A recent failure of a subsurface safety valve (SSSV) component occurred in a North Sea field well that was completed for untreated seawater injection service. The component was manufactured in alloy Unified Numbering System (UNS) (1) N07716 grade 140 ksi SMYS. After detailed investigation and verification testing, the failure mechanism was identified as hydrogen embrittlement (HE). Approximately one year after the well had been completed, a full fracture of the SSSV component caused separation of the tubing string. This required re-completing the well to remove the failed equipment, re-establishing well integrity, and enabling restart of seawater injection to the reservoir. The failure investigation identified that the crack initiation occurred at the outside diameter (OD) of the SSSV component. The failed SSSV component OD was located within the well’s A-annulus and in contact with the carbon steel casing and the packer brine. The packer brine was 1.09 sg NaCl treated with oxygen scavenger and biocide. The maximum temperature the SSSV was exposed to during this period was estimated to be 15°C.
This paper details the well history prior to and during the occurrence of the SSSV failure, and describes the failure investigation findings and the conclusions from the verification work. Finally, the paper summarizes changes in the future use of this alloy for well construction.