Coupling between liquified gas and vessel's motion for partially filled tanks: effect on seakeeping

Gas production projects are today under development or scheduled in a short future. Offshore LNG gas production raises different problems associated with the sloshing of the liquefied gas in the tanks, both for gas carriers and for LNGFPSO’s. New requirements for gas carriers will consider any conditions of filling rates in the tanks, in order to account for emergency departures. Partial filling rates are also mandatory when investigating possibilities to store gas on floating production platforms. Sloshing of liquefied gas in tanks of gas carrier has always been studied in two steps: investigating first the ship motions in waves and secondly applying the calculated or measured motions to the tank in order to obtain the liquid motions. This method is not fully correct representation of reality, as coupling is not taken into account and could play a large role in the ship motion hence the liquid motion itself. A joint effort was launched in 2003 by several companies to study the effect of coupling on the seakeeping. Model tests were conducted in November 2003 with a free sailing gas carrier and a moored FPSO, equipped with two tanks filled with water at various filling rates. The paper will review both the model tests and numerical results obtained for the two vessels with partially filled tanks, taking into account the full coupling between the vessel and its liquid cargo.