Page 65 - Reliability and Maintainability of In service Pipelines

54 Reliability and Maintainability of In-Service Pipelines

Third party interference: One of the common causes of deformations in pipe-
lines is the human interference. When heavy equipment or rocks strike the pipe,
they can make dents or gouges that change the internal geometry of the line.
Changes in the internal geometry alter the distribution of internal pressure by
focusing it on certain sections of pipeline. Over time these deformations may
result in pipeline failure. Deformations are often accompanied by a loss of coat-
ing, which increases the risk of corrosion.
Because human interference is the main cause of deformations, preventative
measures predominantly target public awareness. Pipeline operators participate
in federal, state, and provincial programs in both the United States and Canada
provide toll-free hotlines for property owners planning any excavation projects.
Property owners are required by law to check with these programs before they
begin a project. To increase the general visibility of submerged and buried pipe-
lines, land-based pipelines are often clearly marked and no-anchor zones are
established along aquatic pipelines. Federal law requires that markers be located
“at each public road crossing, at each railroad crossing, and in sufficient number
along the remainder of each buried line so that its location is accurately
known.”
With regards to pipelines, third party interference is a very damaging aspect.
As its name implies this means that there is a contributing third party to the dete-
rioration or failure of the pipeline. This can be in the forms of unauthorized or
mechanical digging, excavation, etc. The more exposure pipelines have to these
third parties the higher the likelihood of failure thus reducing the reliability of the
pipeline significantly.
Landslide: Land movement can be one of the easiest ways to decrease pipeline
structural reliability; especially movements as significant as a landslide or earth-
quake. In the design stage of pipelines, natural disasters can be taken into
account, especially if they are expected within the risky area.

2.2.2 PIPELINE FAILURE MODES

Failure modes can be presented mathematically by using the “limit state” concept.
A limit state is a condition of a structure beyond which it does not fulfill the rele-
vant design criteria. The general formulation for limit state function of a structure
subject to a time varying process (e.g., corrosion) is presented as follows:
GR; S; tÞ 5 RtðÞ 2 StðÞ ð2:1Þ
ð
ð
In the above equation GR; S; tÞ is a limit state function (or failure mode), StðÞ
is the stress affecting the limit state requirement at time t, and RtðÞ is the