| Author | Title | Year | Journal/Proceedings | Reftype | DOI/URL |
|---|---|---|---|---|---|
| Gezer, E.C. | Title in progress: Risk-Aware and Safeguarding Control of Autonomous Ships | 2027 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{egez27A,
author = {Gezer, Emir Cem},
title = {Title in progress: Risk-Aware and Safeguarding Control of Autonomous Ships},
school = {Norwegian Univ. Science & Technology},
year = {2027},
note = {PhD in progress}
}
|
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| Najjaran, S. | Title in progress: Enabling zero-emission high speed passenger vessels along the coast of Norway | 2025 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{snaj25A,
author = {Najjaran, Samieh},
title = {Title in progress: Enabling zero-emission high speed passenger vessels along the coast of Norway},
school = {Norwegian Univ. Science & Technology},
year = {2025},
note = {PhD in progress}
}
|
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| Reddy, N.P. | Title in progress: Intelligent Control of Onboard Power Systems for Zero-emission Autonomous Ships | 2025 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{nred25A,
author = {Reddy, Namireddy Praveen},
title = {Title in progress: Intelligent Control of Onboard Power Systems for Zero-emission Autonomous Ships},
school = {Norwegian Univ. Science & Technology},
year = {2025},
note = {PhD in progress}
}
|
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| Bjørnø, J. | Title in progress: Optimal icebreaker deployment and coordination for effective ice management tactics | 2025 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{jbjo25A,
author = {Bjørnø, Jon},
title = {Title in progress: Optimal icebreaker deployment and coordination for effective ice management tactics},
school = {Norwegian Univ. Science & Technology},
year = {2025},
note = {PhD in progress}
}
|
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| Dahl, A.R. | Title in progress: Nonlinear and fault-tolerant control of electric power production in DP vessels | 2025 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{adah25A,
author = {Dahl, Andreas Reason},
title = {Title in progress: Nonlinear and fault-tolerant control of electric power production in DP vessels},
school = {Norwegian Univ. Science & Technology},
year = {2025},
note = {PhD in progress}
}
|
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| Thorat, L. | Title in progress: Control Methods for Highly Redundant and Energy Efficient Shipboard Electric Power Production Systems | 2025 | School: Norwegian Univ. Science & Technology | phdthesis | |
| Abstract: PhD in progress | |||||
BibTeX:
@phdthesis{ltho25A,
author = {Thorat, Laxminarayan},
title = {Title in progress: Control Methods for Highly Redundant and Energy Efficient Shipboard Electric Power Production Systems},
school = {Norwegian Univ. Science & Technology},
year = {2025},
note = {PhD in progress}
}
|
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| Marley, M. | Hybrid and nonhybrid control barrier functions for constraint satisfaction in dynamical systems: Applied to safe motion control of autonomous vessels | 2024 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Control barrier functions (CBFs) enable constraint satisfaction in controlled dynamical systems, by mapping state constraints into state-dependent input constraints. CBFs may then be synthesized with any nominal control law by solving an optimization problem that finds the safe input that is closest to the nominal control input (by some appropriate measure). First-order CBFs are applicable for systems where the control input appears in the first time derivative of the controlled output. High-order CBFs (HOCBFs) extend the notion of CBFs to systems of any order, following a procedure reminiscent of the recursive design of a control Lyapunov function in backstepping. Augmenting CBFs with logic variables that may change value instantaneously result in hybrid CBF formulations.
This thesis studies hybrid and nonhybrid control barrier functions (CBFs) for continuous-time systems, with contributions to both theory and applications. Two distinct robustness results for CBF-based control strategies are established: robustness towards bounded disturbances, and robustness towards arbitrarily small perturbations when employing discontinuous safeguarding control laws. Robustness of discontinuous CBF-based control strategies for control-affine systems is established, in the sense that the CBF-induced stability properties are retained under Krasovskii regularization. This result alleviates the need of establishing continuity properties of optimization-based control laws frequently employed together with CBFs. The result on robustness towards bounded disturbances is stated in the form of sufficient conditions for input-to-state stability (ISS) of nonhybrid HOCBFinduced systems with respect to the safe set, i.e., the subset of the state space where safety constraints are satisfied. This ISS result is established by constructing a vector comparison system from the worst-case evolution of the HOCBF along the disturbed system. Sufficient conditions for uniform global asymptotic stability (UGAS) of the safe set follow as a corollary from the sufficient condition for ISS. Both the ISS and UGAS results apply to closed, but not necessarily compact, safe sets. The distinction between compact and noncompact sets is important in context of HOCBFs, since the safe set defined by HOCBFs is often noncompact. Hybrid CBFs enable solving control problems that are not solvable by continuous control. Such control problems are frequently encountered in obstacle avoidance problems, since decisiveness with respect to turning direction of the vehicle is required either for task completion or for safety. This thesis proposes to construct hybrid CBFs by combining multiple CBF-like functions defining partially overlapping safe sets. Among the main contributions is a recursive design procedure for constructing hybrid HOCBFs, thereby extending hybrid CBFs to systems with high-order safety constraints. The theoretical results are complemented by several novel CBF-based control designs: Two hybrid CBF designs for safe motion control are proposed. The first ensures robust safety for vessels required to maintain a nonzero forward speed, whereas the second robustly resolves deadlocks that arise for CBF-based control strategies applied to obstacle avoidance. A dynamic maneuvering guidance scheme for path-following and obstacle avoidance is also proposed. The guidance scheme reactively generates a safe trajectory for autonomous for autonomous vessels to follow. Modifying the desired path itself, as opposed to forcing vessels to deviate from the path, avoids integral windup in adaptive control schemes that rely on integrating the tracking error. The thesis also motivates the use of CBF-based control strategies for constraint satisfaction in oscillatory mechanical systems, using a wave energy converter as a case study. |
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BibTeX:
@phdthesis{mmar24C,
author = {Marley, Mathias},
title = {Hybrid and nonhybrid control barrier functions for constraint satisfaction in dynamical systems: Applied to safe motion control of autonomous vessels},
school = {Norwegian Univ. Science & Technology},
year = {2024},
note = {Main Supervisor; Doctoral theses at NTNU;2024:262},
url = {https://hdl.handle.net/11250/3139421}
}
|
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| Dirdal, J.A. | Signal-based sea state estimation: A phase-time-path-difference approach: A new shipboard wave estimation approach | 2024 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: The safety and efficiency of marine operations at sea rely on accurate information about the sea state, which includes the dominant wave height, wave direction, and wave period. Unfortunately, many areas at sea lack this crucial data due to a scarcity of measuring instruments or inadequate measurement resolution. However, ships have the potential to address this issue since they are omnipresent at sea and situated near the waves, making them optimal platforms for both measuring and reporting wave conditions.
