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Proving fitness for purpose [of pipe welds for an offshore structure]

Connect, no.84, March 1997

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 a floating offshore production, storage and offloading 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 send the welds to Abington for evaluation. TWI confirmed the high hardness values but considered that otherwise the welds were perfectly normal. To prove this simulated environment tests were set up. These were slow strain rate (SSR) tensile testing and four-point bent beam tests at elevated temperature and pressure in a hot brine solution (the H2S and CO2 contents simulated field conditions).

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

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