Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering
Proceedings of the ASME 28th International Conference on Ocean, Offshore and Arctic Engineering
OMAE2009
USA
May 31 - June 5, 2009, Honolulu, Hawaii,
OMAE2009
May 31 - June 5, 2009, Honolulu, Hawaii
OMAE2009-79308
OMAE2009-79308
FATIGUE DESIGN OF FLOWLINE SYSTEMS WITH SLUG FLOW
Philip Cooper, Christopher Burnett, Ian Nash
INTECSEA (UK) Ltd
Woking, UK
ABSTRACT
Hydraulic sizing of multiphase flowlines generally leads to
significant potential for slug formation. The dynamic loads
from the passage of slugs through bends are well understood.
Gravity load effects associated with changing contents density
are less widely recognised, but potentially more significant.
Multiphase flowlines can experience several slugs every
minute, which can lead to high cycle fatigue damage.
Slug-induced fatigue damage has emerged as a governing
design criterion in recent projects. Spans, rather than pipe
bends, have become the focus of attention for slug-induced
fatigue damage. These may occur at flowline terminations and
spool-pieces used to connect flowlines in deepwater
developments. Spans introduced by lateral buckling mitigation
features can be particularly difficult to protect against sluginduced fatigue.
This paper defines the slug-induced fatigue problem, and
presents techniques for predicting fatigue damage during
design. It describes analysis of pipeline spans, flowline
connection spools and spans at buckle mitigation sleepers.
Design recommendations are proposed, with suggestions for
follow-up investigations to improve understanding of this
phenomenon.
slug flow conditions apply for the full operating life of a
flowline.
The additional mechanical design tasks required for design
for slug flow conditions are then outlined. This addresses
analysis of free spans, typically associated with seabed terrain,
as an extension to conventional allowable span calculations.
The analysis required to check the fatigue strength of rigid
spools connecting flowline segments is then described. Finally,
examples of detailed analysis of “engineered” spans in a
flowline system, including buckle initiation sleepers, crossing
supports and flowline termination assemblies, are presented.
The paper concludes with design recommendations to be
considered in a multiphase flowline design, emphasising the
need for close links between flow assurance and mechanical
design in the early stages of a project.
NOMENCLATURE
The nomenclature and associated typical values used in
sample calculations are listed in Table 1.
INTRODUCTION
This paper aims to provide an overview of the mechanical
design considerations applicable to flowline systems conveying
multiphase fluids with potential for slug formation. The
conditions for slug flow conditions are identified, and the
requirement for slug flow characteristics in the Basis of Design
is defined. The basic mechanical forces developed by the
passage of slugs are then defined, including assessment of slug
loads for a typical 10” flowline. This is followed by a simplistic
evaluation of allowable fatigue stress ranges, assuming onerous
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Table 1 Nomenclature and Data for Sample Calculations
Parameter
Pipe Outer Diameter
Pipe Wall Thickness
Pipe Inner Diameter
Slug Density
Bubble Density
Flow Velocity
Bend Radius
Bend Angle
Design life
Slug frequency
Fatigue Life Reduction
Factor
Symbol
D
t
d
ρslug
ρbubble
v
R
β
−
−
FLRF
Value
0.324
0.027
0.270
900
100
10
1.524
90
20
100
(varies)
Unit
m
m
m
kg/m3
kg/m3
m/s
m
degrees
years
/hr
-
SLUG FLOW MECHANISMS AND CHARACTERISTICS
Figure 1 Two Phase Flow Regimes
Slug flow can arise in any multi-phase flow involving a
combination of gas and liquid. In practice, most oil reservoirs
will yield fluids containing a mixture of oil, gas and water at
some stage during the life of the field, and there is therefore
widespread potential for slug flow. Slug formation mechanisms
are well established [1]. The hydraulic flow regime alone can
generate slug flow conditions – there is no requirement for
uneven discharge from a well to generate slugs. Indeed,
economic sizing of flowlines tends to lead to a flow velocity
well within the slug flow regime, and hence slug flow under
normal operating conditions. It is therefore very likely that slug
flow will occur in most multiphase flowlines, which typically
operate with flow velocity of around 10 m/s. Accordingly, the
slugging condition is likely to be “chronic”, affecting a major
part of the design life of the flowline system.
Other parameters required for fatigue design are density of
slug and bubble. Again, these will vary widely according to
local flow conditions and fluid composition, but typical
average density for slug and bubble are around 900 kg/m3 and
100 kg/m3 respectively.
