In times of conflict, the deployment of magnetic mines around ports and major shipping lanes can cause great damage to naval and merchant vessels and paralyse essential trade. In order for a ship to pass safely it is necessary to camouflage the vessel from magnetic detection. That is, it must be “magnetically silent”. This is especially important with detail such as mine-sweeping and marine mine evasion but also generally useful in avoiding magnetic anomaly detectors (MADs).

Naval submarines and surface ships are regularly subjected to a treatment called “deperming” that seeks to design the vessel’s permanent magnetisation for optimal magnetic camouflage. However, the incumbent deperm procedure is governed by empirical rules, it is time consuming and there is no existing model for predicting the magnetisation of a vessel during or after deperming. In fact, it has been shown in this project that, because of the empirical rules employed, no theoretical estimate of a deperm outcome can be made a priori.

To address these issues we have established a laboratory scale magnetic treatment facility as a successful simulator of the deperm process on large naval vessels (see Figure 1). Using this facility we developed alternative deperm protocols based on the physics of anhysteretic magnetisation and a mathematical model of magnetic hysteresis applicable to naval vessels.

Figure 1. Oberon Class submarine in the MTF at HMAS Stirling in Western Australia. The laboratory apparatus was approximately 1/100th scale.

The alternative “anhysteretic deperm” was found to produce a more reliable deperm result than the current method: a feature which has the potential to reduce the time requirement for a deperm by 50%. The anhysteretic deperm also had the advantage of being grounded entirely in physical principles. A variation of the Preisach model for hysteresis was used to compute magnetisation changes both for the laboratory model and for a submarine; the agreement with measured data in both cases was excellent (see Figure 2). These results possess the additional synergy that magnetisation during and after an anhysteretic deperm can be easily calculated using the derived model.

Figure 2. Reduced results for longitudinal magnetisation (M) in an Oberon class submarine during a standard deperm and the theoretical predictions from the Preisach model. The values for M follow the sequence of alternating applied fields.

Tim Baynes, Graeme Russell and A. Bailey