Localisation of Submarine Power Cables by Magnetometers on REMUS 100 AUV
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- Institutt for marin teknikk 
Power cables that are exposed to the marine environment are prone to damage, especially if they are located close to, or above, the seabed. Underwater surveys are carried out to ensure that cables are buried and inspect for signs of damage. Today, Statnett SF perform surveys by ROVs and underwater cameras, this is both tedious and expensive. Using AUVs to locate and track the cables based on their magnetic field, could lead to cost reductions. Several commercial products exist for this purpose, however, most of them are developed for telecommunication cables and they are not always applicable for three-phase power systems. In this thesis, a mathematical model is derived for the magnetic field around a three-phase power system, and verified by comparison to finite element analysis. Further, the model is used to design and implement a method to track and locate all cables in a three-phase power system. Both simulations and experiments confirms that the method is accurate when locating the cables. For cable tracking, the method is accurate when on either side of the three-phase power system. When in-between the cables, there is a risk of a wrong direction being given. Nevertheless, it is shown that this do not happen either for simulations or experiments. Today's commercially available solutions for cable tracking, locates the cables in a three-phase power system by assuming the closest is an independent cable. This method give erroneous results when the cables are too close. By simulations it's concluded that erroneous results occurs when the distance between the cables is less than 8 times the AUVs hover height over the same cables. This is, however, dependent on cable specific parameters and the accuracy of the magnetometers. A device for carrying out magnetic-based cable survey is developed and implemented on REMUS 100 AUV. This includes a method to locate single cables, cables in a three-phase power system as well as a calibration routine for the magnetometers. All experiments are carried out with this device and the results from these concurs with that of the simulations. Lastly, it is shown that the calibration routine successfully calibrates for ferromagnetic distortions as well as for misaligned sensors.