By Jerry Fireman

Sensors in advanced anti-ship mines detect a warship’s unique magnetic field signature within the earth’s magnetic field and activate detonation. To counteract these weapons, the Navy outfits warships with coils that generate an equal and opposite magnetic field, which nullifies a warship’s magnetic profile.

However, the direction and magnitude of the warship’s magnetic field signature varies from point to point throughout its surface and as it rocks upon the water. This means that the Navy’s countermeasure challenge is to determine a warship’s magnetic field at any position within the earth’s magnetic field and throughout any wave-induced rocking movement so that the coil currents can adjust and compensate for these variations.

Conventional electromagnetic software requires a huge amount of computational resources to model an object with geometry as complex as a modern naval warship. But a technical team from the Naval Surface Warfare Center in Bethesda, Maryland, Sherbrooke Consulting, and Anteon Corp. has solved this problem by using software with a new edge variable method that calculates the electromagnetic potentials and fields on the edges of the finite elements. “We have reduced the time needed to calculate the magnetic field produced by a rolling gunboat from 19 days to 1 day,” said Bob Pillsbury, of Sherbrooke Consulting.

Here’s a look at how this team developed techniques to model the electromagnetic fields of warships.

The earth’s magnetic field corresponds roughly to that produced by a very large magnet near its center with a north-south orientation. The magnitude of the magnetic field at the earth’s surface is approximately 0.5 gauss. The sizeable mass of iron that makes up most of a warship attains a significant level of magnetization, which generates an electromagnetic field.

The electromagnetic signatures of warships first became important during World War II when sensors capable of detecting a warship’s magnetic field were used to detonate a charge proximate enough to the hull to cause major damage. This development prompted the United States and United Kingdom to develop countermeasures for mines triggered by magnetic field.

To prevent detection by these mines the Navy developed DC compensating coils that degauss or null out the ship’s ferromagnetic signature. However, an additional electromagnetic field is created when electrically conducting materials such as steel, aluminum, and copper move in a magnetic field. Movement of a conducting material in a magnetic field induces currents in the conductor, called eddy currents that generate their own reaction magnetic field.

The most difficult part of modeling a warship’s electromagnetic field is quantifying the reaction magnetic field generated by rocking motion, and the quantification of the reaction field is crucial for developing algorithms that drive the degaussing coils. Since naval ships frequently operate in shallow water, the reaction magnetic field problem grows greater because rolling motion tends to increase as water depth decreases.

The DC curve for the magneto-static or constant field measured versus calculated signatures along a line 8.5 m below the ship.

Since World War II, the Navy’s traditional approach to solving this problem is to use instruments to measure the electromagnetic fields generated by warships in a fixed slip or pier. But this is costly. Due to the high cost, measurements have also been performed on scale models. For example, the Magnetic Ship Models Laboratory at the former Naval Surface Warfare Center in White Oak, Maryland, was used to test physical models up to 12 feet long. The lab was capable of generating magnetic fields to replicate the magnetic environment at any point on the earth’s surface.

Physical models provide useful information and are cost-effective, but are limited by the assumptions required in constructing the model and the capabilities of test facilities. More accurate results can be obtained from testing actual naval vessels. But the cost of bringing ships to fixed facilities where a DC background field can be applied for ferromagnetic signature measurements or an AC background field can be applied to simulate wave motion for eddy current measurements is high.

To address this the Navy purchased an old East German gunboat, primarily for performing full-scale electromagnetic signature tests at the Charleston Naval Station in South Carolina. While these measurements provide a higher level of confidence than scale models, the Navy was still only able to test this single ship under a narrow range of possible motions. The expense of actual vessel measurements and the limits associated with scale modeling led the Navy to seek alternative approaches.

