Simulation and analysis of the magnetic
treatment of naval vessels
Tim Baynes1, Graeme Russell1 and Andrew Bailey2
1
School of Physics UNSW, Sydney, 2052 NSW, Australia
tmb@phys.unsw.edu.au, ph +61 2 9385 4561 fax +61 2 9385 6060
2
Maritime Operations Division, DSTO, Sydney.
PO Box 44 Pyrmont 2009 NSW, Australia ph +61 2 9692 1440 fax +61 2 9692 1560
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. A scaled model of a possible magnetic
treatment facility (MTF) has been used to investigate various aspects of the deperm
process. A steel tube model used in this laboratory MTF produced a good
qualitative simulation of deperm results for a submarine. Simulations of a novel
“anhysteretic deperm” method compared favourably to a standard 3-Stage deperm
protocol used on naval vessels. The influence of magnetic cargo on hull
magnetisation was also demonstrated to be of significance during and after
deperming. 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.
INTRODUCTION
The problem of induced and permanent magnetism in ships was noticed as far back as
J.C. Maxwell (1892). In fact, he included a whole section on “Ships Magnetism” in his
book A Treatise on Electricity and Magnetism Vol. II (1) including suggestions on how
minimise the perturbations on a ship’s compass. More recently the magnetism of vessels
has come under scrutiny from the viewpoint of military security. The deployment of
magnetic sensing marine mines around ports and major shipping lanes can cause great
damage to naval and merchant vessels. 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 important in avoiding magnetic anomaly detectors (MADs).
One defence against magnetic detection is the placement of on-board degaussing coils.
Their basic purpose is to anticipate the ambient magnetic field about a vessel and to
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. This permanent magnetisation accrues over many months
and is difficult if not impossible to anticipate with degaussing coils. Consequently the
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permanent magnetisation of naval vessels is regularly reset to a known value, or deleted,
in a procedure known as deperming.
DEPERMING
The objectives of deperming are to clear the magnetic history of a vessel and also to
establish a known magnetisation that is optimal for magnetic silencing. Note that this
may not necessarily be a demagnetised state. It is frequently useful to have a specific
permanent magnetisation in the vessel that will oppose the Earth’s field at the anticipated
zone of operation. Given that vessels regularly change bearing, the longitudinal aspect of
a ship or submarine will have a varying orientation with respect to the Earth’s field.
Hence, the deliberately retained magnetisation is usually designed to be in the vertical
direction through the keel line.
In preparation for deperming, the vessel is wrapped with a copper cable in a helical
fashion to form a rough solenoid around the boat called the X coil. The vessel is then
berthed inside another rectangular coil, a Helmholtz pair, called the Z coil (See Figure 1).
The berth is aligned North-South so that there’s no need to account for any component of
the Earth’s field athwartships. The whole apparatus is referred to (here) as a magnetic
treatment facility (MTF).
A standard protocol for deperming naval vessels is called “Flash-D” (2). This method
involves applying magnetic fields in the vertical direction through d.c. currents in the Z
Coil as well as applied fields, of alternating direction, in the longitudinal direction. The
latter are generated by pulses of d.c. current in the X Coil called "shots". Flash-D has
three distinct stages.
Figure 1. Schematic diagram of submarine prepared for deperming in a MTF.
2
Stage 1 is a series of alternating shots in the X Coil that decrease in magnitude by a set
decrement. The initial maximum shot is capable of achieving saturation in the vessel.
Concurrent with these X Coil shots is an applied vertical field oppositely directed to the
local magnetic field. The purpose of Stage 1 is to wipe the magnetic history of a vessel
and establish an excessive permanent vertical magnetisation (PVM).