Shipboard sea state estimation uses sensor measurements of the sea surface from a vessel to determine important wave characteristics through model-based or signalbased approaches. Signal-based approaches have several advantages over model-based methods as they estimate waves directly from sensor measurements without the need for any complex ship model. However, these approaches often rely on expensive instruments and expert assistance for installation and maintenance. This doctoral thesis investigates a relatively new and unexplored signal-based approach for shipboard wave estimation that is cost-effective and easy to implement. The approach uses the phase-time-path-differences (PTPDs) between an array of inertial measurement units (IMUs) to infer the directionality and frequency characteristics of waves. Only a few works have considered using a PTPD approach for wave estimation based on shipboard IMUs. However, these studies are restricted to model-ship wave tank testing in regular waves, and its application appears to overlook the differences between sensor delays on a rigid body and those directly obtained from sensors situated on the ocean. Moreover, it is presently unclear how many IMUs are needed, how far they should be separated, and how they should be geometrically arranged to determine the prevailing sea state. The present study proves that the main wave direction and wave number can be uniquely determined from a minimum of two independent phase differences. Although measurements of the latter can be obtained from a minimum of three noncollinear IMUs, this work demonstrates that a single IMU is sufficient by utilizing a rigid-body measurement transformation to generate the other measurements needed. Moreover, a comprehensive theoretical assessment of the validity of the PTPD approach is conducted, determining the conditions under which it may be safely applied to model rigid body sensor delays. These conditions are validated experimentally through extensive testing with a model ship in a wave tank. As a ship moves forward in following seas, it is a well-known problem that each encountered wave frequency can correspond to three distinct absolute wave frequencies, making it challenging to accurately determine the correct wave frequency during movement. However, through the observability results presented in this thesis, we prove that the absolute wave frequency can be uniquely determined while the vessel is moving using the PTPD approach. This interesting result is validated experimentally in a wave tank with a model ship exposed to various regular and irregular waves. An inherent drawback of using measured ship motions to determine wave characteristics is that they are susceptible to distortions caused by the effect of vessel low-pass filtering when the waves are sufficiently short. To address this challenge, a novel analytical expression of the frequency bandwidth of undistorted waves is derived based on the main vessel dimensions. This frequency bandwidth aids in identifying the wave components that are safe to consider and those to avoid. This frequency bandwidth is incorporated into our proposed methodology for implementing the PTPD approach, which comprises a fast Fourier transform and an unscented Kalman filter. Moreover, with our proposed methodology, we are able to yield estimates of the wave direction and wave number/period close to real-time, with updates given every three minutes after an initial six-minute startup period. The validation of the proposed approach is carried out through model-scale and fullscale field experiments. The latter involves a research vessel with a commercial wave radar operating alongside various wave buoys under diverse sea state conditions. The results of these experiments show strong agreement with wave reference systems, confirming the competitiveness of our theory and method against existing wave measurement technology. Notably, our proposed method offers advantages in costeffectiveness, simplicity, and environmental resilience, thereby establishing it as a promising alternative or complementary aid within the field. |
|||||
BibTeX:
@phdthesis{jdir24A,
author = {Dirdal, Johann Alexander},
title = {Signal-based sea state estimation: A phase-time-path-difference approach: A new shipboard wave estimation approach},
school = {Norwegian Univ. Science & Technology},
year = {2024},
note = {Co-Supervisor; Main supervisor: Thor I. Fossen; Doctoral theses at NTNU, 2024:79},
url = {https://hdl.handle.net/11250/3120703}
}
|
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| Cardaillac, A. | Towards autonomous underwater navigation and perception for end-to-end ship hull inspection | 2024 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis presents an innovative and integrated solution for end-to-end underwater ship hull inspection using a small and low-cost Remotely Operated Vehicle (ROV). In a world where maritime activities have significant and distinctive impacts in many sectors, where over 5000 ships operate daily, safety concerns from the structural integrity of the hulls are raised. These concerns are not only related to the ship’s crew safety if there are apparent damages to the hull, but also related to the environmental cause. Indeed, an unmaintained hull will provoke a significant rise of the fuel consumption over time. Although successful, traditional inspection methods in dry docks fail to be fast-paced, which would decrease the ship’s down time and cost. Remote inspections are promising to address this issue. ROV-based inspections enable efficient visual documentation while the ship is still in water and docked. Further automating the process increases the efficiency since it brings consistency and faster data processing. With the inspection culture and regulations in mind, this thesis aims to achieve a fully automated inspection of underwater ship hulls, i.e., from the deployment of the vehicle to the assistance of the surveyor to generate the inspection reports. To achieve this, the drone is equipped with a set of navigation and perception sensors to ensure hull relative navigation and guarantee full visual coverage. Maneuvering based guidance is employed to navigate along the hull. Its relative orientation and distance to the ROV is computed using a forward looking sonar and set as constraints to the guidance mechanism to make sure the vehicle is facing the hull at a constant distance. Additionally, the sonar enables online acoustic mapping of the hull, tracking of the inspection progress, and visual representations of the hull. This relies on a flat surface assumption when operating at close range. When inspecting particular points of interests such as propellers, keels, and gratings, acoustic and optical data are combined to provide a better understanding of the structure through accurate 3D modelling of the scene and improved localisation of the vehicle. The acoustic-visual combination occurs at the feature level based on the relative distances of the detected points to the perception sensors. To constrain the search space of the features that can be matched, the intersection area between the sonar acoustic beams and the camera image plane is dynamically estimated. The acoustic-visual combination is activated when the specific areas of interest are detected. This is done through the use of deep learning models, trained on a tailor made dataset for image classification and semantic segmentation of ship parts and faults. Verified by domain experts, this dataset was made to match the needs of the surveyors and is the first of its kind publicly available for ship hull inspection systems. Sequences of the data collected by the vehicle during the mission are automatically marked based on their relevance for the inspection to further assist the surveyor. These data markers are attached with visual data, models and the ROV telemetry to provide insights and to be compatible with the guidelines from the international regulations. The complete solution was tested in ten harbors and on six ships of different size and strutures to ensure the adaptability of the methods and consistency of the results. By taking advantage of the available sensors, it was possible to move along the hull with high precision at the same time as mapping it. The proposed methods outperformed the related existing one and showed new promising opportunities for future research. Finally, the adaptability of the proposed solution made it possible to apply it for inspection of different structures than ship hulls, including aquaculture fish net pens and subsea structures. | |||||
BibTeX:
@phdthesis{acar24A,
author = {Cardaillac, Alexandre},
title = {Towards autonomous underwater navigation and perception for end-to-end ship hull inspection},
school = {Norwegian Univ. Science & Technology},
year = {2024},
note = {Co-Supervisor; Main supervisor: Martin Ludvigsen; Doctoral theses at NTNU, 2024:68},
url = {https://hdl.handle.net/11250/3121979}
}
|
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| Karimi, S. | Shore-to-Ship Charging Systems for Battery-Electric Ships: Power System Architecture, Performance Analysis, and Future Trends | 2022 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Battery electrification has been of the most promising solutions to achieving zero- or low-emission and sustainable shipping. Battery electrification of the ships refers to installing batteries onboard, in addition to or as a substitute for fuel-based engines, which receive charging from shore terminals. Due to the current limitations on battery technology, among all ship types, mostly short-distanced marine vessels have been developed or planned for
pure or hybrid battery electrification. In this manner, shore-to-ship charging systems play a vital role by bridging land-based sustainable energies and onboard propulsion loads. Nonetheless, charging large onboard batteries within a limited time of docking between two trips is challenging, especially from a weak grid available in remote areas. It even becomes more challenging with the advance of multiple ships receiving charging with diversified charging demands. On top of that, there exist more technical and nontechnical barriers to the implementation of shore-to-ship charging systems facilitating the battery-electric fleet. This thesis is aimed to investigate the main technical challenges, analyze the performance of existing systems, and as the result propose improvements contributing to the further development of marine battery electrification. The first step is establishing an overview of feasible solutions by classifying them with regard to their power system architecture and applications. The battery charging path from the shore-based point of common coupling to the onboard battery terminals involves several components, power electronics converters, transformers, filters, and interconnectors, to name but a few. Utilizing onshore stationary batteries for supporting the weak grids to supply high-power charging loads has been suggested as an effective strategy. However, the integration of onshore batteries into different charging systems might need careful investigation. The most common shore-to-ship charging solutions are ac, dc, and inductive or wireless charging. Besides the charging methods, battery swapping, by which the depleted batteries are exchanged with the fully charged batteries at terminals, has been considered. The importance of energy efficiency has been growing drastically. In fact, improvement in energy efficiency can result in a lower energy cost, less equipment maintenance, and less required space for electrical equipment. Moreover, it can improve the utilization of the available grid power within the critical charging time. Utilizing onshore batteries also introduces energy losses to the system despite their significant benefits. In spite of the importance of energy efficiency, inaccurate and static estimation of energy efficiency has been taken into account for system design and planning. Therefore, a comprehensive and model-based energy efficiency analysis for shore-to-ship charging is carried out in this thesis. To do so, the load-dependent power loss models of the components are used to estimate the energy efficiency of shore-to-ship charging systems including onshore batteries. Various system configurations are compared in terms of energy efficiency. Moreover, the impact of operation scenarios including the onshore battery scheduling scenario on energy efficiency is identified. It is inevitable that the functionality of battery-electric ships relies on the availability of shore-to-ship charging infrastructures. An outage in the charging system can delay or cancel vessel schedules, resulting in significant outage costs and customer inconvenience. Moreover, the shore-to-ship charging system comprises several reliability-critical components, including power electronics converters and batteries. High-power loads and sea conditions also intensify the failure susceptibility of these systems. Thus, this thesis contributes to evaluating the reliability of shore-to-ship charging systems. Then, such analysis is incorporated into the design routine as to how to size and choose the components based on reliability, satisfying the system requirements. Moreover, the impact of operational scenario planning on reliability is investigated. It is shown that by adjusting the operating scenarios and without adding new components, reliability can be improved. By growing the number of battery-powered vessels, the ports must be prepared to be able to provide charging facilities for multiple vessels concurrently. Moreover, due to the lack of standardized design guidelines, the onboard power systems, and their charging interfaces might differ from one vessel to another. Therefore, a tailor-made charging system with related control functions specifically designed for a single vessel is usually required. On top of that, due to the short layover times of the ships, the utilization ratio of these rather expensive charging infrastructures is low. Moreover, nowadays, the concept of interoperability in the maritime sector arises the need for a unified and flexible charging system. To address these challenges, an integrated universal charging system topology – which is compatible with the common charging interfaces – is proposed in this thesis. It can potentially maximize the utilization of the components and improve the long-term revenue of shore-to-ship charging systems. Additionally, control mechanisms for regulating the multi-vessel charging systems are studied. To this end, the charging management systems offering indirect control of onshore batteries, and load management are presented. All in all, this thesis contributes to enriching the design, planning, and operation guidelines regarding shore-to-ship charging systems by shining light on the viable opportunities for improving performance, maximizing utilization, and addressing existing technical challenges of such systems. |
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BibTeX:
@phdthesis{skar22A,
author = {Karimi, Siamak},
title = {Shore-to-Ship Charging Systems for Battery-Electric Ships: Power System Architecture, Performance Analysis, and Future Trends},
school = {Norwegian Univ. Science & Technology},
year = {2022},
note = {Co-Supervisor; Main supervisor: Mehdi Zadeh; Doctoral theses at NTNU, 2022:400},
url = {https://hdl.handle.net/11250/3046425}
}
|
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| Thyri, E.H. | COLREGs-aware Trajectory Planning and Collision Avoidance for Autonomous Surface Vessels | 2022 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis considers trajectory planning and collision avoidance for autonomous surface vessels (ASVs) operating in complex domains in the presence of other vessels. In particular, the task of maneuvering in compliance with the International Regulations for Preventing Collisions at Sea (COLREGs), which are the rules of the road on water, is considered. The contributions are directed towards COLREGs-aware trajectory planning and collision avoidance, where COLREGs rules 8 and 13-17 are addressed. These rules consider the conduct of vessels in encounters where risk of collision is present. The rules address how the maneuvering obligations are assigned to the involved vessels as a function of the encounter geometry and relative velocity. Rules 13-15 are encounter-type specific and consider overtaking encounters, head-on encounters, and crossing encounters, respectively. Rules 8, 16, and 17 address in more general terms how vessels that have either give-way or stand-on obligations are to maneuver to reduce the risk of collision. The main motivation behind the work is to enable electric autonomous passenger ferries as an efficient and environmentally friendly means of transporting pedestrians in urban environments. Still, the concepts and methods are applicable to most surface vessel operations.
