Naval mine are a significant danger to both combat operations and transport of men and equipment over the sea. This is an especially important problem for the coastal navies, due to restricted areas in which they operate.Over 300 types of mines are produced by 30 countries, and many more have them in use. Mines cause damage disproportionate to their price, and anti-mine warfare efforts increase this price even more.
Basic division of mines is into contact and proximity (noncontact, influence) mines. Contact mines have to achieve physical contact with the target ship, while noncontact mines have proximity detectors. Contact mines are typically moored, but can also be drifting and bottom-lying. Moored contact mines are typically used against surface ships, and so are set to float at depths anywhere from right below the surface to five meters below it. Because of this it can be set so that only large ships are at risk, which saves it from being wasted on less valuable targets. In littoral waters the cable is set to account for the tide, so that the mine does not become visible at the low tide. Drifting contact mines are not very effective except in creating fear; moored mines may break the chain and become drifting mines if the deactivation mechanism fails. Since drifting mines are equal-opportunity evil to enemy, friendly as well as neutral shipping, they were banned after World War I. Bottom-lying contact mines are effective against submarines that may decide to hide on the bottom, as well as against amphibious landing ships in shallow water. Limpet mines are manually attached to the target by magnets and left.
Remotely controlled mines have the advantage in that they can be laid down during peacetime and then activated in the event of war. The mines can usually be turned into “normal” mines with a switch (which prevents the enemy from simply capturing the controlling station and deactivating the mines), detonated on a signal or be allowed to detonate on their own. Unlike contact mines, they do not put friendly shipping at risk.
Influence mines are laid down as moored, or, more commonly, bottom-lying mines. They can also be mobile, either drifting or self-propelled. For detonation trigger, they use ship’s signature – acoustic, electromagnetic, hydrostatic, or some combination of those. Modern mines can recognize correct target by comparing signature of a target to inbuilt databases. In order to better mask their presence, mines use techniques and devices such as:
- ship counter, which detonates the mine after a predetermined number of ships passes over it; that way, a mine might damage sixth ship in the line
- target selection, where mine detonates only if a ship of certain characteristics passes over it
- self-protection device, which self-destroys the mine if the enemy attempts to disarm or otherwise remove it
- acoustic paint which reduces mine’s sonar return
Modern magnetic mines use total field magnetometer which responds to changes in magnitude of the total background field, allowing it to detect even degaussed ships. Acoustic mines also use narrow-band hydrophones which are much more reliable and effective than the original broadband hydrophones of World War II. They can also be programmed to listen for highly specific acoustic signatures (e.g. a gas turbine powerplant and/or cavitation sounds from a particular design of propeller) and ignore all others, allowing some level of IFF capability. Mines equipped with several sensors are very hard to detonate as all aspects of intended target’s signature would have to be accurately imitated. Modern influence mines are also programmable, allowing them to quickly load new signatures into fuses, or to program them to detect a single distinctive target signature.
Moored mines have the advantage in that they are effective against both surface ships and submarines and can be deployed in large numbers. They have lifetime of more than 10 years. Usual weight is 200 kg with 80kg of explosive. In excess of 150 kg explosives mine becomes inefficient, as it becomes too large to handle and the extra explosive adds little to mine’s effectiveness.
Bottom mines are used against surface ships in water up to 60 meters deep, and against submarines in water around 200 meters deep. They are hard to detect, utilize multiple types of sensors and typically carry much larger warhead than a moored mine. Typical weight is 150 – 1.500 kilograms with 124 – 1.400 kg of explosives.
The anti sweep mine is a very small mine (40 kg warhead) with as small a floating device as possible. When the wire of a mine sweep hits the mine, it “sinks”, letting the sweep wire drag along the anchoring wire of the mine until the sweep hits the mine. That detonates the mine and cuts the sweeping wire.
Torpedo mines are typically deployed by submarines. They are of a self-propelled variety, which means that the minelayer does not have to approach the target area. This allows the mines to be laid down in normally inaccessible areas, or ones that are too heavily defended to approach. Presence of propulsion unit allows them to lie in wait for target and then pursue (e.g. Mk.60 CAPTOR).
Dummy mines are plastic drums filled with sand or concrete, designed to imitate signatures of real mines. They are laid down alongside the real mines, and are intended to slow down the process of mine clearance as a mine-hunter has to investigate every sonar contact.
