Disclosed is a vessel comprising at least two parallel hulls at a predetermined mutual distance. A deck structure is supported on the hulls. Fluid storage tanks are placed in the at least two hulls, at least partly below the deck structure. The deck structure carries fluid processing and/or production equipment. The fluid is a hydrocarbon. The ratio of length to width of the vessel being at least 3, preferably at least 4. The length of the vessel being at least 150 m, preferably at least 250 m, more preferably at least 350 m.