Minesweeping has not been given high priority during past decades and little effort has been made in the development of new technology within the area. Minesweeping has lived in the shadows of minehunting that by contrast has attracted a good deal of interest and has demonstrated great technical advances.
Therefore when the Royal Norwegian Navy (RNON) established its new project for procurement of new MCM vessels in 1985, it was realised quite quickly that limited information was available for finalising the operational specifications for minesweeping. The magnetic and acoustic signatures from typical merchant ships transiting in Norwegian waters generally were not available.
| The new Norwegian ALTA class minesweeper and Oksøy class minehunter |
It is accepted widely that the modern sea mine represents a cost-effective threat. Furthermore, it is realised that minehunting does not give sufficient protection in many areas. Thus, mine-sweeping is necessary in order to reveal all mines. Areas where the sea bed is covered with stones and/or dense vegetation such as seaweed can be difficult for hunting operations. In sandy sea beds, movements of sand banks that bury sea mines often can be observed. In other areas mines are covered easily by a muddy bottom, and there is of course the self-burying mine to consider.
The development of mines has progressed significantly with the introduction of low-cost signal-processing electronics. And the availability of advanced intelligent mines of shapes and materials that are difficult to detect and classify by hunting operations, can be foreseen.
Modern minesweeping operations can be conducted in two different operational modes. First there is the traditional minesetting mode that requires knowledge of the triggering mechanism of the mines to be swept, and secondly there is the target-sweeping mode that focuses on the signatures of vessels to be protected.
Target-mode operations take into account the fact that the triggering mechanisms of mines are becoming more difficult to predict. An accurate database of signatures of the vessels to be protected is required in order to define the sweep set-up. During sweeping operations sweeping systems are set to copy or simulate the signatures from the vessels to be protected. These vessels can follow safely in the track of the minesweeper that will have triggered all relevant mines, that is those mines that react to the signature from the ships to be protected.
In order to provide sufficient data to finalise the operational specification it was decided that a comprehensive signature-measurement effort had to be carried out. The Norwegian Defence Research Establishment (NDRE) was contracted to perform this programme and based on these efforts, the final operational requirements were established. It was concluded that the performance specifications of the sweeping systems for the new minesweepers had to be improved significantly when compared to existing systems in order to handle modern mines in a target-sweeping mode and to provide sufficient protection against mines along the Norwegian coastline.
In parallel an investigation into available acoustic sweeping systems was carried out. The availability of systems of sufficient acoustic performance was found to be very limited. Low-frequency performance in existing systems was closely linked to unacceptable system weights. As a consequence of these findings, the industry was challenged to propose alternative systems to satisfy Norwegian staff requirements with emphasis on both acoustic performance and system weight.
As a result of this effort GECO Defence was contracted to develop a prototype of the AGATE (AirGun And Transducer Equipment) acoustic minesweeping system.
In parallel RNON developed a completely new vessel concept for minesweeping in co-operation with Kværner Mandal a.s. The outcome was a surface-effect ship (SES). This concept incorporates major advantages when compared to more traditional designs such as a large and spacious working deck for the handling of sweeping equipment; low acoustic signature; low magnetic signature; high transition speeds and a small wet area that provides good protection against mine explosions.
The objective of an acoustic minesweeping system is to generate sound under water and influence the triggering mechanism of mines. If the operation is performed in target-sweeping mode, the sound source must be able to copy the sound image of a vessel or of a class of vessel in order to ensure that the swept route is safe for these vessels.
The most characteristic acoustic features from merchant vessels are within the low-frequency range and propeller and machinery noise are the dominant noise sources. In most cases these sources will generate periodic noise components particularly dominant below 200Hz. These periodic components can be observed easily as characteristic lines or peaks in the noise spectrum. Therefore it can be concluded that, in future, acoustic minesweeping systems must be able to generate significant acoustic energy at very low frequencies in order to operate effectively in target mode. These low frequencies often are referred to as seismic frequencies. So the main acoustic requirements are first that the acoustic power below 200 Hz is particularly important. Secondly, the acoustic spectrum must be controllable; that is, the operator should be able to manipulate the generated noise spectrum to almost any shape. And thirdly the sound pressure level must be high for a wide-frequency range.
These requirements cannot be met using a single acoustic source. In the AGATE system a combination of airguns and flextentional transducers provides an acoustic source of an unmatched performance. AGATE applies air-guns for the low-frequency seismic spectrum, terfenol transducers for medium frequencies and piezo-electric transducers for high frequencies. The acoustic source is towed in a submerged body or tow-fish.
The airgun is a pneumatic source driven by high-pressure air trapped in a chamber. Each impulse is created by the release of a controlled volume of air. The acoustic output level is controlled by the amount of air released and by the air pressure. Air guns produce the right acoustic signatures and they are also much smaller than alternative seismic sources.
The performance of the airgun has proven to be reliable in giving repeatable acoustic signatures. More than 90 per cent of all offshore deep seismic surveys for oil and gas exploration are carried out with such units. For seismic operations the standard airgun is designed to fire every fifth or tenth second but the requirements for minesweeping are quite different.
A minesweeping airgun must be able to fire with short time intervals in order to generate periodic components. GECO Defence has designed a new airgun adapted for minesweeping and it is optimised both with respect to firing interval and weight.
A pattern of repeated firings is used to simulate periodic components and airguns thus provide the acoustic signature's fundamental frequency components while the transducers generate amplified and digitally filtered white noise and spectral lines.
High-pressure air is provided by one or two onboard mounted compressors. Selection of compressor capacity is based on the required acoustic performance and available onboard space and power. The available compressor solutions are designed carefully with weight and magnetic characteristics in mind.
| The AGATE operator console monitoring the performance of a sweeping operation |
The terfenol-D transducers are supplied in a flextensional shell. Terfenol-D provides efficient conversion of electric energy to mechanical energy and makes it possible to design compact equipment with improved acoustic efficiency. At lower/medium frequencies these transducers show considerably higher acoustic power than conventional piezo-electric transducers of the same size.
Terfenol-D is an alloy characterised by a very large magnetostriction. This property makes it a suitable driver in an acoustic, low-to-medium frequency, high-power source in minesweeps. It fills the gap between very low and very high-frequency source transducers.
The main units of the AGATE system are a tow-fish emitting the desired acoustic noise; an operating console; a transducer-control system; a compressor plant supplying high-pressure air to airguns, and an amplifier plant supplying power to transducers.
Each unit is connected to a computer that acts as a central command and control system. The status of all system components is monitored continuously and so as to ensure secure communication under all conditions, the network is based on optical-fibre communication.
The user has flexibility to specify almost any desired noise spectrum selected from a library of ships' signatures in the database. New sweep definitions can be defined graphically by an operator using a roller ball or a keyboard. The man/machine interface is based on modern Windows-type graphical interaction. The screen will monitor continuously the status of all sub-systems and an alarm will alert the operator in case of a malfunction.
The output of the acoustic sweep is monitored continuously by hydrophones mounted close to the tow fish. The hydrophone data is transferred to the onboard system and spectra are computed continuously and monitored with reference to specified levels. As an option, a loudspeaker enables an operator to listen to the audible part of the hydrophone data. The Windows-based user interface has a wide range of menus and access to real-time readings of all sensors, hydrophones and sweep settings.
The prototype of the AGATE system was developed for the Royal Norwegian Navy that took delivery in 1992. Since then the system has undergone operational testing and has been applied in seven major national and NATO exercises.
Based on the operational experience and separate acoustic evaluations performed by the Norwegian Defence Research Establishment (NDRE), the Royal Norwegian Navy has ordered series systems of the AGATE for new minesweepers.
