Louis Diebold

Sloshing Effects Accounting for Dynamic Coupling Between Vessel and Tank Liquid Motion

Sloshing, a violent behaviour of liquid contents in tanks submitted to the forced vessels’ motion on the sea represents one of the major considerations in LNG vessels design over several past decades. State of the art of sloshing analysis relies on small-scale sloshing model tests supported by extensive developments of CFD computation techniques, commonly studying one isolated tank submitted to the forced motion without their mutual interaction. In reality, wave-induced response of the vessel carrying liquid cargo is affected by internal liquid motion, and consequently, tank liquid flow is altered by the vessel motion in return. An efficient numerical model for dynamic coupling between motions exerted by tank liquid (sloshing) and rigid body motions of the vessel (seakeeping) was developed in Bureau Veritas, formulated under the assumptions of linear potential theory in frequency domain. As already experienced with anti-rolling tanks, strong coupling effect is perceived on the first order transverse motions. However, consequences of coupled motions on sloshing loads have not been explored yet. This paper presents comparative analysis of sloshing effects induced by coupled and non-coupled vessel motion, introduced as the excitation to 6 d.o.f. small-scale model test rig. Possible risk of coupled effects is demonstrated on the example of standard size of LNG carrier operating with partly filled cargo tanks.Copyright

Strength Assessment of Membrane LNG Tank Structure Based on Direct Calculation of Structural Response

Expanding LNG market reinforces the demand for new concepts of LNG transportation. Membrane LNG vessel design widely applied until now, encounters new challenges due to requirement for larger vessel’s capacities and more flexible operation in partially filled conditions. Present assessment procedures of LNG tank structure usually combine small scale sloshing loads measurement and containment system structural strength assessment, on a comparative base for the reference and target vessels. For the new LNG design, more rational methods become essential in the assessment procedure. Some improvements in the strength assessment procedure of membrane LNG tank structure is presented in this paper, combining small scale sloshing load measurements with direct FEM calculation of structural response. The complexity of problem is the consequence of: stochastic nature of impulsive sloshing loads, material used for the cargo containment system at cryogenic temperature and strong hydro-elastic interaction during impacts. Disadvantages of small scale testing and limits of today’s numerical methods require that further in the future certain simplifications and assumptions should remain. In the paper, method for the design loads selection from the small scale sloshing measurements is described and discussed. The impulse, transferred over the corresponding impacted surface, is the base for the comparison of successive violent sloshing loads. The stochastic nature and statistics of measured loads are discussed. The structural analysis of LNG tank structure under selected design sloshing loads, using on-linear and time-dependant explicit FE calculations, is described. This paper presents Bureau Veritas recent developments and their applications in the field of sloshing assessment.Copyright

Hydrodynamic Issues of FLNG Systems

Advanced hydrodynamic analyses of floating LNG terminals are presented in the paper. They consist of the complex interaction of multiple bodies and the coupling effect of seakeeping (wave diffraction and radiation around bodies) and sloshing (liquid motions in tanks). Based on the recent development to introduce the dissipation in potential flows and new formulations of boundary element method, the seakeeping analysis is enhanced to be able to make accurate predictions of gap resonances and major dynamic effect of liquid motion in tanks.Copyright