The invention concerns a method for the detection of the discharge of hydrocarbons which are in liquid form at normal ambient temperature, in water, on the ground or in the subsurface, especially for the detection of oil spillage on water or in ground water. The invention also concerns a detector device for the execution of the method according to the invention.

The discharge and spillage of hydrocarbons, above all oil, into the natural surroundings is considered to be a serious problem, with grave consequences for the environment when it takes place. The risk of this kind of accident happening is very great, due to the wide-spread use of hydrocarbons in liquid form in practically every area where mankind travels and lives. The danger of discharge is particularly great during the storage and transport of hydrocarbons. Examples of this kind of discharge can be the leakage of oil from tanks and pipelines into the surface of the ground or the subsurface, thus causing the oil to be deposited in the ground water, or discharges during loading and unloading of vessels and vehicles or in connection with transport of these. Finally, oil spillage in surface water or on the sea is one of the major environmental problems of our time, e.g. in connection with shipwrecks or discharges in connection with oil production at sea.

In order to be able to issue a warning concerning the discharge of hydrocarbons and oil spillage, a number of methods have been developed, both for in-situ detection and remote surveillance. These methods are based on the use of more or less expensive instrumentation or designed for periodic surveillance of vulnerable areas, e.g. from the air.

However, there is a need for a simple detector which is inexpensive to produce and which at the same time can be used for permanent surveillance of areas which are threatened by hydrocarbon discharge such as oil spillage.

Thus the object of the present invention is to develop a simple and inexpensive detection principle and thereafter to use this detection principle in a simple detector device which permits the detection of the discharge of h dro- carbons in liquid form both at sea, on the ground and in the subsurface.

The above-mentioned objects are achieved with a method which is characterized by the features described in the characteristic of claim 1 and by a detector device according to the invention characterized by the features which are described in the characteristic of claim 11. Further features and advantages of the method and the detector according to the invention are presented in the independent claims 2-10 and the independent claims 12-21 respectively.

The invention will now be explained in more detail in connection with embodiments of the detector according to the invention taken in conjunction with the accompanying drawing.

Fig. 1 is a detector device according to the invention, based on the use of ultrasound.

Fig. 2 is a detector device according to the invention, based on the use of light reflection.

Fig. 3 is a second embodiment of a detector device according to the invention, based on the use of light reflection.

Fig. 4 is a detector device according to the invention, based on the measurement of the dielectric constant.

Fig. 5 is a detector device according to the invention, based on the measurement of buoyancy.

Fig. 6 is a detector device according to the invention, based on a combination of two different detection prin¬ ciples.

Fig. 7 is a detector device used for the detection of leakage from a buried oil tank.

The method and the detector device according to the present invention are based on the use of a material which has the ability to attract hydrocarbons in liquid form, e.g. oil or other chemicals with a hydrocarbon basis and which exist in liquid form, while at the same time the material does not absorb or become moistened by water. A material of this kind is used in an absorbent body, and by absorbing hydrocarbons such as oil will alter certain of its physical parameters.

In the present invention it has been preferred to use a polypropylene cloth of the type "ERGON Sorb" supplied by HP Textiles, Oldenzaal, Holland. "ERGON Sorb" is a so-called "melt-blown" polypropylene cloth which is supplied in white in different thicknesses. The cloth can absorb 11.2 times its own weight in oil, but only attracts 0.023 times its own weight in water in the course of 15 minutes. Con¬ sequently "ERGON Sorb" has a relative affinity oil/water of 11.2/0.023 or a selective absorption capacity for oil which is 487 times greater than that for water. If the "ERGON Sorb" cloth is placed under water pressure or compressed under water, it will absorb some water, but the cloth will not become saturated, nor will the cloth's ability to attract hydrocarbons such as oil be destroyed.

It should be understood, however, that other materials of a similar kind which demonstrate a high selective absorption capacity with regard to hydrocarbons such as oil can also be utilized. Consequently, the embodiment using the "ERGON Sorb" cloth is only intended as an example of a preferred embodiment of the detector device according to the invention and is not intended to limit this in any way.

Fig _* 1 shows a detector device based on the use of ultrasound. For this purpose the detector device is equipped with an ultrasound sensor consisting of an ultrasound transducer 1 and an ultrasound reflector 2. The ultrasound transducer and the ultrasound reflector can be a combined device with a piezoelectric element cast in a housing as illustrated in fig. 1. The absorbent body 3, e.g. in the form of an "ERGON Sorb" cloth, is placed either around the transducer 1 or as shown here, between the transducer 1 and the reflector 2. An acoustic jelly 4 is provided between the transducer 1 and the absorbent body 3 in order to improve the acoustic coupling between the transducer 1 and the absorbent body or cloth 3. When the cloth 3 is not moistened with oil, its ultrasound trans¬ missivity is very poor and consequently the ultrasound signal will not penetrate the cloth. If the cloth 3 is moistened with oil, it has good ultrasound transmissivity and a signal transmitted from the transducer l will penetrate the cloth 3, be reflected from the reflector 2 and in turn be detected in the transducer 1. It should be understood that the detector device also may be designed with a transmitter in the form of an ultrasound transducer together with a receiver in the form of a second ultra¬ sound transducer, the transmitter and the receiver respectively being provided on opposite sides of the absorbent body 3, i.e. the "ERGON Sorb" cloth.

