Experimental and Numerical Study on the Resonance in the Narrow Gap Between a Simplified Floating Hydrocarbon Storage Tank System

A novel modular floating hydrocarbon storage facility (FHSF) has recently been proposed to utilize the nearshore sea space of land-shortage countries. The basic modules of FHSF, 14 floating hydrocarbon storage tanks (FHST) and 16 floating barges are loosely connected through flexible connections to reduce the load. This also introduces narrow gaps between the FHSTs and the surrounding floating barges to allow horizontal movement of the FHST. To investigate the potential gap resonance problem, both experimental and numerical study have been carried out. A simplified two-FHST with a surrounding barge system is investigated to reflect the basic problems in the entire system. The experimental study is performed in the ocean basin of SINTEF Ocean with a Froude scaling factor of 1:45. The test conditions include 3 different drafts and 3 different wave headings under both white noise and random waves. The numerical model with the similar setup is analyzed in the frequency domain in WAMIT and HydroStar. The analytical solution on the gap resonance derived by Molin in shallow water is also calculated by assuming the gap has regular shape. The experimental, numerical and analytical results are compared substantially. Results show that the gap resonances are excited in the narrow gaps between the two FHST and between the FHST and the surrounding barge. The potential theory-based codes predict the natural frequencies of the resonances fairly well, although the free surface elevations can be overestimated if no energy dissipation is introduced. The analytical solutions give reasonable predictions to natural period of gap resonances which is useful in the initial design stage. Several resonance modes are identified in the wave frequency range by comparison of numerical and experimental results. The sensitivity study on the gap resonance to the draft of the FHST is further discussed. Some potential solutions to reduce the gap resonance are proposed for the future work.