Corrosion Monitoring Equipment for ROV Interfacing
by Jim Britton (1999) from UI New Orleans
Many of the new deep water production fields are beyond the depth suitable for divers, leaving the ROV as the only viable option for performing inspection tasks. This fact has caused new inspection techniques and new equipment to be developed to meet the challenges. This paper describes and documents some of these new techniques and the equipment used.
Types of Inspection
The main types of inspection which are performed by ROV's are: visual, cathodic protection level surveys (CP) and ultrasonic inspections (UT). The cathodic protection survey - in conjunction with a visual inspection - is the most widely used for subsea equipment corrosion monitoring. This paper will focus on using ROV's to perform meaningful, accurate cathodic protection surveys.
Types of Cathodic Protection Systems
The vast majority of deepwater subsea production equipment is protected by galvanic anodes (usually aluminum alloy) and coatings. Also, many components on these structures are made from corrosion resistant alloys (CRA's). In order to verify that a cathodic proteciton (CP) system and a coating system are providing adequate corrosion control, it is necessary to take some electrical measurements, which can confirm that the steel components are not corroding in the seawater. These measurements are commonly called cathodic protection potentials or "CP potentials."
Basic Potential Measurement - A potential measurement is like any other electrical voltage measurement. It is achieved by making an electrical contact to the protected structure and another connection to a device called a reference electrode. For the potential measurement, one simply compares the two. The reference electrode most commonly used in offshore work is the saturated Silver - Silver Chloride cell (Ag/AgCl). This cell couples a pure silver wire with a saturated solution of its chloride salt. When this couple is exposed to sea water a very repeatable voltage is produced. This known repeatable voltage is compared against the unknown voltage of the steel in sea water. The Ag/AgCl electrode is understandably referred to as the "reference electrode."
Unprotected carbon steel (no anodes attached), will exhibit a potential of approximately (-) 0.570 - (-) 0.620 Volts vs Ag/AgCI. When anodes are added the potential will shift in the negative direction; the greater the shift, the more protection is being provided. The accepted minimum potential level required on steel in seawater is (-)0.800 Volts vs Ag/AgCI to ensure full cathodic protection. However a more practical minimum is (-)0.850 Volts. To go even further, most operators would consider the cathodic protection (CP) design of a system inadequate if minimum potentials of (-)0.950 Volts vs Ag/AgCI were not achieved.
A schematic diagram of a steel to seawater potential measurement is shown in Figure 1. The ground connection can be made above water if there is any above-water structure and if it is electrically continuous with the subsea structure:
CAUTION: Some TLP and other FPS designs deliberately isolate tendons and or risers from the floating section of the facility. Similarly, flexible sections of pipelines cannot always be guaranteed to be electrically continuous.