Subsea Mechanical Dispersion, Adding to the Toolkit of Oil Spill Response Technology

Abstract

Abstract The size distribution of oil droplets formed in subsea oil and gas blowouts is known to have strong impact on their subsequent fate in the environment. Fine droplets are frequently neutrally buoyant and within the full body of water they are available for biodegradation. Subsea Dispersion Injection (SSDI) is an integral part of achieving this goal, lowering the interfacial tension between the oil and water. However, despite their many advantages, the use of SSDI are limited both by the logistical constraints of deployment and legislative restrictions over its use. Adding to the toolkit of methods that achieve subsea dispersion without the use of chemicals would therefore enhance oil spill response capability. There are other ways of reducing the effects of interfacial tension within the oil such as increasing the interfacial shear by introducing more turbulence within the rising oil plume. Using a combination of laboratory experimentation and computational fluid dynamics (CFD) we have explored the potential of three mechanisms – 1) a rotating bladed shearing mixer, 2) ultrasonic cavitation and 3) high pressure water jetting. Physical experiments were conducted at the SINTEF Tower Basin facility in Norway. A scaled-down oil plume of Oserberg blend (1 L/min) was subjected to shear using commercially available rotating and ultrasonic devices both normally supplied for industrial mixing applications and adapted for operation within the tank. Results were compared to chemically dispersed oil under the same conditions. CFD modelling of water jetting was conducted using the BP High Performance Computer facility adopting a Volume of Fluid (VOF) multiphase model with advanced turbulence modelling and automated mesh refinement. Boundary conditions were set to replicate, as close as practical, the dimensions and physical properties used in the tank experiments. Results indicate that all three modes of increasing interfacial shear could be effective in dispersing oil. The ultrasonic device created a broad distribution of oil droplet sizes, spanning 10-100 μm in diameter whilst the mechanical shearing technique dispersed oil droplets into a narrower size distribution, centred on 16 μm. These values fall close to the droplet size of the same oil dispersed using chemicals of 70μm. Estimation of droplet sizes in the water jetting scenarios yielded values <50 μm. These tank scale experiments indicate that a new class of oil spill response technology may be possible using a mechanical device – Subsea Mechanical Dispersion (SSMD). The varieties of techniques that have been shown so far to be effective indicate that a range of devices may be designed and scaled to suit different oil spill scenarios and become a valuable addition to this class of response strategy.