A detector device based on an ultrasound sensor is robust and provides a good signal-noise ratio by using the pulse echo technique. The disadvantage of using an ultrasound

sensor in a detector device according to the invention is that it can only be used in water, while at the same time a certain amount of electronics are required for pulse generation and detection. Moreover, the arrangement of the transducer and the reflector is critical with a view to obtaining a good echo, nor should there be any bubbles in the water. However, an increase in pulse level not only provides a stronger echo, but also makes the transducer's position in relation to the reflector less critical.

A second sensor for use in the detector device according to the invention is shown in fig. 2. This sensor is based on an alteration in the optical reflectance of the absorbent body 3 when the body is moistened with, e.g., oil. If, e.g., an absorbent body 3 in the form of an "ERGON Sorb" cloth is used, this cloth is basically white and is a very good reflector of light. The difference in reflectance with a moistened and an unmoistened cloth will be dependent on the colour of the oil. It is obvious that a black oil like unrefined oil, waste oil or the like will cause a greater alteration in the reflectance than a clear oil. In the detection of the optical reflectance a sensor is used based on an optical transmitter 6, e.g. a photo-diode, and an optical receiver 7, e.g. a phototransistor. The transmitter and the receiver 7 can be installed in the same plane in a sensor housing 5 and shielded from direct light between transmitter and receiver. In connection with the optical transmitter 6 electronics will be provided which may cause a modulation of the light, a feature which is well known in the art. The measurement of reflected light is performed via the phototransistor and the results of the measurement are referred to the measurement conducted with a dry absorbent body, i.e. an unmoistened sheet. In purely practical terms the detector device in this case can be de- signed in such a way that the optical transmitter 6 and receiver 7 are provided in the same plane in the sensor housing 5 which is open at one end, but with the opening

covered by the absorbent body , e.g. in the form of a piece of white "ERGON Sorb" cloth.

As illustrated in fig. 3, the detector device can also be equipped with an optical sensor with two transmitters 6 and receivers 7 respectively, provided in a respective chamber 12a,b in the detector housing 5, and connected to a supply cable 10, an opening of the chamber being covered in turn by the absorbent body 3 and the detector designed in such a way that the absorbent body will be in contact with the surface of the liquid, for instance by providing a buoyance block 11, if the detector is intended for use in water. In this case the respective transceiver pair 6, 7 can be arranged to perform the measurement in different wavelength frequencies. One transceiver 6,.7, for example, can perform a measurement in the UV range, while the other can perform a measurement in the IR range. This can be expedient if the absorbed hydrocarbon has a different reflectance in the two wavelength frequencies. Thus the measurements from each of the transceiver pairs can be appropriately correlated in order to provide a more reliable detection. The measure¬ ments can naturally also be conducted in the wavelength frequency for visible light and there is nothing to prevent an additional pair of transceivers from being used.

If the liquid hydrocarbon is colourless, i.e. clear, the reflectance value referred to that for an unmoistened cloth will show little alteration. In order to provide a more reliable detection the absorbent body 3, i.e. the "ERGON Sorb" cloth can therefore be impregnated with a hydro¬ carbon-soluble dye 13. In practical terms this can be done be providing the absorbent body 3 with a dye-impregnated intermediate layer 9. If it becomes moistened with a liquid hydrocarbon such as oil, the hydrocarbon penetrates into the intermediate layer 9 and dissolves the dye, thus adding colour to the absorbent body 3 and providing a greater alteration in the reflectance signal.

Experiment shows that a detector device based on an optical sensor provides a simple and robust solution. The necessary electronics, which are otherwise well known to those skilled in the art, are also simple as well as inexpensive and in addition the measurement results show a satisfactory difference between a dry and a moistened absorbent body.

A third embodiment of the detector device according to the invention can be based on the measurement of the dielectric constant for the absorbent body 3. There will be a difference in the dielectric constant for a dry and a moistened absorbent .body 3. Oil, e.g., has a relative dielectric constant of 2-3. If the "ERGON Sorb" cloth is used as the absorbent body, it can be assumed that it has the same dielectric constant as air, i.e. a relative dielectric constant of 1. This means that the absorbent body's dielectric constant will alter when it is moistened with oil. In purely practical terms the detector device in this case can be equipped with a sensor as illustrated in fig. 4. The actual sensor consists of a plate condenser 4 with the absorbent body 3 as dielectric medium. The plate condenser 15 is provided in series with an ohmic resistance 13 across the input and output of an oscillator and similarly in series between the input and output of a not shown voltmeter device, the condenser 15 thus being located in a parallel branch 16 between the oscillator 14 and the voltmeter device. The alteration in condenser voltage, i.e. in the condenser's capacitance, will be a measure of the alteration which is obtained in the dielectric constant when the absorbent body 3 is moistened or dry. The problem with a detector device based on the measurement of the dielectric constant, however, is that first of all the detection sensitivity is slight and even only a low absorption of water in the absorbent body 3 will probably lead to a relatively substantial alteration in the measurement values, since the dielectric constant for water is approximately 80 times that for air. The advantage of a detector device according to the invention,

based on the measurement of the dielectric constant, is that it only requires very simple electronics.

