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Sloshing of LIquefied Natural Gas (SLING)

For many years, GTT has been at the forefront of research into the fundamental behavior of liquid motion (sloshing) of LNG. Besides GTT’s extensive laboratory of sloshing assessment equipment, GTT has been involved in a number of industry research projects. The reader may well remember the Sloshel project, where a very large water flume was used to simulate full-scale sloshing on tank walls.

Now, GTT has teamed up with MARIN and others partners for a new research program; “SLING”.

Overview of the research programme SLING

SLING stands for Sloshing of Liquefied Natural Gas (LNG). This major international cooperative research programme began in 2016 at the instigation of MARIN and GTT. SLING is supported by NWO-TTW, the Dutch Technology Foundation. The remainder of the consortium comprises four top Dutch universities, representatives from the LNG shipping and shipbuilding industry, engineering companies, and research institutes (Fig. 1). Through advanced experiments, numerical simulations, and theoretical modelling, SLING aims to plug the gaps in the knowledge of wave impact loads induced by sloshing in LNG tanks.

SLING CONSORTIUM

 

Objectives

Natural gas is in liquid form at -162°C and atmospheric pressure. Vessels which transport LNG (LNG carriers, LNG bunker vessels or LNG feeders), floating units that produce or re-gasify LNG (respectively FLNG and FSRU), and vessels which use LNG as a fuel (LFS), all need to be equipped with dedicated tanks to hold the LNG and minimize the heat transfer. Membrane LNG tanks designed by GTT are widely used because, with their prismatic shapes and with no structure inside the tank except a pump tower, they use the hull space most efficiently. However, sloshing impact loads are the dominant design factor of these containment systems.

The current state-of-the-art methodology to assess sloshing impact loads relies on sloshing model tests. A model tank at scale 1/40 is filled with water and a heavy gas. It is placed on the platform of a hexapod that simulates the real ship motions. The ullage gas within the model tank is a mixture of nitrogen and Sulphur hexafluoride tuned so that the gas-to-liquid density ratio is the same as within a LNG membrane tank. The ship motions are calculated at scale 1 and down-scaled according to the Froude similarity. The model tank is equipped with 300 piezo-electric pressure sensors located in the relevant areas where liquid impacts occur.

However, despite the Froude similarity for the imposed motions and the density ratio similarity, these tests do not fully represent the reality. Some physical phenomena that occur during LNG impacts and influence the impact loads are not present during the small-scale tests (for instance phase change or hydro-elasticity). Some other phenomena involving compressibility and viscosity of the fluids, or surface tension at the interface, are biased because there is no fluid that can fulfil the relevant similarity laws. Nevertheless, dedicated measurements on-board an LNG carrier have shown that the long-term statistics of the impact loads as obtained by GTT sloshing assessment methodology are conservative. The conservatism on the mechanical strength of the containment system is always to the detriment of its thermal capacity. Therefore, GTT’s ambition is to be able to optimize the containment system design, especially for LFS, in order to find, for any project, the right balance between mechanical strength and thermal efficiency. This will enable the operation of partially filled tanks with more flexibility, or to design new tank shapes further maximizing the use of the vessel’s capacity, and obviously to minimize the amount of boil-off gas.

GTT liquid motion laboratory and sloshing model tests

 

This requires a better knowledge of physics of the sloshing impact which lead us to explore the last frontiers of this complex domain: the multiphase dynamics, the variability of sloshing loads and the structural response to very sharp space and time pressure distributions. The objective of SLING is to disentangle this complex physics in order to better master the scaling of the pressure measurements from sloshing model tests to full scale.

Scope of Work

SLING consists of five projects: three thematic research projects, an engineering project, and a physical integrative project.

The three thematic projects respectively study the multiphase dynamics, the variability of the wave impact loads and the structural response through experiments and numerical simulations. Wave impact tests, performed within a new facility designed and built through the engineering project, have been especially designed to ascertain the influence of each phenomenon independently. Moreover, several kinds of numerical simulations contribute to this knowledge acquisition by extending the physics implemented in the numerical models.

The engineering project led to the design and construction of a first-of-its-kind test facility, named The Atmosphere. This facility consists of a wave canal that is operated within an autoclave. The Atmosphere has officially been inaugurated in MARIN on October 19th 2020.

The Atmosphere and the wave canal within the autoclave
Courtesy of SLING consortium

Finally, the integrative project gathers the knowledge gained by the different experiments and numerical simulations in order to develop new theoretical models for liquid impacts. The models are implemented into a software framework, named the Liquid Impact Simulator (LIS). The LIS enables to generate in only a few seconds a space and time distribution of a wave impact load on a wall. The inputs are simply the wave geometry and kinematics that can be derived from either a test, a numerical simulation or even a simple drawing. The LIS calculation can integrate different levels of complexity for the wave impact physics including phenomena inducing variability of impact pressures.

Current status

The new facility The Atmosphere is operational since early 2020. A few test campaigns have already been performed to master the generation of the waves, to study the influence of the ullage pressure and of phase change on the global and local wave shapes. SLING will run until the end of 2021. Many test campaigns are scheduled. No test results have been published yet.

By its complete involvement in SLING research programme which accompanies the development of LNG as a fuel in the shipping industry, GTT is fully in line with the new baseline included in its logo: Technology for a Sustainable World.

 

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