Editing Pipeline (section)

==Implementation==
Pipelines are generally laid underground because temperature is less variable. Because pipelines are usually metal, this helps to reduce the expansion and shrinkage that can occur with weather changes.<ref>{{cite book|title=Oil and Gas Pipelines|last=Saxon|first=Carina|publisher=Gale|year=2016|isbn=9781410317513|pages=636–639}}</ref> However, in some cases it is necessary to cross a valley or a river on a [[pipeline bridge]]. Pipelines for centralized heating systems are often laid on the ground or overhead. Pipelines for petroleum running through permafrost areas as Trans-Alaska-Pipeline are often run overhead in order to avoid melting the frozen ground by hot petroleum which would result in sinking the pipeline in the ground.