Marianne Videm, a renowned expert on materials and corrosion, shares her know-how on material selection for offshore topside process systems.

“Whether you are in the planning phase of a new facility or modifying an existing one, you start by reviewing the entire system; from well streams entering the process facility, through separation into oil, water and gas, and finally export of oil and gas. Knowing the content of CO2 and H2S within the hydrocarbon gas is decisive to the corrosivity of the fluid. 

Carbon steel or corrosion resistant alloys (CRA)?

CO2 levels will determine whether you choose carbon steel or the more expensive CRAs (corrosion resistant alloys). The selection will be based on calculated CO2 corrosion rates. The conditions vary through the system and multiple calculations are required. One must know the system well, to apply the correct process conditions. High corrosion rates call for CRAs, as the use of inhibitors has been proven unreliable topside. If H2S is present in the gas, the system has to be designed according to sour service criteria.

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Water quality is an important factor, as the water is the corrosive phase. As opposed to produced water, condensed water is more corrosive. Only water in direct contact with the steel will cause corrosion. Thus, a dry gas or oil with a very low water content will not cause corrosion because there is no water wetting of the steel surface. The aim is to dry the gas to a low dew point (e.g. -18 °C) to avoid water condensation, and excess water is removed from the oil so that carbon steel can be used for the export pipeline. 

Not only the internal medium has to be considered, the external marine atmosphere is corrosive to carbon steel and lower grade CRAs. If carbon steel is selected, a coating has to be applied, and in addition inspection and maintenance is required. Altogether, using CRAs might be less costly in a long-term prospective.

Consequences from choosing the wrong material

The two main issues faced topside, are corrosion damages caused by the fluid contained in pipework and equipment and corrosion attacks from the outside. Typical internal degradation mechanisms for hydrocarbon systems are CO2 corrosion and sulphide stress cracking (SSC).

Carbon steel is a simple material that is prone to visible corrosion. CRAs are more prone to local damage, which is harder to detect. Pitting and crevice corrosion are likely external failure mechanisms for CRAs. At high temperatures, there is also a risk of chloride stress corrosion cracking (SCC).

A hydrocarbon leak as a result of corrosion is always an unfavourable situation. A leak is somewhat manageable, whereas cracking (e.g. due to SSC or SCC) may result in a complete loss of integrity with a substantial leak into it the environment with a risk of ignition.

A word of advice

Become familiar with the system that you are working with, including its operating conditions. Know all the details – they may be crucial. Also, make sure you know your standards and make sure you familiarise yourself with client specifications.”