This work presents the baseline design for the autonomous subsea vehicle capable of traveling at a lower speed of 1 m/s with an operating range of 400 km. Owing to UiS subsea-freight glider's (USFG) exceedingly economical and unique propulsion system, it can transport various types of cargo over variable distances. The primary use-case scenario for the USFG is to serve as an autonomous transport vessel to carry CO2 from land-based facilities to subsea injection sites. This allows the USFG to serve as a substitute for weather-dependent cargo tankers and underwater pipelines. The length of the USFG is 50.25 m along with a beam of 5.50 m, which allows the vessel to carry 518 m3 of CO2 while serving the storage needs of the carbon capture and storage (CCS) ventures on the Norwegian continental shelf. The USFG is powered by battery cells, and it only consumes a little less than 8 kW of electrical power. Along with the mechanical design of the USFG, the control design is also presented in the final part of the paper. The manoeuvring model of the USFG is presented along with two operational case studies. For this purpose, an LQR and PID-based control system is designed, and a detailed comparison study is also shown in terms of tuning and response characteristics for both controllers.
A planar mathematical model for the analysis of equilibrium glide paths of the UiS subsea freight-glider (USFG) is presented. The model is developed using Simscape Multibody in MATLAB/Simulink to study the ever-changing dynamics of the glider. Motion along the heave and pitch direction is regulated by two separate PID controllers. Controllers are tuned for the optimal bandwidth and phase margin to provide the system with ideal gains which satisfy the system requirements. A wide-ranging sensitivity investigation is carried out on the USFG by changing the two key variables, pump flow rate and ballast fraction. The results reflect the advantages of using higher flow capacity and ballast fraction, which should be preferred according to the application, provided if there are no space and weight restrictions. Finally, different glide paths were simulated to observe that, controller gains obtained from the linear model can be improved to acquire better performance in terms of robustness and stability of the system.
The UiS subsea freight-glider (USFG) is a novel 785 DWT large cargo-carrying underwater autonomous vehicle. It uses variable buoyancy propulsion, i.e., it glides forwards using generated lift and drag forces at the hydrofoils while ascending and descending in water. This propulsion method is ultra-efficient and can transport cargo over long distances with minimal energy consumption. For USFG to operate with maximum speed and enhanced range, a robust, reliable, and accurate control is important as this results in a precise navigation system that would allow for its gliding motion to be extremely efficient. During long-distance voyages, a robust control system based on a feedback loop provides enhanced utility against any external uncertainties or environmental disturbances. The purpose of this work is to implement a control methodology that is based on feedback from the model. This approach removes the need for excessive tuning of controllers for any changes in operating conditions. Moreover, dynamic states, i.e., velocities, can be determined by designing an observer which can be used to predict the motion of the USFG in a planar axis. The gliding paths of USFG in the vertical plane are analyzed along with the observability and controllability of the steady equilibrium glides. For this purpose, the control system is designed with two different controllers, PID and LQR, which control the heave and pitch of the vehicle. Finally, a comprehensive comparison study is presented, which highlights the key differences for the controllers for tunning and rise time. This can also help in designing the ideal control for the required application.
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