The method that is commonly used in Electric Heat Tracing System is self regulating type cables. But when the pipeline to be heat traced will have a distance of 25km, a convenient Skin effect type shall be applied. 

It will have only require a single power point to trace this pipeline.

 The principle of skin effect is based on 2 phenomena, proximity effect and skin effect. 
There is the "heat tube", through which a specially designed skin effect conductor has been pulled.

Following description below is from Thermon, a manufacturer who has a product that used Skin effect as Electric Heat Tracing.

"The  heat  tube  and  insulated  conductor  are joined together at one end, while at the opposite end the heat tube and the conductor are connected across an AC voltage source (typically 50 or 60 Hz). The impressed AC voltage will generate a current in the conductor which will return through the inside surface of the heat tube. The concentration of the return current on the inside surface of the heat tube is due to the magnetic flux linkages originated by the currents in the insulated conductor and the ferromagnetic pipe. This current penetrates  into  the  heat  tube  a  distance  termed  the  “skin depth.”  Due  to  the  phenomena  described,  there  is  virtually no measurable voltage on the outer surface of the heat tube, allowing the piping system to be grounded. The heat generated in a ThermTrac system is the result of the resistance that occurs on the inner skin of the heat tube. 




While the electrical current is concentrated on the inner surface of the heat tube, the heat generated will dissipate from the tube into the attached carrier pipe to increase the surface temperature of the pipe and its contents to a designed level."

The figure below is a typical example and components of Skin effect EHT.

Important Link from Thermon website for their Skin effect Heat Tracing "ThermTrac":

For reference purposes, below are the other competitors of Thermon who had same product.