Subsea Completions

Obviously when dealing with a subsea tree or manifold structure, there is nothing above water to be able to hook a reference ground to. For this reason, the ROV cathodic protection probe was developed (see Figure 2). This probe can be interfaced to the ROV in many ways, the most commonof which is to attach it to a multi-function manipulator. Three conductors are then needed in the ROV umbilical, in order to connect the probe to a topside readout: a twisted shielded pair (for the two reference electrodes) and a ground wire (for the tip). These conductors must run all the way to the surface where they can be interfaced to a voltmeter display or into the video writer A-D converter. To complete the survey successfully, the ROV operator must simply "stab" the item under investigation and read the potential. The second electrode in the probe provides an on-line calibration facility to ensure that the readings taken are accurate.

Probe Features - When selecting an ROV probe, look for the following features to ensure that the device will give accurate, repeatable measurements and will not cause unnecessary system downtime:

Flowlines and Pipelines

Flowlines and pipelines cause an interesting problem for cathodic protection survey. Often in deep water work there is no place to establish a topside reference ground, and even if there were, it would be of little use to us 10 - 20 miles away along the pipeline. We therefore must use a survey technique called the "multi-electrode survey". There are several types. These methods rely on a concept called the "remote potential." The theory behind the remote potential states that if a reference electrode is located far enough away from the structure being surveyed, the potential indicated will be the same no matter where the electrode is located. When this happens, the electrode is said to be at a "remote" location. This is typically no more than 20-30 feet away in seawater, but the further the better. The remote electrode is calibrated to a regular close potential (taken with a tip contact probe, usually on an anode), and then the survey proceeds by comparing the close versus remote cells. Figure 4 shows simply how a 2 electrode remote survey works.

The advantage of this method is that continuous potential profiles can be measured with only periodic contact to the pipe being required for re-calibration of "remote" potential.

Two vs Three Electrode Method - The addition of a third electrode, allows an additional value to be recorded, called the electrical field gradient. This measurement is indicative of the local electrolytic activity around the pipeline. It is useful in predicting remaining life or potential problems. Because of the very small voltages involved, the three-electrode method does require a computer subsea on the ROV. For this reason, the two-electrode survey shown in Figure 4 is more common for most pipeline surveys.

Figures 5 shows a finished survey plot using the three-electrode method. As can be seen, the three electrode survey shows current density, which can give a complete picture of the pipeline cathodic protection / coatings system. Figure 6 shows a typical ROV set up to run a 3 electrode survey.

Two Electrode Technique - In figure 4, above the sequence is:

So, by logging just the offset potential as the line is flown and then adding the remote potential, an accurate profile of the close potential can be generated. An example plot is shown in Figure 7. In this way, anode sites are easily detectable but there is no way of knowing how hard they are working. For most post-lay surveys, this is sufficiently detailed information.

New Developments

A comparatively recent development for deep water ROV's is the move to fiber optic rather than copper cables, wherever possible, for umbilical construction. The advantages in weight and speed are obvious. Another relatively new development is the "subsea toolbox" concept that will have large work class ROV's deployed subsea on protracted dives, with access to all the tools they may need in a rack mounted to the side of the production equipment or on the tether management system.

Deep Water Probe - This lead to the development of a deep water version of an old tool, the bathycorrometer. A bathycorrometer is essentially a self-contained cathodic protection measurement device which contains the reference ground tip, the reference electrodes and the voltage readouts all in one instrument (see Figure 7). The new bathycorrometer-style device can be placed on the ROV, with no umbilical interface necessary.

The advantages of this new probe-and-readout system are:

The unit pictured has a depth rating of 10,000 feet and has recently completed successful trials in the deep water Gulf of Mexico. One disadvantage of the bathycorrometer is its inability to perform remote electrode surveys. This is a development currently under study by the writer's organization.

Old Flops

Just to prove that the corrosion community doesn't always get it right, let me mention a couple of the more notable failures in ROV interfaced corrosion monitoring.

Field Gradient Scanner - The field gradient scanner head was intended to work on platforms to provide multi-directional field gradients and potential information. While conceptually it was sound, practically the difficulty of ROV mounting and accuracy of flying in a platform geometry made the system's data output somewhat difficult to interpret. The system was discontinued in 1988. A picture of the scan head is shown in Figure 8.