Damage Estimation of a Ship's Hull using ICCP system Measurements

Abstract

Most ocean-going ships are fitted with an impressed current cathodic protection (ICCP) system. The currents impressed by this system onto the seawater find their way to corroding parts of the ship’s hull, and prevent the hull from corroding. The electric field resulting from the currents around the ship is called the electric signature. This signature is a threat in seas with mines, as mines explode when they detect this signature. Therefore controlling the signature is important, especially to naval ships. In order to be able to control the signature the location and size of corrosion damage needs to be known. Today no method exists for corrosion damage estimation on a sailing ship. This study aims to develop a method for corrosion damage estimation using only measurements of an ICCP system. The research question is: To what accuracy and with what measurements can the damage distribution on a ship’s hull be reconstructed? The quality of the damage reconstruction is evaluated by how well the signature is estimated. First an analytical model is used to explore the theoretical limits of damage reconstruction. This shows damage reconstruction is limited by the number of measurement electrodes. Then damage reconstruction by numeric model inversion is attempted, but is shown to be unattainable. The proposed solution is damage reconstruction by repeatedly calculating for different damage configurations, comparing the calculated measurements to the true ones, and iterating until an optimum is found. This method works well on a ship geometry if 140 measurement sensors are used. In order to verify the damage reconstruction algorithm a scale model facility is developed. The produced field sensors show shifting offsets that are not explained, but good linearity with the electric field. Simulations and measurements with a calibration source show good similarity, but a factor of 1.6 difference that is not explained. Test with sacrificial anodes show large variations over time and a large dependency on the sailing speed. The results show that corrosion damage reconstruction using only on-ship measurements can be achieved. The resolution of damage reconstruction is limited by the number of measurement electrodes, at least one hundred are needed for the signature to be resolved. Current ships have between one and eight measurement electrodes; it is recommended future ships will have at least one hundred. These results furthermore indicate that time-dependent effects should be included in the physical model used for the simulations.