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Recent Advances in Offshore
Cathodic Protection Monitoring

James N. Britton, John P. LaFontaine,
Grant T. Gibson
Abstract
Offshore production facilities and pipelines are being installed in new, more hostile environments. Advances in Cathodic Protection Monitoring for new structure types in deepwater as well as for high temperatures are discussed. New portable ROV instrumentation as well as permanent monitoring of parameters affecting CP system performance are reviewed.
Introduction
The factors driving the use and design of cathodic protection (CP) monitoring equipment have not changed since first use, however our understanding of the CP process has and so has information technology. Recently proposed new design approaches (1), have increased the importance of area specific steady state polarization data. More complex monitoring systems are being deployed to generate these data. Improvements in remote data transmission have revolutionized collection of and access to the information thus making it more available to the right people. Deep-water structures of today are engineered to the highest standards, leaving little room for error or corrosion. Corrosion resistant and high-strength materials used are often susceptible to damage if exposed to cathodic protection, magnifying the need for accurate, real time monitoring. Ongoing improvements in Remotely Operated Vehicle (ROV) technology affects the way CP systems are being monitored and maintained, and how information is gathered and transmitted. Finally the need to understand how temperature and water velocity affect cathodic protection design is challenging the imagination of monitoring system designers.
Application of New Design Methodology
The slope parameter CP design method will no doubt replace or at least enhance the design of many sea-water immersed CP systems in the future. The method requires an understanding of the polarization curve of the construction material in a particular oceanographic environment. Variation of the curve shape with current density, sea-water flow rate, temperature and resistivity are not accurately known for all values measurable in sea-water. The best way to obtain these curves is to monitor actual installations and develop real polarization curves for a particular area. Thus the use of shunted current density sensors (Figure 1) and anode current monitors (Figure 2) is growing in popularity. The parameters measured from these instruments offer obvious benefits. First, the operator is provided with critical information on the monitored structure. Second, the system will yield information that will almost certainly allow a safe reduction on the size and cost of the next CP systems deployed in that particular area. Monitors can interface directly with using new cable-less diver and ROV tools. New cable-less designs reduce the cost of monitoring by 50%. Because continuous monitoring is not necessary to develop the polarization data needed, the sensors can be interrogated during already scheduled regulatory subsea inspections. Since their introduction in 1998, four systems have been installed offshore.

Figure 1 (below)- Shunted current density sensors
(for more recent versions of this sensor click here).




Figure 2 (below)- Anode current monitors
(for more recent versions of this monitor click here).
Data Gaps
Ironically, the most in depth quality CP data exists in deep water. There is a real gap in our knowledge in shallow water. We know that current CP designs perform well, however most professionals involved in offshore CP design would agree that the designs used are ultra-conservative. The opportunity to save millions of dollars annually on shallow water CP 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.

Figure 3 (below) - Monitored anode pipeline retrofit assembly




New Structure Types
Looking at some of the novel new structure types being used to produce deepwater oil & gas, we can see new reasons for and ways to monitor CP.

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 CP probe is shown in (Figure 5). This probe is rated to 10,000 feet of sea-water and can perform contact CP 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.

Figure 4 (below) - Subsea well head assembly




Figure 5 (below) - Self-contained cathodic protection
(for more recent versions of this monitor click here).




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 / production structures which float on the surface. They are however 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.

Figure 6 (below) - SPAR under construction, laying horizontal.




Floating structures often incorporate Moonpools, which are large apertures extending from above water through the hull of the structure. In these areas deepwater drilling and production operations can be completed in a calm, controlled environment. On a SPAR structure the bottom opening of this "hole" could be 600-700 feet below the surface. In addition, they are often full of drilling and production risers and riser buoyancy structures. The exposed surface areas are very large and access for post-installation retrofit or even routine inspection, are limited or non-existent. Due to the very recent installation of these structures, no long-term CP field data exists. Current density sensors, anode current monitors and dual element reference electrodes (Figure 7) were installed on a recently launched structure in the Gulf of Mexico to provide some answers for future designers.

Figure 7 (below) - Current Density Sensor and dual reference electrode for use with SPAR structures.
(for more recent versions of this monitor click here).



Variable Ballast Tanks

Tethered floating structures must have variable buoyancy to adjust for differing deck loadings and sea state. Large tanks in the hull that can be flooded or evacuated provide control with seawater. These tanks are constantly being polarized and depolarized by the installed cathodic protection system which has to last the entire design life of the structure. If there should be a problem and corrosion were to perforate these tanks or the ballast control piping within the tanks, the structure could be placed in jeopardy. Rugged Zinc reference electrodes located within these tanks provides long term verification of CP adequacy in a cost efficient manner (Figure 8).

Figure 8 (below) - Zinc reference cell.
(for more recent versions of this monitor click here).
Pipeline CP Monitoring

ROV Monitoring

Most offshore pipeline CP 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. CP measurement systems must have some copper-analog conductors to connect remote and close cells in a typical threeelectrode 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.

Figure 9 (below) - Three-electrode set up for monitoring CP on pipelines (courtesy Subspection Ltd.)



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 CP design parameters were determined in the laboratory. To obtain field data, a complex system of on-pipe sensors was deployed to verify CP 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.

Figure 10 (below) - Data acquisition panel



Figure 11 (below) - Sensor array on pipeline
Future Developments
It is probable that the development of new thermally applied metallic coatings will be a part of future deepwater or high temperature CP systems. Large capacity middepth 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.




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