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Use of airborne remote sensing equipment for detecting discharges of oil from offshore installations
Oil & Chemical Pollution 6 (1990) 69-80
Use of Airborne Remote Sensing Equipment for Detecting Discharges of Oil from Offshore Installations
N. H u r f o r d & D. Tookey Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2BX, UK (Received 6 March 1989; accepted 14 April 1989)
ABSTRACT The use of airborne sensing techniques to detect discharges ofoil from offshore installations has been investigated. It is shown that side-looking airborne radar can provide initial detection of oil-like targets from a range of15 km; the presence ofoil can then be confirmed by investigation at close range with a combined infrared~ultraviolet line scanner. During 154 h of flying, 21 oil slicks were detected near offshore installations," however, most of these slicks contained less than 100 litres of oil.
1 INTRODUCTION
In recent years there has been increasing concern about levels of pollution in the North Sea and, through the forum of the International Conference on the Protection of the North Sea, the coastal states have agreed to maintain their efforts to monitor and reduce pollution in the North Sea area. Against this background, pollution from offshore oil exploration and production has come under particular scrutiny. This is hardly surprising as accidents such as the Ekofisk blow-out, which result in large amounts of oil being discharged into the sea, receive wide publicity. Fortunately, such large-scale releases of oil are rare; however, small spills can occur more frequently, and in the UI~ operators are required to report such spills promptly to the Department of Energy. In 69 © 1990 Crown copyright.
70
N. Hurford, D. Tookey
addition to accidental spillages, oil may be discharged into the sea as a result of operational activities such as the discharge of production water, though strict limits on the oil content of any discharges are imposed by the Department of Energy. In order to determine the amounts of oil discharged into the sea from offshore activities a number of North Sea states have carried out regular surveillance patrols (Cormack & Hurford, 1987). The surveys carried out by the Netherlands Rijkswaterstaat in 1985 and 1986 indicated the presence of significant amounts of oil near some installations. Although the aircraft used for these surveillance patrols was fitted with modern sensing equipment, extensive use was made of visual observation and aerial photography to quantify the oil. For this reason, the data presented were somewhat subjective and open to differing interpretations. Nonetheless, there were grounds for suspecting that offshore operators were not reporting all spills to the Department of Energy. Accordingly, the Department recognised the need to commission its own surveys to collect data on the amounts ofoil discharged from offshore installations. Because the Marine Pollution Control Unit (MPCU) of the Department of Transport had recently installed sensing equipment in a light aircraft it was appropriate to seek to use this equipment for the offshore surveys. However, the MPCU had obtained the equipment as an aid to its spillage response capabilities and it was not clear how suitable such equipment would be for detecting discharges of oil from offshore installations. Accordingly, it was decided that Warren Spring Laboratory (WSL) should undertake a study to determine the feasibility of using the M P C U sensing equipment to detect discharges of oil from offshore installations. Operational procedures that would ensure that such surveys could be carried out as effectively as possible were also to be developed.