In Stage 2 some of that excess PVM is deleted but this involves applying alternating
longitudinal fields of increasing magnitude in conjunction with the local field at the
MTF. The end result is the desired PVM but an excess permanent longitudinal
magnetisation (PLM) is a necessary, though unwanted, by product. What is more, the
magnitude of an applied field in the X Coil during Stage 2 depends on an empirical
formula that incorporates previous applied fields and the response of the vessel’s PVM to
those applied fields. (We believe that this second stage adds an unwarranted degree of
difficulty to the procedure and excludes the possibility of a complete theoretical model of
the magnetisation during, or after, a deperm. This problem is discussed in detail shortly).
Stage 3 uses the same principle as Stage 1 but with a finer decrement in the magnitude of
successive shots. The idea here is to remove the extraneous longitudinal magnetisation
but consolidate the PVM obtained during Stage 2 (see Figure 2 below).
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.
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 (3), recommends that
the magnitude of successive X Coil shots in Stage 1 decrease by half the value of their
predecessors. This produces an exponential decay in the magnitude of Stage 1 shots.
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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 many years. In reviewing the deperm process we have identified three key
issues of particular concern.
1. There seems to be a significant redundancy in the Flash-D procedure. The excess
PVM generated in Stage 1 is partially deleted, in Stage 2, by means of the vertical
component of the local Earth’s field and longitudinal applied fields of increasing
magnitude. This then begets a third stage to remove the by-product excess PLM. In
order to attain the desired final PVM is it really necessary to apply such a sequence of
longitudinal fields that are counter to the aims of achieving a minimal final PLM?
Additionally, the efficiency of Stage 2 relies heavily on the local vertical field, which
will of course vary with the location of the MTF. The ambient vertical field may be so
small as to extend the duration of Stage 2 or so large as to reduce the amount of
control the operator has on the decrease of PVM.
2. Perhaps as a result of this redundancy there’s the issue of the time it takes for a FlashD deperm. In the experience of the Royal Australian Navy (R.A.N), this is normally
at least 6 –7 hours (after the vessel has been prepared), but there is also the problem
of repetitions. Although all processes and equipment used at the MTF are automated,
on most occasions, for Australian ships, there has been the need for a manual
override. For a variety reasons, the final PVM has not been satisfactory, and the
whole deperm has to be repeated, effectively doubling the time taken.
3. There is no physical or mathematical model for predicting the final deperm result, nor
even the magnetisation in the vessel after a given shot.
The suggestion put forward here is that by applying a very specific vertical field during
the alternating shots of Stage 1, instead of acquiring excess, it should be possible to get
exactly the desired final PVM. Not only would this make the efficiency of the deperm
independent of the local vertical field but this single stage could also be modified so that
you simultaneously wipe out any magnetic history and reduce longitudinal magnetisation
to zero; potentially reducing three stages into one. This simplified deperm might also
consume less time than Flash-D, providing a solution to both items 1 and 2 above.
Additionally, the removal of Stage 2 alleviates the deperm process of the empirical
regressive algorithms that determine the applied fields in that stage. This permits, perhaps
for the first time, the possibility of using a theoretical model of magnetism to predict
beforehand what the PLM or PVM of a vessel will be at a given stage during the deperm,
including the final magnetic state.
The proposed single stage deperm resembles a well-known process called anhysteretic
magnetisation whereby a constant static field and an alternating field with decreasing
amplitude are applied simultaneously to a ferromagnetic sample (4). If the alternating
field strength is slowly reduced to zero the resultant anhysteretic magnetisation is a single
valued function of the static field. Hence the suggested name “anhysteretic deperm”.
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LABORATORY EXPERIMENTS
Physical Model of Deperming
Though some experiments were carried out directly on naval vessels, the primary tool
used in these investigations was a laboratory scale MTF. The “laboratory MTF” was
designed to be in proportion to the dimensions of the R.A.N’s MTF in Western Australia
on the 1/100th scale.