Mines can be used both offensively and defensively. In the offensive application, they can be laid down by submarines or aircraft to hamper enemy movement, including locking enemy vessels inside harbours. They can also be used in widely spread minefields to cut off merchant shipping – a single mine inserted strategically on a shipping route can stop maritime movements for days while the entire area is swept. When trying to block enemy harbours, there is a problem of actually getting the mines there as any minelaying vessels will be attacked. As a result the effort has to be covert, which prevents laying down large number of mines and thus places emphasis on careful positioning of laid mines. However, as modern minesweepers are expensive and few in number, defending all the ports all the time is impossible, which makes such minefields potentially very effective. Mines can also be used to restrict warships to certain kill zones, and to draw valuable mine warfare ships into positions where other forces can neutralize them. In the defensive application, they can be used to create safe zones and limit possible routes of attack by surface ships and even submarines. This way they can defend critical coastal cities and naval installations, but this makes minefields relatively predictable. They can be laid down by almost any platform: purpose-built minelayers, refitted ships, submarines, aircraft, divers. Main attraction of the mines is their low price. A mine can cost from 1.000 to few million USD, and typically the cost of producing and laying a mine is from 0,5% to 10% of the cost of finding and removing one. It can take up to 200 times as long to clear the minefield as it took to lay it down – parts of World War II minefields still exist. This makes mines attractive to less powerful belligerents in the assymetric warfare, especially when supported by other assets.
In riverine warafre, mines may be simply dropped into river and let to float downstream into the target area. Static mines may be installed into river banks and set to detonate when target passes; such devices need not be mines in strict sense, but can also be minethrowers, rocket launchers, fougasses etc. These techniques are typically used by irregular forces. However, conditions of flowing water prevent the use of acoustic or pressure mines which would be detonated by water and flow conditions, or at least rendered useless. Magnetic mines are not much of a threat to wooden craft, which means wooden riverine craft useful for military purposes. Floating mines are easily spotted visually, and contact mines are likely to be detonated by contact with the riverbed or rocks long before getting to the target area. However, moored contact as well as off-route mines are a very serious problem, and latter are almost insoluble. Restrictions caused by rivers also mean adoption of very small mine hunting ships which, due to limited capability and operating conditions (such as attacks from the shore) are subject to high operational attrition. Best solution, especially due to limited depth of rivers, is usage of manned or unmanned minesweeping aircraft.
Platforms used for mine laying include submarines, aircraft, surface ships, converted merchant ships, combat boats, camouflaged boats and divers. They can also be dropped from the shore. When laying down a mine each one is recorded for the later removal, but these recording may be lost if the ship is destroyed. Therefore many countries demand that all mining operations shall be planned on land and records kept so the mines can later be recovered more easily. In some cases, mines are automatically activated on contact with water. In other cases, a safety lanyard is pulled to start the countdown process, which typically takes some minutes to complete so as to allow the minelayer to clear the mine’s effect area before the activation. In the 1960s, USSR developed torpedo and missile mines which were copied by other navies, and significantly simplified process of mine laying.
The damage that may be caused by a mine depends on the “shock factor value”, which is the combination of initial strength of the explosion with distance from the target. It can be taken in reference to either keel (keel shock factor) or hull (hull shock factor). Direct damage is typically caused only by contact mines, which blow a hole in the ship. Among the crew, fragmentation wounds are the most common form of damage. Flooding occurs in one or two watertight compartments, which is sufficient to sink smaller ships or disable larger ones. Contact mine damage is most common at the waterline in the bow area, but can occur anywhere on the ship (e.g. USS Samuel B. Roberts attack, where mine detonated amidships and underneath the ship). When a mine detonates in the water a short distance away from the ship it causes a bubble in the water. Due to differences in the pressure, this bubble collapses from the bottom while moving towards the surfac. If it collapses onto the ship’s hull, a high energy water jet formed by the collapse can break a meter wide hole straight through the ship, flooding one or more compartments, and is capable of breaking smaller ships apart. If a mine detonates away from the ship, the change in water pressure causes the ship to resonate. This can cause serious damage to the whole ship, causing multiple small cracks across the plating and destroying or disabling engine and power systems; with no way to power the pumps, ships quickly sink.
Ships can be designed so that they are difficult for mines to detect. For this reason, minesweepers typically have hulls of glass fibre or wood to reduce magnetic signature, and limit the acoustic signature by utilizing special propulsion systems. Hull shapes are also designed to reduce pressure signature. But because of this they are expensive, slow and vulnerable to enemy fire. Many modern ships also have a mine-warning sonar, a simple forward-looking sonar which warns the crew if the mines are ahead, but it is only effective when the ship is moving slowly.
A steel-hulled ship can be degaussed using a special degaussing station that contains many large coils and induces a magnetic field in the hull with alternating current to demagnetize the hull. This is a rather problematic solution, as magnetic compasses need recalibration and all metal objects must be kept in exactly the same place. Ships slowly regain their magnetic field as they travel through the Earth’s magnetic field, so the process has to be repeated every six months. This solution is also of limited effectiveness against mines that detect background magnetic signature.
Active countermeasures and counter-mine combat
Naval mines, especially the bottom-laid ones, are hard to notice due to accumulation of seaweed and, in the bottom-lying mines, mud. As mines always have the advantage over the ships, one of main ways of hunting for mines is usage of minesweeping devices which as accurately as possible duplicate ship’s acoustic and magnetic characteristics. For detection of mines, there are onboard sonars, as well as offboard sonars, capable of sailling in front of the minesweeper.
