Oil spill recovery operations have a huge energy cost as well as causing a secondary pollution caused by the use of polluting engines.
Besides the use of antediluvian technologies, the largest contribution to energy consumption growth during the recovery operations is caused by the decrease in the efficiency of skimmers, or what is the same, the increased volume of water in the recovered substance.
For example, a weir skimmer recovers a large amount of water in a range from 50% (layer thickness above 25 mm) and 90% (layer thicknesses between 1 and 8 mm), this means that if we are facing an oil spill of 1,000 m3 we will collect between 1,000 m3and 9,000 m3 of water. Among other things, we are multiplying by 10 the time invested in recovery.
A selective skimmer (brushes, discs, drum) can work at a maximum efficiency of 95% (5% water in the recovered substance), that will be greatly harmed by the state of sea, to reduce it to 50%. In this case we will recover between 50 m3 and 1,000 m3 of water.
The consequence of skimmers inefficiency is an increase in energy cost of recovery operations broken down as follows:
- Increase the operating time of skimmers. In the case of a skimmer working at 50% efficiency we have to operate twice as long compared to an ideal situation (95-100% effectiveness). A selective skimmer working at 50% consumes 66 kWh compared to 35 kWh of a 95% efficient skimmer.
- Increase in the transfer pump total consumption (between skimmers and temporary storage tanks). In the case of a weir skimmer working with a oil layer thickness exceeding from 25 mm, energy required to pump 1,000 m3 of spilled oil plus other 1,000 m3 of water is 1.120 kWh while in the case of thickness lower than one centimeter energy consumption soars to 5.600 kWh. In an ideal situation, considering an efficiency of 95%, energy consumption would be 590 kWh, between a 50% and a 90% lower than real cases.
- Increase in the transfer pumps total consumption (between temporary storage tanks and oily water separator). We’ll be in a similar situation to point 2.
- Energy cost of separation. In the working vessel we will separate most of the water from hydrocarbons by means of an oily water separator. This water should have a hydrocarbons content below 15 ppm. Energy cost of this operation amounts to 1.800 kWh in the first case and 16.200 kWh in the second one. If we had worked with a truly efficient skimmer (95%) we would have spent 100 kWh to 1.000 m3 of spilled oil.
The above calculations are extremely approximate and may suffer significant variations depending on many factors but they offer us a striking confirmation of the importance to improve methods of oil spill recovery, because we are not talking only about an economic cost but a higher cost in terms of operability, responsiveness and autonomy of the media displaced to the spill.