ABSTRACT Offshore pipe-in-pipe systems require high performance thermal insulation to maintain high fluid temperature at arrival and to avoid hydrate formation during the cool-down process that follows a pipeline shut-down. At field joints, it might be difficult to achieve the design insulation performance due to installation challenges. In these cases, the insulation layer partially fills the gap between the inner and outer pipes and thus &quot;cold spots&quot; could potentially arise at field joints during the pipeline operation and cool-down. In this paper the impact on the thermal performance of partially insulated pipe-in-pipe field joints is evaluated through Computational Fluid Dynamics (CFD). Thermal convection is included in the fluid model for the pipe content and the air gap between the inner and outer pipes. Comparison is also made between the numerical analysis and simplified lumped-parameter models. Results from numerical simulations show that for the case considered no cold spot arises due to a lack of field joint insulation and lengthaveraged Overall Heat Transfer Coefficient (OHTC) can be used to predict the pipeline cool-down time. Numerical predictions have been compared to simulated service test results, which confirm the length-averaging effect on the OHTC. Further studies are recommended to assess potential cost savings that could be achieved for uninsulated field joints

Helen Boyd

Luca Chinello

Philip Cooper

Ramon De Haas

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1 Copyright © 2013 by ASME 
COOL-DOWN TIME ESTIMATION THROUGH NUMERICAL ANALYSIS FOR 
PARTIALLY INSULATED OFFSHORE PIPE-IN-PIPE FIELD JOINTS 
  
 
Luca Chinello 
INTECSEA 
London, UK 
Philip Cooper 
INTECSEA 
Woking, UK 
Ramon de Haas 
Heerema Marine Contractors 
London, UK 
Helen Boyd 
Heerema Marine Contractors 
Leiden, The Netherlands 
 
 
ABSTRACT 
Offshore pipe-in-pipe systems require high performance 
thermal insulation to maintain high fluid temperature at arrival 
and to avoid hydrate formation during the cool-down process 
that follows a pipeline shut-down. At field joints, it might be 
difficult to achieve the design insulation performance due to 
installation challenges. In these cases, the insulation layer 
partially fills the gap between the inner and outer pipes and thus 
“cold spots” could potentially arise at field joints during the 
pipeline operation and cool-down. In this paper the impact on 
the thermal performance of partially insulated pipe-in-pipe field 
joints is evaluated through Computational Fluid Dynamics 
(CFD). Thermal convection is included in the fluid model for 
the pipe content and the air gap between the inner and outer 
pipes. Comparison is also made between the numerical analysis 
and simplified lumped-parameter models. Results from 
numerical simulations show that for the case considered no cold 
spot arises due to a lack of field joint insulation and length-
averaged Overall Heat Transfer Coefficient (OHTC) can be 
used to predict the pipeline cool-down time. Numerical 
predictions have been compared to simulated service test 
results, which confirm the length-averaging effect on the 
OHTC. Further studies are recommended to assess potential 
cost savings that could be achieved for uninsulated field joints. 
 
1. INTRODUCTION 
Offshore hydrocarbon transportation in deep-water fields is 
often undertaken via pipe-in-pipe (PIP) systems. The main 
elements of a pipe-in-pipe system are a steel inner pipe and a 
steel outer pipe coaxial to the inner pipe. 
Support by the outer pipe and axial alignment are ensured by a 
series of spacers fitted on the inner pipe at regular intervals 
along its length. 
A layer of insulating material is wrapped around the inner pipe 
so that the gap between inner and outer pipes is almost 
completely filled. A typical cross-section arrangement for a 
pipe-in-pipe system is shown in Figure 1. 
At field joints, thermal insulation is achieved according to the 
arrangement shown in Figure 2. 
 