If the detector device according to the invention has to be used on water, the detection of discharge of hydrocarbons can employ an alteration in the absorbent body's buoyancy. If an "ERGON Sorb" cloth is used as the absorbent body, it will basically have a low specific weight and float easily on the water. At the same time the cloth naturally attracts very little water, but can absorb a large amount of oil, thus causing the buoyancy to alter radically after the absorption of oil. This can be used in a detector device where the sensor is designed as a buoyancy sensor, as illustrated in fig. 5 which shows the sensor in elevation in (A) and (C) and from above in (B) . The detector as shown in the figure is designed as a floating body 18 which is connected via a pipe 20 to the absorbent body 3 in the form of a piece of "ERGON Sorb" cloth. Inside the pipe 20 there is provided a ball 19 which can move freely. The detector device is arranged in such a way that it will float on the surface 17 of the water (Fig. 5(A)) . After the cloth 3 has absorbed the floating hydrocarbon or oil, it will sink. The ball 19 in the pipe 20 is intended to provide a hysteresis effect, but, after a certain amount of oil has been absorbed in the absorbent body 3, it will roll through the pipe 20 towards this and cause the detector to change position, e.g. so that the floating body 3 is tipped up (Fig. 5(C)), giving a visual indication of the oil discharge. The alteration in the position of the detector by means of a change in the buoyancy of the absorbent body 3 can also be indicated by its being detected by a switch mechanism which triggers an electrical signal which is transferred to a receiver station. The electrical signal may also trigger a flashing light mounted on the detector device, or trigger an acoustic signal or activate a radio direction finder.

It should be understood that the different sensor types described here with a view to using them in the detector device according to the invention can be used separately or combined in an integrated detector device. Thus the detector device can, e.g., be based both on an ultrasound sensor 21 and an optical sensor 22, as illustrated in fig. 6, wherein the reference numbers denote parts similar to those in Figs. 2 and 3, and the signals from the two sensors can be correlated in order to provide a more reliable indication of the detection of discharge of hydrocarbons such as oil.

If the method and detector device according to the invention have to be used in the detection of hydrocarbon discharge on water, the detector device can be designed as a small buoy and possibly be anchored at a suitable spot.

Designed in this way, it will be possible to deploy the detector device according to the invention in great numbers and use them to monitor shipping lanes and oil terminals where there is a high risk of discharge of hydrocarbons. The detection signals can then be transferred from each individual detector via a cable system to a central monitoring unit or they may also be transferred by means of a miniaturized radio transmitter provided in the detector. Free-floating detectors can be designed as floating buoys and equipped with a radio direction finder. In this case the direction finder will be capable of transmitting a signal automatically when a discharge is detected, thus enabling the position of the discharge to be determined.

When the detector device is used in the water, it is of course based on the assumption that it must be designed in such a manner that the absorbent body, which itself is basically buoyant, is situated on or near the surface of the water where the discharge of the liquid hydrocarbon will be located. A person skilled in the art will soon realize that there are a number of ways in which a detector

device according to the invention can be designed in practice, in order to ensure that the absorbent body absorbs discharge on the surface of the water.

The method and the detector device according to the present invention can naturally also be used for the detection of the discharge of hydrocarbons on the ground, e.g. in the form of leakage from tanks, pipelines or transport vehicles. This is again based on the assumption that the detector device is so positioned and with the absorbent body arranged so as to enable absorption of the discharge to take place and reliable detection to be achieved. Similarly the detector device can be located in connection with buried oil tanks or the like and detect a leakage from the oil tank as illustrated schematically in fig. 7, showing the oil tank 23 burried beneath the surface level 24 and below the ground water level 25. The detector 26 is provided in a detector tube 27 at the ground water level 25 and connected to signal cable 28, the detector tube 27 providing a passage between the detector 26 and an oil trap 29. The whole arrangement with the tank is enclosed in tarpaulin 30.

Finally the detector can also be used for the detection of oil pollution in the ground water in which case it naturally does not need to be located in the vicinity of the pollution source. On the contrary, such buried detectors connected with a monitoring system can be used for rubbish dumps, industrial sites and the like where the possibility exists of a leakage of liquid hydrocarbons into the subsurface. By using a network of such buried detector devices and recording the times of detection, it will be possible to localize the source of pollution.

In a more comprehensive system for monitoring the discharge of liquid hydrocarbons and oil spills, it will naturally be expedient to be able to connect a number of detector devices to a central monitoring unit, which devices are

arranged so as to transmit detection signals to the monitoring unit. This transmission can be performed automatically or by the central monitoring unit polling the individual detector in the system according to a set plan. Systems of this kind are well known in the art in con¬ nection with alarm and monitoring systems and a person skilled in the art will also soon realize that the method and the detector device according to the invention within the scope of the attached claims can be implemented in other ways than those described here or used in other connections than those mentioned above, without deviating from the spirit and scope of the invention.