Improved Magnetic Treatments for Naval Vessels
T. M. Baynes, J. Smelt* and A. Bailey*
School of Physics, University of New South Wales, Sydney NSW 2052, Australia.
*Maritime Operations Division, Defence Science Technology Organisation, PO Box 44,
Pyrmont NSW 2009, Australia.
Abstract- Military submarines and surface ships are regularly subjected to a
treatment called "deperming" that seeks to design the vessel's permanent
magnetisation for optimal magnetic silencing. We have successfully extended results
from a scaled model of a magnetic treatment facility (MTF) to a working procedure
on a full-scale submarine. A novel "anhysteretic deperm" method tested in the
laboratory MTF compared favourably, in the laboratory and in vivo, to a standard
3-Stage deperm protocol used on naval vessels. Using a Preisach analysis of
hysteresis, a mathematical description of bulk magnetic changes that occur within a
deperm has been developed. With this analysis we propose to predict deperm
outcomes a priori.
Index terms: demagnetisation, anhysteretic, deperming, steel naval vessels.
I.
INTRODUCTION.
The magnetic detection of a naval vessel is of particular concern in relatively shallow
water where marine mines are present. The threat is exacerbated when a ship arrives in
such waters from a significantly different magnetic zone. This was, for example, the case
for Australian ships recently deployed in the Persian Gulf.
One countermeasure against magnetic detection is the placement of on-board degaussing
coils. During operation, these coils anticipate the ambient magnetic field about a vessel
and actively oppose that field, dependent on the orientation of the vessel.
Quite a separate strategy is used to account for the magnetic fields from within the
magnetic material of the vessel itself. Simply from being used, a ship or submarine
becomes magnetised due to mechanical stresses and an incessant exposure to the Earth's
magnetic field. This "permanent" magnetisation accrues over many months and is
difficult, if not impossible, to anticipate with degaussing coils. Consequently the
permanent magnetisation of naval vessels is regularly reset to a known value, or deleted,
in a procedure known as deperming.
II.
CURRENT DEPERM PROCEDURES.
Given that vessels regularly change bearing, the longitudinal or athwartship aspects of a
ship or submarine will have a varying orientation with respect to the horizontal
component of the Earth’s field. It is more useful to set the vessel's own magnetism in
these directions to zero. Regardless of the bearing, there is the constant effect of the
vertical component of the Earth's magnetic field and it is frequently useful to have a
specific permanent magnetisation in the vessel that will oppose the Earth’s vertical field
at the anticipated zone of operation. Therefore, the designed magnetisation is usually in
the vertical direction through the keel line.
A deperm treatment involves wrapping a copper cable longitudinally around the vessel (X
Coil) and “flashing” currents of up to 6000A through that cable. This is essentially a very
large solenoid with the vessel as the core. At the same time, the vessel is berthed inside a
second coil (Z Coil) that is used to apply an appropriate vertical field for the desired final
magnetisation (see also Figure 1). The berth is aligned North-South and the whole
apparatus is referred to (here) as a magnetic treatment facility (MTF).
X Coil
Z Coil
Figure 1. Schematic diagram of submarine prepared for deperming in a MTF.
The current flashes are alternated in direction, producing alternating magnetic fields.
However, there is a definite protocol determining the order and magnitude of these
applied fields called “Flash-D” [1]. In a Flash-D deperm the longitudinal and athwartship
magnetisations of a vessel are always removed. The specific vertical magnetisation is
achieved by first giving the vessel an excess magnetisation and then sculpting away the
superfluous magnetism using 2 extra stages of alternating applied fields in the X Coil and
varying applied fields in the Z Coil. (See Figure 2 next page)
The fact that Flash-D is a competent method for realising the objectives of deperming has
naturally entrenched the protocol in deperming manuals. Some variations in the detail
occur. For example, a technical manual on the magnetic treatment of ships published by
the United States Department of the Navy, Sea Systems Command [2], recommends that
the magnitude of successive X Coil current flashes in Stage 1 decrease by half the value
of their predecessors. This produces an exponential decay in the magnitude of Stage 1
current flashes. However, that procedure continues with a similar format to the Flash-D
protocol. Essentially the basic three-stage structure has remained the unquestioned
foundation of deperming for more than 30 years.
3 Stage Deperm
X Coil Field (A/m)
2000
Stage 1
Stage 2
Stage 3
1000
0
0
150
300
450
-1000
-2000
Time (mins)
Figure 2. Schematic diagram of applied fields in the X Coil during a typical 3 stage
deperm (Flash-D). A positive value indicates an applied field in the direction of
North. The applied vertical fields from the Z Coil are not shown.
III.
THE PROBLEM
Although Flash-D works well enough to satisfy the requirements of the Royal Australian
Navy (RAN), it is time consuming and therefore expensive: after the vessel has been
prepared, it can take anything from a day to a week depending on conditions. Sometimes
the resultant magnetisation from an initial deperm is unacceptable and the process has to
be repeated. Flash-D also involves a number of empirical steps without scientific basis
and the final magnetisation of the vessel can be unpredictable. The research undertaken at
UNSW in collaboration with the Magnetics Division of DSTO, Sydney, sought to address
these issues with the aid of a laboratory scale model of the MTF (see Figure 3).
Figure 3. (a) Laboratory MTF and (b) schematic view of control and data
acquisition system.
IV.
THE SOLUTION
In the laboratory we tested a number of alternative deperming methods. One of these was
based on a process, familiar to physicists, called anhysteretic magnetisation first studied
in the early part of last century. We refer to this here as the "anhysteretic deperm".
The anhysteretic deperm works by allowing a vertical bias field, from the Z Coil, to be
present during a sequence of decreasing, alternating applied fields in the X Coil (See
Figure 4).
