The phenomenon studied in this work is that of an air pocket entrapped by a free surface water wave inside a rectangular tank at a high filling level. The wave, which is a gravity wave, is caused by forced horizontal motion which is constructed in a particular way, in order to entrap an air pocket as it approaches the upper left corner of the tank. As the wave touches the roof, the air is compressed and starts to oscillate. The oscillations resemble, to some extent, the free oscillations of an underdamped mass-spring system, where the mass is related to the generalized added mass effect of the water pressure associated with the air pocket oscillations. The stiffness is due to the compressibility of the air. The reason for the damping or, more generally, the decay of the air pocket oscillations is less understood. Air leakage has been proposed as one possible reason for this decay. In this work, the role of air leakage is found not to be the reason for the decay of the air pocket oscillations, because it is not present during major parts of the impact. However, by drilling holes in the roof of the tank, the effect of leakage during the oscillations is proven to cause decay. To explain the physical source of the decay of the oscillations, damping due to heat transfer to and from the air pocket is investigated through an analytical one-dimensional steady-state model. The damping due to heat transfer is observed to play an important role. The obtained understanding of the mechanisms causing the decay of the air-pocket impact at the upper corner is believed to be relevant to other types of impacts, particularly the entrapment of air pockets on walls by breaking waves.

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Line breakage events have been experienced on moored structures during recent years. These are often occurring in heavy weather and overload is one of the reasons pointed out. The present paper identifies posible physical phenomena that may lead to wave forces higher than predicted by state-of-the-art hydrodynamic tools and procedures, and thereby higher mooring line loads, in high and steep waves. In particular, a need to re-explore wave-group induced slowly varying, low-frequency (LF) drift forces has been identified. Both mobile offshore units (MODU's) and permanently moored floaters are considered, semisubmersibles and FPSOs. Empirical corrections are sometimes being applied in design of mooring lines, while not in general, and there is no established common industry practice on such corrections. More advanced tools and knowledge do exist in research communities, while they still need further development for robust engineering use. A brief overview is given of state-of-the-art methods and tools in modelling of the hydrodynamic forces on large-volume floaters, with particular focus on slowly varying wave forces. Full scale experiences from real sea events and from a variety of earlier case studies including model tests are reviewed. It is found that several items may be critical in the proper prediction of LF wave forces in high seas and combined current and should be investigated further, in particular: –Wave-current interaction–Viscous wave drift forces–Large and nonlinear wave-frequency vessel motions. Based upon these preliminary investigations, the paper gives recommendations for actions and further developments for improved predictions in industry practice.

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The capability of the OpenFoam software to model slamming where air is entrapped between the free surface and the structure is investigated. The test case studied is a previously studied phenomenon, where an air pocket is entrapped between the free surface and the upper corner of a rectangular tank during sloshing. The air is entrapped due to the shape of the wave approaching the roof. The air pocket is compressed and starts to oscillate. The oscillations resemble the free oscillations of a mass spring system. OpenFoam results are compared with experiments and a numerical method based on a boundary element method (BEM) both of which are available from the previous study. In this work a compressible VOF (Volume Of Fluid) Eulerian two-phase mixture flow solver called compressible InterFoam from OpenFOAM package is used to perform the simulations. The sensitivity of the results to numerical parameters is addressed.