4.20.5 Operations 



According to many studies, onshore pipelines are considered the 

 safest means for bulk transport of hydrocarbons. In spite of the severe 

 marine environment, offshore pipelines are reported to be less prone to 

 oil spills than onshore pipelines. 



The largest spills occurred prior to 1970 and when pipeline rup- 

 tures often were caused by dragging anchors. The largest pipeline spill 

 occurred in October 1967 when a vessel 1 dragging its anchor in a storm 

 severed a pipeline about 20 miles west of the mouth of Southwest Pass, 

 Mississippi River delta, Louisiana. The resulting spill went undetected for 

 10 days and released over 160,000 BBL of oil into the Gulf of Mexico. 

 Since 1970, spillage from pipelines has been considerably reduced as a re- 

 sult of several precautions in location, design, and construction of pipelines, 



Although some of the largest pipeline spills have been accidents, 

 such as breaks from anchor dragging, most pipeline spills result from 

 failure of older pipelines [139]. Corrosion by the forces of seawater 

 and shifting bottom sediments, from without, and by hydrogen sulfide, 

 dissolved oxygen, saltwater and fatty acids, from within the pipe, lead 

 to pipeline failure. Failures due to external corrosion (80 percent) 

 exceed those due to internal corrosion (20 percent) [5]. 



Safe operation of a pipeline system requires continuous line 

 pressure monitoring systems with automatic shut down valves or alarms, and 

 regular pipeline inspection for leaks [139]. The primary leak detection 

 system in use (required on all lines built after March 13, 1970) is a set 

 of automatic pressure sensing recorders on both ends of each pipeline 

 system. These devices are equipped to either shut down the flow 

 automatically or to sound alarms in event of an abnormal pressure level. 

 The system is sensitive only to leaks which cause a decrease in line 

 pressure greater than 300 to 500 psi and therefore is essentially a safe- 

 guard against the effects of a catastrophis line break [107]. 



The second system of leak detection, useful mainly in intertidal 

 areas (and onshore), is routine patrolling of pipeline route by boat, 

 aircraft, or wheeled vehicle. Inspection at intervals not exceeding 

 2 weeks is required. This type of monitoring would be of little 

 consequence in preventing losses of a large amount of petroleum in 

 the event of a major break. Regular pipeline patrolling allows detection 

 of small leaks, and therefore, complements the pressure-sensing system 

 described above [107]. 



A third system for detection consists of reading and comparing the 

 volume-recording flow meters on either end of a pipeline system. Because 

 crude oil moves from OCS areas to shore by common carrier lines, meters 

 are required at the offshore pipeline gathering system and again at the 

 onshore pipeline terminal in order that each producer be properly credited 



204 



