The most common method of cathodically protecting fleet moorings involves the use of 150- to 500-pound zinc anodes attached to buoy hulls and each shot of chain. Underwater voltmeters are used to determine the effectiveness of the system by measuring relative electrical potentials of the buoy and chain at certain distances from the anode. The electrical potential of the metal is the charge of the metal compared to a standard reference electrode, typically silver/ silver chloride. Steel that is adequately protected from corrosion should have electrical potentials that fall between -0.80 and -0.90 volt, as illustrated in Figure 3-42.
A portable voltmeter and portable reference electrode can be used to measure the potential on buoy hulls, but due to water depth and questionable electrical continuity, an underwater voltmeter is required for inspection of the cathodic protection system on chain.
A greater potential indicates that the anode is overworking and serious damage could occur to the metal, while a lesser potential indicates that the system is not operating effectively and corrosion may be occurring.
Fleet mooring inspection divers will normally use a self-contained voltmeter, such as that illustrated in Figure 3-23, which consists of a digital display, surface readout facility, and rechargeable battery. Underwater voltmeter readings must be taken at 20-foot intervals on the chain, on each side of each anode, at each end at the continuity cable, and on each side of each swivel. Whenever readings are taken, potentials, depth, and element measured should be recorded. Note that a moored vessel can affect the cathodic protection system on the mooring buoy and chain and cause the readings to be either higher or lower than normal.
Chain measurements are best made with precut "go-no go" gauges, as illustrated in Figure 3-41, calibrated at 90 and 80 percent of original wire diameter. Calipers (24-inch minimum) are also required, with the measurements taken off a ruler attached to a Plexiglas slate. A 100-foot tape and scales 1, 2, and 3 feet long with large numbers suitable for photo documentation will be required. A diver's compass and accurate depth gauge, as well as survey buoys, will also be required.
The effectiveness of cathodic protection systems is measured as follows:
• On mooring buoys - either an underwater voltmeter as shown in Figure 3-23 or a portable voltmeter and portable reference electrode as shown in Figure 3-22 can be used.
• On chain - an underwater voltmeter is used.
To record any findings underwater, a grease pencil and Plexiglas slate are re quired. An underwater camera with extra film is required for photographic documentation and a video recording system may be required. An inclinometer is required for obtaining the angles of mooring chains in nonriser-type moorings and spread moorings. Marker tags are used to relocate or mark links or accessories. Transits and targets are required for locating buoy positions. Because UCT divers need high mobility, and because of the depth of water in which they will be working, the cleaning operations to be performed for inspection work generally require only hand tools, such as wire brushes and scrapers.
At times ROVs may be used to supplement mooring inspections (see Section 2.9).
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