The first step in maneuvering in compliance with the COLREGs is to determine which rules that apply to the ASV. In this work, a COLREGs classification algorithm has been developed, to determine the encounter type and hence the maneuvering obligations of the ASV in a vessel-to-vessel encounter between the ASV and each so called target ship, which is another vessel that the ASV must avoid collision with. Determining the obligations of the ASV is, however, the easy part, whereas maneuvering in compliance with the obligations is a more challenging one. The COLREGs are written by humans and for humans, and its formulation is in some parts qualitative, to allow for humans to assess the situation based on experience and skills. This poses a challenge when it comes to evaluating and acting on these rules through machine code, where quantitative statements are preferred. This thesis presents a novel mechanism for enforcing maneuvering in compliance with the COLREGs. It comprises a target ship domain with broad consideration to the regulations, where the encounter type, encounter geometry, relative velocity and available space to maneuver are considered. The domain is designed such that if the ASV maneuvers as to not violate the domain, the ASV is consequently maneuvering in compliance with the encounter-type specific COLREGs rules 13-15 and 17. By enforcing the target ship domains as strict constraints in the trajectory planning and collision avoidance algorithms, the proposed domain robustly enforces COLREGs compliance independently of other objectives such as trajectory tracking, energy efficiency and passenger comfort. Several reactive collision avoidance methods are also proposed for ensuring safe operation of ASVs in dynamic and unstructured areas with other vessels and restricted space to maneuver. The methods include capacity for COLREGs-aware maneuvering when avoiding collision with target ships, and also collision avoidance with static obstacles with complex geometries. The methods have a varying degree of coupling with the ASV's guidance, navigation, and control (GNC) system, which makes the proposed mechanisms for COLREGs-aware and collision-free maneuvering easy to integrate in an arbitrary GNC architecture. A trajectory planner for path following and collision avoidance with static and dynamic obstacles is also proposed. The trajectory planner is formulated as an optimal control problem, minimizing the tracking error to the path and the induced accelerations. In addition to the COLREGs rules considered by enforcing the novel target ship domain, the trajectory planner includes consideration to rules 8 and 16, regarding making maneuvers that are readily apparent and performed in ample time to stay well clear of target ships which the ASV has give-way obligations to. This is achieved by assigning windows of reduced cost for the tracking error and the induced accelerations in the control horizon. These windows facilitate any maneuver to avoid collision to be performed within them. The windows are parameterized by a small set of intuitive parameters, and enable, if circumstances of the case admit, maneuvers to avoid collision to be conducted in ample time, in accordance with Rule 8 and Rule 16. The work in this thesis has both a theoretical and practical focus, to develop and also test new methods. The proposed navigation algorithms have been tested through an extensive set of simulations in relevant operational domains, where it is demonstrated that the proposed target ship domain robustly enforces compliance with COLREGs rules 13-15 and 17, and that the windows of reduced cost increase compliance with rules 8 and 16. Furthermore, some algorithms have been tested in full-scale experiments with an electric prototype autonomous passenger ferry. In the experiments, a radar- and lidar-based target tracking system has been applied to close the autonomy loop, demonstrating that the proposed methods are suitable for real-time operation, and are robust to a realistic level of noise and uncertainties in the tracking data. |
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BibTeX:
@phdthesis{ethy22A,
author = {Thyri, Emil Hjelseth},
title = {COLREGs-aware Trajectory Planning and Collision Avoidance for Autonomous Surface Vessels},
school = {Norwegian Univ. Science & Technology},
year = {2022},
note = {Co-Supervisor; Main supervisor: Morten Breivik; Doctoral theses at NTNU, 2022:318},
url = {https://hdl.handle.net/11250/3025463}
}
|
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| Sørensen, M.E.N. | Topics in Nonlinear and Model-based Control of Ships | 2021 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This PhD thesis considers topics within automatic motion control of ships, which has been an active research topic since the early 20th century. Specifically, the thesis aims at designing controllers to achieve a good tracking performance by handling actuator constraints, internal uncertainties and external disturbances of the ship’s inner-loop control e.g. controlling the velocity loops in order to achieve robust manoeuvrability.
The thesis proposes improvements to two existing ship models, which have been found by evaluating the steady state velocities for uniformly distributed control inputs. Through this evaluation it is shown that the original ship models give rise to physically impossible motions. It is suggested to add extra terms to the damping matrices in order to overcome the issues with the existing ship models. An overview of existing performance metrics is given. Subsequently, three novel performance metrics are suggested. These performance metrics evaluate the overall energy consumption, wear and tear of the actuators and a combination of these. The proposed performance metrics are used as a tool to compare and evaluate the performance of various controllers. In addition, the use of purely nonlinear feedback strategies and combinations of linear-nonlinear feedback strategies are investigated for pose and velocity control of ships. The nonlinear feedback terms are based on a sigmoid function which limits the effects of the error term. A modification to the nonlinear feedback terms concepts is suggested by changing them from symmetric to asymmetric nonlinear feedback terms. This results in a stepping stone to handle actuator constraints. A novel motion control method is suggested in order to handle magnitude constraints of the actuator. This is based on a simplified version of the collision avoidance algorithm called dynamic window. The dynamic window algorithm was originally developed for collision avoidance for mobile robots. This control method is suggested for 2 and 3 degrees-of-freedom motion control. Here, the benefits and limitations of both the design and results are discussed. Some state-of-the-art adaptive control algorithms have been applied to a mathematical model of a ship to see if it is possible to accommodate for internal uncertainties and external disturbances. The performance of the considered adaptive control algorithms have been checked both in a numeral simulation and experimental environment. Finally, experimental work in the Marine Cybernetics laboratory and onboard the research vessel Gunnerus is described. Here, the equipment and software of the laboratory and ship are presented and discussed. Additionally, all the experimental results from the publications in Appendix A are summarised here. The thesis is organized as a mix between a monograph and an article collection. It includes eight conference papers, two published journal paper. One additional paper is mentioned, but is outside the scope of this thesis. |
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BibTeX:
@phdthesis{msor21A,
author = {Sørensen, Mikkel Eske Nørgaard},
title = {Topics in Nonlinear and Model-based Control of Ships},
school = {Norwegian Univ. Science & Technology},
year = {2021},
note = {Co-Supervisor; Main supervisor: Morten Breivik; Doctoral theses at NTNU, 2021:401},
url = {https://hdl.handle.net/11250/2977595}
}
|
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| Ueland, E.S. | Load Control for Real-time Hybrid Model Testing using Cable-driven Parallel Robots | 2021 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Real-time hybrid model testing (ReaTHM testing) is a method for emulating ocean structures that combines numerical methods with traditional hydrodynamic model testing. This is done by partitioning the ocean structure under consideration into numerical and physical substructures that are coupled in real-time through measurement and control interfaces, for high fidelity emulation of the original ocean structure. The method can be classified as an extension of traditional hydrodynamic model testing since it considers experimental testing of down-scaled models in basin laboratories, and as a subset of hybrid testing since it replaces parts of the down-scaled structure with numerical simulated models.
