Floating liquefied natural gas (FLNG) facilities are a game changer in offshore hydrocarbon development. One of the key challenges associated is offtake of LNG from FLNG to LNG carriers. Side-by-side offloading is considered a promising method, but one which may be sensitive to LNG carrier roll response. During an offload the sloshing of the cargo at intermediate load conditions could couple with the global vessel response i.e. coupling of sloshing with the higher harmonics (double or triple value) of the roll natural frequency. An experimental study is conducted with a barge-like vessel carrying two spherical tanks. These tanks are filled with water to different volumes to simulate different load conditions. For comparison, same tests were performed for which the tanks were empty, but the vessel properties modified to a ‘frozen’ approximation of an intermediate load condition. The effects of the internal liquid cargo motions on global roll motions are clarified, for the first time, for vessels with spherical tanks. The roll response of the vessel with liquid cargo inside spherical tanks is greater than observed for the equivalent ‘frozen-cargo’ case. This study also provides a foundation for improved numerical modelling with relevancy for side-by-side offloading operations.