AUV Integrated Cathodic Protection iCP Inspection System – Results from a North Sea Survey

Abstract

The OFG AUV non-contact integrated Cathodic Protection (iCP) inspection system enables a fast and reliable approach to monitoring the state of cathodic protection systems on subsea pipelines. By measuring the electric field, the system monitors directly the change in electrical currents in the pipeline due to anodes or damage. This approach allows for improved monitoring of anode energy remaining, predicts anode end of life earlier than stab methods alone and pinpoints problem areas on the pipe that need attention. When combined with the camera imaging, multi-beam measurements, synthetic aperture sonar (HISAS), and chemical sensors, the OFG AUV iCP system provides a compelling set of measurements for pipeline cathodic protection monitoring and pipeline inspection. The advantage of the OFG AUV iCP system over traditional ROV CP systems is the speed with which these surveys can be undertaken, coupled with the dramatic increase in sensitivity which is approaching 100 times the sensitivity of the traditional ROV CP survey systems. Ocean Floor Geophysics (OFG) has a successful history of developing numerous magnetic and electric field instruments for ROVs, AUVs and deep-tow systems. Based on the success of these programs, and in collaboration with ISES Technical Services (ISES), OFG & ISES have developed an electric field measurement system which mounts onto a pipeline inspection AUV. Initial testing of the system in 2017 on OFG's 3000m depth rated Hugin AUV "Chercheur" demonstrated that the intrinsic electrical noise of the "Chercheur" with motors and all survey sensors running, was well below the threshold needed to successfully measure CP signals from the AUV. This was further supported through the development of a first principles model of the electrical fields generated around a generic cathodically protected pipeline. These supporting measurements and calculations led to the first operational field test of the system on a North Sea pipeline in April 2018. During these trials we were able to demonstrate the responsiveness of the system to the cathodic protection currents in the pipe at a variety of ranges and orientations up to 10 meters distance from the pipe. Furthermore, by performing repeated surveys of sections of the pipeline in both directions, we were able to confirm the repeatable detection of extremely small electric field gradient signals (less than 0.025μVrms/cm difference). The results show levels of sensitivity and detection hitherto unattainable using any other currently available CP survey method. These measurements were taken concurrently with all the other onboard survey sensors running, including the HISAS, Sub-bottom, Multi-beam, Camera system, and USBL, while running at the nominal survey speed of the vehicle of 3-4 knots.