FLNG sloshing analysis: a fresh approach

April 1, 2011

Class society ABS and Korea's Daewoo Shipbuilding & Marine Engineering (DSME) are concluding a joint development program examining critical wave conditions for sloshing model tests and computational fluid dynamics (CFD) in the cargo tanks of FLNG vessels.
The results of this year-long JDP are expected to offer the industry a new methodology to determine critical sloshing conditions more efficiently than in the past while also helping to reduce the cost and time duration for sloshing test campaigns.

Designing for sloshing impact loads is a key concern in tank containment system design. As cargo tanks have increased in size along with various filling conditions for offshore FLNG operations, the possibility of severe sloshing becomes more likely as well as the probability of structural damage to the containment system. The new ABS-DSME methodology is being proposed as a pre-screening procedure for selecting the most severe critical sea states before the start of a sloshing model test or flow simulation.

According to Yung Sup Shin, head of the Containment System Group at ABS Corporate Technology, model tests cannot cover all possible wave height, period, heading and filling levels since there are just too many combinations to determine dsme the lifetime maximum sloshing loads. ‘However, we have developed a robust way to determine a finite number of critical sloshing conditions consistently and efficiently compared to other existing methods,' he says.

The methodology, explains Shin, consists of two main steps. A sloshing pseudo-response amplitude operator (RAO) is defined based on the total sloshing wave energy and, secondly, the concept of sloshing severity is introduced based on the area under the sloshing energy response spectrum curve. This highly advanced theoretical formulation represents ship motion and sloshing interaction calculating the energy of the liquid cargo in the tank.

The numerical analysis of the fluid motion from this theoretical methodology has been validated by comparing the results to those from conventional sloshing tests for an FLNG cargo tank. The validation is based on extensive sloshing model test data developed from a joint industry project by DSME using Gas Transport & Technigaz (GTT NO 96) membrane containment system for an FLNG with a two-row cargo tank with centerline cofferdam arrangement. ABS was a participant in this two-row JIP along with other class societies and energy majors.

Shin stresses the importance of including any critical sea state in a sloshing test matrix, but believes ‘current state-of-the-art CFD codes have a limited value for evaluating the lifetime design sloshing pressures' due mainly to the lack of computational efficiency. The total sloshing wave energy and severity computation developed jointly by ABS and DSME provides an alternative to selecting relevant sea states in sloshing model testing and analysis, he says.


This new advanced numerical approach is now set to join other methods of sloshing assessment developed over the past decade, such as sloshing flow simulation techniques, scaling laws, fluidstructure interaction, corrugation effects on sloshing and partial filling operation. Shin expects associated software to carry out the sloshing methodology for sitespecific offshore FLNG application to be available in May.


Meanwhile, ABS has also revised its Guide for Floating Offshore Liquefied Gas Terminals, reflecting the growth in size of gas handling, storage and transportation systems and the resulting impact on structural design and analysis requirements. The technical challenges in this sector include the increasing size of terminal hulls, shallow water load effects, frequent partial filling, offloading operations and critical interfaces between the hull and topside structure and between the hull and position mooring system.

Offshore floating liquefied gas terminals (FLGT) concepts have introduced the possibility of hull structures up to 450m in length and 90m in breadth, which would make them the largest ship-shaped units yet built. With the hull structure so large, designs with two cargo tanks abreast are being proposed to minimize the internal load effects, particularly from sloshing in the partially-filled tanks during loading and discharge operations. OE



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