Abstract

Over the past 10–15 years there has been a growing interest among operators of offshore production platforms to utilise more corrosion resistant materials in process and piping systems to save costs associated with inspection and maintenance. High alloy steels such as duplex UNS S31803, super duplex UNS S32750 and the 6Mo alloys (for example UNS S31254) have gradually been replacing carbon steel and cupronickel alloys for process and seawater piping. Further, in recent years other alloys such as titanium and non-metallic materials such as glass fibre reinforced epoxy (GRE) have also been introduced gradually, in response to increased severity of service enviroments associated with acidising in oil production or oil reservoirs going sour.

The new materials and in particular the high alloy corrosion resistant steels were introduced in the Norwegian and UK sector of the North Sea mostly for application in seawater piping, flowline inlet and outlet, process pipe and pressure vessels such as separators. Currently we are experiencing a similar trend for offshore projects in South East Asia.

Det Norske Veritas (DNV) has, through internal R&D work as well as failure investigations, consulting and contract work, gained experience from offshore application of these materials in the North Sea as well as in offshore projects in SEA. Much of this experience was gained through welding procedure qualification testing and from service failure investigations involving piping and pressure vessels in high alloy corrosion resistant steels.

This paper describes some of the DNV experience and presents examples of problems which can be experienced when utilising these new materials for offshore applications.