Corrosion kept under control
BAC has provided corrosion protection to assets in Northumbrian Water’s region for almost thirty years. A programme of refurbishment work is now underway as Ross Fielding reports
When NW, along with other water undertakings throughout the UK, was being pressed by OFWAT to reduce distribution leaks about four years ago, the company began discussions with BAC about the economics of applying CP to other steel mains in the NW region. As the pipelines that were already protected by BAC systems had remained leak-free, the question was being asked internally as to which other steel mains would benefit from the installation of CP systems.
As a result, BAC was awarded a contract to carry out a pipeline audit, refurbishment/upgrading and a cathodic protection programme for all NW’s large diameter steel trunk mains. The pipeline audit comprised a desktop study of buried steel water mains from the Scottish border down to the River Tees in County Cleveland. Working closely with NW asset managers and utility survey teams, buried steel mains were identified and assessed for refurbishment.
The mains were prioritised utilising a custom made criticality matrix, the major factors of which were the mains’ age, its coating specification and carrying capacity together with its strategic importance. Time and repair costs were also considered as was soil corrosivity and of course third party interaction. Also of importance were Sites of Special Scientific Interest (SSSI), leak histories, alternative routes to ensure supply continuity, access difficulties and finally, expenditure. Steel mains ranging from 200mm to 1200mm diameter with a combined length 450km were identified and CP requirements re-assessed, designed and implemented.
From the criticality matrix a formula was devised and a table of about 20 pipelines were identified requiring protection by location, diameter and length and priority of installing CP as funds were made available. To date, the ten pipelines most in need have been refurbished and fitted with corrosion protection.
ICCP and hybrid
Impressed current corrosion protection (ICCP), galvanic anode and hybrid systems using both ICCP and sacrificial anodes have been used to provide protection to the steel conduits, dependent upon factors pertinent to the specific main. Impressed current cathodic protection systems employ inert anodes with an external source of DC power to impress a current from anode to cathode (pipe) through the soil. Voltages are applied to overcome soil resistance to give a spread of protection of up to 50km or more for well-wrapped pipe under cross-country conditions.
Anodes are commonly used in groups and buried in a low resistance backfill called a groundbed. ICCP systems are considered to be the most efficient cathodic protection systems for long distance cross-country lines. ICCP systems used on pipeline installations have monitoring stations at regular intervals along their length, which can be linked into a telemetry scheme allowing pipe condition to be checked from a central monitoring station.
The ICCP systems used for NW employ shallow horizontal groundbeds and the sacrificial systems, installed where ICCP is not practical due to possible interference with other buried structures, such as utility pipelines and AC power lines, use distributed magnesium anodes. These tend to be installed in pairs either close to 3m, or remotely up to 20m distance.
BAC is now into the commissioning phase, the straight-forward, direct pipelines in the Wearside and Tyneside area having been completed. Those serving the industrial Teeside area are more complex in that they form more of a matrix and therefore take longer to commission. However, about 50% of the total NW area has been commissioned.
Working with others
Interaction with other facilities in the area has been put to the test. Railtrack’s fears of electrical interference with signalling systems were alleviated after a full and detailed testing programme indicated that no adverse affects would occur to the train signalling systems at maximum CP system outputs. TRANSCO has a vast array of HP gas mains in the industrial area along the River Tees and to ensure inter-company harmony BAC has maintained close contact throughout the construction phases.
The final test for interference is with Huntsman Petrochemicals and BP on the TSEP Ethylene pipeline. Based on early co-operation between NW/BAC and BP the same successful outcome is expected.
To maintain integrity of the overall system a cathodic control on-line system has been installed on a trial basis. This is a new BAC remote monitoring scheme for corrosion protection systems that makes use of GSM mobile telephone technology to transmit readings back to a control room rather than expensive bespoke equipment. The system allows for various levels of data monitoring, logging, graphing and alarm warnings and uses the cellular network short messaging system (SMS) and the internet for display, monitoring and data manipulation. Each data collection unit (SATcell) comprises a data logger with two channels: one for current and one for potential, a GSM transmitter and terminal board.
Data is accessed through a unique website where information can be viewed and graphed. Maps and documents can be uploaded to the relevant SATcell on the website whilst readings and graphs can be downloaded to a local PC for later viewing and analysis. Alarms for the potential channel can be set and if triggered can be sent to the user either through the SMS system or via an e-mail. Initial results from the trial are very encouraging.
The Tyne and Wear Metro has been extended to Sunderland and any interaction with NW pipeline CP systems and other utilities along the route has been considered. To determine and set up a controlled system of test and monitoring facilities for utilities crossing, or in close proximity to the track a corrosion working party has been established. BAC Corrosion Control, representing NW, is a member of this working party.
Before the contract is complete BAC will finalise all documentary records and hold a number of seminars for the operational and engineering staff to encourage best practice when working on pipelines. This will ensure that no adverse effects to either the pipes or the CP systems are created.
Operational manuals and codes of practice for the protection of pipelines will also be produced to enable NW engineers to operate the systems correctly and carry out on-going maintenance functions on the CP installation thus ensuring system longevity once BAC move away from the site.
There are some very old sections of the network where cast iron pipes have been used and, as they are not electrically continuous due to the type of jointing system, protection is not economically practical. To be effective sacrificial anodes would need to be installed every 6m along the lines. Instead, BAC’s legacy for these pipes is that when repairs are necessary, a sacrificial anode is applied while the pipe is exposed, even if only a repair collar is fitted. As the majority of the cost of repair is digging the hole and the labour, the cost of an anode, about £40.00, is insignificant but it will provide future protection to the pipe 5m either side of the repair.
With continued monitoring and maintenance, the investment made over the last three years is likely to be paid back many times with diminishing costly leaks due to corrosion