The developments presented in this thesis is aimed at ReaTHM testing where the numerically computed load vector is calculated based on measurements of the experimental displacements and thereby actuated onto the physical substructure via a configuration of distributed cabled winches. The experimental platform, together with the actuators, thus constitutes a cable-driven parallel robot. This PhD project’s overall goal is to further improve the ReaTHM testing methodology as part of a research effort to make it a well documented, accepted, and valued practise that accurately identifies and predicts the behaviour of ocean structures in realistic marine environments. One of the major challenges in this regard is to ensure that load actuation is performed with minimal errors and without significant degradation of emulation performance. To this end, the focus of this work is to identify and mitigate issues associated with the actuation of the numerically calculated load vector onto the experimental test platform and to enable more accurate and robust load control. The thesis is organised as a collection of articles. The two conference articles identify and quantify sources of error in load actuation. They serve as the basis for the subsequent journal articles that address specific load actuation challenges and associated good practise control methods. In the first journal article, novel methods for determining each actuator’s appropriate target cable forces are proposed. These methods guarantee continuous differentiability of the resulting cable forces. The article also shows that an implementation of Newton’s method specialised for the resulting optimisation problem can be used for practical real-time applications. The results are beneficial for ReaTHM testing because of the method’s flexibility, and because it is expected that smoother cable force trajectories can be more accurately tracked. The second journal article proposes a procedure for optimal actuator placement that is particularly suitable for ReaTHM testing, for which no such guidelines exist at the time of writing. The third and final journal article demonstrates how position-controlled servo- motors connected to drums via clocksprings can be used for accurate actuator force control. Associated controllers that compensate for both delays and motion-induced forces are proposed. The study emphasises developments for ReaTHM testing by focusing on relevant use cases, force magnitudes, and frequency ranges. For development, problem identification, method validation, and demonstration, the work in this thesis is emphasised by extensive experimental testing. Experiments are presented using both a readily accessible 1 degree of freedom setup for basic testing and development and a more complex ReaTHM test setup of a moored barge in a basin laboratory in which the cabled winches are tasked with actuating loads in three degrees of freedom (surge, sway and yaw). The thesis does not use ReaTHM testing to determine realistic ocean structures’ behaviour, which is the intended end-use of the overall methodology. Instead, simpler test cases are considered to understand, develop, and improve control functions at a more fundamental level. |
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BibTeX:
@phdthesis{euel21D,
author = {Ueland, Einar Skiftestad},
title = {Load Control for Real-time Hybrid Model Testing using Cable-driven Parallel Robots},
school = {Norwegian Univ. Science & Technology},
year = {2021},
note = {Main Supervisor; Doctoral theses at NTNU, 2021:284},
url = {https://hdl.handle.net/11250/2780240}
}
|
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| Værnø, S.A.T. | Transient Performance in Dynamic Positioning of Ships: Investigation of Residual Load Models and Control Methods for Effective Compensation | 2020 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis concerns how the dynamic positioning (DP) control system can better handle transient events, where the loads experienced by the vessel change significantly over a short time frame. The material is intended for DP systems controlling surface ships, but it is also relevant for other DP vessels, as well as other motion control applications for marine vessels. The thesis is a collection of papers, with some introductory chapters to
set the context of the problem. The thesis contains contributions on the fundamental level, where different load models and observer algorithms are fairly compared and analyzed. The study investigates the effect of including nonlinear damping in the model. The results show that when using models where the residual loads are modeled as a current, then nonlinear damping improves performance. For the models where the residual loads are modeled as a superimposed load vector, then the effect of nonlinear damping is less apparent. The different observer algorithms show surprisingly similar performance, which indicate that DP is dominantly a linear process. Two different augmentations of existing observer design have been proposed for better transient performance, while maintaining good steady-state performance. A time-varying model-based observer is presented and analyzed, where aggressive gains are used during transient for responsiveness, and relaxed gains are used in steady state for lower oscillations in the state estimates. The performance is verified through high-fidelity closed-loop simulations and on experimental full-scale data from a cruise with the research vessel R/V Gunnerus. In addition, on the cruise with R/V Gunnerus, a partial closed-loop validation with integrated DP observer and controller was performed. The other design is a hybrid observer combining model-based and kinematic observers. The hybrid observer switches to the kinematic observer in transient conditions, and to the model-based observer in steady conditions. The observer performance is verified through model-scale closed-loop experiments, and on full-scale experimental data from R/V Gunnerus. For the control design, integral action is compared to other ways of compensating the environmental and unmodeled loads, with special focus on transient conditions. The results show that the best solution is to use the estimate of the environmental and unmodeled loads from an observer with tuning optimized to estimate these loads. This method outperforms the other methods in transients, and has equal performance to integral action in steady state. In addition, using an observer to find the estimate alleviates anti windup issues, and contrary to tuning integral action, an observer can be tuned open loop – which is a large benefit. Hybrid integral action is proposed to improve performance in transient conditions and still keep relaxed and satisfactory performance in steady conditions. This is achieved by high gains in the integrator in transients to better compensate the loads in transients, and relaxed gains in steady conditions to not induce unnecessary oscillations. Pseudo-derivative control (PDF) is proposed as an alternative to traditional proportional-integral-derivative (PID) control. The PDF control algorithm does not need a reference filter as the PID does, as the references are generated internally, and the PDF control algorithms is better at mitigating integral windup, compared to the PID control algorithm. Performance of the PDF control law is shown through a simulation study, and through full-scale closed-loop trials with R/V Gunnerus. |
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BibTeX:
@phdthesis{svar20A,
author = {Værnø, Svenn Are Tutturen},
title = {Transient Performance in Dynamic Positioning of Ships: Investigation of Residual Load Models and Control Methods for Effective Compensation},
school = {Norwegian Univ. Science & Technology},
year = {2020},
note = {Main Supervisor; Doctoral theses at NTNU, 2020:355},
url = {https://hdl.handle.net/11250/2685693}
}
|
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| Ren, Z. | Advanced Control Algorithms to Support Automated Offshore Wind Turbine Installation | 2019 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: The thesis presents the research on automated offshore wind turbine (OWT) installation. The aim of the project is to nd innovative and cost-effective methods for installing and maintaining OWTs.
Utilization of wind energy grows quickly in the past two decades. The trend of increasing turbine size reduces the costs of installation and grid connection per unit energy produced. However, the growing installation height challenges the OWT installation. The target of the present thesis is to develop novel algorithms of real-time state estimation and control of the installation of bottom- xed and floating OWTs (including individual components like blades or pre-assembled subsystems), by state-of-the-art automatic control theories, aiming for improved effciency, increased operation safety, and reduced installation cost. In the present thesis, two installation strategies are studied, i.e., single blade installation to a bottom-fi xed OWT with a monopile foundation using a jackup vessel and tower-nacelle-rotor preassembly installation to a spar foundation using a catamaran. Depending on the level of onshore pre-assembly, they are two opposite extremes among all installation strategies. The former strategy has a wider application with the lowest operational efficiency, while the latter has the fewest offshore lifts but requires more specific equipment. The major emphasis is on automated single blade installation. Since blade is the component with the most complex aerodynamics characteristics and its installation is the most time-consuming and expensive, the research can be extended to other components can be easily solved. Furthermore, part of the effort is put on improving the performance of the novel wind turbine installation concept using a catamaran proposed by SFI MOVE. A user-friendly numerical modeling framework for the offshore installation is developed for control design purposes. Various OWT installation models have been investigated. Several controllers and estimators have been developed. Time-domain simulations and sensitivity studies have been conducted to verify the performance of the proposed algorithms. This work was supported by the Research Council of Norway (RCN) through the Centre for Research-based Innovation on Marine Operations (CRI MOVE, RCN-project 237929), and partly by the Centre of Excellence on Autonomous Marine Operations and Systems (NTNU AMOS, RCN-project 223254). |
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BibTeX:
@phdthesis{zren19C,
author = {Ren, Zhengru},
title = {Advanced Control Algorithms to Support Automated Offshore Wind Turbine Installation},
school = {Norwegian Univ. Science & Technology},
year = {2019},
note = {Main Supervisor; Doctoral theses at NTNU, 2019:231},
url = { http://hdl.handle.net/11250/2645775}
}
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| Heyn, H.-M. | Motion sensing on vessels operating in sea ice: A local ice monitoring system for transit and stationkeeping operations under the influence of sea ice | 2019 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: The diminishing sea ice in the Arctic and Antarctic could lead to a higher number of ship operations in these areas, such as cargo transit, resource exploration, fishing, and tourism. However, despite the sea ice reduction, sea ice remains the predominant risk during ship operations. Due to the remoteness and fragility of the polar regions, accidents are difficult to handle and could have devastating effects
on the local ecosystem. Therefore, continuous assessment of the prevailing ice conditions is essential to operating vessels. Individual sensor systems, called ice monitoring systems, provide the necessary information for the ice condition assessment, and by giving early warnings, these systems reduce the risk of accidents. A reliable ice observer system employs several technologies for ice monitoring such as optical cameras, radar systems, drift buoys, and hull strain measurements. Each additional technology increases the chance of early detection of dangerous ice conditions, and additionally adds redundancies to the overall system. The aim of this thesis was to present and validate an applicable ice monitoring system that can increase the safety for vessels operating in polar regions. This thesis presents a series of studies, presented as a collection of journal papers, that lead to an on-board motion sensing based ice monitoring system for ships, which bases on distributed measurements of ice-induced vibrations in the ship’s hull. The results of this thesis are based on field data collected during four Arctic cruises performed between 2015 and 2017. An initial study established a relationship between the prevailing ice conditions and ice-induced vibrations in the ship’s hull. A detailed frequency analysis of ice-induced vibrations concluded that accelerometers in the hull can provide information about the acting ice breaking mechanism, ice conditions around the vessel, and the location of ship-ice interaction along the hull. Two further studies established and validated the application of hull accelerometers as ice monitoring system. A first application study suggested to monitor ice conditions in real time with the help of statistical signal processing and change detection. It was found that the suggested ice monitoring system provides robust real-time information about the local ice conditions and operates independently of weather conditions. A second application study utilised several sensors along the hull and methods from extreme value statistics to find a relationship between the ice drift direction and statistical properties of the recorded signals. The proposed methods in this thesis would allow for an early detection of ice drift changes, which is essential for station-keeping operations in sea ice. The overall research presented in this thesis conclude that motion sensing on vessels operating in sea ice provides a fast and reliable local ice monitoring system for both transit and station-keeping operations. It is further suggested, that motion sensing in the hull of a vessel also provides information about the current sea-state in open water. However, this possible application is outside the scope of this thesis. Besides the main studies, the thesis additionally offers a contribution in form of a model for distributed motion sensing on ships operating in sea ice. |
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BibTeX:
@phdthesis{hhey19C,
author = {Heyn, Hans-Martin},
title = {Motion sensing on vessels operating in sea ice: A local ice monitoring system for transit and stationkeeping operations under the influence of sea ice},
school = {Norwegian Univ. Science & Technology},
year = {2019},
note = {Main Supervisor; Doctoral theses at NTNU, 2019:138},
url = {http://hdl.handle.net/11250/2611143}
}
|
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| Norgren, P. | Autonomous underwater vehicles in Arctic marine operations: Arctic marine research and ice monitoring | 2018 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis considers autonomous underwater vehicles (AUVs) in Arctic marine operations. It focuses on their use as a sensor platform in ice monitoring operations and the use of AUVs for Arctic marine research. Arctic AUV operations pose several challenges compared to standard AUV operations, including the presence of drifting sea-ice and navigational challenges in the polar regions. These are some of the issues addressed in this thesis.
