Investigations of the magnetic fields from ships due to corrosion and its countermeasures

Allan, Peter J (2004) Investigations of the magnetic fields from ships due to corrosion and its countermeasures. PhD thesis, University of Glasgow.

Abstract

Corrosion is one of the main concerns for the shipping industry. The enormous cost associated with the corrosion of ships means that operators are more than ever seeking methods to provide optimal protection. Another problem which results from the corrosion of ships is that the electric currents produced give rise to a corrosion related magnetic field which adds to the ships overall electromagnetic field which can trigger underwater mines. Hence for the Admiralty to protect their fleet, they need to understand, predict and reduce these electromagnetic signatures. This will also be a concern for operators of merchant ships if, as recently suggested, terrorists begin to target them. There are a number of factors contributing to the electromagnetic signature of a ship. The work presented in this study is concerned with the magnetic field arising due to corrosion and the countermeasures employed against it (namely Impressed Current Cathodic Protection (ICCP) systems]. This field is called the Corrosion Related Magnetic (CRM) field. Since this field is directly related to the rate of corrosion of a ship, it may also be used as an indicator as to the efficiency of the corrosion protection measures, This work presents a range of techniques which may be used to model the CRM field. Simple dipole models are used to investigate the general behaviour of a corroding ship in sea water. These investigations result in an expression for an approximation of the CRM field from a dipole-wire configuration which can be used to estimate the CRM field from a corroding ship with an active ICCP system. A second, more detailed, method was then developed using the Boundary Element Method (BEM) to model a ship corroding in a tank of sea water. The BEM uses a large set of simultaneous equations, the coefficients of which are calculated by numerical integration using a new method based around the moments of triangular surface elements which discretise the boundary of the domain. By using a Point Successive Over-relaxation Method (PSOM) to solve the BEM equations, the electric potential and its flux are calculated for each discrete element on the boundary. These potentials are then used to evaluate the CRM field within the domain. Rather than use the Biot-Savart Law for this purpose, a new method is presented which removes the requirement for an internal volume mesh of the domain, allowing the CRM field to be calculated directly from the electric potentials of the surface elements. To test this method, results are presented for a tank of sea water with a hull located on the top surface and an appropriately placed propeller. Calculations were performed for an unprotected propeller and the hull with various degrees of protection from corrosion offered by paint coatings and an ICCP system. Results from the "exact" method are in good agreement with those from the simple dipole model and although there were no published results of the kind produced by this research available for comparison, published results from similar studies of related topics appear to further support the results presented. These results indicate that the method developed is capable of modelling all the key characteristics required for a detailed analysis of an actual corroding ship, with protective paint coatings and ICCP system, in a physical environment.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: A Watt
• Keywords: Materials science, Naval engineering
• Date of Award: 2004
• Depositing User: Enlighten Team
• Unique ID: glathesis:2004-74050
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 23 Sep 2019 15:33
• Last Modified: 23 Sep 2019 15:33
• URI: https://theses.gla.ac.uk/id/eprint/74050