2 DETECTION OF OIL BY REMOTE SENSING The equipment fitted in the MPCU aircraft (a Britten-Norman 2A Islander) was the Maritime Surveillance System (MSS) built by the Swedish Space Corporation (SSC). The MSS comprises side-looking airborne radar (SLAR) and a combined infrared/ultraviolet line scanner (IR/UVLS); detailed technical specifications for the sensors are given in Table 1. It was intended that the SLAR be used for oil detection at long range, with the IR/UVLS used subsequently to provide information on
Remote sensing of oil dischargesfrom offshore hzstallations
71
TABLE 1
Technical Data for SLAR and IR/UVLS SLAR Frequency Polarisation Peak Power Resolution IR/UVLS Frequency
Resolution Field of View Sensitivity
X-band,9.3 GHz Vertical 10 kW 75 × 75 m IR channel 8-14/~m UV channel 0-3-0.4/~m 2 m at 1 000 ft (305 m) 80° 0.5 K (IR)
slick dimensions and estimates of oil quantity. A full description of the MSS has been given by Loostrom (1987). WSL has carried out a detailed evaluation of the use of SLAR and IR/UVLS for detecting oil (Hurford & Tookey, 1987). This evaluation showed that oil targets could be detected by SLAR at ranges up to 15 km. This implied that the SLAR would be very useful for pollution monitoring purposes as large areas of sea could be surveyed in a relatively short period of time. However, the evaluation had also shown that the SLAR was best suited to detecting relatively large slicks (say greater than 5 km X 5 km). The slicks arising from offshore installations were expected to be relatively narrow, and as the spatial resolution of the SLAR was 75 m there was some doubt whether the SLAR would be sufficiently sensitive to detect these slicks. Furthermore, because the SLAR detects oil slicks by measuring changes in surface roughness, there was the possibility that other aspects of offshore activities might lead to an apparent s m o o t h i n g of the sea surface near installations. If this proved to be the case, m a n y installations would give rise to oil-like targets in the SLAR even though no oil was present. Accordingly, it was not clear whether it would be possible to use the SLAR to distinguish between notionally 'clean' and 'dirty' installations. If the SLAR could not be used in this way it would be necessary to overfly all installations and carry out investigations with IR/UVLS. Because of the large number of installations on the U K continental shelf this would be a time-consuming process. Clearly, the first stage of the work was to resolve the uncertainties about the role of the SLAR; this could only be achieved by carrying out surveys and analysing the results obtained.
72
N. Hurford, D. Tookey
3 SURVEILLANCE, PROCEDURES AND ANALYSIS OF IMAGERY
3.1 SLAR survey In order to obtain the best SLAR imagery and maximise the chances of detecting oil it was essential that the aircraft flew as straight a course as possible. This also helped to reduce the number of stray effects caused by the aircraft turning. As the aircraft flew along the survey route, the operator watched the monitor screen for any oil-like targets; these would appear as black areas against the grey background, see Fig. l(a). When an oil-like target was detected the operator would mark it with a lightpen which would result in its position being displayed in the data block at the bottom of the screen. After target-positioning, the SLAR blockexpansion facility would be used to examine the target in more detail, see Fig. l(b).
3.2 Investigation with IR/UVLS After detection by SLAR an oil-like target would be investigated with IR~S; exactly when this investigation was carried out was left to the pilot. Normal practice was to investigate the target immediately with IR/UVLS, but sometimes it was more convenient to continue with the SLAR survey and carry out the IR/UVLS investigation of the SLAR targets at the end of the route. When carrying out the IR/UVLS investigations it was essential to fly along the slick towards the platform so that imagery could be obtained before the IR detector was saturated by the flare on the installation. These IR/UVLS investigations were normally carried out at a height of 1000 ft (305 m). Figure 2 shows the IR/UVLS imagery obtained by investigating the SLAR target shown in Fig. 1; the saturation of the IR detector as the aircraft flew over the installation is apparent.
3.3 Analysis of imagery The output from the sensors was recorded on digital tapes which were subsequently analysed using a data evaluation terminal (DET) manufactured by the SSC. The DET enables the digital tapes to be replayed at five times the recording speed without any loss of image quality; there are also a number of image processing functions which assist analysis of the imagery. Hard copies of the imagery were obtained using a video graphics recorder which was connected to the DET.
Remote sensing of oil dischargesfrom offshore installations
73
(a)
(b) Fig. 1. Detection ofoil target by SLAR. (a) Normal 30 × 20 km mode; (b) block expand.
N. Hurford, D. Tookey
74
•
~'i~i'ii! ~::~ ¸ ~
t
~ilil ~,~ ...... ~i/~:~ ~!~z,ii i !i!~i
Fig. 2. I R / U V L S i n v e s t i g a t i o n of a n oil target.
Remote sensing of oil dischargesfrom offshore installations
75
The volume of oil in each slick was calculated by measuring the areas of the slicks in both the IR and UV imagery. The area in the UV imagery was assigned a thickness of 0.01/~m whilst the area in the IRwas assigned a thickness of 10/3m. These values for slick thickness are based on previous experimental work aimed at determining the minimum thicknesses detectable by IR/UVLS (Hurford and Tookey, 1987).
