PROGRESS IN THE PREDICTION OF TOP OF THE LINE CORROSION AND CHALLENGES TO PREDICT CORROSION RATES MEASURED IN GAS PIPELINES
Yves GUNALTUN Total S.A seconded at PTTEP. Shinawatra Tower III 1010 Vibhavadi - Rangsit Road. Chatuchak, Bangkok 10900 / Thailand Ussama KAEWPRADAP, Marc SINGER, Srdjan NESIC Institute for Corrosion and Multiphase Technology. Ohio UniversityResearch Park 342 West State Street. Athens, Ohio 45701 /USA Suchada PUNPRUK, Matina THAMMACHART, PTTEP/OPS/OMI. Shinawatra Tower III 1010 Vibhavadi - Rangsit Road. Chatuchak, Bangkok 10900 / Thailand
ABSTRACT Bongkot is an offshore gas field in the Gulf of Thailand in operation since 1992. The sealines have been subjected to top of line corrosion (TLC) since the production start-up. Afterdetection of the first TLC case in this field in 1999, different possibilities were investigated and implemented to reduce the corrosion rate to a reasonable value. Recently few leaks were experienced at cold spots like bare metal surfaces around subsea flanges or anode pads welding. It was clear that stabilisation does not take place at cold spots. TLC prediction for the sealines of different gasfields was necessary for prediction of leaks at such locations. The TOPCORP model was selected for this purpose. As a first step, the capabilities of this model were evaluated using data available from Bongkot field. This paper gives a short review of different prediction models, summarises selected model’s main features, compares the predicted water condensation and corrosion rates to inspectionresults and discusses the capabilities of the model. Key words: top of line corrosion, corrosion prediction, CO2 corrosion, modeling
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INTRODUCTION About Bongkot Field and TLC Problems Bongkot is an offshore gas field in the Gulf of Thailand (Figure 1) in operation since1992. The first TLC case was detected in Bongkot sealines in 1999. Chemical treatments were immediately initiated. Chemical treatment efficiency was gradually improved by using better chemicals and tools by 2003 – 2004 following the progress achieved in research and development projects. Inspections, using MFL type tools, showed important thickness losses up to 74 % along the first 1 to 5 km ofsealines as a result of TLC. A UT tool was also used for the inspection of a few pipelines and it showed only 10 – 20 % thickness losses. Further investigations showed that the UT tool was not able to measure the remaining thicknesses accurately due to presence of corrosion products that had accumulated inside the localized corrosion features. It is very likely that these thick corrosion products alsoprevent the corrosion inhibitor (applied by TLCC –PIG, also called spray pig) to be effective. Three leaks were experienced in two of the pipelines in 2008. The first leak was close to a sub sea flange connecting the inlet riser to the dogleg, where there was no pipe coating. Two other leaks occurred on another line at anode pads where the original pipe coating had been replaced by a thin epoxylayer after anode installation. Failure analysis confirmed that the failures were not the result of any parameter related the metallurgy of the pipe, flange or welding. The main cause identified was localized TLC. In these three cases the corrosion occurred on small pipe surfaces where the water condensation rates were high when compared with other pipe surface locations (cold spot corrosion1)....
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