British Gas recently had a requirement to fabricate some UNS3 1803 duplex stainless steel pipe work for an offshore opsides process plant. The pipe work had a maximum diameter of 600mm, with a corresponding wall thickness of 18mm, and it was designed to operate at a minimum temperature of - 40°C. There is a lack of published toughness data for girth welds in duplex stainless steel at this thickness and minimum design temperature. Additionally, toughness requirements for girth welds in current pipe work and pressure vessel codes are based on experience with carbon steels. As a result, a programme of work has been carried out to study the Charpy, CTOD and wide plate toughness of girth welds in 22%Cr duplex stainless steel pipe work. The welds were produced using a typical gas tungsten arc/gas metal arc pipe work fabrication procedure. In addition, non destructive evaluation trials have been carried out on a deliberately defective weld using radiography and ultrasonics. It was demonstrated that double wall single image y-radiography. single wall single image and panoramic X-radiography, and conventional shear wave ultrasonics were all able to detect planar root defects varying from 3 to 7mm in depth. There was good agreement between the sizes recorded by ultrasonics and those measured from macro sections. Small scale mechanical tests demonstrated that welds with overmatching tensile properties, and low temperature toughness properties whichwere acceptable to specification, could be produced. Wide plate tests demonstrated that defect size calculations from BS PD6493 were conservative.
The process plant for a British Gas North Sea offshore platform topsides facility required the use of 22%Cr (UNS3 1803) duplex stainless steel pipe work. The pipe work varied in diameter from 26 to 600mm, with a corresponding wall thickness of 3 to 18mm. The work described in this paper had the objective of confirming that the material properties of this welded UNS3 1803 pipe work were such that the selected non destructive evaluation (NDE) procedures, would allow detection of the allowable defect sizes determined by fracture mechanics with reasonable confidence,
The design of the pipe work was generally to ANS1/ASME B3 1.3', and the material toughness requirements were derived from EEMUA 153*. This specification gives the appropriate Charpy test temperature, known as the materials reference temperature (MRT), for the particular design reference temperature (DRT) of the pipe work and the pipe wall thickness involved. The DRT at full design pressure was -40°C. although in the blow down condition, the temperature could fall to -50°C (in this case the stress would be less than the full design pressure). The EEMUA requirements for service in the as-welded condition are shown in Figure 1. It can be seen that, for the maximum pipe work thickness of 18mm the MRT is -40°C for a DRT of -40°C.