4 RESULTS AND DISCUSSION 4.1 Detection ofoil
Figure 1 shows that the SLAR is capable of detecting narrow slicks near offshore installations up to a range of 15 km. Furthermore, it was found that the SLAR was able to detect slicks even when subsequent investigation by IR/UVLS showed that they contained very small amounts of oil. For example, Fig. 3 shows an oil-like target near an installation; investigation by IR/UV showed that oil was detected by UV but not by IR, which implies that the discharge consisted only of thin sheen. Not surprisingly, SLAR targets were detected most easily when they contained thick oil and were less than 10 km from the aircraft flight path, see Fig. 4. Occasionally, oil-like targets have been detected by SLAR but subsequent investigation by IR/UVLS has not revealed the presence of any oil, see Fig. 5. It is not clear how these 'stray' targets are produced but their existence emphasises the need for investigation by IR/UV-LS to confirm the presence of oil; detection by SLAR alone is not sufficient. Fortunately, these 'stray' targets occur infrequently and do not greatly reduce the effectiveness of the SLAR in identifying installations which should be investigated by IR/UVLS. 4.2 Number and size of oil targets
Surveillance flights took place on a monthly basis and involved some 154 h of flying between October 1986 and September 1987. During that time 34 oil-like targets were detected by SLAR; 23 of these were subsequently investigated by IR/UVLS and 21 were found to contain oil. As can be seen from Table 2, most of these discharges contained <100 litres of oil, and only one slick contained > 1000 litres of oil. These data suggest that the overall standard ofoil discharge control by the platforms is high. Despite the small amounts of oil detected, the surveillance patrols have provided an incentive for operators to improve the reporting
N. Hurford, D. Tookey
76
(a)
(b)
Fig. 3. Detection of thin oil. (a) SLAR; (b) IR/UVLS.
Remote sensing of oil dischargesfrom offshore installations
(a)
(b)
Fig. 4. Detection of thick oil close to aircraft track. (a) SLAR; (b) IR/UVLS.
77
78
N. Hurford, D. Tookey
(a)
(b)
1
Fig. 5. Detection of 'stray' target. (a) SLAR; (b) IR/UVLS (no oil detected).
Remote sensing of oil dischargesfrom offshore installations
79
TABLE 2 Investigation of SLAR Targets
Volume range 0 < 1 litre 1-10 litres 10-100 litres 100-1 000 litres > 1 000 litres Total
Number of targets 2 7 3 6 4 1 23
of spills. As a result of the regular patrols, there has been an increase in the number of spills reported to the Department of Energy (DEn, 1987). In line with our findings, most of the reported spills are believed to contain ~relatively small amounts of oil.
5 CONCLUSIONS 1. Airborne surveillance using SLAR and IR/UVLS sensors can be used to provide effective surveillance of oil discharges from offshore installations. SLAR can be used to detect oil-like targets at ranges up to 15 km; however, subsequent investigation with IR/UVLS is required to confirm the presence of oil. 2. During 154 h of flying, 34 oil-like targets were detected. Twentythree of these targets were investigated by IR/UV, and 21 were confirmed as containing oil; 11 targets were not investigated. 3. Most of the slicks investigated contained <100 litres of oil. 4. Carrying out regular surveillance patrols encourages operators to report all oil spillages.
ACKNOWLEDGEMENTS The work described was funded by the Petroleum Engineering Division of the Department of Energy. The assistance of the M P C U in providing its surveillance aircraft is gratefully acknowledged.
80
N. Hurford, D. Tookey
REFERENCES Cormack, D. & Hurford, N. (1987). Current operational airborne surveillance activities in the FRG, the Netherlands, Norway, Sweden and the UK~ Oil & Chem. Pollut., 3, 245-56. Department of Energy (1987). Development of the Oil and Gas Resources of the United Kingdom, HMSO, London. Hurford, N. & Tookey, D. J. (1987). A detailed evaluation of the maritime surveillance system for oil slick detection. Oil & Chem. Pollut., 3, 231-44. Loostrom, B. (1987). The Swedish remote sensing system for maritime surveillance. Oil & Chem. Pollut., 3, 209-29.
