Investigation of thruster performance in reduced under-thruster clearance during station-keeping operation

Abstract

The capability to maintain position in different operational scenarios is crucial for every offshore vessel, including semisubmersible vessels, which are traditionally designed to operate in deep water conditions. With the transition to new sources of energy production, such as wind turbines, the necessity for these vessels to operate in shallower waters is increasing. This aspect necessitates identifying the impact of such conditions on the vessel’s capacity to maintain its position. This thesis focuses on identifying the governing factor in the reduction of thruster efficiency and, therefore, evaluating the thrust delivered by the thruster to the semisubmersible vessel when operating near the seabed. Specifically, it is relevant to identify which parameters, or combinations, induce an additional reduction in the effective thrust propellers deliver to the vessel in reduced clearance beneath the thruster, thereby affecting the vessel station-keeping capability. The primary goal of this work is to estimate the reduction in the delivered thrust when a thruster operates in proximity to the seabed. Therefore, it is crucial to identify the sources of thrust reduction for a semisubmersible vessel based on the available literature and previous tests on similar vessels. Specifically, the interactions between hull, current, and thrusters have been identified as the most impacting effects in reducing thruster efficiency. Therefore, it was possible to estimate the impact of each of these mentioned types of thruster interactions both with analytical solutions and the suggested formulation from the register class (e.g. ABS). The results indicate that the interaction between the thruster outflow and the hull, combined with the impact of the current on this condition, is strongly modified when compared with deep-water conditions. Approximately 50 computational-fluid-dynamics (CFD) steady-state simulations have been performed to investigate the current effect and the vessel’s geometrical characteristics under different current speed with reduced under-thruster clearance. These simulations were conducted across three different water depths at a single vessel draft, combined with four distinct beam-side current speeds. The primary focus is on evaluating the flow development beneath the hull and comparing it with the deep water scenario. The analysis revealed that the proximity of the seabed partly influences the flow deviation. Contrarily, the flow velocity, and consequently, the frictional resistance, predominantly affects the thruster’s performance. A significant finding is that the thruster’s delivered thrust could decrease by up to 53%, with respect to open water conditions, when operating in a current of 1 knot with 6 meters of under-thruster clearance. These results confirm the hypothesis regarding the impact of shallow water conditions on delivered thrust. The obtained data highlights the importance of accounting for additional thrust loss in situations of reduced under-thruster clearance, as this aspect was not previously considered for dynamic positioning operations. The investigated topic covers a broader scope than the presented results. However, this thesis marks the starting point for future research. It emphasizes the significance of studying the impact on thruster performance under reduced under-thrust clearance conditions and provides the thrust reduction obtained for the specific case of beam-side current condition and the thruster operating in the direction of the downstream floater. Furthermore, the obtained uncertainty indicates that the used CFD approach (steady-state) needs a more comprehensive analysis involving time-consuming numerical solutions to address the vortex shedding phenomena, especially for high current values.