Page 117 - Pipeline Rules of Thumb Handbook

104 Pipeline Rules of Thumb Handbook

Determine buoyancy of bare steel pipe

Multiply the square of the outside diameter of the pipe, in = 162.56 ¥ 0.341 - 33.4
inches, by 0.341, and subtract the pipe weight in lb/ft; the = 55.4 - 33.4 = 22lb/ft
answer is the buoyancy per foot in fresh water.
Since all of the coating materials in common use are
1
Example. 12≤ (12.75≤ OD) 33.4lb pipe ( / 4 ≤ wall). heavier than water (but not as much so as steel), coated pipe
will have less buoyancy than indicated by the rule. For sea
2
Buoyancy = 12.75 ¥ 0.341 - 33.4 water, use 0.350 for the constant.

Determine buoyancy of bare and concrete coated steel pipe in water and mud

To find the buoyancy of bare steel and concrete coated pipe
in water in lb/ft:

D
Buoyancy B () = ( D - 32 t)+11 t 2
3

for bare pipe, and

D 63 - W c
t D
B = ( D - 32 t)+ 1
3 48
for coated pipe

where D = outside diameter of pipe, in.
t = wall thickness of pipe, in.
t 1 = thickness of concrete coating, in.
W c = weight of concrete, lb/ft 3
To give this pipe a negative buoyancy of 100lb/ft:
To find the buoyancy of bare steel and coated pipe in mud
in lb/ft:
20 Ê 63 -143 ˆ
1 t
-100 = [20 - 32 (1 2 )]+ ()
20
3 Ë 48 ¯
Ê DW m ˆ 2
.
Buoyancy B () = 10 7 - t +11 t -100 = 26 7 33 3 1 t
.
.
Ë 2 000, ¯ -
.
.
1 t = 38in of concrete coating.
for bare pipe, and
The error introduced by this method is about 15% but
Ê DW m ˆ Ê W m - W c ˆ would be less as the thickness of coating decreased.
t D
.
B = 10 7 - t + 1
Ë 2 000, ¯ Ë 48 ¯
Example 2. Bottom conditions at a certain crossing that
3
for coated pipe, where W m is weight of mud, lb/ft . require the pipe to have a negative buoyancy of at least
50lb/ft. Let’s check to see if a 10-inch pipe having a 2-inch
Example 1. Find the buoyancy of steel pipe with 20-inch coating of concrete (W c = 143) has sufficient negative
1
OD and / 2 -inch wall thickness in water. What thickness of buoyancy.
concrete coating would be required to give this pipe a nega-
tive buoyancy of 100lb/ft (W c = 143)? 10 Ê 63 -143 ˆ
B = (10 - 32 (1 2 ))+ ()
2 10
3 Ë 48 ¯
20 2
11
B () = [20 - 32 (1 2 )]+ (1 2 )
3 B =-20 - 33.3 =-53.3lb/ft; so coated pipe meets design
.
= 26 7 2 8 = 29 5 lb ft positive buoyancy. requirements.
+
.
.