| .IV. PIPE MATERIAL
SELECTION
A. WALL THICKNESS
-D/T "RULE OF THUMB" - The following table provides generalized
recommendations for the selection of steel pipe wall thicknesses relative
to pipe diameter. These recommendations are meant to be used only as
a starting point in the design. It is recommended that in the final
design, specific stresses be calculated and compared with allowable
limits.
Diameter (D) Wall Thickness (T)
6 in and smaller 0.250 in.
6 to 12 in. 0.375 in.
12 to 30 in.
For 30 in and larger, D/t < 50
(For high-density polyethylene (HDPE) pipe, a standard dimension ratio
of D/t, SDR, of 11 or less is recommended and the pipe manufacturer
should be consulted).
B. STRESS ANALYSIS - In finalizing the design, the stresses imposed
during construction and in-service must be calculated and checked to
be within allowable limits for the grade of material. The stresses at
each stage must be considered acting individually and in combination.
Stresses result due to spanning between rollers prior to pullback, the
hydrostatic testing pressures, pulling forces during installation, radius
of curvature as the pipe enters the ground, the drilling profile curvature,
external pressures in the drilled hole, and the working pressure.
1. Pre-installation
a. Hoop and longitudinal stresses resulting from hydrostatic testing
are calculated.
b. Using the known distance between rollers as the free spanning
distance, the maximum hogging and sagging moments can be calculated.
Considering the greater of these two moments, the maximum spanning stress
is calculated. Note: during hydrostatic testing the pipeline will be
full of water therefore the additional weight of water must be included
in these calculations.
2. Installation
a. The spanning stresses calculated in stage 1.b. also apply in
this installation phase.
b. The theoretical pulling force must be determined in order
to provide the stresses that will result. An assumed down hole friction
factor of 1.0 is recommended to provide conservative results and to
include the effect of the pipeline being pulled around a curve. The
maximum predicted pulling force should then be used in calculating the
resulting longitudinal stress.
c. Allowing for a 10% drilling tolerance, leads to the use of
a radius of curvature 90% of the design radius when calculating the
longitudinal curvature stresses.
d. External pressure from static head in the drilled hole and/or
overburden pressures must be considered. It is recommended that the
static head resulting from the maximum envisaged drilling fluid density
should be used with a factor of safety of 1.5 to provide conservative
estimations of resulting hoop and longitudinal stresses.
3. Post-installation
a. The longitudinal curvature stresses calculated for stage 2.c.
above are used again here.
b. External pressure stresses from 2.d. apply.
c. Hoop and longitudinal stresses resulting from the final hydrostatic
test are calculated.
4. In-service
a. Curvature -see 2.c.
b. External pressure -see 2.d.
c. The maximum working pressure of the pipeline is used in calculating
longitudinal and hoop stresses that will be imposed during service.
C. ALLOWABLE STRESSES - Having determined the individual and
combined stresses at each stage of construction and those for the in-service
condition, they must be compared
with allowable limits.
1. ASME B31.8 -1992, Table A842.22 provides the following limits:
- Maximum allowable longitudinal stress: 80% SMYS.
- Maximum allowable hoop stress: 72% SMYS.
- Maximum allowable combined stress: 90% SMYS.
(Where SMYS is the Specified Minimum Yield Strength of the pipe material).
2. Regulatory bodies may impose additional limits to those specified
above - owner companies should identify any such further constraints
and ensure the adequacy of the design.
|
