Investigation of the influence of tank geometry on a vessel's stability under free surface effect when in coastal waters

Tank sizes on liquid cargo carriers are becoming larger to minimise wasted spaces on-board and maximise payload capacity. This trend can significantly affect a vessel's stability and its design. Therefore, this paper investigates whether spherical tanks provide better stability than a traditional tank formed with vertical sides under a range of free surface scenarios. The motions of a model liquid cargo carrier hull with various sizes of tanks and various fill levels has been investigated in a coastal basin. Regular waves of various heights and frequencies have been generated and the rolling motions of the hull have been documented. Results have shown that spherical tanks offer less free surface effects compared to vertical sided tanks thus create less rolling motion. In contrast, when the hull has four tanks, all of those with free surfaces, the vertical sided tanks provide better stability up to 50% fill levels whereas spherical tanks induce less roll between the fill levels 50% and 100%.