OFFSHORE OIL SPILL AND REMEDIAL MEASURES By

Prof. V P Sharma

Dept of Petroleum Engg.

Indian School of Mines

Dhanbad – 826004

FATE AND BEHAVIOUR OF SPILLED OIL

When petroleum oils are spilled on to the sea, a number of mechanisms come into play.

These are the following:

1. Spreading

2. Movement and drift

3. Weathering

(i) Evaporation

(ii) Dispersion

(iii) Dissolution

(iv) Oxidation & Photo oxidation

(v) Biodegradation

(vi) Sinking

SPREADING

The principal forces influencing the lateral spreading of oil on a calm sea are:

1. Gravitational force (causing decrease in film thickness)

2. Surface tension

3. Inertial forces

4. Frictional forces

The largest actual experiment on spreading of oil and its disappearance was carried by

Warren Spring Laboratory. In this 120 tons of light Arabian crude oil was discharged at sea

and its appearance and dimensions observed for four days. The diagrams given below show

the outline development and subsequent breakup of the oil slick, particularly interesting is the

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fact that they show the difference between the overall picture and the thicker patches

observed.

Slick developments in major and minor dimensions with time

MOVEMENT AND DRIFT

Drift is a large scale phenomenon and is a measure of the movement of the centre of mass of

an oil slick. Nevertheless wind direction and duration can have a dramatic effect on oil spill

drift and trajectory and an example of wind induced drift perturbations of an extensive near

shore spill occurred after the wreck of Amoco Cadiz accident occurred on 17 March 1978.

April 1-2

Wind shift pattern, which occurred on 2nd April, caused an extensive oiling of

previously clean coastal areas, which had not been touched during first eight to ten days after

the spill.

Drift is also strongly influenced by waves and tidal currents. After the Amoco Cadiz

spill waves of 1.5 – 2 meters and 6 – 7 meters tides further affected the overall drift and the

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Hundred of feet

fefeet

0

8

12

16

20

24 Major

Minor

0 2hours

Slick dimensions

Thousands of feet

Minor

Major

2

8

Slick dimensions

0 4days

17 March

10 April

extent of the oiling of area. At the end of the first two weeks of a total of 72 kilometres of

coastline has been covered with oil, then following the dramatic wind shift change of April 2,

a total of 213 kilometres were highly oiled and 107 kilometres were heavily oiled.

WEATHERING

(i) Evaporation

(ii) Dispersion

(iii) Dissolution

(iv) Oxidation & Photo oxidation

(v) Biodegradation

(vi) Sinking

The rate of weathering of oil depends upon a number of factors including oil

type and ambient climatic conditions. Rate of the various processes also vary throughout the

duration of an oil spill with many being greater in the first few hours.

(i) Evaporation: The most significant initial weathering process for oil spills on the sea is

evaporation. Thus the more volatile fractions of an oil (the light ends) are lost within the first

few hours. Therefore, afterwards the rate decreases and less volatile components will form a

residue, which has a higher specific gravity and viscosity than the original oil. The rate of

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evaporation is influenced by factors such as air and water temperatures, state of sea, wind and

rate of spreading.

(ii) Dispersion: Dispersion is of two types: (i) oil in water (ii) water in oil.

Another important mechanism affecting the natural fate of oil on the sea surface is dispersion

on the formation of oil in water emulsions. The rate of dispersion is a function of sea state

and of the nature of the oil.

(iii) Dissolution: Some components of spilled oil are also lost through the process of

dissolution. Whilst dissolution of the most suitable components in the oil will occur in the

first few hours, other processes such as oxidation and biodegradation will constantly produce

additional compounds that are water-soluble.

(iv) Oxidation and Photo-oxidation: Oxidation of certain hydrocarbons can also produce

compounds, which may act as emulsifiers and increase the natural dispersion of the spilled

oil. As the process requires oxygen it tends to be a surface phenomenon and occurs most

rapidly when the oil is spread into a thin film. The precise mechanisms are not well

understood but it is thought that the process may be enhanced by trace metals such as

vanadium but retarded by high sulphur content.

(v) Biodegradation: Biodegradation has a significant effect on the removal of oil from the

marine environment. Many species of marine bacteria, fungi and yeasts oxidise petroleum

hydrocarbons by utilizing them as a food source.

