This invention concerns a method for mine detecting in the waters by using an acoustic wave transmitter and a receiver of acoustic waves reflected from an object in the water, on the bottom or under the bottom of the water, the both of which are placed in a vessel operating in the waters. The invention also concerns an equipment for mine detecting in the waters, whereby the equipment consists of a vessel sail¬ ing on a fairway with an acoustic wave transmitter and a re- ceiver of acoustic waves reflected from an object in the water, on the bottom or under the bottom of the water.

A sea mine is a very effective weapon that enables ob¬ structing of fairways and harbours and destroying big vessels at low costs. Advanced sea mines are extremely difficult to be dis¬ covered and extremely difficut to be cleared. Besides they choose their targets selectively.

The present mine counter measures have obvious weaknesses. The so called Mine Hunting Vessel, the length of which is about 50 m, is a low excited structure. It has a high grade equipment and a crew of 40 men. The weakness is its high price (abt 100 million USD) and the fact that it sails manned into the minefield.

Known is also an unmanned underwater vessel (the so called fish) drawn by a helicopter. Its weaknesses are the high costs and the fact that its ability to discover mines is not good.

An unmanned submarine (the so called ROV: Remote Operated Vessel) that goes in front of the mother ship is only a draft so ^' far. It is technically difficult and has a high price. Additionally its ability to discover mines is not good.

The method in accordance with this invention is characte¬ rised in that the vessel where the transmitter and receiver are placed in is a semi-submersible vessel. The equipment in accordance with this invention is characterised in that the vessel is a semi-submersible vessel.

By a semi-submersible vessel it is generally meant a structure with at least two underwater pontoons. At least

three, preferably at least four pillars extend above the surface of the water from these pontoons, supporting the deck construction over the surface of the water. An advan¬ tage of the stucture is its stability in rough waves due to the small water-line area achieved by using the pillars. Acoustic waves are suitable for mine detecting in the waters because the sound waves damp slowly in the water.

The semi-submersible vessel is a specially suitable base¬ ment for the sonar equipment and ROVs, because the structure acts smoothly in waves with the result that the ability to discover and identify mines is very good.

The vessel also stands pressure waves caused by the ex¬ ploding of mines. It can easily be arranged to drive remote controlled in front of the mother ship, which means that almost any small vessel is suitable as its mother ship. The unmanned vessel can be equipped as desired. The constructio is very cheap, the price of a vessel with operating systems will be only abt. 0,2 million USD. Price of the ROV and the sonars is additionally abt. 0,2 - 1 million USD. So the price is only a fraction of the price of the equipment ear¬ lier used.

The function of a mine detecting equipment is to find mines that are in a sea, on the bottom of a sea or dug under the bottom. The versatile sonar and ROV equipment of the system makes it possible to perform this task.

For identification of a discovered object the equipment can be furnished with a light ROV carrying a TV-camera, manually operated from the mother ship on the basis of the sonarpicture remitted by the detecting equipment. For the purpose of mine clearing the mine detecting equip ment is replaced by adjustable clearing coils to produce a magnetic excitation and by a sound source to produce an acoustic excitation.

The equipment can also be used for other kind of under- water detecting like f.ex. searching of cables and sounding of the sea bottom.

The planning of the equipment was started by assuming that performing of tasks does not nesessarily require a big, expensive and vulnerable, manned vessel. As the efficiency of the performing of tasks totally depends on the charac- teristics of the detecting equipment to be used, it is not necessary to risk a ship as their carriage. Thus the outcome is a small, light and cheap device, that has the same detect¬ ing equipment installed in as it would be in a ship, but goes unmanned, remote controlled in front of the mother ship into the mine field.

Information produced by the detecting equipment (sonars) is wrote out or displayed in the mother ship, where the functions of the detecting device are controlled. The de¬ tecting device is connected with the mother ship by a 200 m long, floating cable. The cable reel is placed on the mother ship.

As a small device it is easily used, cheap and versatile. Requirements made of the mother ship are only the electric energy production and places for the sonar display unit and cable reel of the detecting equipment.

The invention and its details will be described in more detail in the following with reference to the attached draw¬ ings, wherein

-Figure 1 shows axonometrically an equipment in accordance with the invention,

Figure 2 shows the equipment viewed from the side, Figure 3 shows the equipment viewed from above, Figure 4 shows a sectional view A-A in Fig. 1, Figure 5 shows a sectional view B-B in Fig. 1, Figure 6 shows the other half of the equipment viewed from behind,

Figure 7 shows the same viewed from the front, Figure 8 shows a sectional view C-C in Fig. 3, Figure 9 shows a sectional view D-D in Fig. 4 and Figure 10 shows a sectional view E-E in Fig. 4.

