DEGAUSSING

A steel-hulled ship is like a huge floating magnet with a large magnetic field surrounding it. As the ship moves through the water, this field also moves and adds to or subtracts from the Earth’s magnetic field. Because of its distortion effects on the Earth’s magnetic field, the ship can act as a trigger device for magnetic sensitive ordnance or devices which are designed to detect these distortions.

Degaussing is a process in which systems of electrical cables are installed around the circumference of ship’s hull, running from bow to stern on both sides. A measured electrical current is passed through these cables to cancel out the ship’s magnetic field. It could be said that degaussing, correctly done, makes a ship “invisible” to the sensors of magnetic mines, but the ship remains visible to the human eye, radar, and underwater listening devices. The degaussing system is installed aboard ship to reduce the ship’s effect on the Earth’s magnetic field.

In order to accomplish this, the change in the Earth’s field about the ship’s hull is “cancelled” by controlling the electric current flowing through degaussing coils wound in specific locations within the hull. This, in turn, reduces the possibility of detection by these magnetic sensitive ordnance or devices.

The ship’s permanent magnetization is the source of the ship’s permanent magnetic field. The process of building a ship in the Earth’s magnetic field develops a certain amount of permanent magnetism in the ship. The ship’s induced magnetization depends on the strength of the Earth’s magnetic field and on the heading of the ship with respect to the inducing (Earth’s) field.

The Main coil (M) compensates the induced and permanent vertical components of the ship’s magnetic field (Z zone). It is installed in the horizontal plane at the waterline. As the ship changes hemispheres the coil current polarity must be manually adjusted.

The Forecastle permanent – compensate for the longitudinal permanent component of the ship’s magnetic field.

The Athwartship coil (A) is installed in the vertical plane and extends from the keel to the main deck. It compensates the athwartship induced and athwartship’s permanent components of the ship’s magnetic field. The A coil current consists of permanent and induced components.

Modern influence sea mines detect the magnetic disturbance of a vessel in the Earth’s Magnetic Field, this is referred to as the vessels magnetic signature. This Signature is the most significant influences of those used to trigger to the sea mine or torpedoe.

To minimise this threat naval vessels are fitted with an on-board Degaussing System (DG). The DG reduces the signature with a counter-acting field, generated from a coil system, connected to loop-coil current amplifiers. Typical 90- 95% of the ship’s signature can be neutralised with a well designed 3-dimensional DG system.

Naval operational priorities have shifted from blue water to brown/shallow water. This means that the threat is increased as ships comes much closer to the influence mines. Better degaussing systems than previously used are therefore now required. The answer to this increased threat is ADG.

>Typical degaussing equipment

>Systems suitable for the smallest vessel’s to aircraft carriers

>Turnkey capability including system design, equipment supply and proving trials

>Favorable total ownership cost

>Efficient signature handling with control, evaluation and prediction

>Modular and digital open systems, upgradeable for future threats

>Cost effective procurement

ADG is normally controlled by one or more magnetometers to achieve a low signature. Several fallback control modes are available, including Gyro Geomagnetic map and manual, depending on the system layout.

Essentially magnetic ranging facilities for vessels consist of submerged field probes and evaluation equipment which measure the magnetic interference field of the vessel at a specific water depth. In addition, auxiliary sub-systems may be required.

In practical use two measuring range configurations have proven their value:

>Overrun ranges

>Stationary ranges

Typical tasks comprise:

>Check ranging

>Calibration of the permanent and induced channels and, if applicable, also of the eddy current channel of the shipboard degaussing system

>Measurement of the magnetic stray field

>Check ranging to confirm improvements made by calibration of the degaussing system.

In contrast to the overrun method, stationary measurements are carried out whilst the ship is moored over an array of probes. The probes are arranged along the longitudinal and transverse axis of the vessel and thus allow the interference field to be sampled at various points beneath the ship.

Standard measuring depth for ranging of non-magnetic vessels is 9.0 m. The essential feature of stationary magnetic ranging is the possibility also to measure the ship’s interference field caused by eddy currents. Since this effect occurs only during the rolling motions of a vessel, modern ranges for non-magnetic vessels allow roll simulation with the aid of vertical and horizontal magnetic fields generated by a coil system installed on the sea bed.

Another important feature of a stationary range for Mine Countermeasure Vessels is the possibility to directly measure and assess stray fields. Though small by comparison to other sources of ships magnetism, these fields caused by currents in the electric plant of a vessel require attention in MCMVs, due to their low overall magnetic signature

The land range facility generally mainly consist of the following:-

Stainless steel railway with trolley and cable winch.

Deperming coil, consist of either smaller portable coils with a hole size of approximate 75cm or of bigger component the coil will be winded directly on the components.

Zero-field coils (H,Z) are situated in the middle of the range, (where also the magnetometer probes are located in bore holes).

Magnetometer probes. mounted in bore holes, which are connected to magnetometer electronics and compensators situated in the range office.

Analysis capability of the land range lack the use of data acquisition and data analysis tools. only analogue high speed channel writers exist. All analysis work must therefore be done manually and very labour intensive with paper and rulers, this work method was very common until the end of 1970-ties before computerized data acquisition units became more commonly available on the market.

Location of the land range and buildings

The land range is located in N-S direction and located with office building which had offices, training class rooms and separated laboratory office with the magnetometer electronic equipment’s. A separate electric house facility is located beside the land range. The electric house contains all electric generators, high precision current shunt etc. for the land range deperming function.

UMISS© System Description

UMISS© is a family of Multi Influence Sensors Systems designed to measure Underwater Signa-tures. It consists of parts and subparts that have been proved by use with the fixed and mobile ranges of the Royal Swedish Navy, and has been accepted for International market applications. UMISS© is designed for use as a single unit or with several UMISS© sensors, combined in a full range application.

The UMISS© Sea Module is equipped with magnetic, electric and pressure sensors and an op-tionally acoustic sensor. Underwater signatures are measured for fixed & mobile ranging and surveil-lance applications such as for harbour protection.

The UMISS© sensor signals measured by the Sea Module sensors are converted to digital signals using a high precision 24Bit ADC. The sensor out-put data is converted to optical serial data traffic by a media converter and transmitted through an opti-cal cable to the Land Module. The Sea cable carries 2 optical fibres and 2 electrical conductors connect-ing the UMISS© Sea Module to the Land Module. Alternative data communication with W-LAN from a surface bouy or dedicated Ethernet TCP/IP is also available.

UMISS© is designed for easy deployment from a small workboat with minimized requirement for diver assistance. The Sea Module is also equipped with 2-axis inclination sensor unit. Data from this unit and the static 3-axis magnetic sensor enable compensation for non horizontal positions of the Sea Module to be made during subsequent analysis.

When UMISS© is used as a single unit, a laptop computer and a dedicated program permits real time presentation and storage of the raw sensor data. This stored data is available for later analysis. In a full sea range application PC sensors are connected to a hub and a workstation at the shore range station running the software.