Frederick R. Driscoll

Development of a Finite Element Cable Model for Use in Low-Tension Dynamics Simulation

To accurately simulate the motion of slack marine cables, it is necessary to capture the effects of the cables bending and torsional stiffness. In this paper, a computationally efficient and novel third-order finite element is presented that provides a representation of both the bending and torsional effects and accelerates the convergence of the model at relatively large element sizes. Using a weighted residual approach, the discretized motion equations for the new cubic element are developed. Applying inter-element constraint equations, we demonstrate how an assembly of these novel elemental equations can be significantly reduced to pretreat the growth of the system equations normallly associated with such higher order elements and allow for faster evaluation of the cable dynamics in either taut or low-tension situations.

OCEAN TURBINES — A RELIABILITY ASSESSMENT

This paper identifies factors that impact reliability and safety of ocean turbines. We describe how physical and environmental factors will impact the design of its machine condition monitoring (MCM) system. Environmental factors like fouling, corrosion, and inaccessibility of equipment sets this MCM problem apart from those encountered by wind turbines, hydroelectric plants, or even ship hulls and propellers. Fouling constitutes the primary and most persistent source of failure. In addition to compromising turbine efficiency and reliability, fouling reduces sensor data quality — masking faults that will ultimately lead to failure. Unmitigated fouling triggers a form of biological succession known as flocculation that may eventually attract threatened species of tortoises and cetaceans to this rotating machinery. We review and suggest refinements to a class of non-toxic biologically-inspired anti-fouling techniques known as engineered topographies. Advances in this area will enable turbines to operate in portions of the water column that maximize momentum flux while minimizing retrieval cost.

STATIC ANALYSIS OF THE LUMPED MASS CABLE MODEL USING A SHOOTING ALGORITHM

This paper focuses on a method to solve the static configuration for a lumped mass cable system. The method demonstrated here is intended to be used prior to performing a dynamics simulation of the cable. Conventional static analysis approaches resort to dynamics relaxation methods or root-finding algorithms (such as the Newton-Raphson method) to find the equilibrium profile. The alternative method demonstrated here is general enough for most cable configurations (slack or taut) and ranges of cable elasticity. The forces acting on the cable are attributable to elasticity, weight, buoyancy, and hydrodynamics. For the three-dimensional problem, the initial cable profile is obtained by solving three equations, regardless of the cable discretization resolution. This analysis discusses regions and circumstances under which failures in the method are encountered.

Numerical optimization of a cage-mounted passive heave compensation system

Heave compensation systems are used to increase the safe operating sea-states of vertically tethered systems, as well as, decrease the tension in the tether and the motion of the underwater platform. However, the characteristics of the heave compensator must be carefully chosen or operating problems may be exacerbated. In this paper, a discrete representation of a deep-sea remotely operated vehicle system with a passive cage-mounted heave compensator is used within a sequential quadratic programming optimization routine to choose the stiffness and damping characteristics that minimize the tension in the tether and platform motion. Using the optimal values of the stiffness and damping, the rms cage motion and rms tension were significantly decreased over all operating depths and sea-states.

Florida's Center for Ocean Energy Technology

The Center for Ocean Energy Technology (COET) at Florida Atlantic University (FAU) is a timely State of Florida initiative for the research and development of ocean current and ocean thermal energy technologies. The Center is a synergistic partnership among academic, industry, and government organizations focused on developing knowledge, understanding, infrastructure, technology, and policy towards a low-environmental-impact extraction of ocean energy. The Center is a hub that bridges the gap between concept and commercial implementation by fostering the research, design, development, implementation, testing, and commercialization of cutting-edge, clean, and innovative ocean energy technology. It brings together a broad range of unique, enabling, and accessible expertise and physical assets.

Adaptive Disturbance Rejection for the Rapidly Deployable Stable Platform when Transferring Cargo in Seas

In this paper, we will consider an adaptive output feedback control law with direct adaptive disturbance rejection for the Rapidly Deployable Stable Platform (RDSP). The RDSP is a combined spar and catamaran where the spar lifts the catamaran completely out of the water when transferring cargo, creating a much more stable platform than traditional craneships. This vessel is under development for potential sea cargo transfer applications. The controller, developed to minimize the pitch of the platform when transferring cargo at sea, requires only pitch measurements in near real time. The results in this paper show that the adaptive control and disturbance rejection technique can effectively reduce roll in seas when cargo is being lifted or when waves are producing the primary external forces on the vessel.

