The presence of hydrogen sulfide, (H2S)in corrosive oil and gas production poses its own specific threat to the integrity of the production system. Many materials suffer from cracking of various forms when exposed to HJS which may result in the catastrophic failure of equipment with the attendant risk of releasing the contents into the environment. Beside the general risks associated with release of hydrocarbons in terms of pollution and fire, the release of H2Sexposes persons in the vicinity to the risks of poisoning and death.
For these reasons the materials? engineer is mindful of the need to select materials of proven resistance to cracking in H2S-containing environments. There is a clear driving force for appropriate materials? requirements when exposed to H2S.There cannot be any excuse for ignorance of the risks involved.
A EUROPEAN VIEW OF NACE MR0175
A materials? recommendation document for valves was first produced in Canada more than thirty years ago to provide guidance on the type of environments which may cause sulfide stress cracking and to propose appropriate limitations on the strength and thence, hardness, of steel to be used within the cracking regime. Initially restricted to requirements for valves, this document, adopted by NACE in 1975 as NACE MRO175, has been expanded over the years to cover other components used in the oilfield. Changes in the past few years have tended to focus upon the introduction of various corrosion resistant alloys within the text.
In the context of carbon and low alloy steels (CLASS)the guidance given in NACE MRO175has continued to be used practically and is widely applied in the oil industry world-wide.
The less reliable aspects of the document are, arguably, the early additions of corrosion resistant alloys (CRAS) with acceptance based on hardness limits but without further environmental constraints. Some alloys therefore appear to have unlimited resistance to cracking at any level of H2S whilst ?better? alloys are more restricted because of the way the text is written and the document is structured. There is also a fundamental concern about ?mixing ?these two classes of materials within the same document since CLASS and CRAS crack by different mechanisms inH2S-containing environments.
A further concern about NACE MRO175 is that it sets ?prescriptive? requirements: specifically identified alloys are quoted (and therefore allowed). This restricts the application of other alloys and inhibits the development of new alloys. The test data submitted for proving alloy performance and the acceptance criteria applied have also varied over the years. Thus, in the area of CRAS, the guidance given in NACE MRO175is less consistent.