Causes of Hydrocarbon Release from Pipelines
A hazardous substance, either flammable or toxic, is safe till it is fully contained and maintained at desired parameters during storage, operation and transportation. The chemical hazard is materialised only when it loses its confinement and is released to the atmosphere. On release, the chemical disperses in the air and forms a toxic/flammable gas cloud in the downwind direction. Flammable chemical forms a cloud which could be within the flammable range. Any source of ignition within the cloud could lead to fire or explosion.
Incidental or accidental releases of hydrocarbons are bound to occur, even though modern pipelines are safer in construction and have highly integrated leak detection systems.
The predominant causes of hydrocarbon release from the pipelines are as follows:
1. Failure due to external factors
The oil and gas pipelines are often laid belowground at a depth of 1.2 to 1.5 m, but can also be installed aboveground, if necessary. Failure due to external factors is generally caused by third party mechanical interference, which leads to a puncture or a gouge severely reducing the wall thickness of the pipeline. A catastrophic guillotine rupture of the pipeline can also occur as a result of third party interference. The failure may occur immediately or may occur sometime later due to fatigue.
A high probability of ignition is likely when flammable substances are released from a pipeline as a result of the immediate failure. A source of ignition may be provided by the adjacent hutments or vehicles moving nearby. Third party interferences can be either intentional or unintentional. Unintentional interferences occur when the third party is unaware of the presence of the pipeline.
2. Failure due to corrosion
The most commonly employed material of construction for hydrocarbon pipelines is carbon steel. Two types of corrosion are responsible for the failure of pipelines – internal corrosion and external corrosion.
Moisture in the ground and salinity of the soil are major factors causing external corrosion. External corrosion may manifest itself in the form of pinhole failures caused by pitting and more generalised corrosion leading to a reduction in pipe wall thickness over a plane area. External pitting corrosion leads to small leaks that are often difficult to detect. The pinholes caused by pitting gradually grow in size over a period of time. A reduction in wall thickness occurs due to external area or plane defects. This can cause catastrophic failure of the pipeline under pressure, leading to a large scale release.
Internal corrosion is caused by the corrosive nature of the hydrocarbon being transported. This also leads to failures similar to the ones caused by external corrosion. Failures are generally caused by the formation of acids resulting from the presence of water or sulphur compounds in the substance being transported. The corrosion effect caused by the presence of water could be more prominent in the bottom half of the pipeline.
3. Failure by miscellaneous causes
Pipelines can also fail due to a variety of other causes like construction defects, pipe material defects, human error, flood ground erosion, and earthquake. The most common construction defect is the mechanical damage caused by earth moving machinery during backfilling and is often discovered shortly after pressurisation. However, some of the defects such as inadequate welding, inadequate corrosion protection to joints and damage to corrosion resistant coating that occur during the laying of the pipeline will only be identified sometime later.