High density polyethylene pipes (HDPE) are now common place for a variety of purposes and are increasingly being used as a replacement to more traditional materials, such as concrete or iron, thanks to its reliable, long lasting and durable properties. Dr Vasillios Samaras, senior technical engineer at HDPE pipe manufacturer Asset International looks at how the pipe is increasingly being used for projects that subject it to extreme conditions and how the material is able to cope with the most severe forces that Mother Nature can throw at it.
While it is accepted that plastic pipes provide a cost-effective solution for a range of piping applications, HDPE pipe is increasingly being used for a wide variety of purposes in more extreme environments.
The last three decades have seen a significant increase in the use of plastic pipes in innovative engineering solutions such as marine applications, and hydro-projects where temperatures can plummet to extreme lows.
HDPE pipe is the top choice for the growing number of marine applications that require pipelines installed on the sea-bed. Treated ductile iron or concrete pipes do not offer anywhere near the excellent level resistance to abrasion, corrosion and chemicals that HDPE proffers. As a result, the cost does not to need to incorporate extras such as surface coatings or other additional preparations.
Rigid pipe materials are susceptible to cracking or breaking completely; whereas HDPE pipe has a natural ability to flex, which enables it to adjust to different loading conditions, vibrations, stresses and movements without causing any damage to the pipe system while under water.
Flexible pipes are extremely versatile when compared with rigid pipes, and have important structural performance advantages. Unlike rigid pipes, flexible pipes have excellent resistance to differential settlements. When overloaded, the structured-wall pipe will simply deform further to generate greater passive pressures until the system regains equilibrium.
Solid-wall polyethylene pipes reach sizes of up to 1.6 m in diameter and have already been installed in several projects throughout the UK but the need for larger sizes is growing.
Structured-wall polyethylene pipes carry every benefit associated with their older sister, the solid-wall polyethylene pipe, in the marine field. And now plastic pipe manufacturers are able to use high quality structured-wall technology that can reach size of up to 3.5 m diameter.
Larger pipe sizes generally have a decreasing ability to cater for settlements and uneven seabeds. In this respect, structured-wall pipes have an advantage because their pipe wall design will more readily adjust itself.
The pipe not only has to prove strong and durable once installed, it needs to withstand several testing demands during the sinking process.
Throughout sinking, the pipe is subjected to stresses much higher than those experienced during its actual use. A large bending radius capability is needed to allow for the ideal smooth ‘S-bend’ sinking operation.
For pipes with a large diameter, the pulling force needed to uphold the bending radius can be extremely high, so a good bending radius capability is crucial. As plastic pipes can be submerged using open ends, the force on the pipe system during sinking is significantly decreased. The smooth and controlled ‘S-bend’ sinking operation is further enabled by the natural buoyancy of plastic pipes and the bending radius capability of 50 times the outside diameter.
It is also crucial that the pipe must not be over-stressed or moved on the seabed, so a suitable weighting system is required. Thanks to the flexibility of structured wall pipe, it corrects itself following any influence from weighting.
As a high-quality structured wall system, this product has proven itself in several marine installations both for water intake lines, cooling water systems and sewage out-falls.
It is also worth noting that an additional benefit of structured-wall pipe for marine applications is the significant time and cost savings procured during installation. Long piping sections can be prepared and welded on land before sinking, saving manpower once the pipe has been laid. As plastic pipes are laid during using the simple “float and sink method” minimum equipment, boat and manpower is required, providing further cost and time savings.
The Longman Out-fall in Inverness installed in 1998, is a prime example of such product versatility thanks to the decision to use structured wall pipe for a sea out-fall. 500 metres of 1.65m pipe was installed in 100 metre strings, saving the client installation time and considerable costs.
The lower carbon footprint of HDPE (which can be up to 85% less than concrete) is also an attractive prospect for many end-users keen to meet green initiatives, especially for transatlantic projects. As the pipe is lightweight and can be easily nested, the reduction in deliveries significantly reduces carbon emissions during transportation. Asset International regularly delivers fully nested loads to the Falklands Government providing dramatic reductions in delivery costs and carbon emissions.
The HDPE is used in water transportation applications in the Falklands where temperatures can plummet well below freezing, especially in marine environments. HDPE is an ideal choice as steep variations in temperature will not affect the pipe’s structure, unlike concrete and ductile iron pipes which are highly susceptible to cracking or damaging in extreme cold.
HDPE pipe is also used for projects closer to home that expose water transportation pipes to sharp fluctuations in temperature.
A pioneering hydro-power scheme in Glendoe, near Loch Ness has recently used HDPE pipe for transporting vast amounts of water from a reservoir high up in the Glendoe hills down to a power station, covering an area of 15 sq kilometres. Once again the lightweight nature of the pipe meant installation and transportation up the steep mountain-side was quick and easy, saving time and costs. Additionally, it was the ideal choice for an area such as Glendoe where temperatures can reach similar extremes as experienced in the Falklands.
What distinguishes HDPE from its rivals is the diversity of applications for which the pipe is ideal.
While HDPE pipe has proven itself in a number of applications, the cost of delivery and speed of installation places it high above traditional pipe materials. Furthermore, its prominent role in green applications and its lower carbon footprint means HDPE pipe is a highly competitive product and looks set to remain so.
Dr Vasilios Samaras is technical engineer at water management specialist, Asset International Limited.
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