Pipeguard Ltd was awarded multiple cathodic protection contracts for the gas import infrastructure being built by National Grid in 2006. These new projects are the largest investment made by National Grid since the original high pressure gas pipeline system was built in the 1960's.
Cathodic protection system design has changed over the past 30 years, the CP designer has to consider many factors at the design phase, such as:
• Identify and record of the effects of third party CP systems on the new pipeline.
• Identify and reduce hazards during design and construction.
• Identify and reduce the environmental impact of the design and construction.
• Size the CP system to last for the next 40 years without modification.
• Justify using complicated mathematical calculations why the number of groundbeds are needed and reduce the quantity to the minimum.
• Calculate using computer aided modelling how much ac will be inducted into the pipeline from adjacent electric powerlines and accurately predict the correct location and quantity of pipeline earthing required to dissipate the induced ac to the correct levels for the next 40 years.
Key CP equipment and operations for the FM 29 Pipeline System
Soil Resistivity Survey to 50m depth
Computerised induced AC modelling
Coating current drain tests at all horizontal directional drills or auger bore locations.
492 No welded pipeline connections
15,000m of pipeline earthing
20 No connections to 240V power supplies
20 No groundbeds and rectifier locations (impressed current Installations)
492 No AC coupons
492 No cathodic protection test points (posts)
67 No DC-Decouplers.
Third party stray current interaction testing
National Grid now has a 316km, 1220mm pipeline supplying up to 20 % of the nation’s gas imported from LNG via Ships. The gas is depressurised at the Milford Haven terminals via a complex procedure prior to transmission into the countries existing network. The pipeline system runs from south west Wales to Swansea then heads north east and along the way is connected a number of new and existing pipeline systems, compressor stations and pressure reduction installations before its final connection to the existing national transmission system in Gloustershire.
Picture 1 shows the pipes stringed along the pipeline spread. The pipe to soil AC voltages under the 400Kv power lines were monitored and the pipes earthed when required.
Picture 2 shows pipe stringing in full flow, 40 feet trailers constantly delivering pipes to be strung out over the complete pipeline spread.
The test post shown in picture 5 is a special heavy-duty unit fitted with a tamperproof lock. Inside is housed the remote monitoring unit and the DC-decoupler used to isolate the copper pipeline earthing from the pipeline CP system.
Picture 4 shows the view over the beautiful Welsh countryside. Some of the hills the pipeline followed could not be walked down as they were too steep. The routing of this pipeline was the most challenging ever attempted by National Grid
Picture 3 shows the close proximity of the 400kV power lines.
Picture 6 shows the top ammeter reading at just over 30A/m2 and the bottom voltmeter showing the AC potential of the pipeline just over 1 Volt. This demonstrates the high levels of AC current on the pipeline with relatively low voltages. The earth connection on the far right is still to be connected.
Picture 7 All complete, the twin 400kv power lines ever present. No sign left of any construction activities other then the CP posts and pipeline marker posts.