Sloshing in Partially Filled LNG Tanks - An Experimental Survey

Abstract

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.

Outlier analysis of sloshing impact loads on liquid ship cargo

“…The degree of deviation varies with filling depth and impact location. In most cases, comparing the minimum and maximum values of 20 replicates showed a difference of more than twice that of the pressure value at the probability level of 10 −4 . From these results, it can be seen that there is a lot of uncertainty in the results of estimating the sloshing load through the 5-h test results recommended by the existing classification societies and laboratory.…”

“…Numerous studies to investigate the sloshing phenomenon have been continuously conducted based on numerical, [1][2][3] and experimental methods. [4][5][6] Recently, there is also an attempt to identify the risk of sloshing by applying big data-based machine learning techniques. 7 Although the recent development of computational techniques for violent fluid flows, model-scale experiments are generally accepted as the most reliable method to understand the phenomenon of sloshing within a liquid cargo and to predict the sloshing-induced impact loads than analytical or computational methods.…”

Study on tank shape for sloshing assessment of LNG vessels under unrestricted filling operation

“…Figure 1 shows typical barred filling range and sloshing impact load of conventional LNG tank. As shown in the red line in the figure, sloshing impact loads are very high in partial filling depth conditions especially around 0.30 H height [1,2,5,6]. To enable unrestricted tank filling operations, it is necessary to reduce sloshing pressures around 0.30 H fillings, or to increase the structural capacity of the LNG CCS.…”

“…The main reason for filling depth restrictions is the high sloshing pressures around 0.20 to 0.40 H tank filling depths [1][2][3][4][5][6]. Intermediate filling depths cannot be avoided with Floating LNG (FLNG), LNG-Shuttle, and Regasification Vessels (LNG-SRV), because of the production, offloading, and regasification operations.…”