Response characteristics and maneuverability of a small twin screw displacement hull vessel in seas

Abstract This paper presents the response characteristics and maneuverability of a small twin screw displacement hull vessel quantified through a series of full-scale trials conducted in different environmental conditions. The 20-m test vessel is instrumented with actuator, environmental, and motion sensors. Several different maneuvers are performed at different speeds, including steady maneuvers with constant control input and transient maneuvers with varied control input to quantify and characterize the response of small vessels to aid in automatic controller and simulation development. Straight-line runs are performed in both forward and reverse over the entire operating range of the test vessel to investigate the relationship between throttle position, RPM, and surge velocity. Turning maneuvers are conducted over the achievable rudder deflection range to quantify the vessels turning radius and the relationships with surge, sway, and rotational speed. Other maneuvers include stationary rotation with the engines operating in opposite gears, and transient tests when the vessel is rapidly accelerated and decelerated. These actuator tests not only quantify the response of the actuators, but also set guidelines for the minimum dwell times that should be observed when shifting gears. These data found using a comprehensive sensor suite provide valuable benchmarks for several maneuvers that can be used for simulation validation and the actuator response information provides valuable set points and performance characteristics/limitations that should be considered in control development. The data from these tests were repeatable from run to run and thus, with sufficient instruments, at sea maneuvers can be used to collect a comprehensive set of data that can expand on data collected in tow tests.

Mitigation of vortex-induced disturbances for the rapidly deployable stable platform

The rapidly deployable stable platform (RDSP) is a concept vessel that is being developed for missions, such as at-sea cargo transfer, that will benefit from a stable spar-like platform that can transit under its own power at medium speeds (∼5–10 m/s). The stability and transit qualities of the RDSP extend from its two distinct modes of operation. These operating modes are the horizontal transit mode (HTM), where the RDSP is configured like a trimaran and the vertical operating mode (VOM), where the RDSP is configured like a spar buoy. In VOM, the RDSP will use the thrusters that are located on its spar section to manoeuvre at slow speeds, mitigate disturbances and dynamically position itself. When the RDSP is moving through the water in VOM, vortices will oscillate off of its spar section and produce forces that are primarily perpendicular to the direction of oncoming flow. Mitigating these disturbances is the focus of this work and is done using an adaptive control algorithm. This algorithm includes a disturbance rejection component that is capable of compensating for any unknown disturbance and phase information, as well as a phase locked loop (PLL) that improves the disturbance frequency estimate on-line. Using this technique, the vortex-induced motions are reduced to 18% of their open loop values using control inputs that are obtainable for the proposed RDSP thrusting system.

Development of an efficient general purpose cable model and simulation for marine applications

This paper develops a general numerical model and efficient integration routine to predict 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-/spl alpha/ method to integrate these equations because it possesses numerical dissipation. This integration routine is coded into a PC based numerical simulation and the results and efficiency were compared with those from the Runge-Kutta method. Based on the validation test cases, the Generalized-/spl alpha/ method proved to be an efficient and reliable integration method for stiff equations governing the motion of underwater cables.

Preliminary Assessment of the Importance of Platform-Tendon Coupling in a Tension Leg Platform

This paper presents performance metrics that can be used to evaluate the response sensitivity of a Tension Leg Platform (TLP) to its tendons. An uncoupled TLP model ignores the intrinsic dynamics and environmental loads on the cables by treating each tendon as an ideal massless spring. A coupled TLP system, in contrast, considers the effects of distributed mass and drag along the tendon. Under certain operating conditions, an uncoupled dynamics model can produce results comparable to its coupled counterpart. This paper defines the conditions under which it is acceptable to model a TLP tendon as a linear spring, as opposed to one that considers the cable dynamics. The analysis is performed in the frequency domain and, for generality, the results are non–dimensionalized. The findings indicate that a more elaborate set of conditions than the platform–to–cable mass ratio must be satisfied for the two models to provide similar results. To conclude this study, two simulations are performed and compared against the performance metrics derived in this paper.