Page 504 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 18.23
18.5 Ultrasonic metal thickness 499
(a)
(b)
(c)
CP probe types. (a) Drop cell, (b) proximity probe, and (c) contact probe.
T1 T2T3
Paint, dirt, couplant, etc.
Metal
Time
FIGURE 18.24
Operation of a multiple echo metal thickness (sampling is taken at T2 or later).
The ROV-operated CP probe has a much higher accuracy as it is placed directly adjacent to the anode being measured. This type of measurement is required by regulatory authorities to be mea- sured every 35 years, for example, API Recommended Practice 2A Level II and III Inspections.
CP readings are rife with potential errors through instrument miscalibration and equipment dam- age. It is imperative for the technician to become completely familiar with the operation and cali- bration of the equipment before taking the equipment into the field.
18.5 Ultrasonic metal thickness
Further examination of metal corrosion is possible with the ultrasonic metal thickness (UT) gauge. As with the acoustic FMD sensors (above), a pulse of high-frequency sound (in the 25 MHz