The cathodic protection (CP) system for a subsea wellhead is designed to last the entire service life of the structure and subsystems. However, deviations from the initial design philosophy during the operational phase may occur.
These deviations include:
- More current drainage from the CP system to buried sections of the structures or connected systems, such as wellhead casings and suction anchors or connected flowlines and anchor chains
- Inactive anodes due to lack of electric connection to the structure
FiGS® baseline CP survey for a wellhead
The structure below is a wellhead not yet in operation, installed at the seabed, where two of the anodes were found inactive. Performing a FiGS® survey of a new structure installed at the seabed provides a reliable baseline status for the CP system. This can be used as comparison for future inspections and detailed risk based planning of surveys.
The buried section of the wellhead will drain current from the active anodes until these are consumed. Since two of the four anodes installed were found inactive, the active anodes would be consumed much faster than planned for in the design phase.
Results from the FiGS® survey and postprocessing of field gradient (FG) data through SeaCorr® modelling, enabled the client to evaluate replacement of the inactive anodes before the XMT and frame structure were installed, thus saving expensive maintenance in the future.
Current measurements performed with the FiGS® tool can be used to estimate the depletion of active anodes with SeaCorr® modelling as shown below. Estimation of anode depletion is inadequate using visual inspection and conventional CP probes. The anode depletion status is valuable information when planning a retrofitting / maintenance operation before the system starts to corrode and threat the integrity of the facility.
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