Pipeline burial

This case study illustrates how combinations of pipeline burial, insulation, heating, and methanol injection can be used to prevent hydrates. The selection of the hydrate prevention scheme(s) is then a matter of balancing capital against operating costs. 

Siting - The preferred arrangement of bulk transport pipeline systems is for burial underground. This provides for enhanced protection from overhead events. This is even the case for offshore pipelines where there have been numerous incidents of dragged anchors from fishing vessels to pipelines exposed on the seabed. A radius of exposure from a pipeline can also be easily calculated for fires and vapor explosions based on the commodity, pressure, release opening, etc. From these calculations a restricted zone or similar can be designated. 

Line heaters could be installed at the wellhead to increase the inlet gas temperature from 85°F to 125°F. Figure 8.5 shows the pipeline temperature increase caused by the combined prevention methods of burial and wellhead heating. Use of these two methods permitted the methanol concentration in the free water phase to be reduced to approximately 14 wt% to prevent hydrate formation in the line. It should, however, be noted that heating may increase the amount of corrosion in the line. 

In addition to line burial and the addition of heat at the wellhead, insulation of exposed areas near the wellhead maintained higher pipeline temperatures, thereby reducing the amount of methanol needed for hydrate inhibition. Figure 8.6 displays the temperature increase in the buried and heated pipeline when exposed pipes were insulated. A combination of the methods causes the pipeline fluid to be outside the hydrate formation region (to the right of the curve marked 0 wt% MeOH), and methanol addition is no longer needed. 

When a hydrate plug occurs in a pipeline at temperatures above the ice point, the pressure-temperature conditions are illustrated in Figure 8.14. To the left of the three phase (Lw-H-V) line hydrates can form, while to the right only fluids can exist. Because the lowest ground burial temperatures or ocean temperatures (39°F) are usually above 32°F, ice formation (which will also block flows) is not a normal... 

Marine geophysical survey techniques can usually be used to locate such services. Sonar or multi-beam bathymetric surveys can map pipelines lying on the seabed. A magnetometer survey can be utilised to determine the position of buried steel pipelines. If a depth of burial is required, or if the pipeline is constructed of non ferrous material then a seismic technique using a pinger or boomer sound source will be required. 

Standard Test Method for Disbonding Characteristics of Pipeline Coatings by Direct Soil Burial Standard Test Method for Chemical Resistance of Pipeline Coatings... 

FIGURE 4-18 Tailings pipelines follow the slope of the hills and use soil friction produced hy partial burial as an anchor. 

In addition, the formation of craters near the outlet of the pipeline and erosion channels as a result of the flow of the sand - water mixture and the subsequent burial of these features with on-going filhng may further increase the variability of the deposits. 

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