Corrosion protection at natural gas pipeline installation
Corrosion protection at natural gas pipeline installation
Keywords Cathodic protection, Corrosion, Pipelines
On completion of a natural gas pipeline installation in Malaysia the Peninsular Gas Utilization Project a permanent impressed-current cathodic-protection system consisting of several cathodic-protection stations located evenly along the pipeline was commissioned to operate continuously.
One deepwell and five shallow-anode ground beds were constructed to provide cathode protection to the Sector one pipelines. To provide cathodic protection to Sector two and three pipelines, seven shallow groundbeds and one groundbed comprising six deep well anodes in line were installed.
At horizontal directionally drilled crossings and where lateral pipelines from the 36-in. main line occur, 44 monolithic isolation joints were strategically installed.
Electrical interference from the HVAC distribution power lines which traversed Sectors two and three pipelines was encountered following commissioning of the Segari power station at Lumut, which utilizes gas transported by Sector one pipelines.
Induced HVAC voltage on the coated pipeline can pose hazardous step-voltage shock to personnel who may be required to perform tasks associated with the pipeline facilities.
This is predicted at above-ground pipe work associated with scraper stations, main line valve stations, teeoffs, and cathodic-protection test stations. To safeguard personnel working at these locations, the HVAC-induced problem has been mitigated by installation of equipotential mats.
In addition, the test stations have been designed to include fully insulated, Zap-guard-type test points complete with underground zinc ribbon anodes. These devices will dissipate hazardous induced voltages to ground thereby preventing danger to personnel.
To monitor the performance of the cathodic-protection system, test stations have been installed at 1-2km intervals along the pipeline. To mitigate stray current interference, test stations were installed at foreign buried pipeline crossings.
Hydrostatic testing along the coastal lowlands can be characterized as testing with simple pumping requirements and more difficult water sourcing. The maximum pump head on Sector one was 30m compared to 200m on Sectors two and three.
Most large rivers were unsuitable as water sources because of sediment load. Most of these rivers were crossed by horizontal directional drilling resulting in the accessible ends of the pipe being remote from the water. While irrigation canals were sometimes available, the water level was seasonal, depending on agricultural demands.
Some water sources had high acidity as a result of high sulphate levels in the soils, while others were brackish and tidal. To avoid the risk of contamination to water that may be used for domestic or irrigation purposes, chemicals were not used. Instead, the water was used for as short a time as possible, and the best available water sources were used.
Test sections were determined on the basis of location class, with a maximum length of 25 km and elevation changes. Test pressures were between 90 per cent and 110 per cent specified minimum yield stress (SMYS) for 24 hours. Sector one pipe was cold-expanded; testing rarely exceeded 30 hours duration.
For Sectors two and three,spiral welded pipe was used which was not cold expanded with tests regularly extending beyond 30 hr because the test pressure was kept at the same pressure level for the full duration of the test.
To meet contract requirements and deliver a quality product to the customer, all work was conducted in accordance with ISO 9002.