 
Figure 1 - Cross-section of a Typical Subsea Pipe-in-Pipe System 
 
 
 
Figure 2 - Thermal Insulation of a Typical Pipe-in-Pipe Field Joint 
Proceedings of the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering 
OMAE2013 
June 9-14, 2013, Nantes, France 
OMAE2013-10812
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2 Copyright © 2013 by ASME 
The purpose of the insulation layer is to prevent the cool-down 
of the fluid within the inner pipe and to avoid the formation of 
hydrates that could block the wellstream flow. 
 
2. PROJECT CASE 
Unexpected challenges arose during the installation of a 
10.75”/14” pipe-in-pipe system where the insulation layer had 
to be removed in the area around the weld. The resulting 
partially-insulated field joint is outlined in Figure 3. 
 
 
Figure 3 - Partially-insulated Pipe-in-Pipe Field Joint 
 
Following the partial removal of the insulation layer, a potential 
“cold spot” could occur at field joints and therefore numerical 
analyses were required to ensure that the thermal performance 
of the modified field joint was in accordance with project 
specifications during the cool-down process. 
A full-scale test of the pipe-in-pipe system with partial 
insulation was also set-up to validate the results produced by 
the numerical simulations. 
Furthermore, the overall heat transfer coefficient was calculated 
with analytical methods to verify the results obtained with the 
numerical models. 
 
3. COOL-DOWN NUMERICAL SIMULATIONS 
Two thermal analyses were carried out to simulate the field 
joint cool-down: 
1. Axisymmetric finite element simulation (using ANSYS) 
2. 3D CFD simulation (using ANSYS CFX) 
For both analyses, the objective was to simulate the cool-down 
process from operating conditions and to verify that the 
minimum fluid temperature after 12 hours of the cool-down 
was higher than the temperature of hydrate formation (17 °C). 
 
3.1  Axisymmetric Finite Element Simulation 
The geometry of the axisymmetric finite element (FE) model is 
presented in Figure 4. The only heat transfer mechanism 
included in this model is conduction.  
A steady-state analysis was run as initial condition for the 
subsequent transient cool-down simulation. As indicated in 
Figure 4, the boundary conditions for the steady-state analysis 
are: 
− Gas uniform temperature of 32 °C 
− Outer pipe external surface temperature of 3 °C 
 
Figure 4 – FE Axisymmetric Model: Geometry & Boundary 
Conditions 
 
An equivalent thermal conductivity was assigned to the full 
insulation layer, which captures the effects of conduction 
through the layer of Aerogel and natural convection plus 
radiation through the small air gap between the Aerogel and the 
outer pipe. 
Similarly, for the volume of the increased air gap (Figure 3) an 
equivalent thermal conduction was assigned, which includes 
conduction, natural convection and radiation. 
Only conduction was modeled for the gas volume within the 
inner pipe. 
The thermal properties assumed for the steel pipes, the thinner 
layer of Aerogel and the equivalent layer of Aerogel & air gap 
are listed in Table 1.  
 
Table 1 – Thermal Properties 
 
Layer 
Thermal 
Conductivity 
(W m-1 K-1) 
Specific Heat 
Capacity 
(J kg-1 K-1) 
Density 
(kg m-3) 
Steel Pipes 60.5 432 7854 
Aerogel (thickness  
= 6 mm) 
0.02 950 140 
Equivalent Aerogel 
+ Air Gap 
0.022 950 79 
 
The finite element model was meshed using 2D 4-node 
axisymmetric quadrilateral elements.  
The temperature plot resulting from the axisymmetric steady-
state simulation is presented in Figure 5. 
 
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3 Copyright © 2013 by ASME 
 
Figure 5 – FE Axisymmetric Model: Steady-state Temperature Plot 
 
The Overall Heat Transfer Coefficient (OHTC) expressed in W 
m
-2
 K
-1
 for the finite element model was calculated as: 
 
  	     (1) 
 
Where: 
• qr: radial heat flux extracted from the FE model at the outer 
surface of the inner pipe (W/m
2
) 
• ∆T: overall change in temperature (K) 
 
The lack of insulation in the weld area causes the OHTC to 
increase as outlined in Figure 6. 
 
