Mitigating internal corrosion in carbon steel pipelines

April 1, 2000
IFE checking inhibition methods
IFE specializes in the development of techniques to control corrosion such as this 'mesa' attack, which has caused extensive damage to the section of pipeline wall.
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A new phase of work on the use of pH sta bilization as a method of corrosion control in carbon steel pipelines carry ing gas/condensate has begun at the Institute of Energy Technology (IFE) in Norway. The idea originated from and was first applied by Elf, and the method has been further developed by IFE with a more environmentally friendly approach in a joint industry project. It is now used for corrosion control in the Troll Field gas export pipelines.

Whereas the earlier work focused on the problems of corrosion caused by the presence of carbon dioxide in the hydrocarbons, the new project, which will run until early 2001, aims to qualify the technique for pipeline environments with hydro gen sulphide, in addition to carbon dioxide.

IFE's first project ran between 1993 and 1997, says Arne Dugstad, section head for corrosion technology. In late 1997, Statoil applied it with success in the two 36-in gas condensate export pipelines on the Troll field. The problem at Troll was not pipeline corrosion itself, but the buildup of corrosion products from the pipelines in heat exchangers and boilers at the Kollnes terminal, which are used for cleaning the glycol added to the pipeline flow to prevent hydrate formation. The buildup was becoming sufficiently great to affect full running of the transport system.

Environmentally friendly

The pH stabilization technique involves increasing the pH of the water phase of the pipeline flow so as to facilitate the formation of a dense iron carbonate film with good protective properties on the pipeline wall. The stabilization agent used is sodium hydroxide, which reacts with the carbon dioxide, increasing the bicarbonate concentration in the liquid and the precipitation of iron carbonate on the pipeline wall. In the Troll pipelines, the injection of sodium hydroxide into a pipeline flow over a four-week period reduced the proportion of both total iron and dissolved iron in the flow, from over 100 ppm to less than 10 ppm, a level which was maintained after injection ceased.

Sodium hydroxide has the further advantage of being environmentally friendly, Dugstad says - the bicarbonate of soda formed by its reaction with carbon dioxide is widely used as baking powder. In case of spillage, its presence in seawater would be harmless, which is not the case with some other pH stabilizers, which though perform efficiently in controlling corrosion, would have a harmful effect on the marine environment if spilled. This is the case with an amine-based product qualified by IFE in the mid-1990s for pH stabilization on Elf's Lille Frigg and Frøy Fields.

Positive experience

The pH stabilization technique works well even for flows with a high carbon dioxide content. The method is straightforward and inexpensive - once injected into the glycol, the sodium hydroxide lasts for years with only an occasional top-off being required, Dugstad says. In view of these advantages, and the positive experience on Troll, the technique is now being considered for further application on several fields, including Statoil's Huldra.

Since it began tackling pipeline corrosion problems for the offshore industry 20 years ago, IFE has become a leading player in this field, says Liv Lunde, head of the Materials and Corrosion Technology Department. Leading international oil companies such as Agip, BP Amoco, Conoco, Elf, Shell, and Total regularly support its projects, together with the Norwegian oil companies. The new phase of pH stabilization development has attracted an important new sponsor, Saudi Aramco, which also plans to join.

Inhibitor test methods

The institute, based at Kjeller near Oslo, is an acknowledged center of expertise on corrosion inhibitors, working not only on the testing of inhibitors but also developing test methods. "This is important because in practice laboratory tests are not always representative of what happens in an actual multiphase pipeline," says Lunde. Reactions in a clean pipe, as used in a lab test, will not necessarily be the same in a real pipeline, where there will always be something on the wall. As a result, inhibitors which perform well in the lab have sometimes been found to have a diminishing effect in practice.

Valid test methods are now under development in the Kjeller Inhibitor Project, a joint industry project which IFE is running on critical parameters influencing inhibitor efficiency in multiphase flow. One line of investigation which is proving fruitful is that the diminishing effect of a wall-mounted inhibitor may be caused by emulsion forming in turbulent flow and acting as a barrier, Dugstad says. Controlling the pipeline regime, then, could be an important key to achieving efficient inhibitor behavior.

Prediction uncertainties

Another current joint industry project is the Kjeller Field Data Project, in which the institute is evaluating different models for carbon dioxide corrosion prediction using field data supplied by the 11 participants. Almost all commercial and in-house corrosion prediction models used by the oil industry have been available for evaluation, as has IFE's own KSC model.

The exercise has turned up some interesting findings, not least considerable differences in predictions depending on the model used. "We've found a spread of 10-100 times in corrosion rate predictions based on the same data, and large spreads in pH calculations," says Dugstad.

The Kjeller Acetic Acid Project is underway, running for one year and backed by Agip, BP Amoco, Elf, and Statoil. The project investigates the conditions in which acetic acid, often found in oil and gas reservoirs, increases corrosivity. and why.