Cathodic Protection Monitoring of Offshore Pipelines and Structures in Alaskan Waters
by John P. LaFontaine and Jim Britton (2000)
Pipelines and structures located offshore of Alaska face unique challenges to monitoring cathodic protection. Advances in cathodic protection monitoring technology are discussed. New portable ROV instrumentation as well as fixed monitoring of parameters affecting cathodic protection system performance are reviewed. Case histories from the southern coast of Alaska as well as Arctic waters are detailed.
It is common knowledge that cathodic protection (CP) is necessary to limit corrosion on metallic structures in marine environments. Monitoring cathodic protection can provide valuable data to owners and operators regarding:
1. The level of protection.
2. The remaining service life of the system.
3. Improvements for future designs.
In the environmentally sensitive coastal waters of Alaska, it is critical that the performance of the CP (cathodic protection) system on a structure or pipeline can be monitored. From the fast currents of Cook Inlet to the Frozen Beafort Sea the marine environment of Alaska presents many unique challenges from a cathodic protection standpoint. The current density required to achieve polarization on steel in Cook Inlet is over 6.5 times higher than that required in the Gulf of Mexico. In addition the cold temperatures of these waters are as much as 30% less conductive that ambient waters. The nearly year-round ice cover and permafrost make the Arctic one of the most challenging environments yet encountered by corrosion engineers.
CP Monitoring - General
The basic criteria for cathodic protection of steel in sea-water is that it is polarized to at least (-) 0.800 Volts vs. Ag/AgCl (silver / silver chloride) or (-) 0.850 Volts vs. Cu/CuSO4 (copper / copper sulfate). This value can be determined by employing either reference cell, but typically in sea-water silver / silver chloride is used. Measuring the potential will tell you if are currently protected. However measurement of other parameters is necessary to determine the remaining service life of your system. Among these are the current density pick-up on the steel and the anode current output. These values can be compared to design values to determine if the system is operating as expected. On coated structures i.e. pipelines, anode current output can be used to determine the efficiency of the coating.
Several factors make Cook Inlet one of the most corrosive marine environments for steel structures in the world:
1. Extreme tidal ranges create tidal currents as high as 8.7 knots (1). The high velocity water provides constant oxygen replenishment to the steel surface. In addition sand and other particulates are churned into the water column, in effect "blasting" the steel surface, preventing it from forming carbonate layers, which would otherwise decrease current demand.
2. The water temperatures are cold, ranging from 50 °F (10 °C) to 29 °F (-2 °C). The cold water has a high dissolved O2 concentration, which further increases current density demand on the steel (1).
3. The resistivity of Cook Inlet water is as much as twice that of ambient 77 °F (25 °C) sea-water. This effect is a result of the low temperature as well as fresh water input.
It is imperative in such conditions that a comprehensive cathodic protection monitoring program is followed.
Pipeline Surveys in the Cook Inlet - There are a number of critical aging pipelines in the Cook inlet that have only ever been surveyed using trailing wire type remote electrode techniques and some riser drop cell readings. It is now well accepted that these surveys give no detailed information regarding the true pipe potential unless the electrode position with relation to the pipeline is well known, and the system is corrected for the IR errors caused by the impressed current system. Many of the pipelines are installed using pull tubes so that even the drop cell readings are meaningless.
Two or three electrode techniques would provide better data validity providing that periodic pipeline contacts can be made to re-calibrate the true remote pipeline potential (Figure 1). This can be difficult because most of the pipelines are concrete weight coated for stability and mechanical protection, so if the pipe doesn't have anodes (bracelets), there is no way to calibrate unless concrete is removed.