If it’s made of metal, then the chances are that it will corrode, are you prepared? Greg Whitaker investigates.
Could there be a subject that causes as much debate in our sector as corrosion? Countless books, journals and scientific papers have been written on the subject of corrosion, but it is still one of the great variables to be considered by engineers when designing a plant.
Whichever way you cut it, corrosion costs money. A NACE report for the US government calculated that the bill for corrosion across the United States tops $276bn per year, equal to about 3% of GDP.
Admittedly that figure included everything from corrosion in oil pipelines to washing machines but technologies are being developed to maintain and treat metal all of the time.
Stopping corrosion before it starts is preferable to patching it up later. For this, a number of engineered coatings designed for different applications are available to extend the life of components. The choice of coatings can include both synthetic and organic compounds and for obvious reasons are indispensable on anything which has to live below the surface of the sea.
While the exact formula of the paint can vary for oil and gas applications all of them should meet an internationally recognised standard. The two most common standards are known as NACE CIP and Norsok (developed by the Norwegian Technology Centre).
Within the Norsok M-501 standard there are a number of separate systems for different applications, ie for carbon steel operating below 120 degrees centigrade, for insulated stainless steel piping or vessels.
Recently, Shell called on coating manufacturer International Paint to supply a number of these systems as the Dutch oil major was refurbishing a number of its offshore platforms in the North Sea.
As many of the platforms had exceeded their original design lifetime, with some having been in service for almost 40 years, a good deal of work was necessary to address key corrosion, as well as health and safety concerns.
The paint firm specified a coating system using a product known as Interzone 954 with an epoxy primer with the trade name Intercure 202. This was a large contract with 45,000 litres of each coating being applied to the topsides and splash zones of the platforms.
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Shell is confident that even with the harsh North Sea conditions that the coating will be durable enough to last 10-15 years.
“With the high build characteristics, [of the product it] means that virtually one coating system can be used for each area of the platform. Despite challenging weather conditions, we were able to complete the painting of the first rigs on schedule,” Rick Klein, an area manager at International Paint explains.
Corrosion on the outside of metalwork might be hard enough to deal with, but corrosion on the inside of pipes and valves can lead to a sudden, and often catastrophic failure.
In a similar way to how an exhaust system on a car might look as good as new, but rust from the inside out can cause it to disintegrate without warning, so too can the component parts of a pipeline rot from the inside out due to the nature of the gas being carried. One highly corrosive culprit is hydrogen sulphide found in ‘sour’ natural gas. The chemical causes the rapid attack of steel, even in the absence of oxygen.
There are a number of ways to tackle internal pipeline corrosion and the first must be a detailed maintenance plan, which takes into account the amount of metal likely to be eaten away each year.
“Probably the most important element of an effective corrosion management plan is to ensure that all corrosion technologies, whether internal or externa, operate within an integrated, online data management system,” explains Kjell Wold, a commercial manager at Roxar, a business unit of Emerson Process Management.
“Emerson’s corrosion monitoring systems, for example, include an online data logger which can be used with a wide range of wireless communications solutions. Data from intrusive corrosion probes, such as ER, LPR and galvanic probes, can also be accessed online. Such a system must also be able to operate alongside other instrumentation, such as downhole pressure and temperature gauges through to flow lines, sand monitors, and multiphase and wet gas meters.
Together, this instrumentation provides the operator with greater control over production” concludes Wold.
Planning for corrosion is all well and good, but modern technology actually allows plant managers to ‘see’ inside pipes and valves. One of these systems has been developed on behalf of BP and is an ultrasonic and wireless system to help make decisions about managing corrosion. The new equipment is known as Permasense and was developed in collaboration with Imperial College, London.
The wireless corrosion sensors work by detecting unexpected changes in the wall thickness of the pipes. The firm has also set up a lab in conjunction with the University of Manchester to develop smart coatings, for increased protection from the elements and improving a structure’s usable life.
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“The widely adopted approach of periodic manual inspections does not capture the often intermittent, and sometimes accelerated and nonlinear nature of corrosion. It is, therefore, very difficult to use this data to correlate directly with either corrosion drivers or inhibitor applications to enable an understanding of the effects of feedstock and process decisions and the use of inhibitors on plant integrity.
Manual ultrasonic wall-thickness measurement is also subject to repeatability and reproducibility errors, from inspectors, different ultrasound test sets and small differences in measurement locations, which is not the case with permanently mounted systems,” says Kevin Clarke, an engineer at Permasense.
“Permanent corrosion monitoring can represent progress in enabling effective use of corrosion inhibitors and in providing asset and integrity managers with a real-time picture of how the infrastructure is coping with the demands placed upon it.”
However, internal corrosion monitoring, does come with its limitations.
“Corrosion on the probe does not necessarily reflect corrosion at the pipe wall, resulting in difficulties in detecting localised attacks, such as pits or weld corrosion, as well as questions over accuracy and accessibility,” he explains.
“Iron sulphide can cause monitoring problems in sour systems and problems are reported when using probes at very high temperatures. It’s with these limitations in mind that non-intrusive corrosion methods, directly installed on the pipe, have become increasingly attractive to operators,” he says, adding that he believes that one of the most popular non-intrusive monitoring techniques is the Roxar Field Signature Method (FSM) system.
“FSM is based on feeding an electric current through a selected section of the structure to be monitored,” Wold states. “By inducing an electrical current into strategically located pipe sections, the induced electric current creates a pattern determined by the geometry of the structure and the conductivity of the metal.”
Voltage measurements on each pin pair (up to 384 pins can be applied in pairs in a matrix) can then be compared to the ‘field signature’ which provides the initial reference and changes in the electrical field pattern then monitored, according to Wold. These changes are compared against the initial measurement to infer structural changes in the monitored area.
Through this, graphical plots can be generated, indicating the severity and location of the corrosion. Wold says FSM has become particularly popular in sour gas production in the Middle East because of the its ease of installation and because of its strong safety record.
“Direct measurements at the pipe wall can give more reliable measurements and where safety is a concern for probe retrieval operations” he explains.
Corrosion will be a significant issue for the sector, so it is fortunate that modern technology will reduce the amount of invasive maintenance required and that new coating chemistry will increase the lifetime of metal parts. Ignore these at your peril as the cost of having to replace parts will be very expensive.
