A rapid sloshing model for liquefied natural gas carriers

Godderidge, B. and Turnock, S.R. (2009) A rapid sloshing model for liquefied natural gas carriers. Hydrocarbon World, 4 (2), 40-46.

Abstract

The significant rise in demand for liquefied natural gas (LNG) and the economic aspects of its transportation have resulted in an increase in the number and size of LNG carriers. One of the principal design issues for LNG carriers is sloshing as the containment systems have no internal structures to damp out the liquid motion. Furthermore, because the mass of ship and cargo are comparable, the coupling effect between ship motions and LNG sloshing requires carefu investigation. Considerable increases in the capacity of LNG carriers have renewed interest in the assessment of sloshing loads, and analysis of floating liquefaction and re-gasification installations (floating LNG) requires the inclusion of the sloshing dynamics in a seakeeping model. Recent incidents of sloshing damage onboard LNG carriers1 have added further urgency to the improvement of sloshing analysis in LNG carriers and floating LNG design. The costs of repair of sloshing damage to the containment system and consequent loss of revenue can be significant.
Design optimisation or the use of a numerical wave tank to gather statistical sloshing data requires sloshing simulations with long durations. The full assessment of loading times for offshore LNG (approximately 12–18 hours per condition) with computational fluid dynamics (CFD) is not feasible with currently available computational resources and methods. Membrane containment systems are considered to be at greater risk from sloshing damage than spherical tanks, and detailed sloshing studies are required to determine the sloshing characteristics of a new tank design or vessel operating profile.
Model testing at the experimental scale is often used for the comparative assessment of sloshing, but the scaling of impact pressures between model and full scale is often problematic. Full-field numerical techniques such as CFD can capture strongly non-linear sloshing at full scale, but large computational requirements restrict their application and they are unsuitable for the analysis of longer time series due to excessive computational requirements and their susceptibility to growth of numerical errors. Analytical approaches such as multimodal analysis can be computed in faster than realtime, but they are limited to linear and some cases of weakly non-linear sloshing.

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Contributors

Author: S.R. Turnock

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