Applied Field (A/m)
2000
Anhsyteretic Deperm
1000
0
0
150
300
450
-1000
-2000
Time (mins)
Figure 4. Plot of applied field delivered by X Coil during an anhysteretic deperm
with 40 current flashes and using a corresponding decrement of 50 A/m. The vertical
field is not shown but it would remain a constant value throughout the deperm.
At the end of this alternating field sequence, there should be some remnant vertical
magnetisation but no longitudinal or athwartship magnetisation. From theory we know
that the anhysteretic magnetisation is a single valued function of the vertical bias field.
Hence, we can reliably and repeatably choose the vertical magnetisation . Whereas FlashD requires 3 separate stages, the anhysteretic deperm involves only one. The outcome of
an anhysteretic deperm can be calculated beforehand, no Flash-D result has ever been
predicted. (In fact, for the Flash-D protocol, it has been found [3] such a calculation is
mathematically impossible.)
In the lab, the anhysteretic method proved to be much more reliable than Flash-D [4]. For
an anhysteretic deperm, requiring the same number of steps (of the same duration) as a
Flash-D, the standard deviation in the final vertical magnetisation was ~3 times less [5].
This implied that in the full scale trial a shorter duration anhysteretic deperm could be
used to achieve the same stability in the final magnetisation result. That is, no repeat
deperms would be required.
In the recent full-scale test the anhysteretic method did indeed perform as expected - there
was a significant ( > 50%) reduction in time taken to deperm the vessel and, for that
vessel class, a characteristic vertical anhysteretic magnetisation curve was derived
experimentally by repeating anhysteretic deperms for a range of applied fields in the Z
Coil (see Figure 5).
Final Vertical M agnetisation after Anhysteretic Trials
Relative VM after deperm
1.2
1
0.8
0.6
0.4
0.2
0
0
20
40
60
80
100
120
140
N et Applied Field from Z C oil
Figure 5 The (relative) final vertical magnetisation characterising the class of a 90m
submarine following four anhysteretic deperm trials. The net applied field is the
applied field generated by the Z Coil during the deperm, minus the vertical
component of the Earth's magnetic field. All results are displayed as relative to the
maximum vertical magnetisation achieved - actual values are not represented due to
the sensitivity of that information.
V.
MAGNETIC CARGO
One aspect of the anhysteretic deperm procedure that can not be anticipated with the
preceding characteristic curve, is the effect of magnetic cargo. From measurements on
vessel models it appears that the location of a magnetic source inside a vessel, is recorded
into the magnetic material of the hull during a deperm (see Figure 6). In fact, the local
magnetisation created in the hull near the source is such that it automatically compensates
for the field of the source, as measured from the exterior of the vessel. This has
immediate consequences for the order in which maintenance and deperming should occur.
It is preferable that all maintenance that may interfere with magnetic objects inside a
vessel be carried out prior to deperming. Should that order be reversed it is entirely
possible that the removal or replacement of the magnetic source will expose a localised
magnetic “image” in the hull
The Effect of Magnetic Components Inside the Vessel During Deperm.
(a)
Vertical magnetisation prior to anhysteretic deperm.
model submarine
engine block
combined
15000
M (nT)
10000
5000
0
-1
-0.5
0
0.5
1
-5000
-10000
-15000
Longitudinal displacement (m)
(b)
Vertical magnetisation after anhysteretic deperm.
15000
model submarine
engine block
combined
M (nT)
10000
5000
0
-1
-0.5
0
0.5
1
-5000
-10000
-15000
Longitudinal displacement (m)
Figure 6 (a) Magnetic profile along the length of a model submarine alone and
combined with a magnetised "engine block" before a deperm (blue and green
respectively) and the profile of the engine block alone (red). (b) Magnetic profile
along the length of a model submarine alone and combined with a magnetised
"engine block" after a deperm (grey and green respectively) and the profile of the
engine block alone (red). Note: after the deperm, the model alone has acquired a
magnetism that compensates for the presence of the more magnetic engine block.
VI.
CONCLUSIONS
We have tested a simple procedure for the magnetic treatment of naval vessels: the
anhysteretic deperm. In both the laboratory models and on a full-scale military submarine
we have found that the anhysteretic deperm is quicker, more reliable and more susceptible
to easy theoretical description than the incumbent Flash-D deperm. By performing several
anhysteretic deperms on the one vessel, using a range of applied fields, it was possible to
derive a final magnetisation response curve characteristic to all vessels in that class. From
this we can anticipate the result of any future deperm on vessels of that type. The only
impediment to the reliability of this system is the rearrangement of magnetic cargo after
deperm which should never be allowed as the position of magnetic cargo is recorded into
the hull during deperm. Removal or re-orientation of a magnetic source after deperm will
reveal this "magnetic image".
VII.
REFERENCES
1. British Standard BR 825, “Degaussing by Magnetic Treatment, Section 4: Deperming
and Magnetic Theory,” (U.K. Ministry of Defence, Jan. 1975).
2. United States Navy Technical Manual S5475-AL-PRO-010 “Principles and
procedures for magnetic treatment of ships” (Department of the Navy Sea Systems
Command 31st May 1985).
3 Baynes T. M. “Analysis of the Demagnetisation Process and Possible Alternative
Magnetic Treatments for Naval Vessels” PhD Thesis School of Physics UNSW, Sydney
Australia, February 2002.
4 Baynes T. M., Russell G. J. and Bailey A., IEEE Transactions on Magnetics, 38 (4) Part
II pp1753-1758 July 2002.
5 Baynes T. M., Russell G. J. and Bailey A., "Improving the method for demagnetisation
of naval vessels and Preisach analysis" Proc. MARELEC (CD ROM) July 2001.