Page 54: of Offshore Engineer Magazine (Sep/Oct 2024)

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TECH FILE ARGEO LISTEN with anodes can be buried or be exposed above the sea?oor. The electric ?eld measurements and output current estimations work equally well for buried pipelines.

The burial depth and positions of the anodes along the pipeline are determined directly from a combi- nation electric and magnetic data.

Once the electric currents go- ing out and into the pipeline have been estimated, the anode material consumption rate can be calculat- ed. In fact, the anode output cur- rent is proportional to the material loss rate. With this information

Figure 2. A typical AUV electric field measuring configuration together with camera pictures, the along a pipeline.

status of the CP system can be de- termined. Damages are revealed and the amount of consumed an- ode material is determined.

The accuracy i.e. the error in the consumption rate estimation is highly dependent on the SNR. Ide- ally, a 30 dB SNR is necessary for suf?cient accuracy. This require- ment determines the maximum sail line altitude above the pipeline given a typical noise level in the measured data. With the Argeo

Listen system it is possible to ?y the

AUV 13-15 m above the pipeline and still reach an SNR of 30 dB when the output current is 0.1 A as

Figure 3. Noise and altitude requirements for the measured can be seen in ?gure 3. However, electric field data after processing given a 0.1 A anode output to resolve output currents down to electric current amplitude.

0.02 A, the recommended AUV height above the pipeline is set to knots. This is illustrated in ?gure 2 where the CP resulting 5-10 m. This altitude range also allows for good multibeam electric ?eld amplitude and direction are visualized as well echo sounder and camera/laser coverage along the pipeline.

as the AUV on a survey line on top of the pipeline (black line on the sediment). A CP data case

The electric currents going out from each anode into the The electric ?eld data along a pipeline shows the behav- seawater, are obtained from solving a linear system of equa- ior of the CP-system. The anodes generate output electric tions which relates the electric current going in and out from currents into the seawater if they are active. The amount of the pipeline with the measured electric ?eld. The low elec- current depends on several things like the remaining mate- tric ?eld noise and high accuracy of navigation data results rial in the anode, coating of the pipeline, i.e. return points in accurate estimations of the electric currents. The pipeline for the electric current into the pipeline etc. The sacri?- 54 OFFSHORE ENGINEER OEDIGITAL.COM