Recent advances in simulation of naval vessel signatures

By Chris Riley

The simulation of naval vessel signatures has been an important application for a long time, and using simulation software electromagnetic designers can perform different types of calculation for this application. But what is a signature and why do we need to determine signatures at all?

The main reason that magnetic signatures occur is because the large amount of ferrous material, or electrically conducting material, that we find in a ship or submarine interacts with the earth’s magnetic field. The ferrous objects are sufficiently large that they deflect the earth’s field. It’s a static situation, a DC field is coming from the earth and the field gets “sucked in” by the ferrous material in the ship. The difference in the expected earth’s field and the field that is actually measured is what is called the static signature. There is also a signature effect due to eddy currents, called the dynamic signature. The vessel is also moving through the earth’s field, and in particular a surface ship will rock from side to side, so this produces eddy currents in conducting material. Those eddy currents will produce a reaction field and this is also something that will contribute to the signature field.

Finally, we’re dealing with ferrous materials a lot of the time and these gradually becomes magnetized in a particular orientation as magnetic domains align. They start to act like a weak permanent magnet and the resultant external field is, of course, what is known as the permanent signature.  The removal of this signature is called de-perming. There is also equipment on board the ship, especially since there is an increasing drive to produce more electric ships and submarines where some of the traditional propulsion methods are being replaced by electric motors. Large transformers and large generators on board are also necessary.

All these may produce stray fields which will also add to the signature. All metallic vessels, whether or not they are civilian or naval, also need cathodic protection systems fitted. These CP systems protect against the electro-chemical behaviour of the materials which otherwise would result in corrosion: the different materials that are in the ship, and the conducting path through the seawater all play a role. Of course, if there is a conducting path, currents are flowing; current flow produces both electric and magnetic fields, and those contribute to the signature as well. Why designers of naval vessels are so concerned about having a low signature is because they don’t want the signatures to be detected. This is usually achieved either by effective initial design or by addition of mitigation measures, such as degaussing coils. Low signature is usually required from a safety point of view. Most sea mine systems are capable of detecting signatures at the level of determining friend or foe. Even if the ship does not make physical contact with the explosive system, it can still detonate in response to the signature. Signature minimization is therefore a very important part of design.

Why do people want to simulate it? The reason is that to measure the signature by testing is very lengthy and extremely expensive. If we can accurately calculate the signature by computer simulation, that is a great benefit for the designer, particularly of the degaussing system, who is trying to minimize the signature. Simulation also gives a lot of insight: when testing is eventually performed, it tells you what levels of output to expect, where will be the most significant signatures you might obtain, and what it is you should be trying to measure. It gives insight into things that are impossible to separate by measurement.

You can actually see where signatures originate, which may not be possible to look at in testing. Simulation helps to find out what signature is created before any mitigation is performed, to effectively design the degaussing coils, cathodic protection systems (based on the potential distribution around the ship) and the de-perming facilities. It also helps to analyze compatibility issues from the onboard equipment, and to see if there may be a problem with any stray fields either adding to the signature or affecting other items of sensitive equipment onboard. The thing that no-one really wants to test (but needs to find out) is what happens if something goes wrong. If there is a fault, can the ship cope with this? Simulation is of great benefit when dealing with this scenario. Here is a simple model, a typical type of model we might have for a ferrous ship:

cr1

 

It’s a series of thin plates which represent the deck, and the hull, and the bulk-heads, and the interior decks. Some of the exterior surfaces have been hidden so you can see the interior structure – there are several decks and several bulk-heads along the length. This particular model was constructed to use Opera’s mosaic meshing technique where we can have very high aspect ratio elements – a vital feature for economically calculating the field accurately in thin plates. The model has sections of steel plate that are, maybe, ten metres long, but only a few centimetres thick. In these very high aspect ratio structures, the field doesn’t change dramatically in the tangential direction of the plate. Hence, when using volume finite elements, it’s useful if you can also use elements that are high aspect ratio, and the elements available in Opera’s mosaic meshing – hexahedra and triangular prisms – will support that without compromising accuracy. This one is a model constructed in a way so that it can be meshed entirely with hexahedra. Here’s the hexahedral mesh: cr2

 

There’s also a surrounding mesh for the sea and an interior mesh in the free space inside the ship and the free space above the ship as well. All of the surface facets are quadrilateral so we are now using hexahedral elements that have an aspect ratio of, perhaps, up to a hundred to one. This doesn’t compromise accuracy because the hexahedral elements introduce additional terms into the basis functions for the finite elements, which allow for the variation in the field in the tangential directions. This is the typical sort of result we might get from the software – the field that would be measured at the sea-bed and the flux density in the ship structure:

 

cr3

 

This is the same model viewed with some more of the structure cut away so we can also see what’s going on inside. In this simulation, the earth’s field is athwartships (the field comes in from the side of the ship) and we can see the flux density in the ship. But what we’re really interested in terms of signature field is discovering the field that can be measured outside the ship. One of the ways this can be calculated and displayed in simulation is mapping on a two dimensional surface, so all of those grid intersections are sampling points where the magnetic flux density is being calculated. Opera has a powerful tool for this application to maximize accuracy, where we can compute field by integration of the computed magnetization distribution for all the finite elements in the steel plates. Find out more about naval vessel signatures in our webinar, infographic and application page below.