Signatures in Design

MAST2012
SC: Public
MAST-C-002-A
KTO/9/11/12
Pages 6
Signature Assessment
as an Integral Part of
Navy Ship Design Process
Kristian Tornivaara
Surma Ltd, Helsinki, Finland
[email protected]
Abstract
Different signature aspects are considered to be important features when new navy
combatants are designed and procured. The traditional methods to assess different
signature related phenomena are very time consuming and the manual work required to
build up the computer models exposes the process to errors. Separate assessment of
each different signature type is also frustrating when the design changes during the ship
design process.
With examples of above surface radar signatures and underwater electromagnetic
signatures, this paper is an introduction to how ship designers can get information
related to signature levels. This also ensures that the designers can be aware of the
effects the design modifications have on the signature levels. This integration is achieved
utilizing a common ship design product model which serves as an information source to
all signature assessments.
Now all the assessments related to different signature phenomena can be rather quickly
assessed and the certainty that they represent the same design is guaranteed. Avoiding
unnecessary modeling work the signature experts can concentrate on the real issues.
Surma Ltd
P O Box 9 7 0
FI -0 0 1 0 1 H els inki
s [email protected] urvivability.fi
.p: +3 5 8 5 0 5 0 3 5 8 5 0
V AT no: FI 2 0 9 6 8 9 3 2
BI C : N DE A FI H H
I BA N : FI 7 4 1 3 0 9 3 0 0 0 2 1 5 2 5 1
.www.s urvivability.fi
.f: +3 5 8 5 0 8 0 5 1 5 0 0 7
MAST2012
SC: Public
Page 2/6
MAST-C-002-A
KTO/9/11/12
Background
As a natural continuation to the series of papers presented in MAST conferences by
members of Surma personnel this paper widens the scope of utilization of the one model
concept introduced before. Previously the main focus has been in the vulnerability
assessment. Now another crucial aspect of comprehensive survivability assessment,
namely susceptibility, can also be evaluated using information derived from the design
model.
As an introduction to our one model concept it is recommended to go through the paper
presented in MAST Americas 2010 “COTS Ship Product Model as a Backbone of Modern
Naval Design Process” and “A Modern Product Model as a Simulation Integration
Platform”.
Traditional Methods
Currently most of the assessments related to susceptibility or signatures in general
require a lot of labor when each of the different phenomena, RCS, IR, EM or acoustics,
just to name a few, are analyzed using separate computer models. These processes lead
to two main obstacles that inhibit the information from being efficiently usable in modern
ship design process 1. The results from these analyzes come usually in months rather than days. This
kind of time span is too long for a normal iterative ship design process.
2. Due to the complexity of these phenomena the interpretation of the results
requires an expert of each field and the designers cannot really benefit or learn
that much from the assessment data.
Integration into Design Process
To enable a successful and efficient integration of signature assessment into design
process it is necessary to eliminate the above mentioned problems of the traditional
methods. In other words it is mandatory to reduce the turn-around time of the analyzes
and also to receive the results in such a form that a designer working on the ship project
can effectively use them to improve the design. Both of these goals can be achieved by
tuning the working methods towards the one model concept.
The time consumed for analyzes can be reduced enormously by automatizing the model
creation for the analysis software. Currently a noticeable amount of the working time in
analysis process is taken by the building and maintaining these computer models.
Depending on the signature phenomena at hand, this can take up to more than half of
the actual working hours per assessment. Automation can be achieved by creating
interfaces from the design software to the analysis software. In many cases this is quite
an easy task to complete keeping in mind that most of the design software have various
output formats and usually the analysis software support some ascii formatted input.
An alternative method to make the assessment process faster for the designers is to
simplify the assessment calculations. In many cases, the analysis tools have been
created to support a scientific approach and the detailed research of the phenomena.
This approach is good in cases when exact results are needed. However, the
requirements from ship designers' perspective are more or less different. Quite often just
a sophisticated indication of whether one solution is better than another is sufficient, and
usually the exact level of signatures sensed from all directions is not necessary. This
means that if the results are accurate enough with a faster analysis code, this serves the
design purposes better.
MAST2012
SC: Public
Page 3/6
MAST-C-002-A
KTO/9/11/12
To make sure that the designers can improve their design after getting the results, the
produced analysis reports must be short and easy to comprehend. If the designer can get
just a few values - for example, the maximum level of a certain signature or the level of
a signature detected from a specified sector - then it is much easier to aim at lowering
these values and improving the overall design signature-wise.
Radar Cross Section Calculation
As an example of the above surface signatures, RCS is selected for introducing the
solutions aiming to integrate the signature assessments into the design process.
Besides eyesight, the usage of radar is one of the most traditional methods to gain
information on the marine traffic. Even nowadays it still is the most common method to
control the traffic at seas. In almost all the coastal area the traffic is observed with land,
air and sea based radar stations. As remaining undetected is the main purpose of
signature reduction the minimization of radar cross section has also been one of the most
utilized methods when controlling the signature levels. The design variables aiming
towards lower radar cross section levels are usually the outer geometric shape and
selection of absorbent surface materials.
Current methods
There are numerous approaches and methods in use world wide. Most of the navies use
their in-house tools, or ones developed with other governmental institutes, which they
rely on. Generally these tools have also gone through a thorough validation process.
Unfortunately most of these applications, as well as many other software assessing any
of the signature related features, require their own computer models, which are created
only for this specific purpose. This means that as such, they can hardly serve the rapidly
evolving design process. Fortunately however, most of these tools can read in some sort
of ascii or some other format geometry file and more than this, most of the software also
can read the definitions of surface materials from these import files.
