Underwater Electromagnetic Signature Services
Clients can also contract Davis to do the signature modeling and analysis using ShipEM.
ShipEM
It is beneficial to understand the magnitude and spatial distribution of the underwater static electric and alternating electromagnetic fields of new or existing ships so that the ship susceptibility to mines can be analyzed. Davis supplies a software code, ShipEM, which is able to predict the field characteristics in three dimensions by inputting ship geometry and the location of the cathodic protection anodes.
The electric potential between the anode and the cathode (the propeller or other bare metal) creates a Static Electric (SE) field (also known as Underwater Electric Potential or UEP). The magnitude of the field is determined by the level of current flowing from between the electrodes and the distance between them.
The current flows primarily from the anode, through the propeller and the shaft, and finally grounds out to the vessel's hull through the shaft bearings. Electrical contact between the shaft and bearings tends to vary as the shaft rotates and the electrical current becomes modulated at the frequency of the rotating shaft. An alternating current creates an electric field at the same frequency, broadcasting an Alternating Electric (AE) field (also known as Extremely Low Frequency Electromagnetic or ELFE) at the shaft frequency and its harmonics. Additional components to the AE field can be contributed by the propeller blades. As the propeller turns, the distance from the propeller blade nearest the anode varies and the signal becomes modulated with the propeller blade passage frequency. Poorly filtered power supplies on ACP systems can also allow electrical "noise" to contribute to the signal, introducing onboard power frequencies into the AE field.
Active Shaft Grounding (ASG)
Davis supplies an ELFE influence mine countermeasure called Active Shaft Grounding (ASG). As shown in the interactive graphic below, the ASG system, when activated, provides for a low resistance path for the current to bypass the shaft bearings.
Main benefits of ASG system:
- Virtually eliminates ship underwater ELFE signature
- Reduces corrosion to shaft bearings
- Resilient to deterioration in slip ring brush contacts
- Compatible with passive and active cathodic protection systems
- Comprehensively tested to military standards
The ASG system uses electronics to actively ground the shaft to the hull through a slip ring brush assembly. The system measures the shaft to hull potential through one slip ring assembly, and then uses this signal, after amplification, to control a high current power supply which draws a current out of the shaft through a second slip ring assembly. The circuitry reduces the variation in shaft-to-hull resistance, which in turn results in small variation in the current between the cathodic protection anodes and the hull. The ELFE signature is then virtually eliminated.
The primary source of the electric field that emanates from the underwater hull of a vessel is the electric current that is produced by important hull corrosion protection systems. These systems use either passive (zinc) anodes or active (powered) anodes to produce a current that flows either through the propeller and shaft of the vessel or to the hull of the ship where coating holidays exist.
The underwater extremely low frequency electromagnetic (ELFE) signal, which results from the alternating current flow between the cathodic protection system and ship propellers, can be detected by influence mines and trigger their detonation. The propagation distance of an ELFE signal from a typical untreated ship is sufficient to make ELFE-capable influence mines a serious threat.
Influence mines which have the facility to install ELFE signal sensors and signal processing exist today. Examples are the Italian Manta and Russian MDM mines.