 
Figure 6 – FE Axisymmetric Model: Steady-state OHTC 
 
The transient cool-down simulation starting from the steady-
state condition gave a final minimum temperature of the fluid 
after 12 hours of 18.9 °C. This suggested that the temperature 
of hydrate formation (17 °C) was not likely to be reached by 
the gas during cool-down. 
 
3.2  3D CFD Simulation 
The geometry of the 3D CFD model shown in Figure 7 
replicates the geometry of the axisymmetric model (Figure 4) 
and takes advantage of the two planes of symmetry associated 
with the field joint. 
Temperature boundary conditions for the 3D CFD model are 
the same as the axisymmetric model. 
No film coefficient was applied to the outer surface of the outer 
pipe. Therefore, the convective heat transfer contribution due to 
the flow of seawater around the outer pipe was not taken into 
account. 
Since the cool-down process occurs in the absence of gas flow 
within the inner pipe, no gas inlet or outlet surfaces were 
defined in the 3D CFD model. 
 
 
Figure 7 - 3D CFD Model: Geometry & Boundary Conditions 
 
Figure 8 shows the prismatic mesh employed in the 3D CFD 
model.  
 
 
 
Figure 8 - 3D CFD Model: Mesh 
 
Gas within the inner pipe and air enclosed in the enlarged gap 
were modeled as fluids. Therefore, the effect of natural 
convection was simulated more accurately than the 
axisymmetric model. 
The air and gas turbulence was modeled using the k-epsilon 
turbulence model. 
Air and gas properties were taken from the material library 
available in ANSYS CFX. 
The insulation layers were modeled as conductive solids having 
the same properties as the axisymmetric model (Table 1). 
Unlike the axisymmetric model, radiation was not considered in 
the 3D CFD model. 
Figure 9 shows the temperature plot (on the left hand side) and 
the streamlines calculated for air and gas (on the right end side) 
for the steady-state 3D CFD analysis. 
 
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4 Copyright © 2013 by ASME 
 
 
Figure 9 - 3D CFD Model: Steady-state Temperature Plot and Air/Gas 
Streamlines 
 
A transient simulation was set up starting from the steady-state 
condition, and the resulting temperature plots within the gas 
volume at three time intervals are shown in Figure 10.   
 
 
Figure 10 - 3D CFD Model: Transient Temperature Plots within Gas 
 
4. COOL-DOWN FULL-SCALE PHYSICAL TEST 
HMC performed full-scale tests of the pipe-in-pipe system at 
Heriot Watt University. The test set-up was as illustrated in 
Figure 11. The outer pipe was 14” OD by 20.6 mm wall 
thickness and the inner pipe was 10.75” OD by 24.7 mm wall 
thickness. The annulus was filled with 500 mm wide packs for 
insulation except for a 500 mm wide section where the 
insulation was provided by a thermal blanket, and a 150 mm 
wide section where no insulation was provided and radiant 
barrier (tape) was installed on the steel surface. Two centralizer 
rings were also included, equally spaced, approximately 3.25 m 
from the centerline.  
 
 
Figure 11 - Setup for Full-Scale Simulated Service Test  
The 150 mm gap with no insulation was provided to simulate 
the gap in insulation beneath the weld in the outer pipe. The 
500 mm wide insulation blanket was included because the 
insulation blanket can be cut to size in the field to fill up any 
extra space in the field joint area. The maximum width of 
blanket that would be used would be 500 mm as that is the 
width of the insulation packs. 
The test was divided into three zones, Zone Two being the 
actual test zone. Zones One and Three were the guard zones. 
The heating unit was installed on the inside of the pipe. A 
typical photograph of a heating unit is shown in Figure 12. 
 