Figure 1: RCS analysis process
One model approach
For the RCS calculations, the Finnish Navy is using a software tool called CAST. The
algorithms this software use are developed at VTT, The Technical Research Centre of
Finland. These algorithms utilize Antenna Theory applied to Physical Optics (APO) in
combination with the Physical Theory of Diffraction (PTD) to predict the radar cross
section. The method takes into account multiple reflections and material properties. Like
many other similar applications, it can also import models from triangulated geometry
files containing the material properties as well.
MAST2012
SC: Public
Page 4/6
MAST-C-002-A
KTO/9/11/12
There is an existing interface from
a well known ship design software
to this analysis tool. By using this
interface in the design process it
is really easy to get an analysis of
the influence of any change to the
outer geometry of the ship. Also
the usage of absorbing materials
in the outer surfaces can be
optimized – yielding trade offs like
added weight versus the amount Figure 2: FAC Hamina-class triangulated model
imported to CAST
of gained radar camouflage.
Advantages
To support the designers in fast evolving iterative design project, the analysis results can
be made short and clear by stripping the non-relevant detailed information out of the
report. For example, to keep some desired stealth level it can be agreed for example that
an index presenting the total radar signature level is calculated by the analysis software
by weighting different angles and wavelengths and summing up the results to form a
single value. This way the designer is able to check that any change made to the outer
geometry stays below the required acceptance criteria.
Electromagnetic Signatures
In this paper, ferromagnetic signatures are selected as an example of under water
signatures.
When a ship constructed of steel or other ferromagnetic material is moving on the
surface or under water, by its very nature, it develops a detectable local disturbance in
the earth’s magnetic field. This disturbance can be sensed with underwater sensors
attached to a surveillance or mine warfare system. The initial magnetic signature of a
ship can be reduced by means of degaussing; for example, by generating a counteracting
magnetic field of suitable strength and direction based on measured data of the magnetic
properties of the vessel.
Preliminary Analysis
A good estimation of the magnetic signature levels caused by the ship in design can be
calculated with a rather simple
method where the main source for
the ship's magnetic signature, the
hull, is treated as a longitudinal
series of magnetic dipoles. The
results
can
be
made
more
accurate by adding the effect of
larger system main components,
such as main and auxiliary
engines,
gears,
axles
and
armament to the hull dipoles.
The estimation process for the
magnetic signature is based on the
magnetostatic scattering theory. In
case of a steel hull vessel, the hull
Figure 3: Magnetic signature of a simplified model
itself is treated as the scatterer,
enclosing and partially shielding
MAST2012
SC: Public
Page 5/6
MAST-C-002-A
KTO/9/11/12
the shipboard magnetic units and components inside it. In case of a non-magnetic hull,
each of the individual ferromagnetic component is modeled separately as a dipole source,
located in their exact positions in the ship geometry.
For this assessment all the geometric and material parameter values can be fetched
easily from a modern design product model the design being in such a phase that some
kind of structural model has been already created. For example, the hull can be divided
into a number of longitudinal blocks or sections and the parameters for each of the
dipoles can be calculated from the model. After this the calculation of the signature levels
caused by this “set of dipoles” model is rather a straightforward process. When using this
kind of approach one has to keep in mind that the results are not good when the
assessment is done too close below the keel.
For preliminary analysis and for comparing design alternatives, the relatively simple
method described above has proven to be efficient. The acceptance criteria can be boiled
down to a single number, the maximum scalar value of the predicted signature below
keel in some predefined depth.
Detailed Analysis
For further analysis, to assess the ship in detail or construction design phases, it is often
necessary to get a more detailed calculation, for example, to determine the exact
positions and sizes of required degaussing loops. This can be done using a more
advanced analysis tools available in the scientific software market. These tools typically
have a finite element method (FEM) approach for solving physical problems. Defining and
solving a physics problem with the FEM tools require the following steps:
•
defining the problem with partial differential equation(s)
•
defining the boundary values, both internal and external boundaries
•
defining the problem geometry
•
creating the finite element mesh (discretization)
•
solving the problem iteratively in the mesh node points
In the FE analysis of the magnetostatic
scattering problem for the ship, the scalar
and vector potentials defining the
magnetic field can be solved from the FE
model, The ship's magnetic signature or
more precisely, the magnetic flux density
outside the ship hull, can be derived from
these potentials. The external boundary
values are used to define the primary
earth magnetic field, and the distribution
of ferromagnetic material is defined in the
mesh geometry.
For this the information can also be
fetched from the design model. For
example the hull structure, with geometry Figure 4: Magnetic signature level at a given
and material parameters, is outputted as depth
a face mesh and the system components
are outputted as volume mesh. In the calculation the surrounding medium, the sea, is
also modeled as volume. An example of this kind of link between the ship design model
and a commercially available multiphysics FEM solver is created in Finnish defense
industry's common research project Smulan.
MAST2012
SC: Public
Page 6/6
MAST-C-002-A
KTO/9/11/12
Conclusions
As emphasized with the presented examples, the utilization of the described one model
concept in navy ship design process gives many benefits, such as reduced design hours,
decreased risk of errors in the analysis, but also the possibility to get all the aspects
related to comprehensive combat survivability assessment integrated into actual ship
design process. When this becomes a natural and truly integrated part of the process, it
really is easier and more practical to end up with better design yielding increased safety
level and enhanced mission performance capability.
There are several benefits an organization working with novel naval ship designs can get
by the use of the design model they anyway need to create during the ship project. As
shown, this model can
also aid when aiming for a comprehensive survivability
assessment - signature evaluation included. The one model concept can be realized in
practice and this philosophy is already in use in SURMA software. For further information
regarding SURMA, please visit our website survivability.fi.