A cellular reservoir flexible pressure vessel is formed as a series of closely packed tubes fitted into a pair of opposing end caps. The end caps have individual receptacles sized and shaped to receive the tube ends that are secured with adhesive or radio frequency welding. At least one end cap has a passageway for connection of the vessel. The flexible pressure vessel has a pressure relief device comprising a reduction in thickness of one end cap at a predetermined location. When subjected to overpressure it fails at the predetermined location. Other pressure relief devices include: a projecting member on the vessel surface, a weakened section of the passageway, a weakening or an absence of braiding material or hoop winding at a predetermined location on the vessel surface or along the passageway, a weakening or spreading of fibers in either the reinforcing panels or the flexible blankets covering the vessel.