Sloshing is liquid movement in a container excited by motion. The sloshing flow becomes more violent and results in an intense liquid impact as the excitation period gets closer to the resonance period of the internal liquid. This sloshing impact is the most critical load component in the structural design of a Liquefied Natural Gas (LNG) Cargo Containment System (CCS) of an LNG carrier or Floating Liquefied Natural Gas (FLNG). In this study, the structural assessment of sloshing loads was performed for a two-row Mark III membrane-type CCS of an FLNG. The CCS was designed to be installed in a gas field in northwestern Australia where the wind-sea and swell components were simultaneously measured. Two distinct sea states of special interest were considered for the structural analysis. One included sea states on the contour line corresponding to a 100-year return period in a wave scatter diagram. These sea states are usually used for the prediction of the extreme motions of floaters and are termed extreme sea states. The other included sea states with a high probability of occurrence. These sea states are used for investigating the effect of long-term exposure to sloshing loads on CCS structures. An evaluation of the structural safety of the CCS and hull structures was conducted by the use of a sequence of Computational Fluid Dynamics (CFD)-based scenario screenings, sloshing model tests, and statistical fittings of Utilization Ratios (URs). It was found that all probable sea states had to be fully considered when the screening for the sloshing assessment was performed. Even though the sea states with a high probability of occurrence had smaller significant wave heights than the extreme sea states, they might cause the largest sloshing loads and structural responses.
The new global energy strategy is gradually focusing on natural gas due to economic benefits from conventional petroleum resources. A number of offshore gas reservoirs, which were uninteresting in the past, are now being commercialized, and as a result, new Floating Liquefied Natural Gas (FLNG) development projects related to natural gas production are being executed. In the case of FLNGs, it is not possible to impose restrictions on the filling level of the liquefied natural gas cargo during continuous operation; thus, significantly violent sloshing phenomena can occur. This is in contrast to LNG carriers that operate under restricted filling conditions. Therefore, the sloshing resistance of an FLNG CCS and the adjacent hull structure is one of the most important design parameters for the structural safety of an FLNG.
With respect to the structural design, it is not easy to determine the design sloshing loads absolutely because the LNG sloshing is a highly nonlinear impact phenomenon. Although the model tests can measure the impact pressure, the direct use of the measured pressures in the design is still controversial due to uncertainty regarding the scale effects and cryogenic characteristics. This is why the common practice is to comparatively assess the structural capacities of the CCS of a target vessel and those of a reference vessel, which is proposed by Classification Societies (ABS, 2006; BV, 2011a, 2011b; DNV, 2006).