With the trend towards offshore LNG production and offloading, sloshing of LNG in partially filled tanks has become an important research subject for the offshore industry. LNG sloshing can induce impact pressures on the containment system and can affect the motions of the LNG carrier. So far, LNG sloshing was mainly studied using model tests with an oscillation tank. However, the development of Navier-Stokes solvers with a detailed handling of the free surface, nowadays allows the numerical simulation of sloshing. It should be investigated, however, how accurate the results of this type of simulations are for this complex flow problem. The present paper first presents the details of a numerical model, an improved Volume OF Fluid (iVOF) method. Comparisons are made with sloshing model test results. Based on the results, the following conclusions can be drawn: - The dynamics of sloshing in LNG tanks can be simulated numerically using an iVOF Navier-Stokes solver. - Several improvements have been made in the treatment of numerical spikes in the pressure signals, but still more improvements need to be made. - Qualitatively, the pressure pulses resulting from impacts against the LNG tank wall show a rather good agreement between experiment and numerical simulation. - Quantitatively, the differences with the experiment show that further detailed studies with respect to cell sizes and time steps are necessary.

Sloshing in LNG tanks has gained increasing attention over the past period of time. This is mainly caused by developments in the LNG market, changes in the design and operation of LNG ships and an increasing interest in floating gas field exploitation. The issue of sloshing in partially filled tanks is relevant for spot trading and offshore loading/off loading of LNG ships as well as for FPSOs with LNG capacity. DNV has developed a step-by-step experimental procedure to determine sloshing loads for structural analysis of the insulation system and tank support structure. Of key importance for a reliable evaluation is the step-by-step approach, putting emphasis on an accurate treatment of every step. This means careful modelling of operational and environmental conditions, accurate ship motion calculations, a well-defined procedure for identifying design sea states, a proper experimental set-up and an accurate treatment of the statistics involved in every step in order to determine reliable and realistic design sloshing pressures. To study sloshing loads in partially filled LNG tanks irregular sloshing experiments have been conducted for head and beam seas for different filling levels and sea severities. A 1/20 scale model of a tank from a 138.000 m3 membrane type LNG ship was used for the tests. Measurements have been conducted using pressure transducers and pressure transducers mounted in clusters. An overview of the tests is given with an analysis of the impact pressure statistics, the pressure pulses and the associated subjected area. From these analyses a discussion is presented on the effect of different filling levels, sea severity, ship speed and heading. Introduction The gas market is in an upswing and will in the future provide an increasing part of the world energy demand. A large part of the natural gas reserves will be transported by sea from well to customer. Several changes are seen in the offshore production of gas and the sea-borne transportation of LNG. Floating installations to produce offshore gas is an upcoming market. In case of liquefied storage, present filling restrictions will be violated implicitly. In the LNG shipping industry three main changes are seen or expected:The LNG market is expected to develop more into a spot-market instead of long-term contracts. This view is supported by the fact that LNG carriers have beenordered without having a first transportation contract. As a consequence of spot-market trading shipowners prefer to increase their trading flexibility by having the possibility to operate with not-fully loaded tanks, which would imply a reduction of the upper filling restriction.A second change is foreseen in the maximum size of LNG carriers. By a market-push to reduce transportation costs the maximum size of LNG vessels will increase.A third change is foreseen in the location of loading and offloading. With an increasing focus on safety, supported by the threat of terrorism, it is becoming difficult to build land-based terminals for loading and off-loading, especially in the US. Offshore terminals, far away from dense populated areas are the logical solution, but with the implication of more severe environmental conditions when loading or discharging.

A summary is presented on calculation procedures to determine extreme responses of ocean-going structures using nonlinear time domain simulations. As such nonlinear simulations are time consuming full scatter diagram assessments are not feasible. Advanced techniques are therefore required to assist in the selection of response critical sea states. Of particular importance is to what extend a few or even a single design sea state can be used to determine a lifetime design value. An overview of methods, which can be used to determine design sea states, is given and discussed. Secondly, if a design sea state is specified different approaches are possible to assess a response in that sea state. Regular, random irregular and conditioned irregular waves can be modeled, which affect both the accuracy and the computation time. Alternative extreme wave conditions are considered, in perspective to existing techniques, and the application of freak waves for response simulation is discussed. An example simulation is shown of a cruise-ship in a freak wave. In this example different phases are identified and the associated physical phenomena are discussed. Based on this study and the calculations, requirements for future research are formulated in order to use extreme/freak waves in response analyses and secondly to consider the use of freak waves in design procedures.

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