A large volume natural gas storage tank comprises rigid tubular walls having closed tubular cross-sections that are interconnected at opposing ends with two other rigid tubular walls such that interiors of the rigid tubular walls define an interior fluid storage chamber. The storage tank includes bulkhead ring webs positioned in the interior fluid storage chamber across intermediate segments of the rigid tubular walls and closure plates connected between exterior surfaces of successive interconnected rigid tubular walls to define sides of the storage tank. Interior surfaces of the closure plates and exterior surfaces of the rigid tubular walls define an auxiliary fluid storage chamber. The storage tank includes exterior support structures extending through the closure plates and between the exterior surfaces of the rigid tubular walls on some of the sides of the storage tank to reinforce the storage tank against dynamic loading from fluid in the interior fluid storage chamber.