Chapter 2 introduces historic Arctic AUV operations, and it presents experiences and lessons learned through these campaigns. Challenges related to communication, navigation, fail-safes, and deployment and recovery, with a focus on Arctic operations are discussed. This chapter also motivates and assess the use of AUVs as a sensor platform for ice monitoring operations. A conceptual guidance and navigation system for Arctic AUVs is presented at the end of the chapter. Field work and experiments are important to test theory, but also to build experience and acquire knowledge. Chapter 3 presents two Arctic AUV deployments using the NTNU REMUS 100 AUV and the experiences learned from these operations. Two experiments demonstrating the use of unmanned surface vehicles (USVs) as a support tool for AUVs in the Trondheimsfjord are also presented, along with a motivation for the use of such platforms in Arctic marine research. Since Arctic AUV operations are considered as high risk, with significant costs associated, an Arctic AUV simulator environment has been developed, as presented in Chapter 4. The simulator consists of seven modules, where the modules defining the default guidance and control system, as well as the numerical AUV model, are similar to a regular AUV simulator. In addition, an ice drift model is provided to simulate drifting and rotating ice features. A multibeam echosounder (MBE) simulator is used to sense the ice topography, given as a digital elevation map (DEM). To achieve drift and rotation of the sensed terrain, the final module in the Arctic AUV simulator is a relative position and velocity module, which provides input to the MBE simulator. A special consideration has been given to iceberg mapping using AUVs in this thesis, as the detailed topography of icebergs are important to develop iceberg trajectory models, as well as decision support in iceberg management operations (e.g., iceberg towing). Chapter 5 details a guidance system for determining the main particulars of an iceberg that relies on MBE measurements to determine the location of the edge of the iceberg. The guidance system is implemented as a state machine, starting in an iceberg detection mode. Once an iceberg is detected, an edge-detection algorithm is used to determine the location of the edge relative to the AUV, and thereby to online generate a path along the iceberg edge. The line-of-sight (LOS) guidance scheme is used to follow the iceberg edge and circumnavigate the iceberg. Motivated by the need to estimate the relative AUV-iceberg position in order to generate a consistent iceberg topography corrected for iceberg drift and rotation, an iceberg mapping navigation system has been proposed in Chapter 6. A simultaneous localization and mapping (SLAM) algorithm based on the bathymetric distributed particle filter SLAM (BPSLAM) is used to track the position and orientation of the iceberg in the global frame. The iceberg mapping navigation filter, implemented using an Extended Kalman filter (EKF) with the SLAM states as input, provides estimates of relative pose and velocity between the iceberg and the AUV. In addition, the velocity of the iceberg is estimated in the iceberg mapping filter. The iceberg SLAM algorithm provides a real-time estimate of the iceberg topography at a fixed resolution, which along with the iceberg drift velocity estimates are important parameters in an iceberg management operation. |
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BibTeX:
@phdthesis{pnor18B,
author = {Norgren, Petter},
title = {Autonomous underwater vehicles in Arctic marine operations: Arctic marine research and ice monitoring},
school = {Norwegian Univ. Science & Technology},
year = {2018},
note = {Main Supervisor; Doctoral theses at NTNU, 2018:255},
url = {http://hdl.handle.net/11250/2565232}
}
|
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| Yum, K.K. | Transient Performance and Emissions of a Turbocharged Diesel Engine for Marine Power Plants: Numerical Simulation and Experimental Investigation | 2017 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: New marine power and propulsion plants have to meet increasingly stringent environmental regulations and requirements for flexible and efficient operations. Such a multi-objective task requires a systems approach in which the design of components and the operation of the system are optimized in a holistic way in order to provide the overall improvements of the system under a realistic operational profile. In this regard, mathematical modeling and numerical simulation of the power plant in conjunction with the surrounding systems and environmental loads becomes an essential tool. It is even more so with the complex operation of a hybrid power/propulsion plant with energy storage devices. In the majority of marine power/propulsion plants, turbocharged diesel engines sit as main prime movers that determine the dynamic response, fuel efficiency and emissions of the overall plant. Therefore, how the diesel engines are modeled has a significant influence on the overall performance of the total system simulator of the power/propulsion plant.
A turbocharged diesel engine itself is a complex engineering system, and one should therefore model it in a good balance of accuracy and simplicity in order to use it in the power plant simulation. Furthermore, many diesel engines are designed with common components and physical laws. Therefore, reusability of a mathematical model in different contexts is important for an efficient modeling process. This thesis aims to find an effective modeling framework of a turbocharged diesel engine for simulation of marine power/propulsion plants considering these aspects. In order to achieve the goal, research works are carried out in different areas: establishment of the modeling framework for a turbocharged diesel engine, development of the marine vessel and power plant simulators to test the diesel engine models and an experimental investigation of the effects of the transient loads on diesel engines. As a modeling framework of a turbocharged diesel engine, architecture of model libraries is proposed, which has a solid hierarchical structure in terms of levels of abstraction: a technical component level, a physical concept level and a mathematical level. Following the suggested architecture enables a modeler to make a decision at each level distinctively that the decisions are more traceable than when it is done holistically. Therefore, reusability of the model is enhanced. Furthermore, a general interface structure is proposed for the thermodynamic modeling of a diesel engine process which can be used for models with different fidelity. The established framework is used to model diesel engines at different levels of fidelity, namely zero-dimensional (0D) semi-phenomenological and mean-value engine model. The former is used for simulation of marine propulsion in wave, and the latter is used for simulation of a diesel-electric power plant in a dynamic positioning vessel. Total system simulators for both cases are also developed in cooperation with other researchers in order to test the developed diesel engine model. Such a multi-disciplinary system simulator includes first-principle models of environmental loads, a vessel hull, a propeller, a shaft, a diesel engine, electrical power plant and control systems. These system simulators provide realistic loads on the diesel engine in the actual operating environments. Finally, effects of the transient loads on efficiency and NOx emissions of a turbocharged diesel engine is experimentally investigated. A sinusoidal load is one of the particular loads on marine diesel engines and is not well studied in literatures. The aims of this study are, first, to find the effect of the cyclic load compared to a constant load and, second, to validate a quasi-steady mapping method for estimation of efficiency and NOx emissions. Average specific fuel consumption and NOx concentration are measured for various load frequencies. The results suggest that the effect depends on both the mean load level and the frequency. The lower the mean load level is and the higher the frequency is, the more distinguished difference are observed. Moreover, a quasi-steady mapping method provides a relatively good estimation for efficiency in most cases. The established framework and developed simulators and mathematical models can be further used for study of the power plants in different configurations and complexity. Such a study may aim to prototype a power plant concept, to find an optimized design of the concept or to design a control system for it. The work in this thesis can also provide a structured guideline for developing a new mathematical models for diesel engines and provide the necessary a priori knowledge to build the model that are fit for the purpose. |
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BibTeX:
@phdthesis{kyum17A,
author = {Yum, Kevin Koosup},
title = {Transient Performance and Emissions of a Turbocharged Diesel Engine for Marine Power Plants: Numerical Simulation and Experimental Investigation},
school = {Norwegian Univ. Science & Technology},
year = {2017},
note = {Co-Supervisor; Main supervisor: Eilif Pedersen; Doctoral theses at NTNU, 2017:237},
url = {http://hdl.handle.net/11250/2455643}
}
|
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| Miyazaki, M.R. | Modeling and control of hybrid marine power plants | 2017 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Recent advancements in energy storage devices technology, together with increased concerns and regulations on greenhouse gas emissions have been a major factor for the development of new and innovative marine power plants design and operation. This thesis focus on one particular operation of hybrid power plants, which is strategic loading.
