Sour service has long been an important issue faced by engineers in upstream operations. Engineering companies, as well as many organisations like NACE and EFC are still working today on improved regulations.
Dillinger Hütte’s technical expert Dr. Gunter Luxenburger, director, sales & technical marketing, explains the latest developments.
Although the problem of failure of pressure vessels operated in sour services has been faced for several decades, research is nowadays still going on to enable end users to operate their installations safely.
The phenomena of Sulphide Stress Corrosion (SSC) and Hydrogen Induced Cracking (HIC) seems to be understood and there exist several standards which enable the operators to specify the appropriate steel plates for sour service environment.
However the phenomenon SOHIC is still under discussion and several newly developed test methods have to prove their reasonable applicability.
Corrosion in sour service
The first step to hydrogen induced corrosion damages is the absorption of hydrogen by the steel. Basic condition therefore is a corrosion reaction in a wet and sour environment.
Furthermore the presence of H2S as promoter is necessary. The corrosion reaction creates a source of atomic hydrogen moving free in the sour atmosphere.
After atomic hydrogen is absorbed in the steel it will migrate to areas such as imperfections or inclusions where it is able to recombine to H2.
This may lead to enormous pressures causing cracks. Different types of damage can occur with other decisive factors like the amount and the type of stress applied to the steel.
The simplest and mostly understood failure is HIC where no stress is applied. Hydrogen Blistering is a special case of HIC due to the vicinity of the material surface.
The second case is a corrosion attack in combination with stress called Stress Corrosion Cracking (SSC). In service SSC is primarily to be expected in the area of welds correlated with hard zones.
In recent years a further corrosion mechanism which is correlated as well to the presence of stress was found. It is the Stress Orientated Hydrogen Induced Cracking (SOHIC).
This mechanism is explained as a combination of HIC and SSC. All HIC-resistant plates both for pressure vessels and for linepipe are produced following a special route which ensures good material properties in regard to sour service operation.
The production route is applied to obtain a homogenous HIC-resistance level over the whole plate and consists of special treatments and quality assurance measures. The market in HIC-resistant plates is complex.
Almost every oil and gas company, as well as the major engineering companies issue their own specification for HIC resistant plates. Often in these specifications restrictions for several production parameters and often enough the most stringent values out of different specifications are mixed up in one specification.
Specifying production parameters like sulphur and phosphorus properties are inducing difficulties as this limits the possibility of special steel design.
To offer best service to end users Dillinger Hütte works with specification writers in order to give support from the manufacturer’s side.
To allow economical production a standard in production was defined.
Dillinger Hütte offers three different acceptance levels, which are named DICREST 15, DICREST 10 and DICREST 5.
The Digit is standing for the maximum CLR level, which will be reached for the average of all sections, measured after the HIC-test.
For urgent cases Dillinger Hütte together with Ancofer Waldram Steelplates and Dillinger Middle East offers HIC resistant plates from stock with a specification covering the most common requirements.
This makes it possible to get HIC resistant plates even with short deliveries in case of emergency.
