Underwater surveys of sacrificial anodes mounted on complex structures sometimes include field gradient measurements, for example made by stabbing an anode using a field gradient probe that has two reference electrodes.

Typical practice is for survey companies to convert the potential difference between the two reference electrodes of the field gradient probe into an estimated anode output, but there is a risk that estimates may be inaccurate if the method used does not appropriately consider factors such as:

  • The stab having been made away from the middle of the anode
  • The probe not having been oriented perpendicular to the surface of the anode
  • The structure around the anode leading to distortion of the electric fields, for example because there is a coating defect on the structure and to one side of the anode, or the structure is not symmetric around the anode
  • The anode being close to other anodes.

Using field examples and numerical simulation of a digital twin model, this paper investigates the effect of the above factors on measured field gradient. Furthermore, this paper demonstrates how field gradient measurements, made with probe orientations which do not correspond to electric-field fluxlines or which are not central on an anode, can nevertheless be used to estimate anode output. The paper seeks to provide reference material which can be used to simplify the estimation of anode output from measured field gradients.

Key words: field gradient, probe, simulation, digital twin

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