ABSTRACT

During side-by-side offloading of LNG (Liquefied Natural Gas), an FLNG (Floating Liquefied Natural Gas) platform and an LNG carrier must remain close to one another for a considerable amount of time. The water column between these vessels will oscillate with high amplitude if the incident wave frequency matches the gap resonance frequency of the water column. These violent wave motions, coupled with the motion of the vessels, demand extensive laboratory model tests to develop a better understanding of the hydrodynamics. In the present study, an experimental investigation has been conducted on two floating barges in close proximity in beam sea conditions as an attempt to simulate side-by-side LNG transfer operative conditions. The two vessels with unequal draughts can only move freely in heave and roll directions. Based on the relative positioning of the vessels, two distinct configurations named SW (Shallow draught barge at Weatherside) and DW (Deep draught barge at Weatherside) are considered in the study. In the present study, the draught-to-water depth ratio for the shallow draught and deep draught vessels are 0.125 and 0.175, respectively. Wave amplification inside the gap for SW was found to be higher than that of the DW case for most of the waves tested. Heave and roll motions and horizontal force acting on the barges are reported for the most critical case of the waves tested. Higher-order components of the wave elevation and horizontal force are analysed for a better understanding of the influence of gap resonance phenomena on side-by-side offloading.

INTRODUCTION

Operations involving multi-body structures in offshore fields are often influenced by gap resonance. This occurs when the wave elevation between two floating bodies gets amplified when the incident wave frequency matches the natural frequency of oscillations of the water column in the gap. The side-by-side offloading process of LNG (Liquified Natuaral Gas) from FLNG (Floating Liquified Natuaral Gas) platform to LNG tanker is influenced by this phenomenon. It is important to study the influence of gap resonance in detail as this would affect the forces acting on the ship and their motion response.

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