 
Figure 12 - Typical Example of a Heating Unit 
 
The heating unit was provided with bulkheads between the 
zones. Thermocouples were installed on the inside of the pipe 
by means of the spacers shown in Figure 12 and also on the 
bulkheads. Thermocouples were also installed on the outside of 
the insulation packs at the same location as the thermocouples 
on the inside. Heating of the individual zones was provided 
with wire heating elements to which power was supplied.  
The full test assembly was installed in the test rig where the 
water on the outer pipe was cooled to approximately 4 ºC. Once 
an initial steady state was achieved the temperature of the 
internal surface was gradually increased to 95 ºC and the 
system was again allowed to stabilize allowing the OHTC of 
the central zone to be determined. 
On completion of the OHTC determination, a cool-down 
simulation was performed using active cool-down. This means 
that the heaters in the central zone were switched off, but the 
heaters in Zones One and Three remained active. 
The power supply to zones one and three was programmed to 
track the temperature in the central zone with an offset of 
typically -0.5 ºC. 
The results are presented in the graphs in Figure 13 and Figure 
14 and are summarized below. 
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Figure 13 - Average Temperature on the Inside of the Inner Pipe 
 
 
Figure 14 - Average Temperature on the Outside of the Outer Pipe 
 
During the steady state period: 
• Average internal temperature within Zone Two: 95 ºC 
• Average external temperature within Zone Two: 3.3 ºC 
• Average power input to Zone Two: 564.95 Watts 
• OHTC: 1.596 W/m2K 
The time taken for the average temperature of the internal 
surface of the inner pipe in Zone Two to reduce from 32 ºC to 
17 ºC was 13.08 hours. 
 
5. COMPARISON OF COOL-DOWN TEST RESULTS 
WITH NUMERICAL PREDICTIONS 
The minimum gas temperatures predicted at the end of the 
cool-down process by the FE axisymmetric model and the 3D 
CFD model are listed in Table 2, which also includes the 
temperature on the inside of the inner pipe measured during the 
full-scale physical test after 12 hours from the time that the 
temperature of 32 ºC was reached (see Figure 13). 
 
Table 2 - Temperature after 12 Hour Cool-down 
 
Hydrate 
Formation 
Temperature 
Numerical Simulation Full-Scale 
Physical 
Test 
2)
 
Axisymmetric 
FE Model 
1)
 
3D CFD 
Model 
1)
 
17 °C 18.9 °C 19.1 °C 18 °C 
Notes: 
1. Minimum gas temperature 
2. Average temperature measured on the inside of the inner pipe 
The results obtained from the experimental full-scale test are in 
good agreement with both the axisymmetric FE model and the 
3D CFD model of the partially-insulated field joint. Both 
physical tests and numerical models predicted the gas 
temperature after 12 hours to be above the hydrate formation 
temperature. This indicated that a “cold spot” at the field joint 
was not likely to occur and therefore the design thermal 
requirements during shut-down for the pipe-in-pipe system with 
partial insulation were fulfilled. 
 
6. 2D CFD MODEL OF THE FULL AIR GAP 
A 2D CFD model of the full air gap (i.e. with no insulation 
layer fitted between inner and outer pipes) was built to further 
investigate the air recirculation due to natural convection 
occurring at the cross-section of maximum OHTC (see Figure 
6). 
The mesh consisted of a thin layer of hexahedral cells aimed at 
representing a 2D flow simulation. 
In this case, it was assumed that the inner pipe was uniformly at 
the same temperature as the gas and that the outer pipe was 
uniformly at the same temperature as the ambient. In other 
words, the gas temperature was applied to the inner surface of 
the fluid domain (corresponding to the outer surface of the 
inner pipe) and the ambient temperature was applied to the 
outer surface of the domain (corresponding to the inner surface 
of the outer pipe). 
The air turbulence was modeled using the k-epsilon turbulence 
model, and air properties were taken from the ANSYS CFX 
material library. 
Conduction, convection and radiation were included in the 
model. 
Geometry and boundary conditions are shown in Figure 15. 
The air velocity and temperature plots obtained from the 
steady-state 2D CFD model for the full air gap are also given in 
Figure 15.   
 
