Advancing Marine Renewable Energy Monitoring Capabilities
Joslin, James Blackburn
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Marine renewable energy is poised to contribute substantially to electricity generation over the coming decades. Marine resources are abundant, but generation options must harness these resources in an economically-competitive manner at acceptable environmental and societal cost. This economic pressure also applies equally to the environmental monitoring of early demonstration projects that is needed to reduce risk uncertainty and inform sustainable commercial developments. Consequently, a new suite of flexible, yet cost-effective, capabilities are required. This thesis presents applied research underpinning the development of the Adaptable Monitoring Package (AMP) and Millennium Falcon deployment vehicle, a system that can widen the aperture of the observable environmental interactions at wave and current energy sites. The AMP and Millennium Falcon deployment vehicle provide a cabled, yet reconfigurable, instrumentation platform. By incorporating a flexible suite of instrumentation into a shrouded body with a single wet-mate connection, the AMP has the power and data bandwidth afforded to cabled deployments, but maintains the ease of recovery and redeployment associated with autonomous packages. Instrumentation included in the initial AMP implementation allows for monitoring of marine animal interactions, noise levels, current profiles, turbulence, and water quality in the near field of marine energy converters. The Millennium Falcon deployment vehicle, along with the docking station and launch platform, provides the support infrastructure for deployment and recovery of the AMP in the energetic conditions that are typical of marine energy sites. Future potential for instrument integration and algorithm development makes the AMP well-suited to face the evolving needs of environmental monitoring around marine energy converters. Development of the AMP and deployment system requires several pieces of new knowledge across the spectrum of ocean engineering. First, because the instrumentation mix defines the envelope for subsequent hydrodynamic optimization, the size and spacing constraints of the instruments needs to be defined. However, prior to this thesis, the utility of optical cameras to provide quantitative information in tidal energy environments had not been established, nor had the practical constraints on camera-light separation beneath the photic zone. Without this information, the benefits of including an optical camera system in an instrumentation package are uncertain. Consequently, the initial investigation focused on developing and evaluating the performance of a new stereo-optical camera system. With this sub-system defined, hydrodynamic analysis and optimization could proceed, through a series of laboratory experiments and vehicle simulations. These suggests that a deployment system built around the capabilities of a low-cost inspection class ROV can be effective, even in energetic environments. Optical systems have been previously deployed around marine energy converters, but not used quantitatively and are anecdotally described as having poor endurance due to biofouling. However, optical systems can provide real-time stereographic imagery to detect and characterize targets in the near field (< 10 m) of marine energy converters in a manner more compelling and accessible than sonar imaging. Given public and regulatory concerns about the potential for marine energy converters to injure or kill marine animals, readily-interpreted and objective observations of the "last meter" of interaction are essential. A method for optimizing the stereo camera arrangement is given, along with a quantitative assessment of the system’s ability to measure and track targets in three dimensional space. Optical camera effectiveness is qualitatively evaluated under realistic field conditions (i.e., at a tidal energy site) to determine the range within which detection, discrimination, and classification of targets is possible. These field evaluations inform optimal system placement relative to a marine energy converter to satisfy the objectives of environmental studies. These tests suggest that the stereographic cameras will likely be able to discriminate and classify targets at ranges up to 3.5 m and detect targets at ranges up to, and potentially beyond, 4.5 m, provided that significant (e.g., 1 m) camera-light separation is maintained. Results are also presented from a four-month field test of the prototype stereo-optical camera system. A combination of passive (copper rings and ClearSignal antifouling coating) and active (mechanical wipers) biofouling mitigation measures are implemented on the optical ports of the two cameras and four strobe illuminators. Biofouling on the optical ports is monitored qualitatively by periodic diver inspections and quantitatively by metrics describing the quality of the images captured by cameras with different anti-fouling treatments. During deployment, barnacles colonized almost every surface of the camera system, excepting the optical ports with fouling mitigation measures. The effectiveness of the biofouling mitigation measures suggests that deployments of up to four months are possible for optical camera systems, even during conditions that would otherwise lead to severe fouling and occlusion of the camera optical ports. In combination, these studies demonstrate the quantitative benefit of optical camera systems and measures to extend their endurance, suggesting that they can serve a valuable role in an integrated instrumentation package. At the level of the overall package, the critical question is whether an underwater vehicle that can be deployed without a specialized surface vessel is capable of performing the necessary operations to connect the AMP to a docking station on the seabed. Answering this question requires simulation of the system in turbulent currents, including control actions to maintain orientation and heading. However, the AMP and Millennium Falcon deployment vehicle have complex geometries that complicate the determination of the hydrodynamic coefficients needed for simulation. The second part of the thesis focuses on the dynamic equations of motion for a generic underwater vehicle and methods to identify the hydrodynamic coefficients of interest during the vehicle design process. Computational fluid dynamic (CFD) simulations provide a fast and economical method for numerically estimating the lift and drag forces acting on the vehicle. However, experimental validation is required and traditional methods often require prohibitively expensive test facilities. Further, CFD simulations are not able to predict added mass, which can be significant for underwater vehicles. Free-decay pendulum experiments are used to experimentally verify the CFD results (ANSYS Fluent) for drag and quantify added mass. Results are presented for benchmark geometries (cube and sphere), followed by an analysis of a commercially available inspection class ROV for both a quarter-scale rapid prototype model and full-size vehicle. Comparison between analytical solutions, simulations, and experiments provides insight into the applicability of these methods and considerations for the effects of scaling and geometric simplifications. With the hydrodynamic coefficients established, a dynamic stability analysis of the AMP and Millennium Falcon is presented. A commercial code (ProteusDS) simulates the dynamic response of the system using the previously determined hydrodynamic coefficients. Deployment operations are simulated with time-varying, three-dimensional current forcing generated from turbulent current data from a tidal energy site. Control forces required to maintain heading, surge velocity, and depth are compared to system thrust capacity. The maximum mean current for which deployment operations are possible is predicted and system stability evaluated. Sensitivity studies of the model input parameters demonstrate the importance of including turbulence in dynamic simulations and accurately characterizing hydrodynamic coefficients. These simulations predict that the system is deployable in mean currents up to 0.7 m/s, which is sufficient to conduct instrumentation package deployment at a tidal energy site. Each of these sections contributes novel methods and results to the broader field of ocean engineering, as well as establishing the foundation for the development of the AMP. Continued development of the AMP will investigate instrument integration to expand the monitoring capabilities of individual instruments and autonomous deployments to simplify system maintenance. Once deployed around marine energy converters, the AMP will enable a wide range of environmental studies that will advance the sustainable development of marine renewable energy.
- Mechanical engineering