There are a number of standards that address coating and installation of fasteners in high-pressure service. Most of them clearly forbid the common coating application practice of oversizing (also called over-tapping) to enable the nut to fit over a thickly coated bolt. Oversizing virtually removes steel from the nut threads, compromising the tensile strength of the fastener, and greatly reduces its effectiveness in containing pressure and load. (For more information, see Tensile Time Bomb: How Overtapping Compromises Pressure Bolt Safety.)
Formerly a coatings applicator, Ian MacMoy now works in bolt product design for Dox Steel. For the past three years, he has been campaigning to expose the ongoing gap between best practices and current field practices in applying high-pressure bolt coatings. MacMoy says that the practice of oversizing continues because there are very few coating solutions that provide reasonable service life and reliability without a very thick application of paint.
So what should the industry do to reduce risks to safety, health, and the environment?
[An edited transcript of the conversation follows.]
Nuts and Bolts: Small Components Causing Big Problems
Oil and gas refining and chemical processing plants, oil and gas pipelines, and offshore rigs all use pressure bolting to secure valves, heat exchangers, flanges, etc. This type of fastener service is governed by the American Petroleum Institute’s standard API 6A, which is enforced by regulators. Therefore, we must follow the rules for everything that is under the wellhead mandate.
A subsea wellhead would be the highest pressure situation that the industry would face, going all the way up to 25,000 psi. We look at bolts under load and tension as well, such as riser bolting. This is a very big topic right now. In essence, risers are giant flanges that have to be bolted to a 60 foot (18 m) pipe. They hook together and can be 5,000 to 7,000 feet (1.5 to 2.1 km) in length, subsea. A very corrosive environment.
The nuts and bolts are the weakest link in these types of systems. Considering safety, we cannot be too careful. We have to take heavy precautions with nuts and bolts because failures can result in massive oil and chemical leaks and catastrophic explosions. (You may be interested in our INFOGRAPHIC: The El Paso Natural Gas Company Pipeline Explosion.)
A key problem is that any time you have a coating on a set of bolts, you have torque value confusion. What happens is that every nut and bolt you install has a different dry film thickness of coating on it, so the torque value is changed. In pressure bolting, every micron matters.
When joining the bolts to the nuts, the uncoated nut threads get smashed, which further skews the torque value. Those are some of the major problems. The way coating inspectors find them is that they see the gaskets aren’t closing, or they get mashed because we force them on and they don’t expand back. Then the fasteners leak pressure, liquids, or gases—whatever the pipe is carrying.
When coated pressure bolts are used in topsides and pipelines, the coating is most often applied to everything but the inside of the nut threads. This way, they are able to oversize the nut threads by only 10 ml, rather than the 25 ml or more if they coated the interior nut threads too.
Either way, the system fails. Either they oversize by 10 ml and get premature rust on the uncoated interior threads, and also break the ASME/ANSI B1.1 ban on oversizing, or they oversize the nut threads by 25 ml and coat the entire nut, which results in failure of the ASTM A194 Proof Load Test.
The API strongly bans oversizing. This is because in an oversized nut under extreme pressure, the tensile force is concentrated on the peaks of the threads. This causes elongation and deformation in the thread valleys, and a chain reaction of failures resulting in a loss of pressure and containment.
All of the ASTM standards that govern pressure fasteners talk about shear, and ASTM A193 and A194 say that when tensile strength is an issue, you cannot oversize the nut. This is because when a nut is oversized, you’re counting too much on the threads to hold pressure and load. Removing steel to oversize the inside of the nut reduces the amount of contact between male and female threads, and thus will affect the strength of that fastener.
What happens is that some of the standards contradict themselves in the same sentence. They state one thing that they want to have done—oversize the nut to whatever is necessary to stay within the dimensions with coating DFT—but stay within ASME B-1.1. The fact is that you cannot oversize even one ml and stay within dimensions.
Unfortunately, not many people out there applying coatings for salt environment pressure bolting know these facts. There is a huge education gap and that’s why I am speaking out about this.