Use of Airborne Remote Sensing Equipment for Detecting Discharges of Oil from Offshore Installations
N. H u r f o r d & D. Tookey Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2BX, UK (Received 6 March 1989; accepted 14 April 1989)
ABSTRACT The use of airborne sensing techniques to detect discharges ofoil from offshore installations has been investigated. It is shown that side-looking airborne radar can provide initial detection of oil-like targets from a range of15 km; the presence ofoil can then be confirmed by investigation at close range with a combined infrared~ultraviolet line scanner. During 154 h of flying, 21 oil slicks were detected near offshore installations," however, most of these slicks contained less than 100 litres of oil.
1 INTRODUCTION
In recent years there has been increasing concern about levels of pollution in the North Sea and, through the forum of the International Conference on the Protection of the North Sea, the coastal states have agreed to maintain their efforts to monitor and reduce pollution in the North Sea area. Against this background, pollution from offshore oil exploration and production has come under particular scrutiny. This is hardly surprising as accidents such as the Ekofisk blow-out, which result in large amounts of oil being discharged into the sea, receive wide publicity. Fortunately, such large-scale releases of oil are rare; however, small spills can occur more frequently, and in the UI~ operators are required to report such spills promptly to the Department of Energy. In 69 © 1990 Crown copyright.
70
N. Hurford, D. Tookey
addition to accidental spillages, oil may be discharged into the sea as a result of operational activities such as the discharge of production water, though strict limits on the oil content of any discharges are imposed by the Department of Energy. In order to determine the amounts of oil discharged into the sea from offshore activities a number of North Sea states have carried out regular surveillance patrols (Cormack & Hurford, 1987). The surveys carried out by the Netherlands Rijkswaterstaat in 1985 and 1986 indicated the presence of significant amounts of oil near some installations. Although the aircraft used for these surveillance patrols was fitted with modern sensing equipment, extensive use was made of visual observation and aerial photography to quantify the oil. For this reason, the data presented were somewhat subjective and open to differing interpretations. Nonetheless, there were grounds for suspecting that offshore operators were not reporting all spills to the Department of Energy. Accordingly, the Department recognised the need to commission its own surveys to collect data on the amounts ofoil discharged from offshore installations. Because the Marine Pollution Control Unit (MPCU) of the Department of Transport had recently installed sensing equipment in a light aircraft it was appropriate to seek to use this equipment for the offshore surveys. However, the MPCU had obtained the equipment as an aid to its spillage response capabilities and it was not clear how suitable such equipment would be for detecting discharges of oil from offshore installations. Accordingly, it was decided that Warren Spring Laboratory (WSL) should undertake a study to determine the feasibility of using the M P C U sensing equipment to detect discharges of oil from offshore installations. Operational procedures that would ensure that such surveys could be carried out as effectively as possible were also to be developed.
