Experimental Investigation of Non-similarity Slamming Phenomena in Geometrically Similar Tanks
- Zhi-jun Wei (Dalian University of Technology) | Qian-jin Yue (Dalian University of Technology)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- December 2015
- Document Type
- Journal Paper
- 288 - 298
- 2015. The International Society of Offshore and Polar Engineers
- experiment, geometrically similar tank, slamming pressure, Non-similarity
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- 53 since 2007
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In this paper, the non-similarity sloshing-induced slamming phenomena in geometrically similar Gaztransport & Technigaz (GTT) membrane-type floating liquefied natural gas (FLNG) facilities are investigated. The experiments were conducted in three scaled prismatic tanks at scales of 1=20, 1=40, and 1=60 with roll excitation, and the impact pressure was measured. The experimental results demonstrate that the kinematic free-surface behavior, time traces, and spatial distribution of the impact pressure during a single roll wave impact exhibit non-similarity in the three geometrically similar tanks. Furthermore, the quantitative statistical analysis of the impact pressure and impact rise time indicates that Froude’s law yields conservative estimates for the slamming force in the tank by using the same experimental media.
The challenge of designing floating liquefied natural gas (FLNG) facilities has attracted considerable attention from both the offshore gas exploration industry and academia. Floating above the deep water or marginal gas fields, the FLNG system is designed to produce, liquefy, store, and transfer liquefied natural gas (LNG). Consequently, it needs large volume tanks, a large amount of deck space, and easy maintenance. Gaztransport & Technigaz (GTT) membrane-type cargo containment systems (CCSs) have an advantage over other types of CCSs for FLNG facilities. They afford the maximum utilization of the ship’s space, making use of the ship’s support structure. However, due to cost efficiency and low-temperature preservation considerations, one drawback to the GTT membrane CCS is the capability of the thin insulation layer to bear liquid impact. Furthermore, one significant difference between the LNG CCS and FLNG CCS is that the FLNG system has no restriction for filling conditions (American Bureau of Shipping, 2010). Liquid tends to slosh in a partially filled tank. These slosh-generated loads have a considerable influence on the tank and support structure design. The slamming force has long been of interest to designers and researchers because it has resulted in structural local damage and liquefied natural gas leakage during LNG shipping (Abramson et al., 1974; Gavory and De Seze, 2009). Thus, it is essential to determine the slamming force caused by violent liquid movements in partially filled tanks in the design of CCSs for FLNG facilities.
Both theoretical and numerical methods have limitations in predicting rapid overturning or the strongly nonlinear free surface with large amplitude slamming and complicated tank shapes, which is reported by many researchers (Abramson et al., 1974; Lee and Choi, 1999; Faltinsen and Timokha, 2009). The experimental approach is efficient and accurate in revealing the complicated physical phenomena during liquid sloshing. Monitoring the slamming force of CCS prototypes for in-service LNG carriers is expensive, difficult, and dangerous. Furthermore, it is difficult to eliminate uncertain factors that are not relevant to the slamming force in prototype experiments (Malenica et al., 2009). Hence, few prototype tests have been conducted, and there is little public information on such tests because of commercial conservation. The scaled model test is a practical approach for filling in the gaps between theoretical, numerical methods and prototype experiments. The slamming pressure obtained by the sloshing model test is processed to identify the most critical sloshing force on the containment system structure. Although many sloshing model studies have been conducted, the effect of scaling remains unclear (Faltinsen and Timokha, 2009).
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