The souring of pipelines
Published by Isabel Stagg,
Editorial Assistant
World Pipelines,
Ian Kinnear, Product Manager, GPT Industries, addresses the impacts of hydrogen sulphide, particularly on corrosion and electrical bridging, and provides solutions to these challenges.
Hydrogen sulphide (H2S) has become an increasingly concerning issue in oil and gas pipelines around the world. Also known as sour gas, H2S is highly toxic, explosive, and corrosive, posing significant safety and infrastructure risks.
Where is H2S coming from?
H2S naturally occurs in some reservoirs as a component of raw natural gas and crude oil. Additionally, it can be introduced into pipelines through microbial metabolism of sulfur-containing compounds. The oil industry is accessing more challenging reservoirs, with many containing higher levels of H2S than traditional resources. Operators are also maximising efficiencies by processing and transporting multi-phase mixtures with heightened H2S concentrations rather than constructing dedicated natural gas pipelines. These factors mean pipelines are transporting more sour gas than ever before.
Impacts on pipeline integrity
Corrosion is the primary threat H2S introduces to pipeline integrity. When moisture condenses on pipe walls, H2S reacts with the steel, producing various iron sulphide corrosion products. At higher concentrations, H2S corrosion can lead to pitting and cracking, which severely undermine pipe strength.
Another emerging issue exacerbated by the presence of H2S is electrical bridging across pipeline isolation joints. Isolation joints strategically installed at certain points along the pipeline are designed to control electrical current flow from cathodic protection (CP) systems. Stray current leakage reduces CP system effectiveness, wasting power while leaving unprotected pipe segments vulnerable to accelerated corrosion.
Electrical bridging explained
Electrical bridging occurs when an electrically conductive material, often iron sulphide corrosion products, spans the isolation joint to create a current pathway between pipe segments. This provides an easier path for CP current to flow than the intended circuit through the soil back to the rectifier. When bridging happens, CP systems continue pushing current through now-connected sections rather than forcing it to disperse into the ground. This can often be seen by what are called travelling shorts. Travelling shorts occur as a pipeline cleaning pig goes down the line and deposits this iron sulphide at the flange interfaces. When this occurs, you will see a shorts travel down the line as the pig does, being isolated before but potentially no longer after. Iron sulphide, known in industry as black powder, readily precipitates from H2S-tainted condensate within pipelines. Over time, iron sulphide deposition can completely mask isolation joint gaskets. Its extreme conductivity enables sustained electrical connectivity between flanges, bypassing the isolation joint’s function. The iron sulphide sludge apparent in Figure 1 illustrates the severity this bridging phenomenon can attain.
Similarly, errant metal shavings from hot tapping or debris from inline inspection tool runs may lodge across isolation joints. Welding spatter can also introduce metallic bridging paths. However, iron sulphide from H2S-influenced corrosion tends to cause the most persistent, widespread bridging problems over the long term.
Effects of uncontrolled bridging
When designing a piping system, isolation is always put in place for a specific reason. There are several different reasons why isolation would be used, with each of these causing a benefit in terms of corrosion mitigation, safety, protection, cost or a combination of all these. When systems experience electrical bridging, isolation across the bolted flange assembly is no longer maintained, resulting in the opposite effect than what was designed for.
If there is no longer isolation being present between the flanges, a number of direct effects can take place. The first is that there is a significantly higher potential for increased rates of corrosion. Isolation is typically used in conjunction with corrosion prevention methods, and a loss in isolation can mitigate these corrosion preventions. In addition to this, there can be potential large expense losses in capital and operational spend. Wasted cathodic protection current and unnecessary rectifier operation drives up expenses while potentially failing to adequately protect all assets. Large amounts of time and resource also typically then needs to be spent to troubleshoot and provide solutions, which many times comes at the expense of...
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Read the article online at: https://www.worldpipelines.com/special-reports/20082024/the-souring-of-pipelines/
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