This paper discusses and presents duplex stainless steel application and maintenance aspects related to not normally manned offshore installations. The discussion and presentation are partly based on a specific project carried out by Aker Offshore Partner a.s. in Stavanger, Norway. Although the presentation and discussion are based on a specific project, the paper also includes aspects of a more general character and is therefore of interest for designers as well as maintenance engineers. Special attention has been given to designing the wellstream containment part of the installation to provide a high safety and production reliability at a minimum of maintenance and operational life cycle costs.

Special aggressive wellstream gave the designers of the Varg wellhead platform no other choice than to use a better material than the traditional carbon steel. Different kinds of corrosion inhibitors were evaluated, but the conclusion was that the corrosion inhibitors did not give satisfactory protection. After evaluation of different kinds of high alloy steels and the costs related to them, duplex stainless steel was chosen.

The selection of duplex stainless steel for wellstream containment at Varg wellhead platform resulted in a reduced weight compared to ordinary carbon steel. This weight reduction is obtained mainly due to reduction of pipe wall thickness. The reduction of pipe wall thickness is the combined effect from removal of corrosion allowance and the increased strength in the duplex compared to traditional carbon steel. Today’s systems are designed for no corrosion. Still, 1 millimetre erosion allowance is designed for the pipes upstream the production header to make up for minor unexpected sand production. The assumption of no corrosion will be checked by relatively extensive inspections after one year. If the assumption of no corrosion is confirmed at this first inspection, the inspection intervals will be increased significantly and the extent reduced. This inspection philosophy allows for a dynamic inspection and maintenance program that provides a very high degree of freedom.

The maintenance program is constructed in accordance with scientific principles, ensuring that the program may be reproduced by peers. Special emphasis is put on objectivity and documentation, and the program allows for a dynamic updating as new information becomes available.