New Article
Kristian Lund Jepsen
Ramboll Oil & Gas
Use of 316 and 316L stainless steel materials is standard for high-pressure (HP) flare systems on offshore platforms because of their exceptional low-temperature properties. They can withstand service temperatures down to -101 °C (-149.8 °F).
HP flare lines are often subjected to temperatures between -50 °C and -100 °C (-58 °F to -48 °F) during blowdown of high-pressure gas lines due to the expansion of the gas and the Joule-Thompson effect. Additionally, 316 and 316L are selected often for seamless instrument tubing and hydraulic lines.
However, 316 stainless steel (316 SS) and other austenitic stainless steels such as 304 and 304L are prone to chloride-induced stress corrosion cracking in service temperatures of above 60 °C (140 °F). Therefore, most standards for material selection, such as NORSOK M-001, do not recommend 316/316L for such applications unless the pipes are painted. Nevertheless, flare lines normally are required to operate at the temperature of the surrounding environment, and during blowdown at much lower temperatures.
A North Sea operator recently contacted Ramboll Oil & Gas to address a problem in relation to 316SS piping. During the risk-based inspection (RBI), severe external pitting corrosion was identified on some 316SS flare pipes. The pitting appeared to be mainly on horizontal pipes, and on the underside of these pipes, mainly between the 5 and the 7 o’clock positions. In a few cases corrosion was observed also at horizontal lines in positions between 11 and 1 o’clock.
Investigations showed that the problem was being triggered by salt water from the environment and probably also from deluge release during firewater system tests. Droplets of salt water were collecting on the underside of the pipes, followed by evaporation of water and an increase in chloride concentration. The chloride build-up was inducing the pitting corrosion.
Crevices are another form of pitting, although this corrosion occurs at temperatures well below the critical pitting temperature (CPT). Typically, crevices occur where pipes are supported in pipe supports, where tape or strips are fastened to the pipe, or in cases of loosened paint.
In the referred case from the Danish North Sea, several cases of crevice were discovered in the same 316 SS flare lines.
Attacked areas on the piping were afterwards subjected to FFS (fitness-for-service) evaluations per API 579, Level 1. In several cases, the remaining wall thickness was so small that it was not possible to prove any remaining life for that section of the pipe.
Immediate replacement of the pipes would have required a complete (unscheduled) shutdown of the plant. It was vital, therefore, to find a solution enabling the pipes to remain in service until the next planned major shutdown, about six to eight months later.
After further evaluations of the probability, and extent, of leakage, Ramboll concluded that all corrosion attacks needed to be arrested. The following action was taken:
- Pipes and attacked areas were rinsed carefully in high pH alkalic soap water
- Pits and attacked areas were filled with zinc paste.
The most severe attacks were later covered by pipe couplings. Later, the FFS evaluations, the risk assessments, and decisions taken were reviewed and verified by a third-party organization, DNV.
Instrument tubing protection
Problems with pitting in 316 stainless steel pipes have been reported in instrument tubing systems. Pitting is seen mainly on the underside of the instrument tube, but some sort of crevice also is found often under loose tape on the tubes, or where instrument tubes are supported in tube clamps.
Pitting in instrument tubing on, for example, gas compression systems can lead to leaks on offshore platforms which occasionally have resulted in gas alarms and production shutdowns.
For new installations, it is important to consider alternatives for the 316SS piping systems in the outdoor salt water exposed areas. SMO 254 (6 % Mo SS) and super duplex 25% Cr SS are options - neither exhibit the same sensitivity to pitting and crevice as 316SS. For existing installations, another course could be to paint the 316SS piping systems, or at least its horizontal parts.
Where 316/316L piping and tubing already are used or are being installed, zinc paste should be applied at all locations with risk of crevice, for example in pipe supports and tube clamps. The use of tape for labeling piping systems should be considered carefully given the risk of crevice formation.
Shell is known to use Tungum fittings and tubings on some of their UK offshore installations. Tungum is a copper alloy which is not sensitive to pitting or crevice caused by chloride. It is, however, only applicable for relatively low pressures, and therefore this recommendation for SMO, alternatively super duplex, also applies to instrument tubing and fittings.
Reference
Sandvik Steel: Corrosion Handbook. Stainless Steels.