Characteristics of the waves in the surf zone are significantly different from those at the open sea due to interference of the shore. In the shallows, the path of the particles becomes an ellipse. However, at the cliffs, particles collide with the shore, retreat only to collide with ones behind, and then repeat the process. Surf zone in military terminology extends from the shore to some three meters of depth. In about three quarters of shores, surf zone will extend around 350 meters from the coast. This means that it is easy to build very effective defenses against an amphibious assault.
In shallow waters, countermine combat utilizes remotely piloted vehicles as well as groups of divers. These destroy mines by planting small explosive charges designed specifically for the purpose. Marine Mammal System has also very successfully utilized groups of specially trained dolphins. For detecting the mines, unmanned aircraft utilizing the COBRA system can detect mines in 50 meter wide area. Another possibility is utilizing aircraft with laser-based mine detection system. These can then be combined with systems for destruction of mines from the air.
Remotely piloted vehicles allow the ship to carry out tasks of locating, identifying and destroying the mines without placing itself into danger. These can be transported in ISO containers and easily mounted on any ship large enough to acept it (for Sea Sleuth system, requirement is a ship of 300-500 mt displacement, 20-40 m length and 7×10 m deck). Unmaned Influence Sweep System is an unmanned system which fools mines into detonating by imitating ship signatures (both acoustic and magnetic). However, weakness of that approach is that it can only detonate one contact mine, and unmanned systems are not cheap (and it is not operational yet). Another RPV which will be carried on LCS is Knifefish, which uses Synthetic Aperture Sonar to detect and identify floating or buried mines. It then creates a map of mines’ locations and sends it to the mothership, which can then destroy the mines. UISS and Knifefish are intended to operate in tandem.
Air cushion vehicles show significant promise in counter-mine combat. Compared to conventional ships, LCAC has high speed of transit, ability to carry out tasks irrespective of undersea terrain, ability to function at unprepared beaches with minimum of equipment, high resistance to underwater explosions, and large deck area. They are mostly immune to mines and torpedoes, which along with listed characteristics makes them ideal for both counter-mine combat and desant operations. One of reasons for this resillience is the fact that they hover above the sea, which reduces shocks from underwater explosions, but also the fact that hydrodynamic, magnetic and acoustic fields of hovercraft are significantly reduced compared to conventional hull forms.
Types of sweeping differ based on the types of mines that are being countered. Contact sweep is a wire dragged through the water by one or two ships to cut the mooring wire of floating mines, while a distance sweep mimics ship’s signature to detonate the mines. During the sweep, ship has to move slowly in a straight line, making it vulnerable to enemy fire. Minesweepers protect themselves with an oropesa or a paravane, torpedo shaped towed bodies that are streamed from a sweeping vessel. These can also be dragged by helicopters. The distance sweep is fitted with floating coils and large underwater drums to mimic the sound and magnetism of a ship, and is the only type of sweep effective against bottom mines.
Modern influence mines are designed to discriminate against the false input and are thus hard to sweep. They can be programmed to respond to unique noise of a particular ship type, its magnetic signature and typical pressure displacement. As a result, a mine-sweeper must accurately guess and mimic the required target signature in order to trigger detonation. Mines may also be set to detonate themselves only after the trigger has been activated a preset number of times, or to arm and/or disarm themselves after a preset time. During the arming delay, which could be days or weeks, mine would ignore any stimulus – either fake or real.
When laid in a minefield, influence mines may have various combinations of fuze settings configured. These would include different times of arming delay, reacting to different types of signatures, using different sets of sensors to detect ships, having a different number of contacts required to activate the fuze. As a result, it is impossible to be entirely certain that an area has been cleared, as even a seemingly safe area where a succession of vessels has passed without a loss may in reality be mined.
Mine hunting is different from sweeping in that mines are searched for individually via sonar, then inspected and destroyed. As a result it is much slower than mine sweeping, but also far more reliable. Sea mammals – especially dolphins – have also been trained for mine hunting.
Mine breaking is a more drastic method where a cargo ship is loaded with cargo that makes it less vulnerable to sinking (e.g. wood) and driven through the minefield, with protected ship following the same path. Another possibility is using a high-speed shallow draught vessel and driving it through the minefield, generating a pressure wave sufficient to trigger mines, with the minesweeper moving fast enough to be sufficiently clear of the pressure wave so that triggered mines do not destroy the ship itself. These techniques are the only way to sweep pressure mines. Both techniques are easily countered by setting a mine to explode only after a certain number of ships have passed. As a result, modern doctrine calls for mines to be hunted rather than swept. An updated form of mine breaking is the use of small unmanned ROVs that simulate the acoustic and magnetic signatures of larger ships and are built to survive exploding mines. Several sweeps are required in case one or more of the mines had its “ship counter” facility enabled i.e. were programmed to ignore the first 2, 3, or even 6 target activations.
Hrvatski Vojnik, Broj 7, godina VI, Siječanj 1996 (Croatian Soldier, No. 7, Year VI, January 1996)
Hrvatski Vojnik, Broj 8, godina VI, Veljača 1996 (Croatian Soldier, No. 8, Year VI, February 1996)