Due to the lack of readily available models and little understanding on this new emerging area, it was mandatory to study the viability of strategic loading on hybrid marine systems, as well as to evaluate exactly the potential for gas emissions mitigation and fuel consumption reduction due to the presence of the energy storage device. Initially, the effects of the power plant over the DP system were studied, analyzing how much the vessel performance can be improved by guaranteeing a more stable power supply. The energy storage device was analyzed more in depth, with two models being derived. The first model, a high fidelity hybrid dynamic model, captures the fact that the energy storage device time constant is much smaller than the mechanical components, such as engines, thus, being modeled as a discrete event system. The validity of the hybrid dynamic model is validated with experiments conducted in the Hybrid Machinery Laboratory at NTNU, where it is shown that the derived model accurately describes the real system, and can be used to model hybrid power plants with strategic loading. Due to the high complexity and the fact that the hybrid dynamic model is computational demanding, a second model is derived, where the steady state values are taken into account. It is derived such that the resulting average fuel consumption and average gas emissions are a weighted average of the set-points defined by the strategic loading. The hybrid dynamic model is used to validate the steady state model and shows that both models present a high correlation, specially in cases where the transient effects are much faster than the steady state, as expected. With the steady state model, two optimization strategies were derived, that could be used either to minimize fuel consumption or minimize gas emission. The first optimization consists of the active set method, which is proven to converge quickly to the optimum solution in case the discrete mapping from the genset is interpolated linearly. The second optimization strategy, the interior point method, is recommended in case higher order interpolation methods are used to describe the engine characteristic curves, but it has the drawback of being computationally intensive. By combining both optimization strategies, it is possible to have the best performance, being able to operate in real time, while the higher order interpolation methods are respected. It lead to the same result as by utilizing only the interior point method. In summary, it is shown how the strategic loading is able to minimize the gas emissions and fuel consumption, while being modeled with a high fidelity model as well as a fast computational method, which enabled a combined optimization strategy which is viable to be used in real time. It also provides models that are useful in the design process of hybrid power plants. |
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BibTeX:
@phdthesis{mmiy17A,
author = {Miyazaki, Michel Rejani},
title = {Modeling and control of hybrid marine power plants},
school = {Norwegian Univ. Science & Technology},
year = {2017},
note = {Co-Supervisor; Main supervisor: Asgeir Sørensen; Doctoral theses at NTNU, 2017:199},
url = {http://hdl.handle.net/11250/2450905}
}
|
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| Chabaud, V. | Real-Time Hybrid Model Testing of Floating Wind Turbines | 2016 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Offshore wind power is an active research topic motivated by today’s world energy crisis. While bottom-fixed turbines are relatively mature and commercially viable (though dependent on state-based subsidies to green energy), floating platforms still mostly remain at the research stage. The cost of offshore wind turbines is driven by many factors, and has to be reduced. The risk induced by the lack of knowledge of the environmental loads and the structural response of the turbine is one of these factors. Model testing has always played a significant role in decreasing this risk for offshore structures. Yet, for the case of offshore wind turbines model testing suffers from inherent modeling difficulties: The aerodynamic and hydrodynamic loads, which both contribute significantly to the response of the structure, are challenging to model simultaneously. It is impossible to maintain the Reynolds number from full to model scale, leading to erroneous aerodynamic loads if the turbine and environment are scaled upon the Froude numbers (which has to be used in hydrodynamic tests). Another issue is linked to the physical modeling of wind, which is hard to generate in a wave basin and inherently uncertain.
The solution explored in this PhD study is the artificial modeling of the aerodynamic loads by a numerical code (called numerical substructure), which takes as input the online-measured motions of the structure and whose outputs are applied to the structure in real time by means of actuators. The loads are computed in full scale in real time from a numerical wind field. This concept belongs to the vast family of hardware-in-the-loop testing and has been here given the name of real-time hybrid model (ReaTHM®) testing. It has been trademarked and selected by MARINTEK (Norway) as the way to model wind loads in the ocean basin model tests of the CSC NOWITECH semi-submersible platform supporting the NREL 5MW turbine. This thesis aims at using this teamwork as a basis for the suggestion of a design and verification procedure for similar ReaTHM test campaigns. As this represents an innovative testing method in the field of offshore hydrodynamics, the design of the ReaTHM test setup had to be made from scratches and trial-and-error. The thesis relates the experience gained. The definition of the quantities to be measured and the frequency range to be captured turned out to affect the design to a large extent. It first determined which components of the rotor load vector were to be actuated for an accurate modeling of the coupling effects between hydrodynamic, mooring, structural and aerodynamic/ generator loads. The choice was made to focus only on the rigid-body response of the structure, and to focus on frequencies up to the wave range. A dedicated sensitivity study to limited actuation concluded that, for this platform, all components except the vertical tangential aerodynamic force were to be actuated. The actuation system was designed as a set of 6 servomotor-torsion spring-pulley-wire assemblies, pulling on the structure in a carefully chosen way. This actuator design enabled the development of an effective force control strategy, based on the stiffness of the transmission and the relative position difference between the motor’s rotor and the structure. A real-time control system was implemented, acquiring and processing measurements that were in turn used in the numerical substructure (including AeroDyn from NREL) and in the force controller driving the actuators. Observers were designed to estimate and filter the line tensions (to be used in the feedback force controller) and nacelle velocities (to be used by the numerical substructure). The knowledge of the motions of the structure was essential for the performance of the controller. The delay in the position measurement was found to significantly affect the force tracking performance. A dedicated identification method has been developped, and the estimated delay was compensated by polynomial extrapolation. The need for exact modeling of the line kinematics also motivated the development of an emulated physical substructure emulating the basin environement including the modeling of the actuators, implemented in MARINTEK’s SIMA. It communicated in real time with the control system, providing a realistic and flexible simulation environment for design and verification. Among other applications, this emulated physical substructure was used to verify the numerical substructure implementation and settings. The ReaTHM tests were performed by incremental order of complexity, ending with the Ocean Basin tests in October 2015. A post-analysis of the ReaTHM testing method revealed a generally satisfactory performance, through indicators such as the intrusiveness of the method on pure hydrodynamic tests, force tracking errors and effect of delayed inputs. The fully numerical environment provided by the emulated physical substructure played a central role in the post-analysis. Also, an integrated linear model was developed for uncertainty analysis. The linear model was able to accurately capture both structural dynamics, hydrodynamics, aerodynamics and wind turbine control, as well as to model the entire ReaTHM testing control system, with the possibility of adding extra uncertainty. It enabled a flexible and efficient use of the frequency domain. Such a tool can also be used for model-based control design if developed at an early development stage. As expected when developing a novel method, some aspects of the design were not optimal. The most critical issues were linked to the mechanical design of the scale model and of the actuators. Process noise arising from structural vibrations of spurious eigenmodes showing an overly low natural frequency and/or too little damping turned out to have a dramatic effect on the performance of the force controller. This was worsened by the action of time delays. Efforts were made to study these undesired processes and illustrate them, leading to a list of design rules and fixes that should be followed/applied to facilitate the success of future ReaTHM testing campaigns. |
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BibTeX:
@phdthesis{vcha16A,
author = {Chabaud, Valentin},
title = {Real-Time Hybrid Model Testing of Floating Wind Turbines},
school = {Norwegian Univ. Science & Technology},
year = {2016},
note = {Co-Supervisor; Main supervisor: Sverre Steen; Doctoral theses at NTNU, 2016:369},
url = {http://hdl.handle.net/11250/2433010}
}
|
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| Kjerstad, Ø.K. | Dynamic Positioning of Marine Vessels in Ice | 2016 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis is a collection of papers focusing on various aspects of dynamic positioning of marine vessels in ice. Most emphasis is put on the dynamic broken, or managed, sea-ice environment where pioneering operations have shown that conventional dynamic positioning systems are capable given light ice conditions. When the conditions toughen, or the ice drift direction changes quickly, these systems struggle and may fail. Yet, it is reported that manual control renders sufficient stationkeeping possible.