In a slug flow, the liquid and gas phases separate into a
“slug” of liquid (with some entrained gas), and a “bubble” of
gas. The bubble is accompanied by a variable quantity of liquid
in a “film” of (Figure 1). The lengths of slug and bubble are
variable and difficult to measure or predict, but are typically in
the range of 10m – 100m – i.e. comparable to typical span
lengths. Slug flow prediction models often indicate slug
frequencies of 100/hr or more.
The potential for slug flow should be assessed at an early
stage of flowline design. Where slug flow is expected, flow
assurance studies should include careful evaluation of slug
parameters for use in flowline mechanical design. Slug
frequency, slug/bubble length and slug/bubble density should
be carefully estimated for the range of hydraulic conditions
expected during the life of the field. These parameters should
be included in the flowline Basis of Design.
FATIGUE LOADS FROM SLUG FLOW
The passage of slugs generates two forms of unsteady
structural loads, with potential to cause fatigue damage:
1. Momentum forces at changes of direction (pipe
bends)
2. Gravity loads due to changes in contents weight.
Momentum forces at a pipe bend can be expressed as:
Rx = ρ π (d / 2)2 v2 (1 - cosβ)
Ry = ρ π (d / 2)2 v2 sinβ
where
Rx = resulting force in x-direction (N)
Ry = resulting force in y-direction (N)
For a 90 degree pipe bend with the date in Table 1, this
leads to the following resultant bend reaction resultant forces:
Slug
7287 N
Bubble
810 N
Change in force
6477 N
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Gravity loads are simply evaluated as the change in weight
of the pipe associated with the slug or bubble. The data in Table
1 leads to change in weight of 449 N/m, so that a typical span
of 20m length will experience a change in weight of 8987 N,
providing the slug and bubble are at least as long as the span.
This load is not suddenly applied as the bend momentum load,
so dynamic response considerations are less important for
gravity load effects, but may still be significant in some cases.
ALLOWABLE FATIGUE STRESS RANGE
Where slug flow is inherent in the flow regime, slugs can
occur throughout the life of the flowline at a rate of 100 slugs
per hour or more. For a typical 20 year design life, this leads to
a fatigue cycle count approaching 20 million. Typical flowline
design codes (e.g. DNV [5]) require a maximum fatigue usage
of 20%, which equates to a factor of 5 on load cycles. Added to
this, produced fluids often contain corrosive compounds which
accelerate fatigue crack growth rate, typically requiring a
fatigue life reduction factor of at least 10 to be used for carbon
steel flowlines. (CRA cladding or lining can be used to
eliminate this corrosion mechanism and retain the full fatigue
strength.)
Taken together, the factored fatigue cycles for a 20 year
life is as follows:
CRA Clad (FLRF=1.0)
87.6 x 106
Unprotected steel (FLRF=10)
876 x 106
The typical design fatigue cycles defined above can be
converted to allowable fatigue stress ranges using established
S-N curves, such as BS 7608 [6]. This leads to the typical
allowable stress ranges (excluding any stress concentration
factor at girth welds) listed in Table 2. (Note that no reduction
in slope in the S-N curve is claimed in the high cycle fatigue
region, as this benefit can be difficult to justify, particularly
where corrosive fluids are present.)
Table 2 Typical Allowable Stress Ranges
BS 7608 Weld
Class
D
E
Allowable Stress Range (MPa)
CRA Clad
Unprotected steel
(FLRF=1.0)
(FLRF=10)
11.1
5.2
10.6
4.9
It is clear that the high cycle fatigue loads associated with
continuous slug flow lead to rather restrictive allowable fatigue
stress ranges for design of the flowline system.
STRUCTURAL RESPONSE OF FLOWLINE SPANS
Kansao [2] describes a rather elaborate method for
calculation of structural response of spans to slug flow using a
dynamic FE model subject to moving gravity loads. However,
the need to accurately include dynamic effects is questionable
at the early design stage.
The approach to span fatigue in recent projects undertaken
by the authors is to incorporate slug induced fatigue into the
allowable span calculations routinely performed during design.
