Duality in lng tank-pressure behaviour and its relevance for ship-to-ship transfers to floating storage and regasification units

Abstract

Floating storage and regasification units (FSRU) are in high demand globally forming a specialized and growing fleet of ships to service a steadily expanding liquified natural gas (LNG) industry. Ship-to-Ship (STS) LNG cargo transfers are an inevitable part of the FSRU day-to-day operating patterns. It is little more than a decade since FSRU operations began and a lack of knowledge persists concerning their relatively complex tank pressure behaviour during STS transfers. In this work, based on details from more than thirty STS operations, FSRU tank pressure trends and influencing factors are explained. The interactions between the vapor space and liquid LNG are strongly influenced by the thin surface film between those phases in an LNG tank. The saturated vapor pressure (SVP) of the LNG and the volumetric balance of gas and liquid movements in and out of the tank are also influential. These factors control the rate and extent of evaporation and condensation occurring at the surface film, thereby impacting tank pressure changes. This paper presents observations that reveal a duality in FSRU tank pressure behaviour that in over-pressured conditions allows tank pressure to rise significantly and rapidly. On the other hand, in under-pressured conditions tank pressure stabilizes at certain levels and inhibits significant reduction below those levels. Here, we explain the processes involved in determining FSRU tank pressure trends during STS transfers, and, based on observations of actual STS transfers, provide rules of thumb and empirical equations that can be used to estimate tank pressure behaviour in a range of operating conditions. These are supported by several generic cases of tank pressure, temperature and SVP trends reflecting the various tank scenarios commonly associated with routine operating patterns of FSRU during STS transfers. The principles described in this paper apply to LNG Carriers but tend to be disrupted by sloshing and wave movements also in the tanks during transit.