Twist control

UK engineers borrow from the body’s internal geometry in move which could reduce damage
in oil pipelines and cut energy costs on drilling platforms.

A reduction in damage to gas and oil pipelines, and a smoother flow of solids, liquids and gases in water treatment works and chemical plants, could be achieved by copying the geometry of blood vessels, according to Heliswirl Technologies, a spin-off from Imperial College London.

The technology depends on putting a small twist in the pipes, which keeps multi-phase flow within a controllable state. Its potential to help reduce electricity consumption has won it a £600,000 investment award from backers including the Carbon Trust.

Known as small-amplitude helical technology (SMAHT), the system was devised by Colin Caro of Imperial’s department of bioengineering. Caro was studying the way blood flows through arteries. These aren’t straight, but have a slight spiral or helical geometry that sets up a swirling pattern in the blood flow.  Caro said that this prevents stagnation of solid material, such as platelets.

This is proving to be an important medical find, as these stagnant regions are prime sites for the development of cardiovascular conditions; platelet buildup kills the cells in the wall of the artery, leading to thickening of the blood vessel wall.

Caro has recently used his findings to design polytetrafluoroethylene arterial grafts with a twisted geometry, to be used in by-pass operations; it is hoped that these will last longer than conventional straight-tube grafts. However, the same techniques could soon find applications on a much larger scale.

Solid material being carried along in the bloodstream is a multi-phase flow problem, which is also common in industry. Gas and oil pipelines see this phenomenon often: water is entrained in natural gas flow, for example. Flows of pure gases and pure liquids are predictable and relatively easy to handle as their behaviour is governed by well-known fluid dynamics principles. But their behaviour is much more chaotic and problematic.

When the proportion of gas to liquid is low, the situation is fairly simple. But as the gas level builds up, the bubbles tend to join together. The result is known as slug flow, where an expanding bubble of gas pushes a mass of water along the pipe with increasing velocity. This sets up vibration in the pipe, and when the slug of liquid hits a bend, or anything that can disrupt the flow, it can cause serious damage.

Oil and gas platforms are fitted with slug catchers — pressurised vessels which allow the liquid to collect, removing most of it from the pipeline so the gas flow can be smoothed. These are large, heavy and expensive pieces of equipment, and difficult to fit into the confined space of an offshore platform.

Gas compressors are also used to push the slugs along the pipeline and into the catcher and, as compressors are energy-hungry equipment, this pushes up the industry’s power usage.

Caro’s SMAHT geometry is a simple, elegant and relatively cheap way to deal with this problem, said Ross Waring, Heliswirl’s chief executive. As a multiphase mixture flows through the helical tube, the swirl patterns disrupt the flow.

‘What happens is that we suppress the onset of slugging or severe slugging, so the fluid stays within a manageable state for longer,’ he said.

‘This can dramatically reduce slugging problems. What we’re hoping is that platforms using this technology would be able to reduce the size of the slug catchers considerably, or even not have to use them at all,’ said Waring.

This would also cut energy costs, as there would be less need for gas compression. There are no moving or internal parts involved, so there is little risk of the pipes being blocked by sedimentation or other solids, such as gas hydrates; this also reduces maintenance costs.

The key to the properties is the tuning of the pipeline’s geometry — the helix angle, relative amplitude and relative pitch of the helix are all vital parameters.

Changing these can alter the effect of the swirling motion; it can mix the fluids flowing through the tube together, or act like a cyclone, segregating substances of different density in particular positions within the flow, so they can be separated.

Heliswirl has linked up with CFD modelling specialist IFS to produce a design tool for SMAHT piping, to calculate the characteristics of the flow depending on such parameters as fluid density and viscosity.

The recent investment, including £150,000 each from Imperial College’s technology transfer company, Imperial Innovations and from the Carbon Trust, and £300,000 from private investors, is helping Heliswirl to fund extensive development and testing of its technologies. The company is currently developing three products for the oil and gas sector: flow enhancement, to reduce the pressure loss caused by slugging; flow conditioning, to suppress slug formation, which is under test at Cranfield University, in conjunction with Wellstream International; and flow separation, using the cyclone effect to separate the different phases of the stream from gas fields in undersea pipelines, before it even reaches the platform.

The oil industry isn’t alone in its interest in Heliswirl. Thames Water is testing the technology for use in water treatment. The swirling flow induces mixing, making the twisted pipes an efficient way to introduce water treatment and disinfection chemicals into the supply. The cyclonic effects can also be used to remove entrained air from the water, which improves the effectiveness of filter beds.

The petrochemical sector is another target for the technology. Swirled pipes could be very effective as chemical reactors, because the mixing induced by the flow patterns improves heat transfer and the distribution of reactants and products. This could significantly increase reaction yields, said Waring.