2 DETECTION OF OIL BY REMOTE SENSING The equipment fitted in the MPCU aircraft (a Britten-Norman 2A Islander) was the Maritime Surveillance System (MSS) built by the Swedish Space Corporation (SSC). The MSS comprises side-looking airborne radar (SLAR) and a combined infrared/ultraviolet line scanner (IR/UVLS); detailed technical specifications for the sensors are given in Table 1. It was intended that the SLAR be used for oil detection at long range, with the IR/UVLS used subsequently to provide information on
Remote sensing of oil dischargesfrom offshore hzstallations
71
TABLE 1
Technical Data for SLAR and IR/UVLS SLAR Frequency Polarisation Peak Power Resolution IR/UVLS Frequency
Resolution Field of View Sensitivity
X-band,9.3 GHz Vertical 10 kW 75 × 75 m IR channel 8-14/~m UV channel 0-3-0.4/~m 2 m at 1 000 ft (305 m) 80° 0.5 K (IR)
slick dimensions and estimates of oil quantity. A full description of the MSS has been given by Loostrom (1987). WSL has carried out a detailed evaluation of the use of SLAR and IR/UVLS for detecting oil (Hurford & Tookey, 1987). This evaluation showed that oil targets could be detected by SLAR at ranges up to 15 km. This implied that the SLAR would be very useful for pollution monitoring purposes as large areas of sea could be surveyed in a relatively short period of time. However, the evaluation had also shown that the SLAR was best suited to detecting relatively large slicks (say greater than 5 km X 5 km). The slicks arising from offshore installations were expected to be relatively narrow, and as the spatial resolution of the SLAR was 75 m there was some doubt whether the SLAR would be sufficiently sensitive to detect these slicks. Furthermore, because the SLAR detects oil slicks by measuring changes in surface roughness, there was the possibility that other aspects of offshore activities might lead to an apparent s m o o t h i n g of the sea surface near installations. If this proved to be the case, m a n y installations would give rise to oil-like targets in the SLAR even though no oil was present. Accordingly, it was not clear whether it would be possible to use the SLAR to distinguish between notionally 'clean' and 'dirty' installations. If the SLAR could not be used in this way it would be necessary to overfly all installations and carry out investigations with IR/UVLS. Because of the large number of installations on the U K continental shelf this would be a time-consuming process. Clearly, the first stage of the work was to resolve the uncertainties about the role of the SLAR; this could only be achieved by carrying out surveys and analysing the results obtained.
72
N. Hurford, D. Tookey
3 SURVEILLANCE, PROCEDURES AND ANALYSIS OF IMAGERY
3.1 SLAR survey In order to obtain the best SLAR imagery and maximise the chances of detecting oil it was essential that the aircraft flew as straight a course as possible. This also helped to reduce the number of stray effects caused by the aircraft turning. As the aircraft flew along the survey route, the operator watched the monitor screen for any oil-like targets; these would appear as black areas against the grey background, see Fig. l(a). When an oil-like target was detected the operator would mark it with a lightpen which would result in its position being displayed in the data block at the bottom of the screen. After target-positioning, the SLAR blockexpansion facility would be used to examine the target in more detail, see Fig. l(b).
3.2 Investigation with IR/UVLS After detection by SLAR an oil-like target would be investigated with IR~S; exactly when this investigation was carried out was left to the pilot. Normal practice was to investigate the target immediately with IR/UVLS, but sometimes it was more convenient to continue with the SLAR survey and carry out the IR/UVLS investigation of the SLAR targets at the end of the route. When carrying out the IR/UVLS investigations it was essential to fly along the slick towards the platform so that imagery could be obtained before the IR detector was saturated by the flare on the installation. These IR/UVLS investigations were normally carried out at a height of 1000 ft (305 m). Figure 2 shows the IR/UVLS imagery obtained by investigating the SLAR target shown in Fig. 1; the saturation of the IR detector as the aircraft flew over the installation is apparent.
3.3 Analysis of imagery The output from the sensors was recorded on digital tapes which were subsequently analysed using a data evaluation terminal (DET) manufactured by the SSC. The DET enables the digital tapes to be replayed at five times the recording speed without any loss of image quality; there are also a number of image processing functions which assist analysis of the imagery. Hard copies of the imagery were obtained using a video graphics recorder which was connected to the DET.
Remote sensing of oil dischargesfrom offshore installations
73
(a)
(b) Fig. 1. Detection ofoil target by SLAR. (a) Normal 30 × 20 km mode; (b) block expand.
N. Hurford, D. Tookey
74
•
~'i~i'ii! ~::~ ¸ ~
t
~ilil ~,~ ...... ~i/~:~ ~!~z,ii i !i!~i
Fig. 2. I R / U V L S i n v e s t i g a t i o n of a n oil target.
Remote sensing of oil dischargesfrom offshore installations
75
The volume of oil in each slick was calculated by measuring the areas of the slicks in both the IR and UV imagery. The area in the UV imagery was assigned a thickness of 0.01/~m whilst the area in the IRwas assigned a thickness of 10/3m. These values for slick thickness are based on previous experimental work aimed at determining the minimum thicknesses detectable by IR/UVLS (Hurford and Tookey, 1987).
