Modeling Waves to Keep the Sea Clean

In 2013, over 4400 million tonnes of crude oil was extracted, which caters to roughly 33% of the global need for energy. Most of this oil is extracted from offshore sites and transported to shores for further processing. During this production and transport, if an accidental release of the crude or processed oil occurs, it is called Oil Spill. With the advancement of technology, volumes of oil spilled have reduced over last few decades, however, factors of human error and natural calamity can never be completely ruled out.

Since the oil spill severely damages the ecosystem of the sea, a quick clean up is necessary. However, such cleanup and recovery of oil are generally difficult and time consuming. It depends upon many factors including the type of oil spilled, the temperature of the water and the sea conditions.

Prof.Dr.-Ing. G.F. Clauss and his group of researchers at Technical University of Berlin have developed and patented a Sea state independent Oil Skimming System (SOS) and optimized it using experiments and numerical simulations. The skimming concept of SOS is very robust as it works without any moving parts and is based on purely hydrodynamic principles.

image of SOS schematics for wave modelling

Schematic of SOS

A bow segment glides over the sea surface. The oil layer flows underneath the SOS bow and meets a separation blade, which separates oil from water. The leading portion of the bow is designed to damp the waves and the trailing edge is designed such that a vortex develops that, in addition to the buoyancy, assists the oil to reach the top of the free surface inside the moon pool. Wave induced sloshing effects transfer the oil over adjustable slopes into the oil sumps from where it is pumped off. The main objective of the optimization procedure is to improve the skimming efficiency especially in variety of sea conditions. This figure shows skimming efficiency of two different models at different conditions.

SOS efficiency for various conditions (Sea state is defined by significant wave height Hs and time period T0)

SOS efficiency for various conditions (Sea state is defined by significant wave height Hs and time period T0)

Authors use CFD simulations extensively and mention that the key advantages of simulations are the possibility of cost- and time-effective geometrical variations, modeling variety of sea states and ease of including various oil types of different density and viscosity. For this analysis, ANSYS Fluent was used along with a User Defined Function (UDF) to generate arbitrary irregular sea states.

With ANSYS Fluent 15, it is now possible to generate virtually any state of the sea using inbuilt “Open Channel Wave Boundary Conditions” in conjunction with the Volume of Fluids (VOF) method. With variety of wave theories like Airy Wave, Stokes Wave, Cnoidal/Solitary Wave etc. and possibility of modeling superposition of these waves, the simulation set-up would be a lot easier and the simulations more accurate. So, researchers can make faster progress in their design optimization and ANSYS can claim a higher share in the efforts of keeping the sea clean.

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