Maximizing the value of underwater cathodic protection surveys
by Jim Britton (1994)
In the Gulf of Mexico and many other operating areas, the measurement of cathodic protection potentials on offshore structures is a regulated activity. Thus, since it has to be done, it makes sense to get as much benefit from the survey as possible. In the majority of cases this is not being done. I will describe here some fundamental requirements for accurate surveys as well as some new approaches to the task which greatly enhances the survey value at little or no additional cost.
Types of cathodic protection survey
For the purposes of this paper, only platform surveys with galvanic anode CP systems will be considered. The types of survey performed may be broadly categorized as follows:
1. Surface-deployed potential survey - Often referred to as 'drop cell' surveys. A portable reference electrode is dropped in even depth increments through the structure. The data is displayed on a topside meter and hand recorded. This is the least accurate of the survey techniques and is also the least expensive. This type of survey is performed annually and is used as a general potential survey only.
2. Diver or ROV-assisted potential survey - This type of survey – part of a level II or level III inspection – is the main topic of this paper. The reference electrode is positioned with divers or an ROV. Signals are normally displayed on a surface meter and may be recorded manually or on a computer. This type of survey is performed periodically (every 3 to 5 years) depending on the structure type and location.
3. Diver or ROV detailed survey - Some operators require more than just cathodic protection potentials; they want anode current output and sometimes cathodic current density data. In these cases, more complex gradient-type survey systems are available . This paper will not go into much detail on these types of surveys since they are normally restricted to special cases in deep water and are considerably more expensive.
Purpose / goals of cathodic protection surveys
Before we can fully appreciate the critical elements which produce a successful cathodic protection survey, we must first appreciate why the survey is being performed in the first place. The reasons why are listed below, in no particular order:
• Regulatory compliance
• To verify adequate CP system performance
• Retrofit planning and budgeting
• Hot spot identification
• Design calculation verification
In order that these goals can be achieved, the cathodic protection survey must be planned and conducted with the following philosophy in mind. It has often been stated that cathodic protection is a black art. This is not true but A.R.T. does describe the key elements of a successful survey:
The A.R.T. of the cathodic protection survey
Accuracy - The accuracy of the cathodic protection survey is of paramount importance. Misleading data can Iead to costly decision errors. The accuracy of the survey is controlled by:
• Equipment quality
• Calibration procedures
• Operator proficiency (diver or pilot)
Repeatable - Repeatability and accuracy go somewhat hand in hand. However, repeatability of a survey is largely controlled by:
• Survey procedure quality
• Survey technique
• Equipment design
Thorough - The need to be thorough is obvious. We don't want to overlook any problem. It should, however, be stressed thal being thorough does not mean that every single member and anode on the platform has to be checked, but a representative sample at each elevation must be looked at. Thoroughness Is controlled by:
• Survey specifications
• Data recorder proficiency
Now we have the basic requirements identified. Let's get into the details a little more.
The main basic pieces of equipment which must go offshore are:
• Reference electrodes (CP probes)
• Electrode cables (umbilicals)
• Ground wires and clamps
• Calibration equipment
The cardinal rule is 100% REDUNDANCY; never go offshore with only one of anything. If it can go wrong, then it probably will.
Reference electrodes (cathodic protection probes)
The reference electrodes are the key pieces of equipment on the spread. There are many points concerning reference electrodes about which a large percentage of offshore inspectors are misinformed. I will attempt to focus on some key points regarding cathodic protection probes.
What is a reference electrode? - A reference electrode is an electrochemical device, which as the name implies, is a reference (known) voltage source against which unknown voltages are compared. The unknown voltages in our case are those at points, between the surface of the platform and the sea water. The (known) reference voltage used in offshore work is that between pure silver (Ag), and a saturation of its chloride salt (AgCl). thus when a cathodic protection potential is reported it will normally be written:
(-)0.935 Volts vs Ag/ AgCl or 935 mV (-) vs Ag/ AgCl
A schematic cathodic protection reading hook-up is shown in Figure 1 below. Note that the polarity of the reading is negative; this is a convention which is observed by all corrosion engineers. it is much more important when dealing other types of cathodic protection systems where polarity reversals may occur under certain conditions. Systems should always be connected as shown.