Page 499 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

494 CHAPTER 18 Ancillary Sensors
18.3.1 Acoustic FMD
With acoustic FMD, a high-frequency transducer is placed directly on the metal of the member to be examined. Air has a much lower sound conductivity/attenuation potential than water. If the member is flooded, the sound will propagate through the metal and water and then “bounce” off of the metal on the opposite side of the flooded member. This time of flight is measured to a high degree of accuracy, thus allowing for a highly reliable test of member air integrity. If the member is air-filled, the sound will simply resonate or dissipate within the single wall of the pipe or member.
Some advantages of the acoustic method over the radiographic method are:
1. Typically, acoustic FMD is much simpler and less expensive than the radiographic technique.
2. The acoustic method does not require the HAZMAT precautions needed for the handling of the
radiographic source.
3. Technician qualification is much simpler.
Disadvantages of the acoustic method over the radiographic method are:
1. The acoustic method typically requires some surface preparation on members with excessive marine growth.
2. Orientation of the transducer is critical in order to achieve sound energy propagation perpendicular to the axis of the pipe or flooded member.
3. False negative or positive readings are possible without proper data interpretation.
18.3.2 Radiographic FMD
As with acoustic FMD, the radiographic method of FMD works on the principle of differing attenuation through the medium of air versus water. With the radiological method, a low-grade gamma ray source is positioned on one side of the member or pipe while a gamma ray detector is positioned on the other side. A known metal thickness produces known gamma ray attenuation through the metal. A simple computa- tion of the theoretical reading on the back side of the pipe will produce a very highly reliable measure of total attenuation. If the member is flooded, the total gamma ray reception will be of a much lower value than through air due to the water’s attenuation of the gamma ray energy within the pipe or member.
18.4 Cathodic potential sensors
Salt water is often termed “the universal solvent.” That solvent potential has an extremely high effectiveness in the corrosion of submerged metal structures in a salt water environment. To under- stand the principals of operation of a Cathodic Potential (CP) sensor, one must first understand the corrosion process (Figure 18.20)—specifically for structural steel in salt water.
Metal corrosion is defined as the deterioration of a metal resulting from a reaction with its envi- ronment. Three elements must be present for metal corrosion to take place:
1. Two dissimilar metals
2. An electrolyte (such as salt water)
3. A conducting path between the dissimilar metals for electrical flow