Page 258 - Trenchless Technology Piping Installation and Inspection

222 Cha pte r F i v e

where d refers to the diameter (in.) of the steel drill rod. For example,
it is possible to say that a 4-in.-diameter drill rod would correspond
to one 90° bend every 1000 ft.
The above linear dependence of (unplanned) curvature on rod
diameter is consistent with maintaining an equivalent stress level in
the steel rod, and corresponds to approximately one-third that typi-
cally allowed by bending specifications provided by drill rod manu-
facturers. Although, in principle, this same rule may be extrapolated
to Maxi-HDD, using corresponding large diameter drill rods, it is
considered excessively conservative for such well-planned, well-
controlled installations.
Buoyant Weight In order to apply Eq. (5.18), it is necessary to deter-
mine the buoyant weight, w , of the portion of the polyethylene pipe
b
submerged in the drilling fluid in route segments L , L , and L , illus-
2 3 4
trated in Fig. 5.20. ASTM F1962 provides general formulae for calculat-
ing the effective buoyant weight of the pipe under various conditions,
including empty, filled with water, and filled with drilling fluid. For
Mini-HDD case of interest, for which the pipe is empty, and, as sug-
gested in ASTM F1962, the specific gravity of the drilling fluid (mud),
γ , is conservatively assumed to be equal to 1.5, the buoyant weight
b
may be conveniently determined by
2
w (lb/ft) = 0.5 · D – w (lb/ft) (5.22)
b a
where D is the outer diameter (in.) of the product pipe. The value of
w may be obtained from the manufacturer, specifications for each
a
specific product pipe (diameter and DR rating).
Pipe Collapse
The critical pressure, P , as given in Eq. (5.14), may be expressed in
cr
terms of an equivalent head (ft) of water, for idealized conditions in
which the ovality reduction factor, f , and tension reduction factor, f ,
o R
are assumed equal to 1.0. Since the (effective) material stiffness, E,
and Poisson’s ratio, μ, are dependent upon the load duration, the
critical pressure is also dependent upon duration. Table 5.1 is based
upon Eq. (5.14), and industry provided pipe characteristics (PPI,
2008), and is applicable to any HDPE pipe diameter.
Since the drilling fluid is of significantly greater density than
water, the indicated pressure head (ft) values of Table 5.1 must be
reduced by a factor of 1 divided by γ . The values must be further
b
adjusted (reduced) for possible initial elevated temperature (PPI, 2008)
as well as aforementioned ovality and tensile load considerations.
There are two phases to be considered with regard to possible
collapse of the pipe. During the installation phase, 1- to 10-hour
strength would be appropriate, in combination with anticipated val-
ues of f and f during this period. For the post-installation phase, a
o R
1000-hour collapse strength is employed as the maximum period