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11/152 EQUIPMENT DESIGN AND SELECTION
For design purposes, condition s h # s y should be con- temperature profile in oil- and gas-production pipelines
firmed, and increasing wall thickness or reducing test pres- are essential to designing and evaluating pipeline oper-
sure should be considered in other cases. For offshore ations.
pipelines connected to riser sections requiring P h ¼ 1:4P i ,
it is recommended to consider testing the riser separately 11.4.2.2.1 Insulation Materials Polypropylene, poly-
(for prefabricated sections) or to determine the hydrotest ethylene, and polyurethane are three base materials
pressure based on the actual internal pressure experienced widely used in the petroleum industry for pipeline insulation.
by the pipeline section. It is important to note that most Their thermal conductivities are given in Table 11.4 (Carter
pressure testing of subsea pipelines is done with water, but et al., 2002). Depending on applications, these base materials
on occasion, nitrogen or air has been used. For low D/t are used in different forms, resulting in different overall
ratios (<20), the actual hoop stress in a pipeline tested conductivities. A three-layer polypropylene applied to pipe
from the surface is overestimated when using the thin surface has a conductivity of 0.225 W/M-8C (0.13 btu/hr-ft-
wall equations provided in this chapter. Credit for this 8F), while a four-layer polypropylene has a conductivity of
effect is allowed by DnV Clause 4.2.2.2 but is not normally 0.173 W/M-8C (0.10 btu/hr-ft-8F). Solid polypropylene has
taken into account. higher conductivity than polypropylene foam. Polymer
syntactic polyurethane has a conductivity of 0.121 W/M-8C
ExampleProblem11.6 Calculatetherequiredwallthickness (0.07 btu/hr-ft-8F), while glass syntactic polyurethane has a
forthepipelineinExampleProblem11.4assumingaseamless conductivity of 0.156 W/M-8C (0.09 btu/hr-ft-8F). These
still pipe of X-60 grade and onshore gas field (external materials have lower conductivities in dry conditions such as
pressure P e ¼ 14:65 psia). that in pipe-in-pipe (PIP) applications.
Because of their low thermal conductivities, more and
Solution The wall thickness can be designed based on the more polyurethane foams are used in deepwater pipeline
hoop stress generated by the internal pressure Pi ¼ applications. Physical properties of polyurethane foams
2,590 psia. The design pressure is include density, compressive strength, thermal conductiv-
ity, closed-cell content, leachable halides, flammability,
P d ¼ P i P e ¼ 2,590 14:65 ¼ 2,575:35 psi: tensile strength, tensile modulus, and water absorption.
The weld efficiency factor is E w ¼ 1:0. The temperature Typical values of these properties are available elsewhere
de-rating factor F t ¼ 1:0. Table 11.3 gives h ¼ 0:72. (Guo et al., 2005).
The yield stress is s y ¼ 60,000 psi. A corrosion allowance In steady-state flow conditions in an insulated pipeline
1 segment, the heat flow through the pipe wall is given by
⁄ 16 in. is considered. The nominal pipeline wall thickness
can be calculated using Eq. (11.116) as Q r ¼ UA r DT, (11:131)
(2,574:3)(6) 1 where Q r is heat-transfer rate; U is overall heat-transfer
t NOM ¼ þ ¼ 0:2413 in:
2(1:0)(0:72)(60,000)(1:0) 16 coefficient (OHTC) at the reference radius; A r is area of
the pipeline at the reference radius; DT is the difference in
Considering that welding of wall thickness less than 0.3 in.
requires special provisions, the minimum wall thickness is temperature between the pipeline product and the ambient
taken, 0.3 in. temperature outside.
The OHTC, U, for a system is the sum of the thermal
resistances and is given by (Holman, 1981):
11.4.2.2 Insulation Design 1
Oil and gas field pipelines are insulated mainly to conserve U ¼ n , (11:132)
heat. The need to keep the product fluids in the pipeline at 1 þ P ln (r mþ1 =r m ) þ 1
A r A i h i 2pLk m A o h o
a temperature higher than the ambient temperature could m¼1
exist, for reasons including the following:
. Preventing formation of gas hydrates Table 11.4 Thermal Conductivities of Materials Used in
. Preventing formation of wax or asphaltenes Pipeline Insulation
. Enhancing product flow properties
. Increasing cool-down time after shutting down Thermal conductivity
In liquefied gas pipelines, such as liquefied natural gas, Material name W/M-8C Btu/hr-ft-8F
insulation is required to maintain the cold temperature of
the gas to keep it in a liquid state. Polyethylene 0.35 0.20
Designing pipeline insulation requires thorough knowl- Polypropylene 0.22 0.13
edge of insulation materials and heat transfer mechanisms Polyurethane 0.12 0.07
across the insulation. Accurate predictions of heat loss and

Table 11.5 Typical Performance of Insulated Pipelines
U-Value Water depth (M)

2
2
Insulation type (Btu=hr ft F) W=M K Field proven Potential
Solid polypropylene 0.50 2.84 1,600 4,000
Polypropylene foam 0.28 1.59 700 2,000
Syntactic polyurethane 0.32 1.81 1,200 3,300
Syntactic polyurethane foam 0.30 1.70 2,000 3,300
Pipe-in-pipe syntactic polyurethane foam 0.17 0.96 3,100 4,000
Composite 0.12 0.68 1,000 3,000
Pipe-in-pipe high efficiency 0.05 0.28 1,700 3,000
Glass syntactic polyurethane 0.03 0.17 2,300 3,000