Oscillations in Moonpool at Static Platform in Waves

摘要

Most of the offshore platforms at sea are static and considered as rigid. This paper presents a series of towing tank experimental research to understand the wave and structure interaction for moonpool installed at static platform. It also gives a comparison of results between static platforms in waves and model moving in calm water with forward speed. Study comprised of circular and square shaped moonpool, while square shaped was further investigated for different attack angles varying from 0～45 degrees with an interval of 15°. Time period of incoming waves was increased from 1 s to 3 s with an interval of 1 s. To have a variety of situations to cater for the actual state at sea, wave height was also increased from 40 mm to 80 mm with an interval of 20 mm. For wave height and time period different combinations are presented to get variety of results. Results are given in terms of 'dB' for water oscillation pressure, which governs the water motions inside the moonpool, and for that matter it directly relates resonance in moonpool as the natural frequency of water inside the cavity does not change with a change in the velocity of incoming waves rather it is dependents upon the physical properties of the fluid medium, shape of cavity, size of cavity and the draught of water inside the cavity. With an increase in the time period of incoming waves, sound level 'dB' in terms of oscillation pressure frequency generated by the circular moonpool also increases. The same phenomenon is observed once the wave height is increased from 40～80 mm; except for one condition I.e., T=3 s and λ=80 mm. This shows that longer and higher waves may not always produce higher noise level for circular moonpools; this is not the case for square shaped moonpool. At increasing attack angles for square shaped moonpool, the acoustic level increases slightly for similar wave conditions.Furthermore, while comparing moving model in calm water with forward speed and static model at in coming waves; it was revealed that, square moonpool at different attack angles in calm water with forward speed condition, produces greater noise level once compared with waves-only condition. This phenomenon is unlike in circle moonpool. The results given in this paper are very important structural elements of the offshore platform during the design process.