Support assemblies for attaching the walls and top of a semi-membrane tank to surrounding support structure include interlocked angled sliding surfaces, whereby movement perpendicular to the walls and top is permitted as well as movement along one line or two perpendicular lines parallel to the tank sides and top. Arrayed on the surfaces of the walls and tops, these support assemblies provide the necessary support for the tank walls and top while minimizes thermal stresses in the tank. Support systems comprising such arrays, surrounding support structure, and tank insulation can be assembled outside a ship and engagingly lowered into a compartment therein, facilitating construction.