Page 121 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

• Thruster location to counterbalance CG (center of gravity) shifts
• Magnetism and its effect on sensors
• Through-frame lift and required certifications
• Capacity to add buoyancy should a heavier than normal payload be required
• Logical integration point for the tether
• Frame integration into a TMS and/or LARS
• Water ingress into frame (a metal frame holding salt water invites rapid corrosion)
• Anticipated frame contact with bottom sediments will hold these particles and possibly affect
buoyancy
• Hydrodynamic characteristics of frame (e.g., shape vis-a`-vis water flow).
Assuming the designer has the above topics under control, the types and characteristics of the buoy- ancy material, discussed in the next section, must be considered.
5.1.2 Buoyancy
Archimedes’ principle states: “An object immersed in a fluid experiences a buoyant force that is equal in magnitude to the force of gravity on the displaced fluid.” Thus, the objective of underwater vehicle flotation systems is to counteract the negative buoyancy effect of heavier than water materi- als on the submersible (frame, pressure housings, etc.) with lighter than water materials (i.e., buoy- ancy material with specific gravity less than that of the ambient water conditions). A near neutrally buoyant state is the goal. The flotation material should maintain its form and resistance to water pressure at the anticipated operating depth. The most common underwater vehicle flotation materi- als encompass two broad categories: rigid, lightweight materials such as polyurethane or polyvinyl chloride (PVC) foams, for shallower depths, and syntactic foams that can support full ocean depth systems. For the deepest trenches, innovative techniques for buoyancy such as ceramic spheres have also been used. All three approaches will be discussed in the following sections along with a summary of things to consider when choosing a buoyancy material.
5.1.2.1 Lightweight foam
The term “rigid polyurethane foam” comprises two polymer types: polyisocyanurate formulations and polyurethane formulas. There are distinct differences between the two, both in the manner in which they are produced and in their ultimate performance.
Polyisocyanurate foams (or “trimer foams”) are generally low-density, insulation-grade foams, usually made in large blocks via a continuous extrusion process. These blocks are then put through cutting machines to make sheets and other shapes. ROV manufacturers generally cut, shape, and sand these inexpensive foams and then coat them with either a fiberglass covering or a thick layer of paint to help with abrasion and water intrusion resistance. These resilient foam blocks have been tested to depths of 1000 ft of seawater—“fsw” (330 m of sea water—“msw”)—and have proven to be an inexpensive and effective flotation system for shallow water applications (Figure 5.1).
Polyisocyanurate foams have excellent insulating value, good compressive-strength properties, and temperature resistance up to 300F (149C). They are made in high volumes at densities between 1.8 and 6 lb/ft3 (2996 kg/m3) and are reasonably inexpensive. Their stiff, brittle consis- tency and their propensity to shed dust (friability) when abraded can serve to identify these foams.
5.1 Vehicle design 109