On 6 January 2018, at about 8 pm, the tanker Sanchi which was carrying a full cargo of 136,000 tons (960,000 barrels) of condensate natural-gas for South Korean petrochemicals company Hanwha Total collided with the Hong Kong-flagged cargo CF Crystal in the East China Sea, 160 miles (300 km) off Shanghai. It had left Asaluyeh port in Iran for arriving to Daesan-South Korea.
Few days after the accident an explosion caused severe fire, shipwreck and large sea pollution. Condensate natural-gas is a highly flammable type of ultra-light crude oil. Sanchi was a double-hulled oil tanker, with an overall length of 274 metres and 41 tons and deadweight tonnage of 164 tonnes. The fire burned for two weeks and the condensate polluted a huge sea portion for several days.
In addition, strong winds rapidly pushed the tank away from the Chinese coast, where the incident happened, into Japan's exclusive economic zone (EEZ).
The radar remote sensing contribution and risk assessment SAR (Synthetic Aperture Radar) satellites are the most widely used sensors for disaster response and recovery (oil spill, landslides, flood mapping) because of their active sensors which measure the backscattered signal at a certain frequency and certain polarization of the electro-magnetic transmitted signals. Moreover, SAR sensors offer all-weather and time imaging acquisition capabilities.
Indeed, for oil spill monitoring at sea, SAR can detect at night, through clouds or fog. Waves on the ocean reflect radar energy, producing a "bright" image known as sea clutter. Since oil on the sea surface dampens some of these capillary waves, the presence of an oil slick can be detected as a "dark" response or as an absence of the sea clutter.
The figure shows how much the polluter slicks expand on sea surface (ordinate) and how long they maintain their superficial floating (abscissa), relating to oil spill typology. Sanchi incident produced the worst environmental risk situation (IV)
The e-GEOS activation and monitoring results
Immediately after the ship explosion, to help and provide the local Administrations with emergency information and monitoring tools e-GEOS tasked COSMO-SkyMed constellation starting from January 14th for acquiring the first image available on 15th in the morning. The accident monitoring has been carried out until January 22nd.
After the first acquisition, two Clean Sea reports (no pollution detection) have been produced both on 15th and 16th. This was due to a drift of the tanker along days; the coordinates for a new and different satellite feasibility and tasking were therefore changed.
As expected, over the successive image on January 17th, oil slicks were heavily visible, despite the bad weather conditions with strong wind.
To increase the potentiality to detect the correct area of interest, the coverage has been enlarged and a stripline of two consecutive COSMO-SkyMed ScanSAR Huge images (30m resolution) have been acquired.
On the same day, during COSMO-SkyMed evening passes, the presence of a large oil spill was highlighted, measuring around 85 km length and 4 km width. Meanwhile, processed images clearly showed that this huge spill entered the exclusive Economic Zone -EEZ of Japan since day 17th. Through the image analysis, the oil spill has continued to move towards East and new COSMO-SkyMed tasking on new updated coordinates was performed.
COSMO-SkyMed acquisitions in the morning of January 18th and 19th still permitted well visible oil spill reports, which unfortunately maintained its dangerous length of 70-80 km. According to the COSMO-SkyMed detection capability (position and delineation), the slick movement towards east was again confirmed, always drifted by meteorological conditions.
On day 20th also a SAR Sentinel 1-S1 image has been acquired and analyzed, detecting a huge oil spill, with additional small slicks in the surround. The location of this slick was found at north with respect to the detected spill acquired by COSMO-SkyMed in the previous days. Following to this new location, coordinates for planning and tasking have been updated accordingly.
Further combinations of S1 image acquired on 20th integrated with COSMO-SkyMed ScanSAR Huge acquired on January 19th , have shown again a slick portion well visible on both scenes.
Through the successive COSMO-SkyMed acquisitions (21st and 22nd January) the slick appeared as scattered and fragmented oil spills, probably already mixed with water, slowly disappearing at sea level and indicating the time for the monitoring closing.
SAR Satellite sea oil spill detection main benefits
Remote sensing by satellite SAR sensors, like COSMO-SkyMed, appear fundamental to react and monitor any accidental or volunteer oil discharge for immediately evaluate the scale and severity of an incident and to plan the appropriate responses and combating actions. Such information is also required as sometimes-unique source for recovery eventual guilty vessels.
Satellite SAR based pollution detection methods offer real effective services to prevent, mitigate and assess oil marine disasters and provide national/international authorities with suitable and timeliness alerts for slicks propagation. In addition, through integrated meteo-ocean and diffusion/dispersion models, satellite data indicate accurate pollutant drifts and their evolution, identifying potentially damaged areas, both for population alert (coast settlements, fishing zones) and environment safeguard (coast-line and bottom sea oil flocculation damages, loss of animal life and bio diversity, etc.), not only on waters of prime interest of media.
The following figure shows the several SAR satellite images acquired along the observation period. Finally, in red the Japanese Exclusive Economic Zone and the affected area.
COSMO-SkyMed -January 17th in the evening