spills from pipelines during operation. Locating a pipeline in a vital 

 area, such as a coral reef, shellfish bed, or fish spawning area, will 

 have an adverse impact. In addition to preempting or disturbing prime 

 habitat, large volumes of sediment may be suspended during pipe burial. 

 Oil spills from rupture of the pipeline or leakage from joints and 

 connections may cause disturbances from direct hydrocarbon pollution or 

 pipelines may emit a continual trickle of oil, which can result in long- 

 term accumulation in sediments. 



While recognizing that longer pipelines consume more raw materials, 

 all efforts should be made to detour around geologically hazardous or 

 environmentally vital areas. The initial avoidance of vital areas while 

 siting a pipeline corridor may avert many short and long-term disturbances 

 to fish and wildlife. The avoidance of areas subject to geologic hazards, 

 such as mudslides, sediment liquefaction, or earthquakes, will result in 

 a lower risk of oil spills and their environmental consequences and 

 expense to oil companies. 



4.20.3 Design 



The design of a pipeline system depends on factors such as the 

 characteristics of the oil or gas well stream (its pressure, temperature, 

 and specific gravity) and the distance between the producing field and 

 shore. Variables include the production rate, whether separation takes 

 place on the platform or onshore, and desired operating pressures for 

 the pipeline. These factors and variables influence the pipe diameter, 

 pipe thickness, and number of intermediate pressure booster stations 

 required, if any [7]. 



The design of a submerged oil or gas pipeline is a complex engineer- 

 ing problem. A design is sought which will meet all operational 

 necessities and safety requirements and still be environmentally accept- 

 able. Pipe selection is an important consideration. The proper pipe 

 diameter must be chosen first (usual o' fi^om 14 to 44 inches) and then to the 

 pipeline and its expected contents must be designed to be stable. If the 

 pipeline is too light, it will gradually work its way up through the soil 

 and become exposed to the water forces. If it is too heavy, it may 

 gradually sink into the bottom and impose additional stress in the line. 

 Design procedures for determining the vertical stability of the line in 

 sands and clays are available [107]. The pipe wall and coatings must 

 withstand the bending stresses encountered during pipe laying. The 

 coatings and pipe must be designed to resist corrosion for the expected 

 lifetime of the pipeline, usually 20 years. 



Of the various design procedures that can make a pipeline more 

 environmentally sound, first and foremost is that the pipeline must be 

 engineered to be safe. This implies that the design life of the pipe 

 should exceed the period during which it will be used. Second, the 

 largest of feasible alternative pipeline diameters should be chosen. 



202 



