Sammendrag
The Direct Electrical Heating (DEH) system which has been qualified for flow assurance of subsea pipelines is now in use on several pipelines on the Norwegian Continental Shelf. Electrical heating has proven to be an attractive alternative for preventing plugs (hydrate) and wax deposition compared to the traditional methods that use chemical inhibitors, which are expensive and represents a risk to the environment. For design and rating of the DEH system, the supply current, the power loss (heat generation) in the pipeline and the total system impedance are the governing parameters. They are calculated by a computer program based on finite element (FE) analyses. Electrical and magnetic properties of steel pipe materials are essential data for determining heat generation and efficiency of the DEH system. These data are not available from the pipe manufacturers and have to be determined by measurements. Experience has shown that between individual pipejoints the resistivity is approximately invariant, but the magnetic properties may vary significantly even in pipejoints from the same production batch. A difference in magnetic characteristics of pipejoints results in a variation of the heat generation. Measurements of impedances have been carried out on a large number of pipejoints in a laboratory setup by supplying current directly to the pipejoints. A comparison is then made to FE simulations calculated with both linear and non-linear magnetic steel materials assuming eddy current losses only. FE simulations show good accordance for 13Cr steels (Steel material of 13% chromium content). For carbon steels the calculations deviate from the measured values. Modification of the simulation model is required in order to achieve acceptable agreement for both the system resistance (power loss) and the system reactance when using carbon steels. This will give reliable data for rating the topside and subsea DEH equipment. Copyright © 2007 by The International Society of Offshore
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