New ceramic pressure hull design for deep water applications

Machines for deep-water application require pressure vessels in order to accommodate various type of units or human in the vessel securing them against the deep-water high pressure. In order to keep the machine to move freely in the water or on the bottom, the weight in the water should be as light as possible, and the pressure vessel must be fabricated as light as possible, keeping the pressure resistance capability. Formerly, high strength steel was used for the material of the pressure vessel, but nowadays titanium alloy is the major material for pressure vessels of deep-water vehicles. Recently, ceramics has been applied for pressure vessel of ultra-deep water vehicle, namely, WHOI's HROV Nereus. However, as ceramics are very brittle material, it must be designed with the greatest care, especially for the penetration. Nereus is reported to have no ceramic penetration ports, instead, it has a hemispherical dome of titanium alloy which has several penetration ports. If the pressure vessel with penetration ports is made from ceramics, the underwater weight of the pressure vessel must become very light. In order to solve this subject, a new concept was introduced for the design of ceramic pressure vessel, and ceramic pressure vessels were fabricated following this concept. Also one of them was laboratory-tested up to 118Mpa with success. The basic configuration is a cylinder with two hemispherical caps at both ends, connected with ceramic flanges. There are two versions of the design; the first one with a transparent glass dome at an end of cylinder and the second one is with two hemispherical ceramic caps with penetration ports. The key concept of the structural design under pressure is to let each part deform or shrink in natural condition without any restriction. Each part is connected with its partners, and when the deformation or shrinkage at the contact line with the partner is the same as that of the partner, no bending stresses shall appear anywhere. To avoid the emergence of bending stress is critical for the brittle material as ceramics. This concept is applied not only on the connection of each part but also for the design of penetration port. Glass dome application requires other great care to connect with ceramic cylinder. It is nearly impossible to adjust the displacements of contacting parts the same as each other because of the large displacement of glass dome due to the small Young's modulus, and slip happens or bending moment appears. In order to avoid bending moment to appear, a slip mechanism is introduced. This paper discussed these new concepts for the design of ceramic pressure vessels.