Application in deep vertical groundbeds of linear flexible anodes

. This paper is about a case of application of a new anode system in a vertical groundbed in Italy for the impressed current cathodic protection of gas distribution pipelines. This system offers significant advantages for the installation of impressed current cathodic protection groundbeds with easy and timesaving procedures, accurate Quality Check on the factory-made conductive connections and optimization of the current distribution. The reduction of anode resistance versus soil enables a relevant energy saving. The product is flexible even in current output and expected design life. It is possible to select parameters, like the Mixed Metal Oxide titanium anode diameter, the feeding cable insulation, the distance and number of joints between the anode and the cable to realize a specific product fit for purpose. The reduced diameter of this anode system will enable to design new wells with consistently reduced borehole diameter. Further cost optimization can be reached by concentrating the length of the active part of the anodic system, this resulting in a reduced perforation depth. Deep Vertical Groundbed (DVG) calculations for the design of new groundbeds are presented in details with examples of a traditional string of MMO titanium anodes and linear flexible anode as alternative options.


Introduction
Linear flexible anodes were designed for the cathodic protection of pipelines, tanks (Above Surface Tanks or mounded LPG storage bullets) in soil or sand and reinforced concrete structures and had considerable success in Horizontal Directional Drilling applications.
This system is the ideal solution when the anode has to be installed very close to the structure to protect in confined spaces. The anode is assembled inside a protective sock that prevents accidental contacts with the cathode and is surrounded by calcinated petroleum coke to optimize the current distribution and to reduce the resistance at the anodesoil interface of the groundbed, either horizontal or vertical. The product core is a pure titanium wire Grade 1 according to ASTM B 863 thermally coated with mixed metal oxides of group VIII. A protective braid is offering additional mechanical resistance without interfering with the current output.
This new product offers significant advantages for the installers of impressed current cathodic protection groundbeds: • easy and timesaving installation procedure, • accurate QC on the factory-made conductive connections, executed leveraging on over 40 years of De Nora experience, • optimization of the current distribution. This is a green product, with zero discharge of pollutants in the soil. The coating consumption rate of mixed metal oxide anodes may be considered negligible with respect to the effects on the environment and can be indicated as 1 mg per Ampere per year [1]. Moreover the reduction of anode resistance versus soil enables a relevant energy saving.

Technical Specifications
The product is flexible even in current output and expected design life (Table 1). It is possible to select parameters, like the MMO titanium anode diameter, the feeding cable insulation and cross section, the distance and number of joints between the anode and the cable to realize a specific product fit for purpose. The typical applications include cathodic protection of above ground tank bottoms with secondary containment, cathodic protection of pipelines with horizontal or vertical groundbeds or close-by cathodic protection of pipelines in congested areas of plants and cathodic protection of mounded bullets.  The active part was 15m long with 3mm mixed metal oxide wire anode inside. The 16mm2 PVDF/HMWPE cable tail was 47m long.
Experience of over 40 years of groundbed installations suggested selection of a PVDF/HMWPE 1x16mm2 cable. This double insulation cable is resistant to environments in which hydrochloric acid and gaseous chlorine may be present. The installation was performed on March, 3rd 2015 in Northern Italy in a vertical well. The well casing was a plastic pipe with holes in the active part. The water table was located at 6m depth. The anode, hanged to a dead weight, was very easily installed without the use of special tools. It was connected to a transformer rectifier equipped with a remote monitoring system.

Fig. 2. Installation of linear flexible anode in deep vertical groundbed
This system allows the daily reading of minimum, medium and maximum current output and DC voltage values to monitor the evolving groundbed resistance. This data collection is aimed to assess the presence of seasonal effects on the anode performance. Moreover the chemical analysis of the water flooding the well allows to check other parameters that can influence the anodic system durability like pH, chloride content, and the concentration of potential pollutants like ammonia, sulfates and nitrites.

Results
As you can see in Figure 3, after the installation of the new system, it was experienced a sudden reduction of groundbed resistance (Ω Av.) from 0,95 Ohm to 0,91 Ohm. Notably the average cell voltage (V Av.) decreased from 6,25V to 5,9V in the early stages and subsequently since the installation date experienced an average cell voltage increase from 5,9 V to 6,1 V and the resistance value stabilized at 0,94 Ohm. The average current output (A Av.) was stable as reported by the remote control system. The test is still running with minor modifications in the groundbed structure performed to increase mechanical resistance and make that suitable for an expected lifetime of 30 years. After this first installation more than 20 were executed in the same area and in all of them the linear anodes are still working perfectly.

Other projects in Italy
Linear anodes have now been installed in Italy for different projects. Between the more significant we need to mention the installation in 12 horizontal groundbeds for a gas distribution company in North West Italy and the cathodic protection of steel sheet piling in 2 industrial chemical plants near Milan. At the time of publication of this article all these installations are running smoothly.

Design of new groundbeds
The reduced diameter of this anode system will enable to design new wells with consistently reduced borehole diameter. This will allow big savings in the time and cost of installation. Further cost optimization can be reached by concentrating the length of the active part of the anodic system, this resulting in a reduced perforation depth. This represents the minimum number of anodes to supply the requested protection current. Conservatively this minimum number is rounded from no. 2.4 to no. 3 anodes.

Anode spacing
In calculation of actual anode spacing the parameters below listed have been used: -anode length, LA: The maximum linear anode current output is 940 mA/m at 100 A/m2 current density. Therefore the minimum required length of the anode is 12/0.940= 12.8 m, conservatively this minimum length is rounded to 15m.7.4.2 Groundbed

Conclusions
Linear flexible anode if properly designed can be an effective solution for the replacement of existing anodes in deep well groundbeds. Installation of the linear flexible anodic system was fast and easy without the use of special tools. The average cell voltage and groundbed were initially lower than experienced with the previous anodic system and remained stable during the test. Selecting this system since the beginning it's possible to design new groundbeds with reduced borehole diameter and perforation depth.