Fitness for purpose of duplex stainless steel risers for sour service

Floating production, storage and offloading (FPSO) disconnectable turret (Courtesy Single Buoy Moorings Inc.)

Simulated environment tests are used to predict in-service test results. TWI provided this facility to Single Buoy Moorings (SBM) Monaco when they requested help.

SBM had been commissioned to design and build an offshore floating production, storage and offloading (FPSO) vessel. This was to be used in the Wanaea and Cossack fields off NW Australia.

During pipework welding, unexpectedly high hardness values were discovered in the weld zones. Duplex stainless steel had been used for the rigid arm and riser piping and it was these weld areas which were giving hardness measurement values outside the design requirements.

SBM contacted TWI and sent the welds to Abington for evaluation. TWI were able to confirm the high hardness values, but considered that otherwise the welds were perfectly normal. To prove this, simulated environment tests were set up. These consisted of slow strain rate (SSR) tensile testing and four-point bent beam tests at elevated temperature and pressure in a hot, sour brine solution. The H ₂S and CO ₂ contents were used to simulate field conditions.

The tests established suitability of the welds for service. Cause of the high hardness levels was related to the strain induced by welding - which was not likely to be a problem under service conditions. The welds were passed fit for purpose.

Copyright © 2001, TWI Ltd