Study On Measuring And Preventing Methods For Corrosion Electromagnetic Field Of Ships

Vessels in the seawater, a strong corrosive medium, will inevitably suffer a corrosion problem. In view of this, a current cathodic protection system is then commonly adopted to handle this problem onboard. However, the application of the cathodic protection system unfavorably creates a corrosive electrostatic field, and an ELF (extremely low frequency) electromagnetic field around the vessel due to the current of protection systems. The ship corrosion electromagnetic field brings the threat to the ship security. Therefore, it is important in military significance to study characteristics of corrosion electromagnetic field of ships.The main work of the dissertati physical physical on consists of study on characteristics extraction of shaft-rate modulated electric field and method preventing from corrosion electrostatic field and ELF based on the measurement system with virtual instrument developed in the lab.Because the strength of electric field with ELF is getting weaken very quickly as the propagation distance increases, it is easily disturbed by the ambient noise. A new method by using a spectral line enhancer on harmonic wavelet is put forward to improve the long-range detectability of shaft-rate modulated electric field with weak strength. The new method is applied to processing the measured data of the shaft-rate modulated electric field collected in the tests of the physical scale model of ships.Regards of shortcoming of harmonic wavelet with slow decay speed in time domain, a method of smooth processing the harmonic wavelet in the frequency domain is proposed to get improvement generalized harmonic wavelet with better properties. The characteristics extraction of shaft-rate modulated electric field is realized by using the improvement generalized harmonic wavelet. In order to "indefinitely subdivide" the whole frequency band self-adaptively to pick up some valid frequency components, a self-adaptive harmonic wavelet algorithm with optimum wavelet packet basis is studied in this dissertation.In addition, the effectiveness of hull corrosion protection is related to the design of cathodic protection system .Therefore, the double-zone cathodic protection system is proposed in the dissertation. The optimization of the double-zone cathodic protection system is carried out in order to realize the hull corrosion protection as well as reduce the strength of corrosion electrostatic field. Furthermore, base on active earth grounding system using singnal processing voltage compensation is proposed to reduce the strength of the shaft frequency electric field.In the light of similarity theory, a physical scale model of vessel electromagnetic field is studied. A 1:100 physical scale model is designed and various characteristics of the shaft-rate modulated electric field under different r.p.m. are extracted based on the model.At last, according to concept of virtual instrument an electric field measurement system by using three dimensional electric field sensors is developed in the Lab such that the accuracy and speed of measurement for corrosion electrostatic field and the ELF shaft-rate modulated electric field of ships is reasonable.