Cathodic protection (CP) comes in many forms. Zinc is often the metal of choice used for CP sacrificial anodes, which are attached to welding on the substrates of structures. In subsea and along pipelines, we provide cathodic protection in the form of an electric pulse to negate the resistivity flow in metals, which would otherwise allow them to rust. We use zinc electroplating on fasteners and we also use zinc-rich prime coats on structures.
Incident Findings from Offshore Spill
An offshore spill in late 2012 has brought the bolt safety issue to the forefront with regulators and the industry. (For more on bolts and other fasteners, read High Pressure Fastener Coating Practices Under Fire: Ian MacMoy Speaks Out.) While performing drilling operations in the Gulf of Mexico, a Chevron-operated rig lost approximately 400 barrels of drilling fluids. It was soon determined that the blowout preventer had separated from the lower marine riser package due to failure of the bolts joining the two segments.
Chevron reported the incident to the Bureau of Safety and Environmental Enforcement (BSEE). In a subsequent report, the BSEE called for ASTM International and the American Petroleum Institute (API) to join forces in determining the best way forward to prevent such disasters in the future.
After two and a half years of investigations and talks, most of the parties involved agree that the connector bolts failed as a result of hydrogen embrittlement (HE) and stress corrosion cracking (SCC).
But what were the root causes?
In an industry meeting on May 17, 2015, petroleum-industry representatives, bolt manufacturers, corrosion and fastener engineers, and members of the ASTM F16 Fastener Committee gathered to compare notes and hear recommendations.
ASTM F16 determined that this was an environmental hydrogen embrittlement (EHE) failure and said the contributing factors were the inappropriate use of zinc plating and improper cathodic protection.1
The experts’ answer, in short, was that the cause of HE and cracking of the subsea bolts was one where the zinc plating was transferring hydrogen to the substrate of the bolt and was able to do this because of the CP pulse that was present in the water. They felt that any cathodic protective system that combined these two would be heading for trouble and that zinc, aluminum and manganese were poor designs with the use of a CP pulse in the presence of electrolytes.
Discrepancies in the current standards were also discussed at the May 2015 meeting. BSEE is calling for the industry to develop a consistent set of standards for connections and connection fasteners used in all offshore subsea systems.
ASTM recommends the following criteria for subsea bolting systems.1
1. Limited in hardness to HRC 35 (A193, B7)
2. Finished with ASTM F1137 phosphate and oil, no baking required
3. Painted thoroughly for added protection
ASTM F16 also recommends that the industry stop using zinc electroplating for subsea applications, and use phosphate and oil, or a barrier coating like polytetrafluoroethylene (PTFE) for protection.1 (Learn more about PTFE coatings in the article Understanding Corrosion in Pumps and How to Deal with It.)
This is groundbreaking because we have used zinc electroplating for decades on fasteners and structures for subsea protection, especially riser bolts, which are the issue in this particular failure.
We were also told that forming a new sub-committee in ASTM F16 for oil and gas was another option they condoned. In order to do so, a new battery of tests would need to be developed for the subsea environment instead of ASTM B117, which tests oxygen, humidity and salt spray. We would actually be testing fastener systems under load in electrolytes with CP pulse. This test is new and does not have an ASTM designation yet, but it will take into consideration all of the variables that are present for subsea.
This is important news for fastener manufacturers, coatings formulators, standards organizations, and the oil & gas industry alike. This new understanding may also apply to buried pipelines in coastal regions where there is water present just a few feet below grade. We will be watching these issues carefully in the coming months and providing periodic reports. (You can read another case study in A Failure Analysis of Hydrogen Embrittlement in Bridge Fasteners.)