Experimental and Numerical Study on the Flow Reduction in the Moonpool of Floating Offshore Structure

This paper presents the efforts of experimental and numerical studies to reduce internal flow of moonpool. Drillships are equipped with a vertical opening at the center of the hull called ‘moonpool.’ Recently, the moonpools are getting longer and wider for the higher operability. With this trend, violent internal flows are getting more concerned in terms of the safety and operability, which have been reported during the operations even in mild seas. Also, it is well known that the internal flow gives higher resistance during the transit of drillship. Therefore, there have been a number of motivated researches on the reason and the source of excitation, the pattern of the internal flow and the way to suppress it. Typically an internal flow of the moonpool has two types of oscillation modes: piston- and sloshing-mode. The excited oscillations of water and resultant internal flows are highly dependent on the shape of the moonpool, partly due that the resonant periods are varied with the size and shape of the moonpool. However, since the shape and size of the drillships are quite standardized, there may be no room for the change of shape to reduce the inflow from the bottom of moonpool. Therefore, more efforts have been made to develop the damping devices such as splash plates, which can be easily installed inside of the moonpool. In this study, to see the effect of larger damping devices, a series of experimental and numerical study was carried out for the four moonpool designs; the ordinary plain moonpool, the moonpool with a recess deck, the moonpool with an isolated recess deck (island deck) and moonpool with a combination of island deck, splash plates and wave absorber. From the model tests, it was found that the internal flow of the moonpool was significantly reduced by the application of the wave absorber. In case of the moonpool with the island deck, the sloshing mode oscillations was not observed due to the gap flow between the inner wall of the moonpool and the recess, while the piston mode oscillations were remained unchanged. For the in-depth understanding of the flow behaviors and characteristics, the internal flow of the moonpool has been investigated using RANS based CFD code. The various moonpool designs were simulated to identify the effect of each device for the internal flow reduction of the moonpool. The CFD analysis results with regular waves, the water surface responses inside moonpool such as the flow pattern and resonance frequency were compared with model test results and showed reasonably good agreements.