ARAMCO Standard for Cathodic Protection of Buried Pipelines
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Descripción: OLGA
Full description
Buried Pipeline analysis and design
Operational pipelines
Design of buried pipeDescripción completa
Full description
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Where Rizal Was Buried
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Manual pipelines
PLEASE FIND AWWA A45 BURIED PIPE DESIGN DOCUMENT
The standard OLGA model assumes that heat transfer flows with a constant flux in the radial direction. A series of concentric concentric wall layers with given thicknesses and heat transfer properties are assumed for the pipe. For a buried pipeline, pipeline, the heat flux is not symmetrical. To simulate a buried buried pipeline in the standard OLGA model, a pseudo-thickness of the soil is needed to account for the asymmetries of the system. It is also possible to simulate simulate the heat transfer in a buried buried pipeline with OLGA using FEMTherm, but this discussion will center on the use of the standard OLGA model. The equation for heat transfer in a buried pipeline is: h soil
k soil
2 H cosh 2 D
D
(1)
1
where: hsoil = = heat transfer coefficient of soil = thermal conductivity of soil k soil soil = D = outside diameter of buried pipe H = distance between top of soil and center of pipe
The term cosh-1(x) can be approximated by: 1 2 cosh 1 ( x) ln x x 1 2 for x > 1
(2)
For heat transfer for a series of concentric layers, the value of the heat transfer coefficient for the soil is given by:
h soil
k soil
D ln 2 2 D
D
(3)
where D2 = = Equivalent diameter of soil layer Equating the values of hsoil from from equations (1) and (3), and substituting the expression in -1 equation (2) for the cosh (x) gives: 1 2 2 H 2 2 H D2 D 1 D D
(4)
This expression gives the following values for D2 /D as a function of H/D: H/D 1.0 1.5 2.0 2.5 3.0 4.0 5.0 6.0
D2 /D 3.73 5.83 7.87 9.90 11.92 15.94 19.95 23.96
The equivalent thickness of the soil layer for use in the concentric layer calculation would be: t equiv 0.5( D2 D )
(5)
where tequiv = equivalent thickness of soil layer for concentric layer calculations It is useful to look at at a ratio of the equivalent equivalent thickness to the burial depth. The burial depth, BD, is defined as the distance from the top of the soil to the top of the pipe. Solving equations (4) and (5) with the t he added relationship BD BD H 0.5 D
(6)
gives the following table: BD/D 0.5 1.0 1.5 2.0 2.5 3.5 4.5 5.5
As the burial burial depth increases, the ratio of the equivalent thickness of soil soil for the concentric layer calculation to the burial depth approaches a value of 2.
An alternative approach to modeling buried pipelines would be to assume that the thickness of the soil layer is equal equal to the burial depth. An equivalent value of the thermal conductivity of the soil would be calculated from equations (1) and (3) to account for the asymmetry of the soil soil layer. A comparison of of the predictions done done with this method vs. the equivalent soil thickness method shown above indicated that the two methods gave the same steady state results. The equivalent equivalent thermal conductivity conductivity method, however, however, showed much more rapid cooling for shutdown cases, due to the decreased mass of th e soil layer. We recommend that the equivalent soil thickness method method be used used if the concentric layer heat transfer model is used.