A variety of vessel models were tested in the laboratory MTF and, to accommodate these
with equanimity, we used a solenoid 0.5m long, diameter 0.11m with 200 turns/meter as
the X Coil. This was a departure from a strict simulation of the MTF but the advantages
were a more homogenous applied field in an arrangement that permited a far simpler
calculation of the fields applied in the longitudinal direction. The laboratory X and Z
Coils were arranged as at the MTF, so that a model vessel (made from steel tube) placed
inside would sit in the horizontal plane with its long axis orientated along magnetic
North-South (see Figure 3).
Figure 3. Schematic diagram of laboratory MTF with steel tube showing the
direction of longitudinal, athwartships and vertical axes and orientation relative to
magnetic North in the lab.
The magnetisation of a model vessel was measured remotely by a magnetometer passing
directly under the “keel” of the model. The raw data was then used to mathematically
model the magnetic source as an “extended point source” from which it was possible to
deduce magnetisation (see (5) for details).
A Flash-D deperm was performed on a model made from CA2S-E grade steel tube and
the resultant magnetisation measured after each step was compared to that measured
during a Flash-D deperm on a submarine.
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In consideration of the military significance of the results I am restricted here to
describing deperm results for a submarine with reduced data i.e. results for magnetisation
or magnetic moment have been divided by the maximum value measured in a given
deperm. Data from the deperms on steel tube were also reduced to this format so that
results from vessels and the vessel models could be compared.
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Longitudinal magnetisation
submarine
CA2S-E
M/Mmax
0.5
0
0
10
20
30
40
50
60
70
-0.5
Shot number
-1
Figure 4. PLM results from a Flash-D deperm on a CA2S-E grade steel tube
compared with the PLM response of a submarine to the similar applied fields.
Applied Field (A/m)
2000
Anhsyteretic Deperm
1000
0
0
150
300
450
-1000
-2000
Time (mins)
Figure 5. Plot of applied field delivered by X Coil during an anhysteretic deperm
with 40 shots 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. See text on
following page.
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Anhysteretic Deperm Trials
Given the successful laboratory simulation of the deperm process demonstrated in the
previous section, it was decided that the anhysteretic deperm could be first tested on the
steel tube models.
The outcomes of a number and variety of anhysteretic deperms were compared with
results for a number of deperms using the standard Flash-D method, on the same vessel
model. The scheme of applied fields delivered by the X Coil during an anhysteretic
deperm is shown in Figure 5 (previous page). The total number of shots in an anhysteretic
deperm depends on the decrement in the magnitude between successive shots in the X
Coil.
Final PVM
(a)
Standard deviation  (A/m)
250
200
150
100
50
0
0
20
40
60
80
100
Number of Steps
Final PLM
(b)
Standard deviation (kA/m)
5
4
3
2
1
0
0
20
40
60
80
100
Number of Steps
Figure 6. Standard deviation of (a) final PVM and (b) final PLM results with
respect to the duration of deperm in numbers of steps or shots taken. Black dots
show the standard deviation of the anhysteretic deperms and the open square that
of the Flash-D data. A line has been fitted to the anhysteretic data as a visual cue.
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Shown in Figure 6 are data for the standard deviation () in the final PLM and PVM in
the model vessel after 30 repetitions of each type of deperm performed. The results
indicate two things: that an anhysteretic deperm taking approximately 40 shots could
produce a final PLM with equivalent certainty to a Flash-D deperm but, for the same
anhysteretic deperm, the variation in the final PVM is improved threefold by comparison
with Flash-D.
Based on the positive results from these tests on the laboratory scale, a full-scale trial was
performed in May 2002 at the MTF in Western Australia. The specific results from the
five deperms executed there are a matter of military confidentiality but we can confirm
that the anhysteretic deperm does work at least as well as the Flash-D protocol and there
was an unexpected reduction in the time taken to complete the anhysteretic deperm.
THEORETICAL MODEL OF RESULTS
Apart from the possibility of the new deperm technique being more reliable and
technically simpler, there is also the advantage that established models of magnetism in
materials can be applied to the problem of predicting the vessel magnetisation during and
after a deperm. One such model was first developed in 1935 by Preisach (6). The
Preisach model begins by assuming that a given magnetic body is composed of parts that
are characterised by a square hysteresis loop with specific switching (or coercive) fields,
and these parts are distributed throughout that body.
