Page 206 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 193

Coefficient of friction for pipe coating materials

Tests indicate that previous values appear to be valid for thinfilm epoxies and conservative for coal tars
J. B. Ligon, Assistant Professor, Mechanical Engineering-Engineering Mechanics Dept., Michigan Technological
University, Houghton, Mich., and G. R. Mayer, Project Engineering Manager, Bechtel, Inc., San Francisco
A major factor in the stress analysis of buried pipelines is where F = longitudinal soil friction force (lb)
the movement that pipe undergoes in the presence of m = coefficient of friction (dimensionless)
temperature and pressure differentials during its life. This p = normal soil pressure acting on the pipe
movement is highly dependent upon friction resistance of the surface (psi)
soil. dA = soil to pipe differential contact area
2
Although ample information is available on the static coef- (in. )
ficient of friction for many materials, there is a lack of data on Ú A pdA = total normal soil force on pipe surface
friction between soils and various coatings used in the (lb)
pipeline industry. In the past, friction coefficient informa-
tion was extrapolated from data in the literature that were The above relation is independent of the pressure per unit
believed to have a similarity to the external pipe coating to area of the mating surfaces as long as very high contact pres-
soil interface. sures are not encountered, which is the case for a buried
However, with the development of thinfilm epoxy resin pipeline.
coating systems and the increasing use of these systems in the In terms of a buried pipeline supporting a soil burden up
pipeline industry, a change from a conventional coal tar felt to three pipe diameters of depth, the soil force relation
coating to a thinfilm epoxy coating would indicate a signifi- becomes:
cant change in the friction coefficient design criteria due to
the extreme contrast in the surface texture of the two È Ê Dˆ ˘
F = m 2 Dg H - + Wp
materials. Í Î Ë 2 ¯ ˙ ˚
To evaluate the effect of the difference in surface texture
on a pipeline system, test procedures were developed to where Wp = weight of pipe and contents (lb/in.)
determine the coefficient of friction for both coal tar felt and D = pipe diameter (in.)
thinfilm epoxy to various soils and to obtain more reliable H = depth of the pipe centerline (in.)
information for future pipeline designs using these coating g = specific weight of the soil (lb/in. )
3
materials.
Static friction tests were conducted to find the coefficient The sensitivity of the soil friction forces to the coefficient
of friction between coal tar felt and thinfilm epoxy pipe of friction is readily apparent from the above relation. Since
coating and eight representative backfill soil samples from this force is inversely proportional to the active length that a
typical locations along a pipeline right of way. The results indi- pipeline moves as a result of temperature and pressure expan-
cate that the friction coefficients are significantly larger than sion, the coefficient of friction becomes a major factor in
those previously extrapolated from literature and that coal tar pipeline stress design.
has a higher friction resistance in respect to anchorage of a
pipeline. Test system. The theoretical soil friction force acting
For the coal tar felt coating, the coefficient varies from 0.59 on the surface of a coated plate can be calculated from the
to 0.91 depending on the soil and moisture content. The thin- relation:
film epoxy coating varies from 0.51 to 0.71 under the same
conditions. F = mN

where N = normal force acting on the plate surfaces (lb)
Definitions W = total weight of plate and external load (lb)

Static coefficient of friction. The theoretical longitudi-
nal soil force acting on the pipe surface can be calculated from
the relation:

F = m Ú A p dA