Sammendrag
Application of AC direct electrical heating (DEH) on subsea pipelines requires a special design of the corrosion protection system. The first electrically heated pipelines in the North Sea were supplied with anodes banks at the ends for AC current transfer in addition to single anodes distributed along the heated pipeline according to CP-design requirements from standards and class societies. Anodes were also installed at pipejoints where pipes sections with different magnetic properties are connected, as current transfer between pipe and seawater will occur at these locations.In case of buried pipelines high temperatures for the anodes implies reduced cathodic protection. For these installations modified solutions for cathodic protection are required. Measurements have been made on a scale test installation without distributed anodes in a project initiated by an oil company. In this case the anode banks at the ends must be designed both for cathodic protection and AC current transfer to seawater. For Cr13 (13% Chromium content) pipelines welding of the anode connections should be avoided related to hydrogen embrittlement. A design with clad steel carbon pipes for the current transfer zones at the ends where anodes are connected has been chosen to solve this problem. For economical and practical reasons, the length of the clad steelsections should be as short as possible. This is especially true for a reeled installation method. It has been verified by tests that the minimum length of the clad steel section depends on the materialcharacteristics of both the clad steel carbon pipe and Cr13 pipes. When the magnetic and electrical properties of Cr13 pipes are known, carbon steel pipes can be selected to obtain a minimum currenttransfer length. These material data are not available from the manufacturer and must be determined by measurements. Model tests indicate that for power supply of 50 Hz a transfer zoneless than 30 m can be achieved. The tests have shown that in ge
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