Page 126 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

114 CHAPTER 5 Vehicle Design and Stability
3.6 in. (9 cm) in diameter and capable of providing buoyancy to 36,000 ft (11,000 m). In fact, Woods Hole Oceanographic Institute’s Nereus vehicle relies primarily on these spheres for its buoyancy. One shortcoming is the potential for sympathetic implosion. To date there is no known testing of that potential failure mode. This phenomenon is also a potential for large glass balls.
5.1.2.4 Summary
Regardless of the material chosen, the following should be taken into consideration when choosing the type of material:
• Specific gravity of the material
• Crush point and safety factor
• Shrinkage due to pressure, i.e., loss of buoyancy with depth
• Abrasion resistance, brittleness
• Potential protective coatings
• Available shapes and machinability, including hazardous material requirements
• Water absorption and thus loss of buoyancy
• Placement and stability considerations
• Ability to modify in the future
Historically, deepwater foams have been encased in a fiberglass shell. It was thought that the fiberglass would protect the syntactic from damage due to impact while being handled and trans- ported aboard ship, which is true. What is becoming apparent, however, is that while the glass pro- tects the foam during transit and slight impacts, in fact it may be causing further damage when the units are deployed. When the compressive modulus is considered, 2.8 million psi (2 billion kg/m2) for fiberglass versus 400,000 psi (281 million kg/m2) for the syntactic, the delta is extreme.
The buoyancy material needs to be able to expand and contract, that is “swim,” during diving operations. Unfortunately, the modulus of the fiberglass is not compatible with the modulus of the syntactic and causes stress and strain on the foam systems. Whereas it was thought the glass was protecting the systems, it may have actually been transferring the impact load deep into the syntac- tic where it cannot be seen until the unit goes deep and stress relieves.
Another area of concern, when designing and machining the buoyancy materials, is to be sure to leave no sharp edges or corners. While these look great for the design, they set up stress risers (Figure 5.7) which can ultimately lead to cracking of the buoyancy system.
Recent developments in high-build urethane materials have allowed the incorporation of these materials to relieve the stress on the buoyancy system. They are flexible, easy to install, available in most colors, off the shelf items, and provide for the buoyancy material to “swim” during the dive cycle. Manufacturers of ROVs and AUVs are leaning more toward the urethane materials for coating the foam. With fiberglass weighing 128 lb/ft3 (2.0 g/cc), using the urethane material is not only saving the syntactic but also regaining buoyancy.
Most buoyancy materials are inflammable. Due to where the vehicles operate it has not been necessary to include brominates in the resin matrix to reduce or inhibit the potential to burn. Only one instance has been documented: In the early 1980s, DSV 3 Turtle had a fire when an oil-filled line ruptured and a spark hit it.
When considering the crush depth of the buoyancy material, it is usually 1.1% times the service depth, except in the case of man-rated materials. Manned vehicles such as the Alvin require