Causes of Hy­dro­car­bon Re­lease from Pipelines

Chemical Industry Digest - - 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.

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