Historically, to get the corrosion protection we needed in the ocean, or topside on rigs, or in hydrochloric acid service, we had to put on more and more coating. Like Winston Churchill said, “It may not be the best system, but it’s the best one we have so far.”
Hot dip galvanizing is great for utilities, but you don’t use zinc within 10 miles of the shoreline because it’s going to rust quickly. Nonetheless, zinc is still used in salt environments with a top coat of PTFE (polytetrafluoroethylene) for lubricity and added corrosion protection. Aluminum-filled ceramics are very good corrosion solutions to use along the shoreline or at sea. They give you 5,000 salt spray hours or about five years of service life. And that’s great.
But these coatings are really the crux of the problem. You have the oversizing problem with all of them because they must all be applied thickly to work in offshore environments.
For example, with the PTFE lubricant topcoat, you have to oversize the nut by 10 ml. The ASME B1 standard says you’re not allowed to cut the substrate of this nut away to make room for more protection. With aluminum ceramics, you have to cut the inside of the nut to 25 to 29 ml, depending on the manufacturer of the coating. When you do that, the nut is no longer strong enough to hold tensile force and it will fail.
Oversizing Leads to Failure
Really, it doesn’t matter what coating you use out there, oversized fasteners are going to fail.
That’s an issue we’re up against. Everyone wants to hear about a giant failure, and then they’ll do something about it. We’re stating that all these problems are there. A valve leak is a safety issue in itself. There have been failures where it’s highly likely that the root cause was fastener failure, and in my opinion, these have not been properly diagnosed.
Just recently, a massive explosion occurred on a Pemex rig in the Gulf of Mexico. Four people died and 16 more were injured on this very large processing rig. It happened in the wee hours of the morning when everyone was sleeping, the worst possible thing. The cause of the explosion in the dehydration facility is still up in the air.
An End to Oversizing
The cost of maintenance is a key issue here. If your price of oil is $90 per barrel, you have the identical maintenance cost in producing that same barrel of oil when the price falls to $60. And at that price, you really can’t make money.
Why the need for this heavy maintenance? It’s because their coating solution is hot-dip galvanizing with failure after failure. This is where Dox Steel is succeeding. People who have been in the field and who have seen the problems want our solution right away. But the use of other types of coatings still continues, along with oversizing.
Our nickel/cobalt plating solution goes much further than that with a life expectancy of 20 years. Having reliable thread stripping calculations and measurements that the industry can agree on is another important step.
We recently had an independent tensile strength analysis done to answer this key question: “How much can oversizing of 10 ml affect the strength of the nut?”
The answer was: “If the pitch diameter of that bolt is 10 ml less than nominal, it will have less than half the rated strength; at 20 ml, it will have only a quarter of its rated strength.”
Another important action is to continue canvassing and educating the engineering, petroleum, and regulatory communities. Any large fastener distribution company is going to have a mechanical engineer on staff. When engineers look at the thread stripping analyses we've done, they’ll say this makes perfect sense.
To maintain fastener integrity, the nut should never fail before the bolt. But if you look at the thread measurements, you'll see that the nut threads will fail under a lot less load than the bolt.
There are two main reasons we must change our approach to pressure bolt coatings:
- This problem is costing the industry too much for maintenance. If we want to make money at $60 a barrel, we have to have innovative ways of looking at this.
- We are breaking the law. API 6A, which states the measurements (ASME B1.1 thread dimensions), is subject to the law and we need to get into compliance. A lot of people don’t realize they are breaking the law.
Many in the industry have not heard this information, and even so, we’re slow to change in this industry. Regulators are seeing that the industry has put off addressing this problem. For the last 20 or so years, we’ve used hot dip galvanizing under tensile strength and you’ve got to go 25 or 30 ml oversized to get that stuff on. We cannot continue to put on corrosion protection coatings that are more than 18 microns (less than 1 ml). And there are very few coatings that are going to provide the robust protection needed in highly corrosive salt environments.