwater systems behavior prediction has become an important success factor in the design and implementation of marine systems. Most marine systems involve cables for mooring, deployment, recovery, or towing; however, estimating the response of these systems is difficult because of their non-linear behavior. Thus, numerical models are used to simulate submerged cabled systems. At FAU, many mission specific cable simulations have been developed, but no single, all encompassing software package exists. This thesis develops a Windows(c) based software package to quickly and easily create FEA models of underwater cabled systems and simulate their response. The model is based on a discrete finite element analysis using linear elements. The software provides fully integrated and interactive Graphical User Interfaces with a 3-dimensional graphical display of the model, and integrates adapted data analysis and visualization tools. The software provides an easy and efficient way to simulate an underwater system involving cables.

Technology movement toward deeper waters necessitates the control of vertically tethered systems that are used for installing, repairing, and maintaining underwater equipment. This has become an essential ingredient for the future success of the oil industry as the near-shore oil reservoirs are nearly depleted. Increased operation depths cause large oscillations and snap loadings in these longer cables. Research on this topic has been limited, and includes only top feedback control. The controllers developed in this thesis utilize top, bottom and combined top and bottom feedback. They are implemented on a discrete finite element lumped mass cable model. Comparison between PID, LQG and H infinity for all feedback combinations reveal that the Hinfinity controller with both top and bottom feedback has the best performance, while LQG has a more consistent and reliable performance for all feedback cases.

The "C-Plane" is a submerged ocean current turbine that uses sustained ocean currents to produce electricity. This turbine is moored to the sea floor and is capable of changing depth, as the current profile changes, to optimize energy production. A 1/30th scale physical prototype of the C-Plane is being developed and the analysis and control of this prototype is the focus of this work. A mathematical model and dynamic simulation of the 1/30th scale C-Plane prototype is created to analyze this vehicle's performance, and aid in the creation of control systems. The control systems that are created for this prototype each use three modes of operation and are the Mixed PID/Bang Bang, Mixed LQR/PID/Bang Bang, and Mixed LQG/PID/Bang Bang control systems. Each of these controllers is tested using the dynamic simulation and Mixed PID/Bang Bang controller proves to be the most efficient and robust controller during these tests.

This thesis develops a novel variable length cable model to simulate the behavior of submerged cables with variable unstretched length and a PC based simulation that integrates the governing cable equations. The general model is developed from continuous cable equations that are discretized using a finite element method with linear elements. Two systems of equations were developed, one for a variable length elastic element and the other for a constant length elastic element. A cable transition model is developed to ensure dynamic compatibility when a variable length element is divided or combined. The model proved to be an efficient and reliable tool to predict the behavior of underwater cables with variable length.

Aquantis, LLC, Santa Barbara, California intends to construct and deploy an ocean current turbine (C-Plane) to extract electrical power from the kinetic energy of the Florida Current. This study characterizes the physical oceanographic environment of the Florida Current flow, at a proposed deployment site located at 26.11 North Latitude, 79.50 West Longitude, which will influence the physical and mechanical design of the C-Plane. Local characteristic features of the Florida Current were determined from data collected during a 19 month in situ study using an Acoustic Doppler Current Profiler (ADCP) moored at 330 meters, a ship-mounted, inertial-correcting ADCP, and data culled from adjacent studies. Principles of physical oceanography and direct observations are applied to characterize the velocity structure of the current and its variability. This thesis presents the motivation behind the study, methods of data collection, statistical and numerical analyses, and available energy analyses.

This work develops, a general numerical model and efficient integration routine to calculate the response of the underwater cable that connects the Lockheed Martin remote minehunting vehicle to its variable depth sensor. The general model is developed from continuous cable equations that are discretized using a finite element method with linear elements. The resulting discrete system of equations is nonlinear and stiff. Thus, we chose the implicit Generalized-alpha method to integrate these equations because it possess numerical dissipation. This integration routine is coded into a C++ based numerical simulation and the results and efficiency were compared with the results and efficiency of the Runge-Kutta method. Based on the validation test cases, Generalized-alpha method proved to be an efficient and reliable integration method for stiff equations governing the motion of underwater cables.