Numerical Flow Models in Oil Boom Design

Why?
Oil booms are an important part of the equipment used in dealing with accidental oil spills. Whilst being easy to handle and deploy, booms should be able to retain as large an amount of oil as possible, in rough weather as well as calm, and facilitate the use of equipment to remove the oil from the water.

The design of booms has so far been based largely on experience and laboratory experiments. Numerical models are likely to add to our understanding of the flow of oil and water inside and round towed booms, and can be a cost-effective supplement to laboratory tests.

NOFO has financed a preliminary study on the feasibility of numerical flow models as design tools when creating new and improved oil booms.

How?
Oil and water are immiscible fluids, and it was considered important to use a model capable of representing both fluids and the sharp interface between them. The CFD code FLOW3D was chosen on account of this, being based on the Volume of Fluid (VOF) method which is good at capturing surfaces. It was considered important to have the oil phase represented in the simulations, because an oil – filled boom will produce a flow different from a water – filled boom.

Can numerical models be useful?
Most likely, yes. Our findings so far are promising. As a benchmark, the radial
spreading of a small laboratory oil release was predicted with good accuracy (see figure below).

Radius of oil release as a function of time, lines are simulations and symbols are experiments. The diagram to the right is for an oil with non – Newtonian behaviour.

Simulations with two-dimensional cross sections with simple towed booms (an example is shown in the figure to the left, top) display a number of observed features: The front of the oil forms a raised “head”, characteristic for a gravity current, followed by a long trailing section with waves moving along the oil/water interface towards the boom. Local effects are predicted in the vicinity of the boom.

Also shown is an example result from a three–dimensional simulation of oil held in a U – type oil boom(only one half of the boom is calculated due to symmetry). The picture series shows how the front of the contained oil is pushed back and deepens, and then escapes under the boom, as the towing velocity increases.

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