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Title: A phenomenological rapid sloshing model for use as an operator guidance system on liquefied natural gas carriers
Author: Godderidge, Bernhard
ISNI:       0000 0004 2708 1883
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2009
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A concept for a non-intrusive sloshing guidance system based on a phenomenological Rapid Sloshing Model is proposed to reduce the operational risk of sloshing damage to LNG carriers. A numerical sloshing model is implemented in a commercial Navier-Stokes Computational Fluid Dynamics (CFD) code which uses a volume-of- fluid approach for the simulation of multi- fuid problems. The effect of spatial and temporal discretisation and turbulence is investigated using systematic variation. Dimensional analysis of the multiphase flow regime and examination of the relative velocity at the fluid interface show that an inhomogeneous multiphase model is appropriate for the simulation of a violent sloshing flow. This is conrmed by the good agreement with the experimental data of Hinatsu. The effect of fluid compressibility is investigated for sloshing impacts and a criterion based on wave propagation is developed to assess the importance of compressibility. When modelling sloshing with large air bubble entrainment, the choice of fluid compressibility model is shown to have a significant influence on pressure magnitude and frequency of oscillation required for structural assessment and a thermal energy model is required. The Rapid Sloshing Model (RSM) is based on the observation that the centre of mass of a sloshing fluid tends to follow a particular trajectory. Using a phenomenological modelling approach, the forces affecting the sloshing response are approximated with mathematical functions for restoring force, damping and sloshing impacts. Calculation times for the resulting equations are typically 0.1% of real time on a desktop PC. A case study of sloshing induced by periodic rotation and translation of two-dimensional longitudinal and transverse sections of membrane LNG tanks is carried out using RSM. RSM is set up using one CFD simulation not considered in the case study and the RSM solutions are then compared to the independent CFD solutions. The fluid momentum from RSM is usually within 5%{15% of the CFD solution for excitation at and near the first resonant period at a filling level near the critical depth. An irregular surge motion profile from an ITTC two-parameter spectrum is applied to the tank and the mean error from the RSM solution remains below 15% when using momentum and transverse force. When applied to sloshing with a 10% filling level excited by an irregular seaway a mean error of 9.6% is obtained. Compared to existing phenomenological modelling approaches the RSM methodology reduces the error by an order of magnitude in sloshing scenarios of practical interest. A non-intrusive sloshing guidance system based on the Rapid Sloshing Model which is suitable for installation on existing and newbuild LNG carriers can be implemented by applying motion data measured onboard to the RSM to provide operator guidance on the sloshing severity in partially filled LNG tanks. The RSM is set up for a particular LNG carrier with existing sloshing data from the design and class approval stages.
Supervisor: Tan, Mingyi ; Turnock, Stephen ; Earl, Christopher ; Shenoi, Ramanand Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: VM Naval architecture. Shipbuilding. Marine engineering