This blog was co-written by Mirosław Wołoszyn, Associate Professor of Electrical Engineering and Dean at Faculty of Electrical and Control Engineering at Gdańsk University of Technology.
Building ships for sea and particularly for the navy has multiple technical challenges. One of them is protection from sophisticated detection devices, which are using multiple methods. Each vessel has its own unique, electromagnetic characteristic given by the various sources of magnetic and electric field. These sources produce a distinctive magnetic and electric ‘signature’, hence the desire by the ship designers to modify and ideally reduce this below the level of sensitivity of detection devices. The signature reduction is performed using two separate processes: degaussing and deperming.
The magnetic ship’s signature contains two main magnetization components: permanent and induced. The permanent type of magnetization depends on ship “magnetic history” (production and storage of ship’s sheet metal, and ship’s building technology), ferromagnetic properties of sheets, or even mechanical strikes and temperature stresses during exploitation. The induced magnetization is directly related to the current geographical position and orientation (course) of the ship in the Earth’s magnetic field – current heading of the ship.
Modeling ship signatures is one of the areas of strength of SIMULIA Opera, a finite element analysis software for low frequency applications. These fields could propagate several meters away around the ship and have complex distribution in space. As a ship sails, it moves in the Earth’s magnetic field, causing its signature to change. Additionally, interaction of permeable materials in the structure of the ship with the Earth’s magnetic field gradually magnetizes these materials. Eventually the remanent magnetization becomes high and has to be removed using the deperming process. Whereas degaussing is done using onboard equipment, a set of electrical coils and controlling devices, deperming is performed at specialized facilities with the ship wrapped by coils or with coils lying on the seabed.
The strength and shape of the magnetic field near the vessel depends on the Earth’s magnetic field, geometry of the ship, magnetic properties of the materials used by ship designers and operating regime of the onboard equipment designed to change the field outside the vessel. It may not be obvious, but the magnetic signature of a commercial vessel may also depend on the nature and layout of the cargo that it carries.
Opera-3d was initially developed at a research facility with an intention of predicting very accurately electromagnetic fields in accelerator physics. This is exactly what is needed to measure ship signatures, where magnetic fields are in the nT range. It is not surprising, therefore, that Opera has been used for evaluating maritime vessel magnetic signatures for many years. The results for the computation and reduction of magnetic signatures has been thoroughly validated over many years against both analytical methods and measurements.
Opera-3d can help designers to simulate and optimize electromagnetic fields around the ship. However, even with the computational power of modern computers it is often not practical to simulate the complete details of a real vessel. A typical case for the finite element analysis approach is to consider/prepare a simplified model of the ship for simulation by de-featuring a CAD model of a real ship or building a representative model from scratch. Even so, meshing of the principal components (hull, decks and bulkheads) is cumbersome due to high aspect ratio of lateral dimensions to the thickness. This is where the modelling capabilities in Opera-3d offer a viable solution: the thin plate boundary condition – a method for finite element mesh construction suitable for magnetic signature simulations. Thin plates with finite thickness are replaced by a two dimensional surface between neighboring free-space volume elements. During equation generation, the surface is expanded to equivalent hexahedral or prism elements, while the free-space elements remain unmodified. This alleviates many of the issues associated with mesh construction in high aspect ratio geometry. The calculation time of the ship’s magnetic field with the thin plate boundary condition method is significantly less compared to the ship model with layers of steel. The other big advantage of using thin plate boundary conditions is the reduced model building and meshing time.
When the degaussing process is performed, the magnetic signature of the vessel is reduced using 3d sets of coils installed onboard. Creating multiple sets of relatively thin coils may significantly slow the analysis if the coils require meshing. This however is usually not necessary and another Opera feature, Biot-Savart conductors, becomes extremely useful. Biot-Savart conductors must be embedded in reduced (scalar or vector) potential (i.e. air regions), but as their fields are calculated analytically the benefit in terms of analysis speed is significant. In the webinar “Modeling Magnetic and Electric Signatures Using SIMULIA Opera,” we will discuss practical aspects of building models for degaussing and show results of more complex analysis that includes optimization using a coupled workflow between Opera and SIMULIA Isight.
Electric signatures of ships can also be easily modeled using Opera-3d. We will show that cathodic protection, a standard procedure to fight corrosion in seawater, can produce an undesirable side effect by creating an electrical signature, which also needs to be taken into consideration. We demonstrate how to set up the analysis for different types of cathodic protection.
The webinar will be taking place on July 7th. Click here for more details and to register.
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