No single microbial species can utilise more than two or three hydrocarbon type

and most preferentially consume the light ends.

(vi) Sinking: Several accounts of oil spills in recent years have reported instances of oil

sinking as a result of increased specific gravity either to a midwater position onto the seabed.

By the nature of the phenomena, the precise mechanism of such sinking is often impossible to

establish but several contributory factors have been cited.

RESPONSE STRATEGIES AND TECHNIQUES

The various response strategies are:

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Monitor and evaluate

Dispersants

Containment and recovery

Shoreline clean-up

Emerging technologies

Burning

Monitor and evaluate: All oil spills must be maintained, preferably by air, to determine their

location and evaluate their potential threat. Oil spill which enters sensitive shoreline must be

properly monitored to prevent environmental damage.

Chemical dispersants: Chemical dispersants can be used to supplement the natural energy

and help in the break-up of oil spills by breaking down the interfacial tension of oil layer.

Dispersants can be applied in a number of ways-

(i) Boat mounted spray sets

(ii) Helicopter systems

(iii) Fixed wing systems

Containment and recovery: This is the ideal strategy for oil spill response. To remove the

pollutants from the environment has to be the most optimum response but unfortunately it is

the most difficult strategy to achieve. The main problem being that the strategy is in direct

conflict with the natural forces driving the oil spill.

Booms systems are used but their efficiency is limited to the conditions of

sea, current speeds etc. They are very useful in the near shore environment to protect

sensitive areas.

Floating booms have four basic components-

1. A floatation chamber filled with air or some buoyant material.

2. A free board to prevent waves from washing oil from the top.

3. A skirt to prevent oil from being swept underneath and a longitudinal support member

to allow the boom to withstand the forces of winds, waves and current.

4. Booms should have weight to keep them perpendicular to the water surface.

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Floating booms can be deployed in four different ways.

There are four basic principles of operation of oil recovery device, these are:

Weir skimmers

Oleophilic skimmers

Vacuum systems

Mechanical skimmers

WEIR SKIMMER

The weir principle relies on using the skimmer to separate the oil and water phases by placing

a weir separator at the oil water interface. To ensure that the skimmer can operate there are

three main requirements:-

Controlling the weir height

Inducing a flow

Pump performance

OLEOPHILIC SKIMMER

The oleophilic (oil loving) skimmer is a device which relies on the property of oil to stick to

materials. This skimmer is capable of optimum performance in the mid range of viscosity.

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It is of three types: -

Disc (most common)

Rope mop

Belt

VACUUM SYSTEM

Vacuum systems provide one of the most readily available spill response tools in the

industry. An air pump is used to reduce the pressure in a storage tank,a suction hose is

connected to the tank which is then used to collect the oil.

MECHANICAL SKIMMER

Mechanical skimmers rely on physically picking up the oil from the surface of water by some

mechanical means. They can be divided into four main categories:-

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Belt

Tooth disc

Grab bucket

Drum

Shoreline clean-up: - Shoreline clean-up is very labour intensive, low technology process.

There are generally three stages to the oil recovery operations: -

Stage 1-to deal with the free floating or gross contamination at the shoreline.

Stage 2-to deal with the intermediate moderate contamination which involves techniques

ranging from flushing techniques to manual collection.

Stage 3-final clean-up only when there is no danger of re-oiling.

Burning: -The burning of oil spills has long been considered as a response strategy from the

earliest days of oil spill accidents. Weathering of the oil tends to make it more difficult to

ignite.

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It can be seen that there are a whole host of options available to the responder in

dealing with a spill situation. It is important to fully understand and anticipate the future fate

of the oil when selecting the response strategy. Once the desired option has been selected the

next stage is to ensure that the logistical support is available to sustain the strategy to enable

it to be carried out along with the management structure to organize the response in such a

way that objectives are clearly defined. Trained and experienced personnel are an absolute

requirement for any response activity.

Government- Industry Co-operation

• Exemption of income tax on income received.

• To move equipment from bonded custom area.

• Approval on the transfer of nominated equipment from oil companies.

• Approval of entry points for external OSR equipments.

• Work permits.

• Constantly reviewing and updating its oil spill response capability.

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