Data Gaps

Ironically, the most in depth quality cathodic protection data exists in deep water. There is a real gap in our knowledge in shallow water. We know that current cathodic protection designs perform well, however most professionals involved in offshore cathodic protection design would agree that the designs used are ultra-conservative. The opportunity to save millions of dollars annually on shallow water cathodic protection is being missed because we don't have the field data. New monitoring approaches can be used to cost effectively monitor new installations and perhaps more importantly anode retrofits (Figure 3). Figure 3 shows a monitored retrofit anode assembly recently deployed on a pipeline retrofit in the Gulf of Mexico.

New Structure Types

Looking at some of the novel new structure types being used to produce deepwater oil and gas, we can see new reasons for and ways to test and monitor cathodic protection.

Subsea Equipment - This broadly classifies equipment located on the seabed, often with no on surface component. A typical subsea well head assembly is shown (Figure 4). These structures are maintained against corrosion by a combination of cathodic protection, coatings and corrosion resistant materials. When there is no surface structure, the operator will typically use portable, usually ROV deployed sensors, to monitor cathodic protection. A new generation self-contained cathodic protection probe is shown in (Figure 5). This probe is rated to 10,000 feet of sea-water and can perform contact cathodic protection measurements; the potential and check potential being displayed on the integral LCD readouts. The instrument can be adjusted to interrogate fixed stab, cable-less sensors (Figure 3). These sensors contain an encapsulated shunt that allows current and current density information to be measured. One internal readout displays the shunt reading and the other displays potential. This instrument has been specifically designed for deepwater ROV use. The power is controlled by a photo sensor, which powers up only when illuminated by the ROV video lighting. The instrument has no electrical interface to the ROV and can thus be included in the subsea tool rack for periodic use when opportunity presents itself.

New systems being utilized in the North Sea and other parts of the world are using high tech subsea modems that are able to modulate signals through the pipelines and flow-lines themselves. Limited usage for corrosion monitoring has proven to be largely successful. As these systems develop, there will likely be more and more application in the corrosion and integrity monitoring sectors. Some of the most critical cathodic protection monitoring for these structures happens on dry land, particularly, verification of electrical continuity on the structure. This subsea equipment is easily the most mechanically complex equipment ever deployed on the seabed for periods of 20 years or more continuous immersion. Mechanical fastenings, connections and other linkages present possible areas of concern. Many parts have to be either continuous or isolated by design. Among the problems that arise is the false assumption that a threaded connection will ensure continuity. Continuity inspections are critical. There are no industry recommended practices for continuity inspections as of this issue date, however an informal procedure is available (2).

Floating Production Systems - Floating production systems are, as the name implies, self contained drilling and production structures which float on the surface. They are tethered to the seabed by a variety of methods. Structure types include Tension Leg Platforms (TLP's), floating SPAR structures and semi submersible type drilling rigs which have been converted for permanent mooring. A SPAR structure is shown floating on it's side prior to tow out in a Texas fabrication yard (Figure 6). These complex structures present a number of new CP environments that have rarely been encountered. In order apply cathodic protection efficiently, we have to monitor.

Pipeline Cathodic Protection Monitoring

ROV Monitoring - Most offshore pipeline cathodic protection monitoring has to be performed by portable means. ROV's inspect offshore pipelines routinely and the three-electrode inspection method (3) is still the most widely used. One recent advance is the modification of these systems that allows for interfacing with fiber optic data telemetry. Cathodic protection testing systems must have some copper-analog conductors to connect remote and close cells in a typical three electrode set up (Figure 9). However the subsea control computer on the ROV keeps this requirement to a single conductor which need extend only as far as the ROV tether management system. All other signals are optically encoded subsea transmitted absolutely error free through the ROV umbilical, and then decoded topsides.

Fixed Monitoring - Some pipelines are so critical that extra ordinary measures are employed to ensure that cathodic protection is adequate. A recent pipeline installed in the Gulf of Mexico was required to operate at 300°F, because this was an industry first, most of the cathodic protection design parameters were determined in the laboratory. To obtain field data, a complex system of on-pipe sensors was deployed to verify cathodic protection system performance. The monitoring package included current density sensors, temperature probes, anode current monitors, coating efficiency monitors and a variety of reference electrodes. The sensors were deployed at critical locations on the pipeline and cabled back to the surface platforms on either end. Data acquisition systems on either end of the pipeline collected the 58 channels of information. Figure 10 shows one of the two data acquisition panels, and Figure 11 shows a typical sensor array attached to one of the pipeline risers.

Information from this system as well as some point data from diver stabs have been used to verify a predicative model which was developed as part of the scope on this project. The predictive model can now be used with a much higher degree of certainty, and will thus save the operator from costly over-designs and frequent monitoring.

Future Developments

It is probable that the development of new thermally applied metallic coatings will be a part of future deep water or high temperature cathodic protection systems. Large capacity mid depth systems will certainly shift more toward impressed current, as cost and flexibility become more important factors. The future success of these systems will depend largely on information gathered from monitoring systems installed on the early deployments of the technology.

References

1. Townley, D.W., "Unified Design Equation for Offshore Cathodic Protection", paper no. 97473 presented at CORROSION/97.
2. "Inspection Methods to Verify Electrical Continuity on Subsea Structures", Deepwater Corrosion Services, Inc., In-House Recommended Practice-020, 1993.
3. Britton, J., "Continuous Surveys of Cathodic Protection System Performance on Buried Pipelines in the Gulf of Mexico", paper 92422 presented at CORROSION/92.