Experimental Study on the Hydrodynamic Forces acting on Objects in a Moonpool

Abstract

This master thesis describes and discusses an experimental study on the hydrodynamic forces acting on objects in a moonpool, carried out at MARINTEK in 2013. The experiments were done using a simplified 2-dimensional moonpool model with two different configurations; one base case with a sharp inlet and no perforated walls inside the moonpool, and one configuration attempting to model the moonpool of the vessel Seven Viking, which has a rounded inlet and perforated walls. Both moonpools were tested without objects, while the Seven Viking-model was also tested with two objects inside the moonpool, both of the same geometry (square cross sections) but of different sizes.In the past, a considerable amount of research has been devoted to the wave elevation in an empty moonpool, a problem which has proven difficult to describe with sufficient accuracy unless the most complicated numerical methods are applied. However, very little is known regarding the hydrodynamic forces acting on objects in moonpools. Simplified methods, recommended by DNV (Det Norske Veritas), for determining these forces exist (DNV 2011). These methods are based on the following assumptions:- The presence of an object does not alter the fluid motions in the moonpool to a significant degree.- The forces on the object can be determined using Morison?s equation with correction factors based on the level of blockage, as for an object in a tube or channel.The main motivation for carrying out the current model tests was to compare the measured forces with the forces predicted by such a simplified method, and thereby attempting to determine the validity of the method and its assumptions.The experiments revealed that introducing objects in a moonpool changes the wave elevation dramatically, even for quite small objects. The overall wave elevation amplitude is strongly reduced and the resonance period is altered. Therefore it is no surprise that the simplified calculation methods fail to predict not only the magnitude of the hydrodynamic forces on the objects, but also the trends.If, on the other hand, the true fluid motions (found from the experiments when the object is inside the moonpool) are used as the basis of the force calculations, the predicted forces turn out to be quite close to the measured forces. This shows that, at least in this particular case, the assumption about the validity of Morison?s equation may be justified to some extent. This scheme is however not very useful in real life, as the fluid motions when an object is inside the moonpool are generally not known unless model tests have been performed beforehand, in which case the forces are likely to be measured directly anyway.Throughout the current model tests, the technique of PIV, particle image velocimetry, has been used for visualizing the flow field inside and in the vicinity of the moonpools. The results have shown how the formation of vortices are considerably less triggered by the rounded inlet of the Seven Viking-moonpool compared to the base case with the sharp inlet. This is also reflected in the transfer functions of the wave elevation in the moonpools where it can be seen that the Seven Viking-moonpool has less non-linear damping than the base case. The PIV results also show that there seems to be only a small amount of flow through the perforated walls of the Seven Viking-moonpool.