Q12570 : Computing the pipeline coating stress voltage

Question
How to compute the pipeline coating stress voltage

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Answer

As you know, what Right-of-Way computes is the induced pipeline ground potential rise (GPR), either for load conditions (run single SPLITS) or for fault conditions (Monitor Fault), not the pipeline coating stress voltages, which is greatest concern to the pipeline industry. Of course, there is little difference between the induced pipeline GPR and the inductive component of the pipeline coating stress voltage if no mitigation (gradient control wire) is applied and the pipeline has a reasonably high coating resistance. Otherwise, the pipeline coating stress voltage can be only a fraction of the pipeline GPR. Obviously, if your mitigation design is based on the pipeline GPR rather than the coating stress voltage, it could be conservative, but also very costly.

For a safe and cost-efficient mitigation system design, the true pipeline coating stress voltages should be considered. To obtain the inductive component of the pipeline coating stress voltage, the following steps should be followed:

Step 1: Compute the pipeline GPR using Right-of-Way;

Step 2: Model a unit length (e.g., 1 km) pipeline in MALZ with the same coating, soil resistivity, etc. Make sure that a unit energization (e.g., 1 A or 1000 A) is applied to the pipeline and a computation profile just outside the pipeline is specified. The touch voltages and the pipeline GPR are calculated by MALZ. The touch voltages (which are also the pipeline coating stress voltages) expressed as a percentage of the pipeline GPR can be computed in this way.

Step 3: Apply this factor to the pipeline GPR obtained from Right-of-Way in the first step.

Please note that the above procedure provides only the inductive component of the pipeline coating stress voltage, i.e., the component of the pipeline coating stress voltages caused by magnetic field induction between the transmission lines and the pipeline. The conductive component of the pipeline coating stress voltages, i.e., the component arising from fault currents injected into the earth by nearby faulted transmission line structures during fault conditions must be computed separately. Then, the inductive and conductive components must be added together in order to obtain the total pipeline coating stress voltages during fault conditions.

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