A guide to dispersion modelling for oil spills

Preparing for an oil spill incident is complex. There are a diverse set of environmental factors that affect how a slick might behave, as well as how the characteristics of the oil itself may affect the incident. Factors like the weather conditions can change hour by hour and sway
the course of the slick, and the impacts it may have.

Understanding the factors that will affect an oil slick when an incident occurs is imperative to the successful development of any response strategy. Modelling
tools help users take them into account to react appropriately to the evolving nature of an oil spill incident.

What is a dispersion model?

A dispersion model (normally implemented as software) aims to predict the movements of
materials – in this case, oil from a spill. Based on the dominant transport and other dispersion processes acting on the oil, the model will forecast where and how the oil will travel. The increased awareness that dispersion modelling provides supports authorities and response teams to mitigate the potential impacts of an oil spill incident.

Our MarineAware solution is a modelling and visualisation tool that allows users to detect, record, locate, and track an oil spill incident. It combines dispersion modelling with source term identification to understand a slick’s movements. Starting from the oil’s source or last known location, MarineAware can
model its dispersion and determine the potential impact on nearby coastlines, marine life, and ecologies.

The following MarineAware capabilities enable the creation of targeted response strategies and the effective deployment of clean-up activities:

  • Trajectory and dispersion modelling
  • Assessment of potential impact
  • Source estimation and identification of vessels potentially responsible
  • Risk assessment for hazardous material transportation

How do dispersion models work?

The processes acting on spilt oil are varied and complex. Dispersion models simulate the primary processes that affect how they behave based on the environment. For example, ocean currents,
wind, and changes to the properties of the oil will all be considered to determine how the oil moves, spreads, and reacts with its environment.

However, modelling the oil slick as a whole is very complex and not always the best approach. Different areas of the slick may behave differently, meaning they may move in different directions or impact different locations. This is why the MarineAware platform, and other dispersion models, use statistical analysis to model particles individually whilst estimating the properties of the overall slick.

MarineAware approximates the whole slick into many “parcels”, where each parcel has a location and a state. Each parcel is processed individually to simulate its trajectory and changes in state. Analysing it in this way provides an efficient and reliable method for modelling a complex real-world situation.

However, oil spills in the real-world are not made of parcels of oil, so the MarineAware interface displays them as a contoured body more representative of the entire slick. From this visualisation, the user can easily see the predicted trajectory and impact of the oil.

Find out more about the impacts of an oil spill

What factors determine the spread of an oil spill?

The following factors represent the processes applied during oil spill modelling:

Current advection

This factor represents one of the largest influencers of oil spill movement – that being the speed and direction of the ocean currents. These influence both oil on the surface and oil that has sunk beneath the surface.

Wind advection

Wind can have significant impacts on the movement of oil on the surface of the ocean, especially when the winds are strong.

Wave advection

Waves have a lesser effect than wind and current on the oil’s trajectory, but still play a role in the direction and speed of oil slick movement.

Spreading

Gravity is one of the initial factors that influences an oil slick’s movement. When oil is released, it naturally spreads across the surface of the water. The speed of the spread is further affected by the viscosity and thickness of the oil itself. Thick oil spills can spread significantly due to gravity, although this will happen more slowly for oil with a higher viscosity.

Diffusion

Diffusion in MarineAware combines conventional diffusion, where oil moves from higher to lower concentrations, with the effects of many ocean processes which are too small to be modelled
individually. The net effect of this is to cause the oil to spread out over the sea surface.

Evaporation

A certain percentage of the oil slick will evaporate, but the levels will depend on the type of oil. In high concentrations vapours from evaporation present a high risk of explosion and can cause health impacts from inhalation for people working nearby to control the spill

Emulsification

As the oil is weathered, water may become mixed into the oil to form a viscous water-in-oil emulsion. This emulsified oil behaves very differently to fresh oil and can be challenging for clean-up efforts.

Deposition and washback

The characteristics of the coastline will affect how much oil will collect there. Using satellite-derived coastline datasets, MarineAware can estimate
the rate at which oil that reaches the coast will either be deposited, seep into the ground or be washed back out to sea.

Dispersion

Wind and waves at the sea surface can cause an oil slick to be broken into droplets which may then become dispersed into the water column. The
amount of oil which becomes dispersed will depend on environmental conditions and the properties of the oil.

Resurfacing

Some of the dispersed oil will reach the seabed while some may return to the surface. Our model continues to simulate the location of sunken oil in case it resurfaces.

Mitigating the impacts of an oil spill

MarineAware incorporates the factors listed above to help responders more accurately locate oil spills, predict their movement and spread over time, and
determine their potential impacts.

By understanding how an oil slick will likely travel, it allows users to appropriately target their mitigation tactics – from control and containment methods to clean-up efforts – prioritising areas with the greatest consequences associated. For example, areas that are ecologically sensitive or areas where local economic activity would be highly disrupted.

For more information on MarineAware download our product sheet, or find more insights here.

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