To understand the operational environment the vessel-ice interactions are studied using model scale experiments and numerical simulations. It is found that the multi-body interactions of the vessel-ice interaction contain complex processes that may introduce a significant and highly varying disturbance. To handle this it is concluded that the core control system must be reviewed with focus on increasing reactiveness to external perturbations together with an operation strategy complying with the ice dynamics. Increasing reactiveness is approached in three ways; by extending conventional model based design methods to capture the ice dynamics, by introducing hybrid control theory to allow for instantaneous change of estimated variables, and nally, by incorporation of inertial measurements to form an acceleration feedforward in the control system. All are investigated theoretically and experimentally and show varying feasibility. For closed-loop experiments in managed ice, a framework using a state-of-the-art high fidelity numerical model is developed and applied. Weather-vaning coupled with the reactive algorithms is investigated for operating compliantly with the ice dynamics. It is advantageous as the optimal vessel heading is found through the vessel motion response, and not an explicit ice drift measurement. Finally, motivated by the oblique heading and ice load coupling a novel recursive thrust allocation algorithm for prioritization of selected degrees of freedom is proposed and investigated. |
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BibTeX:
@phdthesis{okje16C,
author = {Kjerstad, Øivind Kåre},
title = {Dynamic Positioning of Marine Vessels in Ice},
school = {Norwegian Univ. Science & Technology},
year = {2016},
note = {Main Supervisor; Doctoral theses at NTNU, 2016:168},
url = {http://hdl.handle.net/11250/2391724}
}
|
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| Bø, T.I. | Scenario- and Optimization-based Control of Marine Electric Power Systems | 2016 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Diesel electric propulsion has become the industry standard for e.g., oil and gass vessels, cruise vessels, ferries, and vessels with dynamic positioning (DP) systems. Diesel engines are paired with generators to produce electric energy, which is used by electric motors for propulsion of the vessel, and also by other consumers, such as hotel loads, drilling drives, cranes, and heave compensators. This system is reliable and efficient due to the flexibility of the electric grid. DP is often used as a motivating example in this thesis. The thrusters of a vessel using DP is used to fix the position and heading of the vessel. The power plant is operated with redundancy, as a single fault should not lead to loss of position. However, this redundancy decreases the efficiency of the power plant. This thesis presents new ideas and results on how to increase the efficiency of a hybrid power plant with diesel generator sets and batteries while maintaining the required safety level.
A model of a marine vessel is presented in Chapter 2. This model includes the power plant, a hydrodynamic model, and control systems. The power plant includes generator sets, batteries, switchboards, thrusters, and hotel loads. Environmental loads are included in the hydrodynamic model, such as first and second order wave loads, mean and gusting wind, and ocean current, along with the hydrodynamic model of the vessel and the thrusters. The included control systems are a power management system, a DP-controller, thrust allocation, and low level controllers of producers and consumers. Earlier marine vessel simulators mainly focused on the hydrodynamic model or the power plant. However, the present model combines the three models, to investigate the complex integration and interaction effects between the models. These interaction effects are especially important when investigating the DP performance after faults in the power plant. Chapter 2 presents the models needed for this integration. Three simulation cases are presented, to shows that the simulator can capture the interaction effects. A simulation-based dynamic consequence analysis is presented in Chapter 3. The tool uses the simulator from Chapter 2 to simulate several possible worst case scenarios. This tool can be used by the operator to optimize the electric power plant configuration, and to show that no single failure lead to loss of position. The dynamic consequence analysis is necessary when stand-by generators are considered, as the vessel may lose position during the time from when the fault occurs until the plant fully recovers, even if the vessel maintains its position after recover. A scenario-based model predictive controller (MPC) is presented in Chapter 4. This controller uses fault scenarios, internally, to constrain the nominal trajectory, which is an alternative to conventional static safety constraints. The control of generator set speed of a marine power plant is used as a case study. Simulations show that fault scenarios can replace static safety constraints by using this controller. Chapter 5 presents a method to control peak-shaving. Peak-shaving by batteries is used to cancel out power fluctuations, which cause variations in the electric grid’s frequency. However, the batteries may get too hot if power demand is too large. The proposed controller, based on a power spectrum analysis and MPC, reduces the power fluctuations as much as possible without letting the battery get too hot. Simulations using data generated by the simulator in Chapter 2 showed that the controller can achieve these objectives as long as the characteristics of the load does not change too rapidly. Use of the vessel itself as energy storage during DP operation is explored in Chapter 6. A vessel oscillates about its mean position by reducing the thruster power when the total power demand of the vessel is high and increasing it during periods of low power consumption. An analytical formula for motion amplitude given by power amplitude is calculated in this chapter. The formula is compared with simulations, and the simulation results agreed with the formula. It is also shown that the resulting deviations in position from variations of several megawatts are no larger than typical position deviations from the dynamics of ocean waves and wind. The proposed models and controllers are demonstrated through simulations using MATLAB/SIMULINK. The MPC-based controllers are implemented in ACADO. |
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BibTeX:
@phdthesis{tboe16A,
author = {Bø, Torstein Ingebrigtsen},
title = {Scenario- and Optimization-based Control of Marine Electric Power Systems},
school = {Norwegian Univ. Science & Technology},
year = {2016},
note = {Co-Supervisor; Main supervisor: Tor Arne Johansen; Doctoral theses at NTNU, 2016:47},
url = {http://hdl.handle.net/11250/2382342}
}
|
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| Zhang, Q. | Image Processing for Ice Parameter Identification in Ice Management | 2015 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Various types of remotely sensed data and imaging technology will aid the development of sea-ice observation to, for instance, support estimation of ice forces critical to Dynamic Positioning (DP) operations in Arctic waters. The use of cameras as sensors for offshore operations in ice-covered regions will be explored for measurements of ice statistics and ice properties, as part of a sea-ice monitoring system. This thesis focuses on the algorithms for image processing supporting an ice management system to provide useful ice information to dynamic ice estimators and for decision support. The ice information includes ice concentration, ice types, ice floe position and floe size distribution, and other important factors in the analysis of ice-structure interaction in an ice field.
The Otsu thresholding and k-means clustering methods are employed to identify the ice from the water and to calculate ice concentration. Both methods are effective for model-ice images. However, the k-means method is more effective than the Otsu method for the sea-ice images with a large amounts of brash ice and slush. The derivative edge detection and morphology edge detection methods are used to try to find the boundaries of the ice floes. Because of the inability of both methods to separate connected ice floes in the images, the watershed transform and the gradient vector flow (GVF) snake algorithm are applied. In the watershed-based method, the grayscale sea-ice image is first converted into a binary image and the watershed algorithm is carried out to segment the image. A chain code is then used to check the concavities of floe boundaries. The segmented neighboring regions that have no concave corners between them are merged, and over-segmentation lines are removed automatically. This method is applicable to separate the seemingly connected floes whose junctions are invisible or lost in the images. In the GVF snake-based method, the seeds for each ice floe are first obtained by calculating the distance transform of the binarized image. Based on these seeds, the snake contours with proper locations and radii are initialized, and the GVF snakes are then evolved automatically to detect floe boundaries and separate the connected floes. Because some holes and smaller ice pieces may be contained inside larger floes, all the segmented ice floes are arranged in order of increasing size after segmentation. The morphological cleaning is then performed to the arranged ice floes in sequence to enhance their shapes, resulting in individual ice floes identification. This method is applicable to identify non-ridged ice floes, especially in the marginal ice zone and managed ice resulting from offshore operations in sea-ice. For ice engineering, both model-scale and full-scale ice will be discussed. In the model-scale, the ice floes in the model-ice images are modeled as square shapes with predefined side lengths. To adopt the GVF snake-based method for model-ice images, three criteria are proposed to check whether it is necessary to reinitialize the contours and segment a second time based on the size and shape of model-ice floe. In the full-scale, sea-ice images are shown to be more difficult than the model-ice images analyzed. In addition to non-uniform illumination, shadows and impurities, which are common issues in both sea-ice and model-ice image processing, various types of ice (e.g., slush, brash, etc.), irregular floe sizes and shapes, and geometric distortion are challenges in sea-ice image processing. For sea-ice image processing, the “light ice” and “dark ice” are first obtained by using the Otsu thresholding and k-means clustering methods. Then, the “light ice” and “dark ice” are segmented and enhanced by using the GVF snake-based method. Based on the identification result, different types of sea-ice are distinguished, and the image is divided into four layers: ice floes, brash pieces, slush, and water. This then makes it possible to present a color map of the ice floes and brash pieces based on sizes. It also makes it possible to present the corresponding ice floe size distribution histogram. |
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BibTeX:
@phdthesis{qzha15C,
author = {Zhang, Qin},
title = {Image Processing for Ice Parameter Identification in Ice Management},
school = {Norwegian Univ. Science & Technology},
year = {2015},
note = {Main Supervisor; Doctoral theses at NTNU, 2015:340},
url = {http://hdl.handle.net/11250/2372997}
}
|
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| Metrikin, I. | Experimental and Numerical Investigations of Dynamic Positioning in Discontinuous Ice | 2015 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This thesis studies dynamic positioning (DP) operations of a conceptual Arctic drillship in discontinuous sea ice conditions. The stationkeeping behaviour of the vessel under the influence of dynamic ice actions is investigated in both intact and managed ice environments, with an emphasis on DP operations in broken ice. The problem is approached by a combination of experimental and numerical methods. The experimental work was performed in the large ice tank of the Hamburg Ship Model Basin in 2011 and 2012, where almost 250 different scenarios were tested in various ice conditions using a scale model of the conceptual Arctic drillship. The governing characteristics of the global ice load signals were identified from the model testing data, and a connection was established between the major physical processes occurring in the ice cover and the ice loads acting on the vessel. Then, these findings were used to analyse
the limitations of conventional open-water DP control systems in ice. It was found that conventional systems require improvements for successful stationkeeping in tight ice conditions. Finally, it was concluded that model testing is a promising method for studying and analysing DP operations in both intact and managed sea ice conditions. The numerical portion of the thesis presents a novel approach to high-fidelity simulations of the vessel-ice interaction process. This approach is based on a 3D formulation of the nonsmooth discrete element method. A physics engine middleware is used for collision detection, and an iterative multibody solver is employed to calculate the contact forces among the simulated objects. The numerical model enables the simulation of progressive failure and fragmentation of the ice floes, together with the submergence and sliding of the broken ice pieces around the vessel, which makes it possible to simulate both intact and managed ice conditions within a single software framework. The outcomes of the numerical simulations were compared with experimental data, and the results confirmed that the model is able to capture the major physical processes identified in both full- and model-scale experiments with reasonable fidelity and computational performance. Furthermore, the model was successfully applied to a wide range of engineering challenges and novel DP solutions, including DP in managed ice, DP in level ice, physical ice management, automatic heading control of a vessel in managed ice, DP-ice capability plot derivation and DP in curvilinearly drifting managed ice. Although DP is clearly a promising stationkeeping technology for Arctic offshore operations, more full-scale data are needed to qualify the experimental and numerical techniques for predicting the global sea ice loads on DP vessels. |
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BibTeX:
@phdthesis{imet15A,
author = {Metrikin, Ivan},
title = {Experimental and Numerical Investigations of Dynamic Positioning in Discontinuous Ice},
school = {Norwegian Univ. Science & Technology},
year = {2015},
note = {Co-Supervisor; Main supervisor: Sveinung Løset; Doctoral theses at NTNU, 2015:329},
url = {http://hdl.handle.net/11250/2374724}
}
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| Zhao, B. | Particle Filter for Fault Diagnosis: Applications to Dynamic Positioning Vessels and Underwater Robotics | 2015 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Safety remains an important consideration in control system design. This is particularly true for the control systems that are employed by the offshore petroleum and maritime industries, in which authorities continually claim to have developed more and more rigorous methods and processes to ensure safety and reliability. In addition to the use of more traditional approaches to conducting redundancy design, backup system design, robust design, and failure mode and effect analysis, these methods often involve the use of fault tolerant design to enhance the safety and reliability of the control system.
