Fuel gas will be transported out to Seal Island via an identical 10-inch pipeline, which is bundled to the oil pipeline in the offshore section of the pipeline route. The first time use of such technology in this environment required a conservative corrosion protection system design. As an added measure of this approach, a cathodic protection monitoring system was designed and installed on the oil line to confirm that the pipelines are adequately protected for the life of the project.
It is common knowledge that cathodic protection (CP) is necessary to limit corrosion on metallic structures in marine environments. In addition to conventional CP potential monitoring, valuable data can be provided to owners and operators regarding:
1. The level of protection
2. The remaining service life of the system
3. Potential for improvements to future designs
Field data on the performance of cathodic protection systems for offshore structures and pipelines in the Arctic Ocean is very limited. As a result, it was helpful to monitor a range of parameters affecting the Northstar system performance. These include, anode current output, anode potential, temperature, pipeline potential, and current pickup by the pipeline.
Corrosion Protection Design
Corrosion protection is incorporated in every aspect of the Northstar pipeline design. Both the gas line and oil line wall thickness are 0.594 inch (15.1 mm) to handle the unique environmental loads of the shallow water Arctic Ocean. The coating system on both pipelines is a dual-layer fusion-bonded epoxy (FBE). The inner coating was a 12-mil (305 mm) anticorrosion layer, the outer coating was a 28 mil (711 mm) abrasion-resistant layer. Each layer was a different color, which allowed mechanical damage after holiday detector inspection to be easily found by visual inspection.
An identical sacrificial anode cathodic protection system was used on both pipelines. Impressed current was not a viable option due to the difficulties in establishing an effective ground bed in the permafrost, as well as permafrost’s high ohmic resistance. The CP system was designed according to Det Norske Veritas (DNV) recommended practices for submarine pipelines (1). The system incorporates aluminum-zinc-indium bracelet type anodes designed to incorporate 5% mean and 7% final coating breakdown over the 20 year design life of the pipeline. The low temperature anode chemistry employed for this design is a modification of the standard used for ambient temperature applications (2). This was necessary to account for the effects of cold temperatures on activation of aluminum. The anode design is one 104 lb. bracelet spaced 360 feet apart, or 1 every 9 joints (3).
Cathodic Protection Monitoring
The strategy for monitoring the Northstar CP system was to design and install fixed monitors on the oil pipeline, supplemented by periodic (3-5 year) portable instrument-survey monitoring. The monitoring system was implemented on the oil line only due to the pipelines being identical. Due to a yearly average of seven months of ice cover, fixed monitoring was the primary focus of the program.
The fixed instruments were arranged between 500 and 600 feet north of the shore crossing. The instruments included a monitored anode, current density monitor, and reference cells for the pipeline. Each instrument was hardwired to a monitoring panel located at the shore crossing. The pipeline in this region was laid in a trench and backfilled. Safety and logistical concerns required that the instrumentation be installed on the pipe prior to the laying operation. In order to achieve this requirement, each fixed instrument had to be designed to a high degree of redundancy and ruggedness. Accordingly, the signal cable was designed with extra steel armor, protected by an outer non-metallic jacket capable of maintaining its mechanical integrity during pipe lay and backfill.
1. Reference Cells - In order to verify the electrochemical potential, each fixed instrument was fitted with a dual reference electrode array. The array is comprised of Ag/AgCl (silver / silver chloride) and Zn (zinc) reference cells. The dual cell package provided a measure of redundancy, as well as a calibration capability. The reference cells, particularly the Ag/AgCl electrodes were designed with a high degree of ruggedness, without compromising contact with the mud. In order to achieve this, the cells were fitted with large semi-permeable membranes, which retain a constant chloride concentration around the Ag element. The bulb was protected from mechanical damage by a non-metallic sleeve.
2. Temperature Cells - Temperature sensors are also part of each fixed instrument. These sensors are integrated circuit temperature transducers that produce an output current proportional to absolute temperature. The data acquisition panel supplies power, and output current is passed through a resistor. The voltage drop is measured across the resistor and then translated to temperature.
One anode was interfaced with an instrument package to verify performance. The package was designed for measurement of anode current output, potential and temperature. The anode was assembled around the pipeline with a layer of neoprene between the anode and the pipe (Figures 2 and 3). The anode was grounded to the pipe through the instrument, allowing for the current to pass through a shunt before returning to the anode. The signal wires for the shunts as well as the other sensors were bundled in a single, heavily armored cable, which was bundled with the pipeline and run back to the monitoring panel at the shore crossing.