Pressure Hulls and Exostructures

Pressure Hull – the structure that provides the occupants of an underwater submersible with a dry, pressure resistant habitat.

Pressure Hull

Shape

The pressure hull consists of spherical or cylindrical shapes in various combinations. Table 1 illustrates the advantages and disadvantages of various pressure hull shapes.

Table 1. Advantages and Disadvantages of Submersible Pressure Hull Shapes

Shape	Advantages	Disadvantages
Sphere	1. Most favorable weight to displacement ratio 2. Thru-hull penetrations easily made 3. Stress analyses more accurate and less complex	1. Difficult and inefficient interior arrangements 2. Large hydrodynamic drag
Ellipse	1. Moderate weight to displacement ratio 2. More efficient interior arrangements than in Sphere 3. Thru-hull penetrations easily incorporated	1. Expensive to construct 2. Difficult to perform accurate structural analysis
Cylinder	1. Inexpensive to construct 2. More efficient interior arrangements than in Ellipse 3. Low hydrodynamic drag	1. Least efficient weight to displacement ratio 2. Interior frames required to increase strength

Materials

The selection of an appropriate pressure hull material is based on the following criteria:

1. Corrosion Resistance – the ability of a material to resist its deterioration by chemical or electrochemical action within its environment.
2. Resistance to Stress-Corrosion and Cracking – the ability of a material to resist failures caused by the combined action of a flaw (e.g. a crack) and tensile stress.
3. Resistance to Low Cycle Fatigue – the ability of a material to withstand localized fluctuating stress.
4. Creep Resistance – the ability of a material to withstand permanent deformation over time.
5. Stress Relief Embrittlement – the process by which localized residual stresses in a metal are reduced by the reduction in the normal ductility when a metal is heated to a suitable temperature and slowly cooled.
6. Resistance to Brittle Fractures – the ability of a material to resist failures with little or no plastic deformation.
7. High Strength to Density Ratio – the material must be strong and light.
8. High Ductility – the ability of a material to deform plastically without fracturing.
9. Fracture Toughness – the ability of a cracked material to resist catastrophic propagation of cracks.
10. Weldability – the property of a material which allows it to be welded under normal conditions.
11. Formability – the ability of a metal to be shaped through plastic deformation.

Submarine pressure hull are usually made of steel, aluminum, titanium, acrylic plastic and glass. However, the most widely used material is steel, because of a high degree of knowledge available to designers and manufacturers as well as of its outstanding performance in the ocean.

There are several problems with Aluminum and Titanium:

1. Problem: Aluminum is generally considered unacceptable as a pressure hull material because it is unweldable and is subject to stress-corrosion cracking.

Solution: Aluminum pressure hulls can be bolted together instead of welding; it can be anodized to resist stress-corrosion cracking.

2. Problem: Titanium is unacceptable as a pressure hull material because it is susceptible to stress-corrosion at high tensile stress levels.

Solution: Titanium graphite alloys do not exhibit this problem.

Glass: Despite of its weaknesses, such as its brittleness, high sensitivity to surface abrasion and considerable strength degradation at joints, glass and glass-reinforced plastic have a low weight/displacement ratio. Figure 1 clearly shows that titanium, glass and glass-reinforced plastics are the top three pressure hull materials.