 
Figure 15 - Steady-state 2D CFD Model of the Full Air Gap 
 
7. VERIFICATION OF CFD MODELS 
Analytical formulas for the OHTC were compared with results 
from the steady-state 3D CFD model described in Section 3.2 
and with a steady-state 2D CFD model of the full air gap 
described in Section 6. 
The analytical formula for the OHTC expressed in W m
-2
 K
-1 
is: 
 
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6 Copyright © 2013 by ASME 
  

 ∑
	


    (2) 
  
Where: 
• Dref  reference diameter (m) 
• R2i outer radius of layer i (m) 
• R1i inner radius of layer i (m) 
• ki  thermal conductivity of layer i (W/mK) 
• n number of layers 
 
For the CFD models, the OHTC expressed in W m
-2
 K
-1 
was 
calculated as: 
 
  	     (3) 
 
Where: 
• qr: radial heat flux extracted from the CFD model at a 
specified surface (W/m
2
) 
• ∆T: overall change in temperature (K) 
 
Figure 16 presents the variation of OHTC as a function of gas 
temperature obtained with the 3D CFD model for the 50 mm 
long section without insulation (full air gap) and the 200 mm 
long section with reduced (6 mm) Aerogel layer, along with the 
corresponding results from analytical formulas. 
 
 
Figure 16 - OHTC Comparison: Steady-state 3D CFD Model versus 
Analytical Method 
 
The chart in Figure 17 highlights the difference between OHTC 
values calculated with analytical formulas and the results 
obtained from the steady-state 2D model for the full air gap.  
 
 
Figure 17 – OHTC Comparison for the Full Air Gap: Steady-state 2D 
CFD Model versus Analytical Method 
 
Results from 2D and 3D CFD simulations were in good 
agreements with analytical formulas for the gas temperatures 
considered. 
 
8. CONCLUSIONS 
The case presented in this paper shows that no cold spot is 
anticipated during the pipeline shut-down due to a partial 
removal of field joint thermal insulation. This was 
demonstrated through numerical simulations, which were 
successfully validated against full-scale physical tests. 
Therefore, the length-averaged OHTC can be used to predict 
the cool-down time for pipe-in-pipe systems with partially 
insulated field joints. 
Furthermore, the OHTC calculated with 2D and 3D CFD 
models was verified against analytical formulas and good 
agreement was found. 
 
9. FURTHER WORK AND RECOMMENDATIONS 
Further studies are recommended to assess the potential cost 
savings that could be achieved if field joints are not fitted with 
thermal insulation layers. 
We observed from the CFD models that a small convection cell 
started up within the air gap. However, this convection cell was 
not powerful enough to reduce the system cool-down time. The 
full scale tests also showed that where the centraliser rests on 
the outer pipe at the 6 o’clock position, a relative cold spot 
occurs. On all tests this spot cooled down quicker than the field 
joint (i.e. the centraliser location is a “colder spot” than the 
field-joint location). This result could be replicated by an 
analytical model that included the centraliser. This finding 
should be considered in thermal analysis for future pipe-in-pipe 
systems. Further CFD work is recommended to show alignment 
between CFD work, tests and the analytical model. 
 
10. ACKNOWLEDGMENTS 
The authors would like to thank Mr Josef Navarro of 
INTECSEA UK for his contributions to the preparation of this 
paper. 
 
11. REFERENCES 
[1] Taxy, S., 2004, “Use of Computational Fluid Dynamics to 
Investigate the Impact of Cold Spots on Subsea Insulation 
Performance – Application to Deep Water Field”, OTC 16502 
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COOL-DOWN TIME ESTIMATION THROUGH NUMERICAL ANALYSIS FOR PARTIALLY INSULATED OFFSHORE PIPE-IN-PIPE FIELD JOINTS

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COOL-DOWN TIME ESTIMATION THROUGH NUMERICAL ANALYSIS FOR PARTIALLY INSULATED OFFSHORE PIPE-IN-PIPE FIELD JOINTS

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