.
Figure 7. In the Preisach model, magnetism is described by a distribution of parts
that are represented by square hysteresis loops. A given part will be switched to
magnetisation in one direction (+) or the opposite (-) depending on whether the last
applied field H > a or H < b respectively. Otherwise it remains in its initial state.
The distribution function, p(a, b), of parts defined by the values (a, b) may be found
experimentally for a specific magnetic body (7, 8). This distribution is then characteristic
of uniaxial magnetisation in that object whether it is a steel tube or a submarine.
Experimental distribution functions for both a steel tube and a submarine were generated
and used to determine the magnetisation of those respective objects in response to a
sequence of applied fields beginning with a saturating field. Retrospectively, it was
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possible to predict the PLM of a submarine after each shot of a prior Flash-D deperm (see
Figure 8 below).
Modelled Flash-D deperm
1.2
0.8
submarine
theory
M/Mstart
0.4
0
0
25
50
-0.4
-0.8
-1.2
Shot number
Figure 8. Reduced results for PLM in a submarine during a Flash-D deperm and
the theoretical predictions from the Preisach model derived experimentally.
Note that the above calculation was performed with the hindsight of knowing what fields
were applied and when. Actually during a Flash-D deperm it is a non-trivial problem to
know what the applied fields of Stage 2 will be beforehand. However the applied fields of
an anhysteretic deperm are entirely predetermined and so, it is theoretically possible to
predict the outcome of an anhysteretic deperm on a submarine a priori given the
characteristic Preisach distribution function for that vessel. This has been done for a steel
tube and the extension of the theory to the full-scale is the focus of our current research.
MAGNETIC CARGO
One aspect of the anhysteretic deperm procedure that can not be anticipated with the
preceding theoretical model, is the effect of magnetic cargo. From preliminary
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. In fact, the
local magnetisation created in the hull near the source is such that it automatically
negates 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.
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CONCLUSIONS
We have demonstrated the effective simulation of the deperm process on laboratory scale
equipment. This has subsequently been used to investigate a number of aspects of the
deperm process. Simulation of an alternative deperm protocol based on anhysteretic
magnetisation demonstrated that the “anhysteretic deperm” was both practicable and that
it has a comparative advantage over the Flash-D method. Full-scale trials concur with
these results. The anhysteretic deperm is also available to existing theoretical analyses
whereas (Stage 2 of) a Flash-D deperm involves a regressive algorithm to calculate some
applied fields – this is a serious impediment to any predictive model. The Preisach model
of magnetic hysteresis was employed to calculate the response of both a steel tube vessel
model and a submarine to deperming. There is excellent agreement between the
theoretical results and actual deperm data from a Flash-D deperm on submarine.
Magnetic cargo has also been shown to generate a localised magnetisation in a model
vessel hull during deperming. Consequently it is recommended that deperming only
occur after any maintenance that might interfere with magnetic cargo.
REFERENCES
1. Maxwell J.C., “A Treatise on Electricity and Magnetism”, Vol. II 3rd Ed. Article [441]
“Ship’s Magnetism” (Oxford University Press, 1892) pp74-78.
2. British Standard BR 825, “Degaussing by Magnetic Treatment, Section 4: Deperming
and Magnetic Theory,” (U.K. Ministry of Defence, Jan. 1975).
3. 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).
4. Néel L., Cahiers de Physique Vol. 17 1943, p47.
5. Theobold A.G., "On the numerical modelling of a magnetic source" ARE TR 86113
Accession Number 73823, (Admiralty Research Establishment U.K., July 1986).
6. Preisach, F., Zeitschrift für Physik, Vol. 94 1935, pp 277-302.
7. 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, Chapter 6.
8. Del Vecchio R. M., IEEE Trans. Mag., vol. 16, 5, (1980), pp 809-811.
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