Quality Assurance of Aluminum Sacrificial Anodes for Offshore Applications
by Jim Britton (1993)
In recent years there has been a far greater emphasis placed on quality assurance / quality control within the oil and gas industry, this has filtered down to the cathodic protection supply sector within the offshore market. The need for quality aluminum anodes is obvious especially when deep water projects are being considered. This trend toward higher quality control levels has resulted in more third-party inspection work and better defined quality-test procedures . This paper will focus on the practicalities of quality control, and will highlight some of the problems which have been encountered.
When offshore aluminum anodes are manufactured for pipelines and platform applications, there are four basic critical inspection areas. The first is the quality of the raw materials, core steel, primary aluminum and master alloys. The second is the chemical composition of the finished anode casting. The third is the electrochemical performance of the anode alloy. And the fourth is the physical quality or the product. A typical anode inspection will address all four or these areas to some degree. The anode manufacturing specifications are typically different for each major purchaser; the tolerances of the specifications are still quite varied. This paper will concentrate on the specifications which the writer considers to be the most important.
CRITICAL PERFORMANCE ISSUES
When a galvanic anode cathodic protection system is designed, there are certain performance related parameters which are assumed for the anodes. Obviously, it is these parameters which must be checked and verified during the quality control process. The specific areas of Concern vary slightly between anodes designed for platform protection versus those designed for pipeline cathodic protection systems.
Platform Anodes - When a galvanic cathodic protection system is designed for a platform using sacrificial anodes, the following points regarding the anodes themselves are critical.
1. Core Design - The core, usually a pipe, has to he able to keep the anode reliably attached to the structure. The design will normally be reviewed by civil engineers to ensure that the strength of the attachment is sufficient for the weight of the anode as well as the projected launch forces and in service hydrodynamic loads. The location of the core within the casting is also important, because it will affect anode utilization factors used in the design calculations.
2. Driving Potential - Clearly the operating potential of the anode directly effects how much current the anode is capable of delivering to the cathodic protection effort. If there is a significant shortfall in the actual potential of the anode alloy, then there could be a major deficit of available current for polarization. This could lead to an under-protected structure or a poorly polarized structure.
3. Galvanic Efficiency (Capacity) - This value is critical in the design process since it dictates how many ampere-hours of protective current will be available for each unit-weight of anode material consumed. This is a very important property which should command high priority in the quality assurance process.
4. Weight - Obviously the weight of the unit anodes is important since it translates directly to design life of the cathodic protection systems. While some tolerance exists, it is important to monitor weight very carefully.
5. Dimensions - The final dimensions of the produced anode are important, especially the length. The dimensions of the anode govern its electrical resistance to the seawater, length of course being the MOST critical dimension. Again there is some tolerance here, but since the dimensions also control the anode weight they are somewhat self-governing.
6. Casting Integrity - The cathodic protection system designer needs to know that the anode utilization will not be affected by large chunks of anode material falling off prematurely. This can be prevented by core design and by control of major cracking of the casings. Many anode specifications pay (in the writer's opinion) too much attention to cracks, where common sense often reveals cracks to be much less important.
There are a number of less critical issues which could effect the final performance of the system but they are either obvious or relatively unimportant, and therefore have not been mentioned.
Pipeline Anodes (Bracelets) - While the designs of galvanic cathodic protection systems for platforms and pipelines share many common elements, there are some major, important differences regarding anode design and quality control. The potential, current capacity and weight are equally important in both types of system, but there are some concerns unique to pipeline anodes:
1. Core Design - Bracelet anodes are manufactured in two basic configurations. The segmented bracelet is comprised of a number of small segments which are welded to hoops attached to the pipeline. The semi-cylindrical or half-shell anode is comprised of two pieces with integral cores which can be welded or bolted together onto the pipeline. The material quality and the location of the cores for these anode types is critical to utilizing anode efficiency and also in some cases because of the abuse that can be sustained during the pipeline installation.
2. Anode Coatings - Unlike platform anodes, pipeline bracelets, like any other close or flush-mounted anode require a dielectric coating between the anode surface and the cathode to which it is attached. This is to prevent corrosion of the anode in the confined space which could build tremendous pressures with the formation of corrosion bi-products. It is also common to coat the supporting cores, especially on segmented bracelets. Since all underwater / subsea pipelines are coated, the anode core, if it were left bare would represent a fairly significant holiday. Failure or omission of these coatings could lead to major problems with the cathodic protection system down the road.
3. Anode Attachment - Because it is not common practice to weld the anode cores to the pipeline, the electrical attachment is usually made by a pair of cables. Better specifications call for solid welded or bolted bars. In either case this is a weak point in the system. When cables are used it is common practice to powder weld the cables to both the anode core and the pipeline; this requires very close attention. The loss of this electrical connection will of course render the anode useless. The writer has experienced many cases where failure at this connection has lead to failure of the cathodic protection system.
4. Dimensions - Clearly the dimensional tolerances are far tighter on a bracelet anode, which must fit snugly onto a pipeline.