Numerical Simulation and Analysis of Electromagnetic Fields Induced by a Moving Ship Based on a Three-Layer Geoelectric
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Numerical Simulation and Analysis of Electromagnetic Fields Induced by a Moving Ship Based on a Three-Layer Geoelectric Model SHAO Guihang1), and LI Yuguo1), 2), * 1) College of Marine Geoscience, and Key Laboratory of Submarine Geosciences and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao 266100, China 2) Evaluation and Detection Technology Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China (Received September 18, 2019; revised February 26, 2020; accepted April 18, 2020) © Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2020 Abstract In this paper, we present a numerical simulation method of electromagnetic (EM) fields induced by a moving ship (EMFMS), which consist of both the shaft-rate EM field and the static EM field. The shaft-rate EM fields in the frequency domain are first obtained by solving the partial differential equations together with suitable boundary conditions, and then they are transformed into the time domain by using the inverse Fourier transform. Finally, the static fields are added to obtain the EM fields of a moving ship. The effects of the source current intensity and the source position on the EM fields of a moving ship are discussed in detail. A field example of EM response of a moving ship is presented and its characteristics are analyzed. Key words moving ship; shaft-rate EM field; static EM field; numerical simulation
1 Introduction In order to prevent seawater corrosion, ships are often equipped with cathodic protection devices. The currents produced by cathodic protection devices usually form two circuits as shown in Fig.1 (Jeffrey and Brooking, 1999). The one flowing through the ship’s propeller is modulated by the varying bearing resistance, and generates shaft-rate electromagnetic fields (Holtham et al., 1999). The other flowing through the ship’s coating damage point generates static electromagnetic fields (Nain et al., 2013). Thus, the electric and magnetic fields induced by a moving ship (EMFMS) consist of both the shaft-rate field and the static field. The study of ship’s EM fields began in the 1960s (Zolotarevskii et al., 2005), and many studies on EMFMS have been conducted since then (Holmes, 2006). In these studies, however, simulation problems are often simplified. For instance, the geoelectric model is designed as an airsea two-layer model (Sun et al., 2003; Lu et al., 2004; Liu et al., 2004; Zhang and Wang, 2016), in which the current source of a moving ship is assumed to be equivalent to a horizontal electric dipole (Lu et al., 2005; Ni et al., 2006), or the shaft-rate EM fields are neglected (Bao et al., 2011; Li et al., 2012). Although these simplifications can reduce the complexity of numerical simulation, they donot sufficiently simulate the real situation. Therefore, three types * Corresponding author. E-mail: [email protected]
of problems can be caused by the simplifications. Firstly, in shallow water areas, the seafloor sediment
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