At first glance, it may seem as though the petro-chemical industry and the renewable energy industry are mutually exclusive. However, this is not the case. In fact, there are several areas where the former can benefit the latter. Of great significance, microgrids, a renewable energy technology, can protect buried oil and gas lines from corrosion.
The Problem: Maintaining Oil and Gas pipelines
Over time, buried oil and gas pipelines undergo corrosion. This is a critical problem. If left unattended, corrosion problems can have significant consequences. In turn, the Petro-chemical industry must find a cost-effective solution to keep the pipelines from degrading. One option is to make thicker pipes, which must be monitored, and then excavated when replacements are installed. Needless to say, costs can become substantial. However, what if companies could pay a little more for prevention equipment up-front with lower operating costs, ultimately saving money in the long-run? If you haven’t guessed it, many companies are choosing the latter approach.
There’s a relatively simple approach to inhibiting corrosion called “Cathodic Protection.” Basically, a reverse current is applied to the pipelines to counteract the usual effects of ground-current-based corrosion. One of the most cost-effectives modes of providing this Cathodic Protection is by using microgrids that are positioned along the line at regular intervals. The microgrids are made of the standard elements: PV, batteries, controller, and are an ideal implementation of the microgrid concept.
But first, the basics: What even is corrosion?
Corrosion is an electro-chemical phenomenon in which a metal is naturally changing its composition to return to the low energy oxide state – otherwise known as rusting. The basic chemical process is that an electric current will flow from the metal atoms in the pipeline, through ions in the surrounding soil or water and cause chemical changes to occur in the metal. This current is set up by the fundamental properties of the mixed elements in the presence of a charge conductor (the ground or water), and the atoms that will reduce the metal (the air which has oxygen).
Corrosion in Underground Structures
The principle of cathodic protection prevents this natural process from occurring. It operates by passing direct current, opposite to the corrosion current, continuously from electrodes (anodes) which are installed in the soil or water surrounding the structure to be protected. The corrosion is stopped when the protection current is of sufficient magnitude and is properly distributed to offset the intrinsic chemically derived current. Although the corrosion current is based on thermodynamics, and occurs automatically, the protection current requires an external power source. This is where a microgrid would come into play. The figures to the left show the basic principles of this effect.
Corrosion of buried metal structures is mainly dependent on the soil composition, resistivity, pH, water and oxygen content, etc. The most aggressive soil types are clays, mixed clays, muds and sands with high salinity. The most severe corrosion effects can occur on a pipe that passes through soils of different compositions. Traditionally, corrosion prevention of underground pipelines has been done by coatings (bitumen, coal tar and epoxy powder or polyethylene) or applied tapes. However, in corrosive environments coatings or tapes cannot totally prevent corrosion because of porosity caused during their manufacturing, transportation or installation. Therefore, although the standard less expensive up-front approach will reduce the corrosion effects, they only reduce. They do not stop the problem and the pipes will eventually need to be dug up.
Costs of corrosion
It has been estimated that somewhere between 3 and 5 % of the gross national product (GNP) of industrialized countries goes to to corrosion damage control [See the Handbook of Corrosion Engineering; Pierre R. Roberge; 1st Edition]. Corrosion of metals costs the US economy almost $300 billion per year and it’s estimated that one third of this value could be saved by better selecting corrosion prevention techniques, including cathodic protection.
A Solution: Cathodic Corrosion Protection
In cathodic protection (CP) systems, anodes transmit the protective current from the power supply to the structure to be protected. The electrochemical potential of the structure becomes more negatively charged, and eventually the level is adequate that it offsets completely the chemically-driven charge movement, and the unit gets the cathodic protection. For “impressed current” cathodic protection systems, the supplied current is controlled by constant voltage or microprocessor-based potentiostatic control units. Microprocessor-based control units can also be remote controlled. In order to give the best protection, the type, amount, and location of anodes is customized for specific environments. The location of the anodes is very important because there has to be equal protection current provided to all parts.
The Solar Powered Cathodic Protection System (CP)
Diagram of a typical system
For a while now, there’ve been installation of CP systems based on solar panels to generate DC voltages. These solar-powered cathodic protection systems are basically DC microgrids in that they act as a stand-alone station which provides energy in the form of an electric current. (Note that AC modes can be employed and then converted.)
Typically, a project is started by analyzing the specific conditions such as the soil, pipeline, depth, etc., and then designing the system suited to fit the requirements. The microprocessor-based control units and remote control facilities are used to optimize and monitor the operation.
The complete system providing cathodic protection at remote sites consists of the following items:
- Solar modules with support structures
- Solar charge controller
- Battery bank (deep cycling)
- Cathodic protection regulator (control unit)
- System cabling and mounting hardware
- DC array junction boxes
Takeaways?
This is an ideal microgrid application: costs of the microgrid easily offset the corrosion costs. The various components of the microgrid can be scaled and optimized to address the unique requirements of each installation, and even across the installation.
This solution has now been in play for a relatively short time period (almost 10 years) but it’s getting increasingly more attractive as the overall costs of PV and storage drop. It’s anticipated that this application will become a sizable market for microgrids in the next decade.
Learn more from HeatSpring’s Sustainable Scholar Dr. Andy Skumanich by enrolling in his Microgrid Design and Implementation Course!