As a common practice, segregation and redundant design are used for dynamic positioning vessels to isolate faulty components and prevent the propagation of faults. However, many incidents still occur as a result of fault escapes from the segregation on a torpid detection. In more severe cases, false detection can actually cause faults and may result in an even more dangerous situation that has more catastrophic consequences. Hence, precise and timely fault diagnosis is necessary for the operator or the automation system to take appropriate action. This thesis presents a brief overview of the existing fault diagnose methods, with a particular focus on particle-filter-based framework for fault diagnosis. The paper commences with a brief review of the background, theory, and typical features of the particle filter before progressing to examine the relationships and differences between the particle filter and other traditional stochastic filters. Switching mode hidden Markov model were employed to model a system with potential faults and a new methodology that uses a particle filter as fault diagnosis filter was developed. This method was then applied on an underwater robotic, which worked in complex environmental disturbance and suffered from different failure modes. Experimental results from ROV sea trails verified that the new fault diagnosis design is effective and reliable. |
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BibTeX:
@phdthesis{bzha15A,
author = {Zhao, Bo},
title = {Particle Filter for Fault Diagnosis: Applications to Dynamic Positioning Vessels and Underwater Robotics},
school = {Norwegian Univ. Science & Technology},
year = {2015},
note = {Main Supervisor; Doctoral theses at NTNU, 2015:160},
url = {http://hdl.handle.net/11250/296033 }
}
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| Veksler, A. | Optimization-based control of diesel-electric ships in dynamic positioning | 2014 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: Recent advances in computer hardware and algorithms make it possible to consider more computationally demanding control methods, allowing more effective exploitation of the equipment under control.
This thesis explores new ways of controlling ships (or other marine vessels) that are designed to keep a pre-determined position and heading automatically exclusively by the means of their thrusters – a task called “Dynamic Positioning”, or DP. Special attention is given to the interplay between the thruster system and the power plant that supplies it. A DP control architecture typically consists of at least 1) a DP control algorithm that considers the current position and velocity of the vessel against the DP setpoint, and calculates the total forces and the moment that the thruster system should produce, and 2) a thrust allocation (TA) algorithm that calculates the forces to be produced by the individual thrusters to match the command from the DP control algorithm. Chapter 2 describes a TA algorithm that enables centralized control over the power consumption in the thruster system. It achieves that by allowing the TA to make short-term deviations from the command it receives from the DP control algorithm; the resulting deviations in position and velocity of the vessel are carefully monitored and constrained, and are usually small due to the large inertia of a typical marine vessel. This enables the thrusters to counter-act load variations from other consumers on the ship, reducing the total variations on the power plant. The TA algorithm is tested on a simulated marine vessel, which includes a realistic marine power plant. In Chapter 3, a more efficient version of this algorithm is described. The improvement in efficiency is achieved by positioning the vessel against the slowlyvarying component of the environmental forces in a way that increases the acceptable deviation margins in the likely drift-off direction. In Chapter 4, the capabilities of the thruster system to control its power consumption are examined from a theoretical perspective. Much of the work above required a mathematical model of the power output from a diesel engine; a model that is well-suited for controller design and verification purposes was designed based on first-principle models in the literature. This model was then used to design an improved diesel engine governor (controller) algorithm, which is described in Chapter 5. The TA algorithms that are described in the literature usually focus on solving one or a few aspects of the TA problem at a time. In Chapter 6, functionality from several earlier publications is gathered into a single TA algorithm. The singularity avoidance functionality is given additional theoretical treatment. Implementing a DP control algorithm that is aware of thruster limitations such as saturations and rotation rate constraints involves largely heuristic adaptions. Chapter 7 introduces a DP control architecture that avoids having separate DP and TA algorithms, and is instead based on a single MPC-based controller. This allows better coordination between control of the thrusters and control of the ship. |
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BibTeX:
@phdthesis{avek14B,
author = {Veksler, Aleksander},
title = {Optimization-based control of diesel-electric ships in dynamic positioning},
school = {Norwegian Univ. Science & Technology},
year = {2014},
note = {Co-Supervisor; Main supervisor: Tor Arne Johansen; Doctoral theses at NTNU, 2014:302},
url = {http://hdl.handle.net/11250/229730}
}
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| Haugen, J. | Autonomous Aerial Ice Observation | 2014 | School: Norwegian Univ. Science & Technology | phdthesis | URL |
| Abstract: This work is concerned with autonomous aerial ice observation. Ice observation is a supporting activity in cold regions marine operations that are disturbed by various ice features. This supporting activity is motivated by the requirement of maintaining an awareness map of the surrounding ice conditions in order to execute an operation in a responsible manner. It is desired that the ice monitoring occurs both efficiently and as autonomously as possible. A part of the ice monitoring is thus to construct frameworks that are capable of executing various monitoring tasks without, or with minimal human intervention.
Chapter 2 covers viable instrumentation configurations for remotely sensing different ice features from unmanned aerial vehicles. The chapter also motivates the use of unmanned aerial vehicles together with other sensor platforms, so that the strengths and weaknesses of the various sensor platforms can be exploited when maintaining the ice condition awareness map. The task of monitoring moving surface objects, often called target tracking, is examined in Chapter 3. We make use of nonlinear programming to construct feasible continuous trajectories for mobile sensing agents. The proposed framework uses each object’s Riccati differential equation, which is based on the continuous extended Kalman filter, in feasibly guiding the mobile agents between the objects. The framework is validated by a full-scale hybrid experiment where a singular fixedwing aircraft monitors three simulated objects in a constricted region of operation. We also explore the nonlinear programming approach in solving the dynamic coverage problem in Chapter 4. Here, the task is to remotely monitor a dynamic process in a planar region with mobile sensor agents. As in Chapter 3, the mobile sensor agents have maneuverability constraints that the framework takes into consideration when finding paths that the sensors should follow. The framework employs a simpler model, compared to the Riccati equation in Chapter 3, in describing how the possible information reward changes in space and time. The machinery of control theory and Lyapunov functions is investigated in Chapter 5 as a more computational efficient alternative to the nonlinear programming approach. |
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BibTeX:
@phdthesis{jhau14C,
author = {Haugen, Joakim},
title = {Autonomous Aerial Ice Observation},
school = {Norwegian Univ. Science & Technology},
year = {2014},
note = {Co-Supervisor; Main supervisor: Lars Imsland; Doctoral theses at NTNU, 2014:291},
url = {http://hdl.handle.net/11250/261450}
}
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