A datum feature is usually an important functional feature that needs to be controlled during measurement as well. How Datum Features are Shown on a Drawing The datum features on a drawing are denoted with a series of capital letters. These letters are in boxes and tied to the datum feature with a black triangle.
BigInch Petroleum 29 Jan 10 I will try to add what little else I can. Soil can set up restraints, virtual anchors, through friction and cohesion when sufficient contact length is possible. I've found that it generally takes between and feet of buried pipe length Dt coursework specification build up sufficient anchor force to restrain most pipe sizes, but it does vary with diameter and temperature, so be especially carefull where reverse angle changes in Dt coursework specification are at short intervals.
Note that these anchors occur in both horizontal alignment and in the vertical profile too. At horizontal changes in direction pipe is forced into the sidewall of the trench and moves upward.
In the profile, pipe is anchored at sag bends and tends to move upward into backfilled soil above the overbend areas. Where movements are extreme and soil cover does not provide enough weight to hold the pipe down at these bends, pipe can rise through the surface and become quite an embarrassment, so they are esp desireable to avoid.
What I've done in the past is make an installation table, first taking the standard burial depth and then finding out what maximum change in direction can be made without overstress, then doing the same for increasing degrees of bend showing an increasing burial depth needed to prevent liftoff, until the line is reaching stress limits.
Above that, special flexibility methods will be required, such as foam blocks or whatever, wrapped in geotextiles to try to prevent soil intrusion and compaction, etc. Be carefull where you place these soft areas, since they might make excellent farm tractor traps and you probably will want to fence them to prevent entry.
Sometimes these tables begin at 7 to 8 degrees of overbend, which isn't really very much bend until extra soil depth becomes necessary. You can reach a maximum depth where soil forces on an unpressurized line can cause too much ovaling in thin pipe walls.
In regard to setting up virtual anchors, its important to have good values for cohesion from tests on soil samples taken at frequent intervals along the route.
Designate some extra samples at points of large changes in direction. The samples should be from whatever material is to be used as backfill, either natural soil from the trench, or local sand backfill sources, if natural soils are not desireable. Friction and cohesion tests should be conducted on remolded specimens.
I am somewhat distrusting of providing underground flexibility, since I think that after a relatively small number of temperature cycles, much flexibility can be lost to soil sag and recompaction, so at places near valve, pig launching stations, and pump stations, I put a premium on even slight above ground flexibility.
Between everyone's comments above and Pengs paper, the only thing I may be able to add is related to the thermodynamics. Last, be careful with the startup and shutdown of hot lines.
In many cases a product's viscosity, such as heavy crude, can be very sensitive to temperature changes and required inlet pressures at lower startup temperatures can be significantly higher.
I've even had to change pump configurations on the fly from series to parallel as the soil around the pipeline finally heated up and reduced the crude's viscosity to a point where we were able to reach 4 and 5 times the initial flowrate.
You might want to conduct a "restart time to full flowrate" after allowing for various shutdown and line cool-off times. Maybe for something like shutdowns of 1,2,3,5,7,10,14, 30 day or even longer periods. Have a good project.
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