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http://hdl.handle.net/1947/4085
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| Title: | Laboratory simulation of the deperm process |
| Report number: | DSTO-TR-1340 |
| AR number: | AR-012-438 |
| Classification: | UNCLASSIFIED |
| Report type: | Technical Report |
| Authors: | Baynes, T.M.
Russell, G.J.
Bailey, A. |
| Issue Date: | 2002-08 |
| Division: | Maritime Operations Division |
| Abbreviation: | MOD |
| Release authority: | Chief, Maritime Operations Division |
| Release limitations: | PUBLIC RELEASE |
| Announce limitations: | Public Announce |
| Task sponsor: | DGMD |
| Task number: | NAV 00/039 |
| File number: | 490/6/124 |
| Pages or format: | 18 |
| References: | 13 |
| DSTORL/DEFTEST terms: | Mine countermeasures
Naval mine warfare
Magnetic signatures |
| Abstract: | The practise of demagnetising or “deperming" ships and submarines in order to evade magnetic detection has been around since World War II. The methods used rely somewhat on empirical knowledge and little research has been done to analyse the magnetostatic processes occurring during a deperm. We have constructed a scale model of a possible magnetic treatment facility for performing deperms on small (<1 m) steel samples and measuring magnetostatic quantities that relate to the deperm procedure. The apparatus was found to be capable of measuring fundamental magnetostatic quantities remotely, with a comparable accuracy to direct magnetometric equipment. Additionally, a favourable comparison with deperm data from real naval vessels confirms that the scaled apparatus permits a valid simulation of conditions during an actual deperm. The system of measurement presented can thus be employed to investigate the magnetostatics of the current deperm method and possible alternatives. Important qualitative differences between the deperm results for ships and those for submarines were noted though it was still possible to achieve effective simulation of both of these in the scaled deperm apparatus. |
| Executive summary: | In 1994 the RAN Magnetic Treatment Facility (MTF) was commissioned into service following the successful deperm of the Oberon Submarine HMAS Orion. Since then the MTF has been used to successfully deperm several other classes of vessels including a Collins Class submarine, ANZAC Frigate, Fremantle class Patrol boat and a decommissioned Hydrographic ship. Deperming is a one-off treatment, performed perhaps on an annual basis that involves altering the permanent magnetism in ships or submarines. The aim of deperming is to erase any magnetic history, minimise permanent longitudinal (bow to stern) magnetisation (PLM) and to optimise permanent vertical magnetisation (PVM) for the anticipated region of operation. The latter objective involves donating a specific PVM that is designed to counteract the induced vertical magnetisation (IVM) in the relevant magnetic zone of the Earth's magnetic field. This will aid those degaussing coils concerned with minimising vertical magnetisation, and consequently reduce the amount of power required by them.
Despite the proven capability of the MTF, it was obvious that the whole deperm procedure had been formulated over the years by an empirical process with little knowledge of the underlying physics behind the procedure. In fact the procedure of deperming is still considered to be a "Black art'. In practice, there was often little confidence at the start of each deperm that the final signature of the vessel would match the desired criteria to any degree of accuracy. This was especially so for the initial treatment of a 'First of Class' ship or submarine. In practice, past experience was used to estimate the necessary treatment schedule for each new class of vessel and if the resultant signature was not of an adequate nature, then adjustments were made to a few parameters based on the results of the previous treatment. Although this does seem to be quite unsatisfactory, it normally would only take two treatments for each new class of vessel to reduce the magnetic signature to a level that was at least 70% lower than that of the untreated value. Despite the obvious success of the MTF, there was an obvious requirement to set up a program of work to look at the whole deperm procedure on a fundamental basis and hopefully gain some insight into the physics of the process with the ultimate aim of developing a more reliable, predictable deperm procedure. To this end a scholarship was set up with the University of NSW where a suitable student would study deperming on a laboratory scale leading to the production of a PhD thesis.
In this report, a detailed description will be given of the set up of the laboratory MTF that was designed to simulate accurately the layout and procedure of the RAN MTF but on a much smaller scale «1m). Initially, it was important to establish that the apparatus was capable of measuring fundamental magnetostatic quantities remotely, with a comparable accuracy to direct magnetometric equipment. Additionally, a favourable comparison with deperm data from real naval vessels confirmed that the scaled apparatus permits a valid simulation of conditions during an actual deperm. The system of measurement presented can thus be employed to investigate the magnetostatics of the current deperm method and possible alternatives. Important qualitative differences between the deperm results for
ships and those for submarines were noted though it was still possible to achieve effective simulation of both of these in the scaled deprm apparatus. |
| Appears in Collections: | DSTO Formal Reports
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