BACKGROUND
In the following, observations made by Mr Lars Lundin based on practical work, and information gathered by him during two winters on the behavior of ice and oil in the Gulf of Finland, is summarized.
GENERAL
Proper construction of an oil collector requires that there is a good understanding of the behavior of raw oil and refined oil, in other words it is important
- to distinguish between raw oil and refined oil;
- to understand how oil recovery should be carried out with oil having variable properties;
- to consider how the properties of ice change from month to month in winter; and
- to consider how the oil collector operates in ice and oil
It should be pointed out that it is not merely a question about recovering ”oil” but of recovering two liquids that are as different as night and day. They differ from each other with regard to specific gravity and viscosity which are properties that determine the technical construction of the oil collector and its capacity for oil recovery in ice. Further, the brush system of the oil collector must prevent aggregation of small ice cubes and brash ice during collection. It also needs to be considered that only a tenth of a floating ice raft protrudes above sea level.
RECOVERY OF RAW OIL IN WINTER
Raw oil differs from heavy refined oil during transportation because it is much lighter. Raw oil expands rapidly and mixes easily with snow and brash ice. In brief, already a small amount of raw oil is capable of making the environment look devastated. Fear caused by such images may give rise to wrong decisions when starting oil recovery because already a few barrels of raw oil is readily likened to a large volume of heavier oil.
If there has been a rupture in the bottom of a vessel or if vessels have collided, it can be expected that large volumes of oil remain hidden, floating below the ice. In both cases, the management of the operations has to decide on the specific oil recovery methods and how the ice has to be treated. It has to decide whether and where ice has to be broken up, and how large the ice blocks have to be in order to achieve efficient oil recovery.
The properties of raw oil which floats below the ice change very little in the absence of oxygen and only
minor changes in the structure take place because the oil will remain light-flowing and liquid, capable of rapid expansion and therefore difficult to locate. In such a situation, erroneous decisions on ice breaking may lead to complete failure of the oil recovery operation resulting in heavy contamination of large sea areas.
As said, raw oil has variable properties, and in winter its structure can vary from a light fluid to a granular, porridge-like putty that adheres poorly to recovery brushes because raw oil generally contains wax.
RECOVERY OF RAW OIL IN SOLID ICE
When collecting raw oil from broken ice, the water surface must not be disturbed because movement of water is bound to push raw oil away from the collector. The vessel must not either disturb collection of raw oil. The collector should never be installed in a vessel at a place where propeller slipstreams have an impact on water movement, in particular it should not be installed in the aft region.
If solid ice is broken up the time available for oil recovery is limited, in particular at windy conditions, because the ice rafts will, with great likelihood, drift towards each other, resulting in the free water surfaces between the ice blocks rapidly shrinking and oil collection becoming increasingly inefficient.
Personally, I have reached the conclusion that in situations such as these it is important that the collector body is so heavy that it is capable of sinking the ice rafts, and inside the brush drum there must be means for water removal in order to conduct the oil onto the brushes of the collector.
RAW OIL RECOVERY IN EARLY SPRING
When an oil accident takes place in early spring sun will make the ice porous and this will promote the penetration of raw oil into the ice. This was what happened in the case of M/T Anthony Gramsky in 1988 which gave rise to a very difficult situation. Expansion of oil stopped when oil seeped into the ice without spreading any further. If the ice is broken up in a situation like this, millions of minute oil ice cubes or ice slush is generated. These will disturb the collector to the extent that it is questionable whether it is rational even to start oil recovery. This has also to be considered when designing the collector.
In cases such as the one described above it is actually often better to leave the oil untouched and to allow the oil evaporate while monitoring how the situation develops and preparing for oil collection in the open sea. As always, there are two sides of the coin: although the evaporation rate of raw oil is great, there is still a danger that raw oil emulgates into the water, thus giving rise to a mass which is very difficult to treat and collect when it is washed up on the shore.
ABOUT RECOVERY OF REFINED OIL
The greatest difference between raw oil and refined oil is that transportation of heavy refined oil takes place at a higher temperature which means that when there is an accident the refined oil freezes very quickly upon contact between water and ice. When the temperature of water is about 0 degrees heavy oil spills will spread very slowly. Basically, in such a situation, only winds and sea currents will contribute to greater diffusion of the oil. After breaking up of the ice, a thick, floating mass can be found which at heavy frost freezes to the ice.
Being based entirely on the viscosity of the oil, recovery of refined oil is less complicated than recovery of raw oil in winter. Temperature changes influence greatly collection of heavy oils. At heavy frost the oil has to be rinsed from the ice, whereas at temperatures of +5 to +10 degrees C, oil becomes more fluid. For collection of all heavy oil fractions, rinsing with water is to be employed for dispersing oil into small oil droplets and to achieve a uniform collection of oil. At heavy frost it is not possible to separate oil from ice or ice blocks from each other by momentary immersion of ice mixed with oil.
REFINED OIL IN EARLY SPRING
The difference between raw oil and refined oil is that in early spring oxygen in air has but a small influence on the evaporation of heavy oil and changes in its state. Sea water achieves much greater changes in the composition of the heavy oil because oil will absorb the water. Rather than giving rise to heavy emulgation in water, refined oil tends to form very small oil spheres on the surface of water. When ice melts, such oil spheres will float on large areas of the sea surface while emitting thin oil films. Visually, this view makes the extent of contamination of the sea area look much worse than it actually is. Very heavy oil tends to sink if it cannot be found on the surface of the sea in time. Due to strong evaporation, diesel oil is recovered in winter only in exceptional cases.
RECOVERY OF OIL IN DIFFERENT TYPES OF ICE FORMATIONS
The expression “ice formations” stands for ice which is formed along the path of vessels. This form of ice presents the greatest challenges for the development of collectors for recovery of raw oil or refined oil in winter.
A starting point for collectors that work in winter is generally that they are able to operate in ice formations having a thickness of up to 1 m. By contrast, equipment for oil recovery in arctic areas, where the ice has a thickness of two to three meters, needs to have a different construction than such collectors. In arctic conditions, the influence of the tide should also be considered.
When heavy refined oil is collected among ice floes it is advantageous to employ water rinsing for example using sea water having a standard temperature of ± 0 degrees and having a good capability of melting snow. Larger oil lumps can then be released by breaking up the ice floes during oil recovery.
Lars Lundin