The enthusiasm of the market for liquefied natural gas pushes ship owners, charterers and shipyards to develop "tailor made" projects adapted to the new constraints of the LNG chain. This evolution involves, not only increased cargo capacity, but also specific operating conditions well suited for off-shore applications. To assess the viability of membrane design, dedicated liquid motion studies are requested. The initial step of a complete sloshing assessment consists in a full seakeeping analysis to anticipate the behaviour of the ship during her whole life. Gaztransport & Technigaz (GTT) continuously improve this first step of their methodology supported by a large R&D program.
The three main fields of recent improvements deal with:
Environmental conditions;
Coupling effects;
Off-shore projects.
This paper describes the main improvements on these different items integrated into GTT's methodology for sloshing assessment.
As the initial step of each liquid motion analysis, ship motions calculations are carried out. They result from the combination of two parameters: ship project characteristics and environmental conditions. The ship project is defined by the hull form, a good knowledge of the mass distribution over the different loading cases describing the operations of the vessel, and all the contributors to added damping in roll (rudders, bilge keels, …). The sea state is described by the significant wave height (Hs), the associated period (peak period Tp or zero up-crossing period Tz), and the wave spectrum, representative of the energy distribution S(ω). Up to very recently, it was commonly admitted that sloshing phenomena were induced by the worst sea states the vessel will have to face, regardless of all the other intermediate contributors. This approach has been first modified for off-shore projects, for which all contributors are considered.
