Roll Performance of an LNGC with Partially-Filled Spherical Tanks

Abstract

Abstract Objective/Scope Roll performance of an LNG carrier (LNGC) plays a critical role in side-by-side offloading. Liquid cargo motions can couple with the global motions of an LNGC, affecting the roll response and affecting the operational window for side-by-side offloading. In this study, a series of model tests has been conducted in a wave basin, and the roll characteristics of the LNGC are studied with liquid cargo in spherical tanks. Methods/Procedures Scale model tests were conducted with an LNG carrier-like model under waves in an artificial wave basin, whereby the model was positioned in the centre area of the wave basin through four horizontally arranged spring lines. The time series of the vessel motions and the internal liquid cargo motions were measured. To quantify the liquid cargo effect, several sets of tests were performed for which the tanks were empty, but the vessel properties modified to a ‘frozen’ approximation. Decay tests were conducted with the model oscillating freely in calm water, excitation by a given initial heel angle. Results/Conclusions Spectral analyses have been conducted to obtain the response amplitude operators (RAO) from the test results. Based on the obtained RAOs, short-term spectral analyses were conducted to understand the most probable maximum roll response in side-by-side offloading sea states. For the first time, the effects of cargo motion have been clarified for LNGCs with partially-filled spherical tanks. The findings reveal that the operational window may not be estimated robustly using a simplified method which ignores the liquid cargo and considers only the ballast condition. Based on the results in five different loading conditions, the most responsive loading condition is examined for spherical cargo tanks. It is suggested that intermediate load conditions with explicit inclusion of the effects of cargo motions to be utilised in side-by-side operability studies to robustly ascertain offloading availability. To facilitate potential numerical simulations, suggestions on selection of linear or nonlinear roll damping coefficients are provided as well. In conclusion, this study provides a better understanding of roll performance of an LNGC with liquid cargo motion in spherical tanks. Novel information The hydrodynamic performance of a ship with partially-filled spherical tanks has been clarified, for the first time. The findings in this study provide a foundation for improved numerical modelling with relevancy for side-by-side offloading operations, one of the key challenges associated with FLNG facilities.