The Navy has overcome these problems by using software that can calculate the electromagnetic field generated by any object under any set of conditions to higher accuracy and at less expense. Using this software they can determine ferromagnetic and eddy current signatures and aid in the development of the algorithms needed to drive electromagnetic countermeasures to protect their ships. The key advantage of electromagnetic simulation is that it greatly reduces the time and expense required for each test, a critical consideration when the Navy’s ultimate goal is to countermeasure a ship’s electromagnetic signature during every possible wave-induced motion anywhere.

Two major issues are of special interest in electromagnetic simulation. One is ensuring that the simulation matches the physical world, as assumptions and generalities must also be accounted for in computer models. The second issue is the enormous computational effort involved in simulating extremely complex structures represented by naval vessels.

Sherbrooke Consulting has been working with the Navy to overcome both these challenges. “The first order of business was to ensure that we could replicate the actual magnetic fields generated by a ship using software,” Pillsbury said. “We used the FEM (finite element method) OPERA and ELEKTRA software from Vector Fields because this software was built from the ground up for electromagnetic simulation, so it offers many advantages over codes that were originally designed for structural or thermal modeling. For example, we like the fact that the geometries of the coils are independent from the structure of the (finite element) mesh so we can mesh the ship without even thinking about where we are going to put the coils that represent the earth’s magnetism.”

The Navy technical team began by modeling the East German gunboat because it had reliable physical measurements that the team could use to verify the accuracy of the simulation. A warship’s complexity and shape make it difficult to model, according to Pillsbury.

“An automatic modeler is essential to handle the many and varied structural elements in a warship, yet automatic modelers tend to resolve a ship into elements with high aspect ratios, which means they are also long and narrow. This kind of element tends to be very difficult for the analysis module to obtain a converged solution,” he says.

A team of people from the Navy, Anteon, and Sherbrooke with years of experience modeling Navy ships was tasked to overcome these issues. Team members worked with detailed drawings and simplified the geometry by including only those elements with a significant impact on the magnetic field, such as the hull, decks, and other major structural details. “These major features can be more easily meshed with elements whose aspect ratio is less extreme,” says Pillsbury. “The biggest technical problem was determining the permeability and conductivity of each element, which is a necessary prerequisite of the simulation.”

“While the thickness of the hull is known,” Pillsbury continued, “making it easy to estimate conductivity at this point, in many other areas where many different pieces are coming together, it’s much more complex. For example, it may be difficult to determine from a drawing whether a joint is welded and providing a closed path or whether there is a small gap that would prevent current from flowing. Another concern is that there are many gratings in many areas of the ship that needed to be simplified in order to reduce the complexity of the model. Instead of modeling each little hole, we typically adjust the material properties, such as by defining a grating as 10% metal. The complexity of the model was increased by the fact that it needs to be analyzed multiple times to account for the rolling motion of the ship at a period of 3 to 12 seconds for a complete roll.”

When originally solved, the model results correlated very well with physical testing. But using the traditional nodal finite element formulation within Elektra, the model took 19 days to converge. With this method, scalar and vector electromagnetic potentials are calculated at the nodes of the elements by the analysis module. This approach works very well for static fields but tends to be relatively slow for time-varying fields such as those produced by a rolling ship.

While this work was under way, Vector Fields upgraded Elektra by providing the alternative of using the edge variable finite element method, which the company had previously used in its high-frequency electromagnetic analysis module called Soprano. In an edge variable finite element, the potential or field is no longer solved at the nodes of the element but on its edges. This makes a formulation based entirely in vector potential viable, as only one value per edge is required and the vector direction is implied by the edge.

Switching to the edge variable method reduced the solution time to only one day, making it possible to dramatically increase the speed with which results were obtained and analyzed. “The Navy has been very pleased with both the speed and accuracy with which electromagnetic simulation is able to determine the electromagnetic signatures of its ships,” Pillsbury concluded.

Jerry Fireman is a technical writer based in Lexington, Massachusetts. You can contact him through e-mail c/o Desktop Engineering Feedback.

Companies Mentioned in the Article

 

Anteon Corp.
703-246-0200

Sherbrooke Consulting, Inc.
703-812-8774

Vector Fields, Inc.
630-851-1734