4 RESULTS AND DISCUSSION 4.1 Detection ofoil
Figure 1 shows that the SLAR is capable of detecting narrow slicks near offshore installations up to a range of 15 km. Furthermore, it was found that the SLAR was able to detect slicks even when subsequent investigation by IR/UVLS showed that they contained very small amounts of oil. For example, Fig. 3 shows an oil-like target near an installation; investigation by IR/UV showed that oil was detected by UV but not by IR, which implies that the discharge consisted only of thin sheen. Not surprisingly, SLAR targets were detected most easily when they contained thick oil and were less than 10 km from the aircraft flight path, see Fig. 4. Occasionally, oil-like targets have been detected by SLAR but subsequent investigation by IR/UVLS has not revealed the presence of any oil, see Fig. 5. It is not clear how these 'stray' targets are produced but their existence emphasises the need for investigation by IR/UV-LS to confirm the presence of oil; detection by SLAR alone is not sufficient. Fortunately, these 'stray' targets occur infrequently and do not greatly reduce the effectiveness of the SLAR in identifying installations which should be investigated by IR/UVLS. 4.2 Number and size of oil targets
Surveillance flights took place on a monthly basis and involved some 154 h of flying between October 1986 and September 1987. During that time 34 oil-like targets were detected by SLAR; 23 of these were subsequently investigated by IR/UVLS and 21 were found to contain oil. As can be seen from Table 2, most of these discharges contained <100 litres of oil, and only one slick contained > 1000 litres of oil. These data suggest that the overall standard ofoil discharge control by the platforms is high. Despite the small amounts of oil detected, the surveillance patrols have provided an incentive for operators to improve the reporting
N. Hurford, D. Tookey
76
(a)
(b)
Fig. 3. Detection of thin oil. (a) SLAR; (b) IR/UVLS.
Remote sensing of oil dischargesfrom offshore installations
(a)
(b)
Fig. 4. Detection of thick oil close to aircraft track. (a) SLAR; (b) IR/UVLS.
77
78
N. Hurford, D. Tookey
(a)
(b)
1
Fig. 5. Detection of 'stray' target. (a) SLAR; (b) IR/UVLS (no oil detected).
Remote sensing of oil dischargesfrom offshore installations
79
TABLE 2 Investigation of SLAR Targets
Volume range 0 < 1 litre 1-10 litres 10-100 litres 100-1 000 litres > 1 000 litres Total
Number of targets 2 7 3 6 4 1 23
of spills. As a result of the regular patrols, there has been an increase in the number of spills reported to the Department of Energy (DEn, 1987). In line with our findings, most of the reported spills are believed to contain ~relatively small amounts of oil.
5 CONCLUSIONS 1. Airborne surveillance using SLAR and IR/UVLS sensors can be used to provide effective surveillance of oil discharges from offshore installations. SLAR can be used to detect oil-like targets at ranges up to 15 km; however, subsequent investigation with IR/UVLS is required to confirm the presence of oil. 2. During 154 h of flying, 34 oil-like targets were detected. Twentythree of these targets were investigated by IR/UV, and 21 were confirmed as containing oil; 11 targets were not investigated. 3. Most of the slicks investigated contained <100 litres of oil. 4. Carrying out regular surveillance patrols encourages operators to report all oil spillages.
ACKNOWLEDGEMENTS The work described was funded by the Petroleum Engineering Division of the Department of Energy. The assistance of the M P C U in providing its surveillance aircraft is gratefully acknowledged.
80
N. Hurford, D. Tookey
REFERENCES Cormack, D. & Hurford, N. (1987). Current operational airborne surveillance activities in the FRG, the Netherlands, Norway, Sweden and the UK~ Oil & Chem. Pollut., 3, 245-56. Department of Energy (1987). Development of the Oil and Gas Resources of the United Kingdom, HMSO, London. Hurford, N. & Tookey, D. J. (1987). A detailed evaluation of the maritime surveillance system for oil slick detection. Oil & Chem. Pollut., 3, 231-44. Loostrom, B. (1987). The Swedish remote sensing system for maritime surveillance. Oil & Chem. Pollut., 3, 209-29.