The direct electric heating system is used to keep the temperature above the hydrate temperature during shutdown of pipelines and leads to significant AC transfer to the surrounding sea water at the pipeline end zones. The pipes are sorted in groups with the same magnetic permeability and there is a potential risk for high AC density at permeability group transition point. Along the pipe within a permeability group there is also a potential for high AC densities due to minor changes in average magnetic permeability. AC corrosion testing has been undertaken and the AC density limit is found to be 250 A/m(exp2). This applies only for a pipeline with full cathodic protection. At a reduced cathodic protection potential of - 600 mV the results indicates that resistance towards AC corrosion is reduced considerably. The AC density limit of 250 A/m(exp2) has been used in calculations to determine the maximum allowable distance between sacrificial anodes. This distance is found to be 2000 m for low alloy carbon steel and 3000 m for supermartensitic stainless steel A standard cathodic protection (CP) design is normally based on the ISO standard 15589-2. However, for a pipeline with direct electrical heating, the anode spacing shall never exceed the lengths defined by the AC density limit. Anodes shall be installed at all transition points between permeability groups. Moreover, all locations where diameter or wall thickness is changed shall be defined and anodes shall be installed.
Direct electric heating on subsea pipelines a challenge to corrosion protection
Die direkte elektrische Erwärmung an Unterwasserrohrleitungen als Herausforderung an den Korrosionsschutz
Corrosion 2006, 61 ; 06178/0-06178/13
2006
13 Seiten, 9 Bilder, 4 Tabellen, 8 Quellen
1-13
Aufsatz (Konferenz)
Englisch
kathodischer Korrosionsschutz , Wechselstrom , direktes Heizen , Elektroheizung , Kohlenwasserstoffproduktion , Rohrleitungskorrosion , Korrosionsgeschwindigkeit , Off-Shore-Anlage , Stromdichte , Spannungsrissbildung , martensitischer nichtrostender Stahl , magnetische Permeabilität , Opferanode , Abstand , Wanddicke , Korrosionsprüfung , Kohlenstoffstahl
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