Page 69 - Pipeline Rules of Thumb Handbook

56 Pipeline Rules of Thumb Handbook

• Move convoy of sidebooms 250ft, lift the line, remove however, by use of Young’s Modulus of Elasticity (E = stress
6-in. block at beginning and end supports and 12in. of over strain).
blocks at inner supports. The new pipe length can be calculated from the formula:
• Repeat until total transition length is lowered 12in.,
except for several end supports, which are lowered less Arc Length = pq 180
R
than 6in. to match required deflections. Then come
back and do it again, repeating several times until the where q = sin X/R
-1
pipeline is completely lowered. Note that each repeat A comparison of the new pipe length with the original pipe
becomes shorter. Check final pipeline elevations if length will result in a figure for strain as a result of the new
desired. profile. Knowing the strain, additional longitudinal stress can
• Check tape wrap for possible damage, add 12in. of sand be easily calculated.
or pre-mix around pipe, backfill and compact.
ft
,
.
In Example1,q = sin - 1 175 ft 4 350 = 2 3056∞
Surprisingly, this operation goes very quickly, but must be Arc length AL) = p( 4 350 )( . 180 175 047 ft
(
ft 2 3056∞)
,
=
.
well coordinated. One person should signal the sideboom 4(
.
=
operators to lower the pipeline concurrently, so as to elimi- Length of new line = AL)+ 50 750 19 ft
nate excessive bends and sags. The pipeline is heavy when full
of liquid, and if not handled carefully, it could easily rupture. So the pipeline was actually stretched 0.19ft or 2.28in.
Pressure should be reduced, and extreme caution exercised )
.
.
-
.
.
during the actual lowering process. Strain = D L L = (750 19 750 00 750 00 = 0 000253
¥
,
=
E Stress Strain so longitudinal stress E Strain or
=
=
¥
,
LS = 29 10 6 ¥ 0 000253 7 347 psi.
.

Strain calculations This is well within the 35,000-psi yield strength of Grade
B pipe.
A new profile means the pipeline has been stretched a
certain amount, assuming line was horizontal originally. (If the
pipeline was convex, lowering it will put the steel into com- Source
pression.) Fortunately, steel is very forgiving either way, and
the added strain is generally not a problem. It can be checked, Pipe Line Industry, July 1986.
WELDING
When should steel be preheated before welding?
From the chemistry of the steel determine the carbon It is not necessary to preheat this particular steel before
equivalent: welding it.
But for another example:
Mn
=
Carbon Equivalent C+
.
4 Carbon = 020
.
Manganese = 160
If it exceeds 0.58 the steel may be crack sensitive and . 160
should be preheated before welding in ambient temperatures Carbon Equivalent = .20 + = .060
4
below 40°F.
This steel should be preheated, particularly for early
Example. If steel pipe having a carbon content of 0.25 and
morning welding.
manganese content of 0.70 is to be welded in springtime tem-
Why does preheating prevent cracking? It slows the cooling
peratures, ranging from 40°F to 80°F, is preheat necessary?
rate and reduces the amount of austenite retained as the weld
cools. This prevents microcracking. Other alloying elements
.70 and pipe wall thickness may also influence when joints of high
Carbon Equivalent = .25 + = .25 +.175 = .425
0
4 strength pipe should be preheated.