Restoration of gas pipelines in the metropolitan area of Milan using a no-dig technology with polyethylene PE 100 RC pipes: THE CASE OF AEM MILANO
The basic premise of this work was represented by the presentation in January 2003 of a campaign, launched in partnership with the City of Milan, for the replacement of cast iron pipes, still present in the city gas distribution network. The plan, in the replacement of 250 km of pipes in a span of five years, has seen the network operator of the AEM (now A2A) actively engage in the systematic selection of trenchless technologies (in particular, slip lining and pipe bursting) for over 30% of the section to be replaced, with the aim of reducing as much as possible the environmental and social impact of building sites.
In relation to the available technologies, the effort put in place was aimed at identifying some possible solutions able to meet the different maintenance needs (critical factors that can affect the choice consisting of size and pressure of the pipeline, the availability of space, amount and type of derivations, type and location of the problems, the gas-delivery suspension possibilities).
The experimental intervention of AEM consisted in the insertion of a polyethylene pipe DN 400 SDR 11 in a DN 500 pipe for a total length of about 160 meters in a one-way street, where the alternative of the excavation would have resulted in the ‘disruption of traffic with also significant urban impact for the presence of periodic events connected with trade and a strong influx of people.
The choice made was set through the use of an apparatus for pipe bursting (destruction of the existing pipeline), the realisation of three holes (in the head for the insertion of the new pipe, in the middle for the insertion of the tow, at the end for the towing of the pipe in a second phase) and the assembly of the pipe made at installation time, to minimise the footprint of the construction site (the shutter pace was about 36 meters per day).
The replacement of large diameter pipes has always represented an operation of great complexity, due to the dimensional characteristics of the pipe to be inserted into the existing pipe, with the inconvenience directly connected to the realisation construction sites of limited size and to the handling of the pipes.
Furthermore, the construction of pipes of large diameters to be installed by means of slip lining require special properties of the polymer used, such as a high notch resistance on the outer surface as a result of insertion in an existing pipeline, with an adequate resistance to the rapid spreading of the fracture, high flexibility for the containment of the spaces during the phase of insertion and proper resistance from the mechanical point of view to withstand the stresses during dragging.
Idrotherm 2000 was selected as the producer of polyethylene pipes in possession of the most suitable processing technology to the production of pipes to be used in this context, thanks to the availability of machinery equipped with automatic controls on the production line, able to ensure uniformity and consistency in the dimensional characteristics and absolute absence of surface defects. The product with the best performance has been identified in a type of bimodal PE80 yellow resin, which combines good processability with excellent mechanical properties (in particular, greater resistance to the onset and to the spreading of brittle cracks). The extrusion of polyethylene pipes with an elevated diameter and thickness, in fact, usually results as a challenge for the tendency to failure on the part of the polymer, i.e. for the propensity of the “particular” material down before the cooling is complete (sagging). Thanks to the experience acquired by Idrotherm 2000 over the years, the construction of the pipe has been accompanied by the overcoming of any kind of difficulty and the product pipe was characterised by very low values of deviation from circularity. At the end of the production phase, the pipe was exposed to a laboratory test cycle (geometric characteristics, of fluidity index in the mass, thermal stability, elongation at break, hydrostatic resistance) the exceeding of which has allowed the initiation of the subsequent stage being installed by AEM.
The positive outcome of the experiment put in place by AEM revealed a substantial lack of complications from the operational point of view, a running speed of the construction similar to that obtainable with more established technologies and above all the low environmental impact, thanks to the characteristics of the raw materials used, encouraging the inclusion of trenchless technologies among those available for any replacements on gas distribution networks.