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Numerical and experimental investigation of the effect of moonpool positioning on the hydrodynamics of floating drilling production storage and offloading vessel

ABSTRACT The Floating Drilling Production Storage and Offloading (FDPSO) Platform is functionally a non-propelled vessel and belongs to a subset of a wider group of vessels called Floating Production Systems (FPS). The FDPSO, besides its unique attribute of drilling capability, has all the functionalities of FPSOs. Given the remote offshore locations of oil fields, FDPSOs provide viable economic solution for drilling, production, storage and offloading to smaller vessels for transportation of oil. This paper addresses the designer’s concern in the critical role of positioning of the moonpool since it has a direct bearing on the dynamic effects experienced by the platform. The investigation provides valuable quantitative insights into the important effects on drag during towing, sloshing effect within the moonpool and motion dynamics due to the liquid oscillation. The results of this investigation using numerical and experimental tools, is expected to give key inputs for the designer towards the design of better performing FDPSO platform and benefit the industry. This paper investigates the vessel dynamics including the liquid oscillation behaviour inside the moonpool as a function of the moonpool location in the vessel. The results of the study are important inputs to a designer for consideration of new designs. The FDPSO is a dedicated design of a large capacity platform, different from the conventional FPSO which is usually a retrofitted ship for the purpose. The investigation is based on computational simulation using a commercial RANSE solver namely, STAR-CCM+. Comparison with results from towing tank tests serves to initially validate the numerical simulation-based results. The study performs hydrodynamic diffraction analysis using a potential flow-based solver, namely ANSYS – AQWA. A simplified hull form represents the FDPSO, considering that it is a platform predominantly stationary in operation. However, drag and interactive effects with the moonpool as well as a liquid oscillation in the moonpool are important dynamic conditions for investigation in stationary as well as transit conditions during the tow. The focus of the investigation is on moonpool dynamics in calm water and regular sea conditions and ship motion in waves. The moonpool is vertical circular cylindrical shaped and the investigation considers three moonpool locations namely, at the forward, the midship and the aft, respectively. The water column in the moonpool experiences large oscillatory motions in the piston mode or the sloshing mode. The analysis captures the flow physics, amplitude of oscillations, hydrodynamics resistance and the vessel motion response. The results obtained establish that the location of the moonpool contributes to the augmentation of the cavity drag in the FDPSO. For the selected parameters, the study indicates that the moonpool located in the forward region gives better performance as compared to responses in the other locations.