New shape helps to resist pressure

By forming special shaped grooves into tubes, it is possible to make lightweight pipes that can withstand immense external pressures.

Although using grooves increases the surface area of a pipe, it yields a much higher buckling strength to weight ratio meaning less material is actually needed to produce radially strong structures. The base technology has been proven in food cans, where grooves are used to significantly reduce the amount of steel used.

The technique, which can be applied to pipes made from virtually any material, has potential in a host of other applications, most notably for heat exchangers, novel heat engines under development and boiler fire tubes where one fluid, typically at near atmospheric pressure, is passing through a vessel to which it has to transfer heat at a much higher pressure.

Professor Carl Ross from the University of Portsmouth has developed methodologies and finite element analysis (FEA) software to optimise the grooves for particular materials and applications.

He has spent much of his working life designing pressure hulls for submarines. By using the specially written FEA software – which has been fully validated with physical testing - he has demonstrated that using corrugations in tubular structures increases the maximum buckling resistance, while reducing wall thickness in some cases by nearly 60%.

Along with principal lecturer Dr Andrew Little, Ross continues to optimise both steel and carbon fibre tubular structures that can withstand very high pressure by using corrugated constructions.

Although his ultimate goal is to come up with submersibles that can work at extreme depths – up to 11km down and 1100bar external pressure – the technology shows much greater industrial promise.

Making heat exchanger pipes thinner helps because they allow heat to be transferred more easily, which is the crucial barrier that prevents machines such as Stirling engines, into which development has been going for nearly two centuries, from being as efficient as they should be, and could even lead to a revival of Rankine cycle engines, beloved by many engineers but too fuel inefficient for modern small to medium scale applications.

Ross sees the designs as crucial to very deep diving submarines, which have both civilian and military applications, since he says that a solid steel pressure hull capable of diving to the most extreme depths would have to be 2.5m thick and would, "Sink like a stone", unless buoyed up by something else. The technology can also be applied to underground pipes, oil and gas installations and transportation infrastructure and further reduces the amount of material used to make food containers. And the methodology can equally be applied to withstanding internal pressure, although the optimum shapes and forms are different.

Ross's research has now resulted in a totally novel types of corrugations for tubes, the most notable of which has been given the name "Swedges", which in some cases more than doubles the pressure withstanding capability compared to a ring stiffened tube of the same thickness. Using the Swedge concept with carbon fibre composite offers that increase while reducing overall material content and Dr Little told us that: "These tubes can withstand five to six times the pressure compared to a smooth walled cylindrical tube made of the same material."

A manufacturing method has already been proven. Ribs and corrugations are increasingly being used in the manufacture of underground infrastructure pipe work, because less plastic is required. Using the new shape forms should reduce this still further.

Author
Tom Shelley

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