Experimental Study of Anti-Sloshing Foam Layers for FLNG Tanks

Zhang, Chongwei (Dalian University of Technology) | Su, Peng (Dalian University of Technology) | Ning, Dezhi (Dalian University of Technology) | Wang, Zhen (Dalian University of Technology)

OnePetro 

ABSTRACT

An anti-sloshing concept for FLNG tanks is investigated by taking advantage of floating foam layers. Physical experiments are conducted in a rectangular tank to investigate the sloshing properties. Effects of the layer thickness on the sloshing dynamics are analyzed. It shows that the floating foam layer can efficiently damp the free surface oscillations and reduce the pressure amplitude in a tank. Higher-order frequency components of the hydrodynamic pressure in the tank gradually vanishes as the foam thickness increases. Considering the economy in practice, the thickness of the foam layer should be as small as possible, as long as the sloshing mitigation effect can be guaranteed. For the present situation, a suitable choice of the foam thickness is recommended to be one-tenth the mean liquid depth.

INTRODUCTION

A proper design of the anti-sloshing technique for FLNG tanks is important for the operation safety. At present, various anti-sloshing techniques have been investigated for liquid cargo tanks. A great proportion of these techniques was based on energy dissipation effects of internal structures (e.g. fixed baffles, flexible baffles, screens, blocks, bulkheads, etc.) in a liquid tank, which has been comprehensively reviewed by Faltinsen and Timokha (2009). There are also some membrane-based techniques used in space technology, and inflatable membranes in trucks (e.g. Accede company, the Netherlands).

Some more recent studies for FLNG tanks include Jung et al. (2012), Wang et al. (2013), Lu et al. (2015), Cho and Kim (2016), Xue et al. (2017), Yu et al. (2017), Sanapala et al. (2018), and so on. The above baffle-based anti-sloshing techniques have been widely adopted by oil or chemical tankers. However, these techniques still require further verification for the special case of LNG tanks. Because the internal surface of an LNG tank is fully covered by thin invar membranes to prevent gas leakage at an extremely low temperature, installing baffles through the membrane surface may affect mechanical properties of the membrane and bring safety risks. Thus, alternative anti-sloshing ideas keep emerging in the recent decade, trying to avoid damaging the membrane surface.