The base of the mine detecting equipment is formed by a semi-submersible platform the main dimensions of which are f.ex. the following:

Length 9,00

Width 5,00 m

Side height up to the deck 4,00 m

Total height 6,00 m Operating draught 2,50 m

Draught when moved (in tow) 0,55 m

So the platform consists of two underwater pontoons 1, with a length of 9 m and the diameter of 1 m, where the propulsion devices 2, transmitting antenna of the sediment sonar 3, ballast water tanks 4 and ballast water pumps 5 are placed in.

Narrow pillars 7 rise on the surface of the water 6 from the pontoons 1, broadening up to the diameter of 1 by the deck 8. The deck 8 consists of a framework 9 joining the pillars and of a foot grating 10 on top of it. The height of the deck above the water level, with operating draught, is abt. 1,5 m. Winches 15 and 16 of the sonar 11 to be lowered and the ROV 12 are placed on the deck; cables 15 and 16 separate from the winches. The pillars extend above the deck increasing the draught and the water line area in some special situations. Inside the pillars there is room for electric supply and electronical equipment.

The pontoons are connected under the water with eachother by three horizontal fins 17, 17' and 17'', that damp move- ments of the platform in roll of the sea. The fin most forward 17 includes the receiving antenna 18 of the sediment sonar and in connection with the middle 17' there is the cover 19 of the sonar 11 to be lowered. The shed 20 of the ROV 12 is situated between the middle fin 17' and the back- most fin 17' ' .

Aluminium is used as structural material allover in order to minimize magnetic excitations. Also propulsion devices and reels are made of aluminium.

For the purpose of detecting tasks the platform will be furnished with sonar equipment.

High-frequency round-scanning sonar 11 discovers objects in middle water and on the bottom. Its sensing module can b lowered on the cable 15 when the platform is (nearly) stand¬ ing still.

Sediment sonar 3 discovers objects buried in the bottom mud.

Thus the sensors of the sonars 3 and 11 are situated on the platform but the display units in the mother ship, where also the operation of the sonars is controlled from.

The identification requires that there is a small ROV 12 installed in addition to the sonar equipment. ROV 12 carries a TV-camera and it is directed to the object manually from the mother ship, in accordance with the sonar picture. Naturally the TV-picture of the ROV is also transmitted to the mother ship.

For the purpose of clearing the sonar and ROV-devices are removed and three clearing coils with different axial direc¬ tions are fixed to the frame of the platform. They are fixed to the frame but outside it. A noise sweeper is installed into the shed 20 of the ROV.

Other detecting tasks can be performed with equipment used for detecting and identification. The sediment sonar 3 is suitable for charting of the bottom. Hereby the vessel can also be manned and the cable 21 to the mother ship removed.

The mine detecting platform is furnished with two electric propulsion devices 2, the propulsion of which can be turned independently 360 degrees. Thus all plane motions are avail¬ able. The device is controlled from the mother ship by using a joystick. The device is within sight from the mother ship, on an abt. 200 m long cable 21 for transmission of elec¬ tricity and data. In addition the mine detecting device and the mother ship can be equipped with a device transmitting information about their mutual direction and distance.

Thus, when the exact location of the mother ship is known, also the exact location of the detecting device can be found out.

In order to minimize acoustic excitations all noise making operations have been eliminated, as far as possible, on the platform. All operations are effected electrically, no noisy hydraulics is used. Propulsion devices 20 have been instal¬ led "upside down" at the stern of the platform in order to prevent changes in the current of water, caused by them, from

In order to minimize magnetic excitations antimagnetic materials have been used, as far as possible, as structural material. Magnetic excitations are caused by electric drives (electric motors as fairly large magnetic material concentra- tions and magnetic fields from electric currents). Anyway, the share of magnetic materials is relatively small and the magnetic field can be restricted by arranging special compen¬ sation circuits.

Propulsion devices have been situated at the stern of each underwater pontoon 1. The devices have been installed "up¬ side down" and at an angle of 15 degrees with respect to horizontal. The reason for the angle is that the propulsion of the device in this way is directed horizontal in the main propulsion direction. Propulsion of the propulsion device can be totally directed 360 degrees, so the installation of two devices enables all plane motions, also the parking.

The propulsion devices are powered by electric motors, the power of which enables a maximum speed of at least 4 knots for the platform. Maximum operating speed for the high- frequency, round-scanning sonar is abt. 1 m/s.