In this way, slug fatigue is introduced as an additional
constraint on allowable span length, alongside the conventional
VIV, static strength and bar buckling criteria [3]. Static FE
analysis is used to evaluate fatigue stress ranges for isolated
idealized spans, taking account of seabed stiffness and effective
axial force effects. Momentum loads generated at out of
straightness features associated with the nominally straight pipe
route are not considered significant in this assessment, and
have been ignored. Iterative solutions are performed to develop
a graph of stress range vs. span length for various effective
force levels, from which the allowable span length can be
determined (Figure 2). Note that the effective axial force has a
significant influence on fatigue damage. This aspect does not
seem to be considered in [2].
7
6
5
Stress range (MPa)
This is a significant force, and one that changes rapidly as
the slug arrives at the bend (around 0.25sec for a 5D bend in
the above example). This rapid variation of force has potential
to generate significant dynamic response if the pipe bend is not
rigidly restrained.
4
3
2
1
0
1000
1500
2000
2500
3000
3500
4000
4500
5000
Effective force (kN)
10m Span
12m Span
14m Span
16m Span
18m Span
Allowable 1.3 SCF
Figure 2 Typical Span Fatigue Results
It is expected that analytical formulations based on span
structural models such as those in DNV-RP-F105 [3] might
also provide an adequate basis for determination of allowable
slug fatigue span lengths for screening purposes.
Recent applications of this method have indicated
significantly reduced allowable span lengths, even when
compared to the in-line VIV onset criteria, which is normally
regarded as rather conservative. Slug induced fatigue has
emerged as the governing criteria for span lengths, where
chronic slug flow conditions are expected.
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STRUCTURAL
RESPONSE
CONNECTION SPOOLS
OF
40
FLOWLINE
30
Deepwater flowline systems typically feature rigid spools
connecting adjoining flowline segments. These can connect
using either vertical or horizontal diverless connection systems
– a typical vertical connector spool configuration is shown in
Figure 3. It is convenient to design these spools so that they
span from hub to hub, in order to eliminate an interface with
the seabed, which complicates design, metrology, fabrication
and installation. However, this has the effect of introducing a
large span in the path of slug flow.
Stress Range - MPa
20
10
0
-10
-20
-30
-40
Figure 4 Typical variation of stress along length of spool
Clearly, if the spool is unable to sustain this level of fatigue
loading (i.e. stress ranges below limits defined in Table 2),
there is little point in progressing to a more sophisticated
dynamic analysis. However, where acceptable performance is
indicated, detailed design should include consideration of both
gravity loads (accounting for expected ranges of slug and
bubble lengths), and momentum forces at bends. Rigorous
analysis of these combined effects is not straightforward, since
commercially available software tools generally used for this
type of analysis lack features for convenient definition of this
type of dynamic load history.
SLUG FATIGUE AT “ENGINEERED” SPANS
Figure 3 Typical Deepwater vertical spool
Free-spanning spools must therefore be designed to
tolerate stress ranges associated with slug flow. Unlike flowline
spans, slug loads will include both momentum and gravity
contributions, and a dynamic time history analysis is required
to characterize the response of the spool. However, an initial
screening check can be performed using static analysis of the
spool, with a gravity load applied corresponding to the change
in submerged weight during slug flow (449 N/m in this
example). This very simple analysis will directly indicate the
stress range generated by the slug gravity load, assuming the
slug and bubble are sufficiently long to envelope the complete
spool. Figure 4 shows a typical variation of stress range along
the length of a vertical spool such as shown in Figure 3,
considering gravity effects alone. It is clear that significant
fatigue stresses are acting, requiring, at the very least, a CRA
clad construction in order to avoid the FLRF penalty that might
otherwise apply to a carbon steel spool.
A similar approach is used to assess fatigue stresses at
“engineered” spans introduced at pipeline end structures,
crossing supports and buckle mitigation sleepers. In these
cases, the known geometry of the specific features is modelled
using FEA to provide an accurate estimate of fatigue stress
ranges due to slug flow. Static analysis is used, with a selection
of slug/bubble configurations selected to generate the most
severe stress range at critical locations along the span.
Figure 5 shows load cases selected for analysis of pipe
spans either side of a sleeper, such as that often introduced to
deliberately trigger formation of a benign lateral buckle. A
typical stress response from such an analysis is shown in Figure
6. In this case, the pipe weight is modelled with slug contents
density, and the effect of the bubble modelled as a moving
buoyancy force.
Step 1
Buoyancy before
sleeper
Step 2
Buoyancy over sleeper
Step 3
Buoyancy far side
sleeper
Figure 5 Schematic of FE model of Slug Flow over Sleeper
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1.000
0.000
Stress Range (MPa)
-1.000
-2.000
-3.000
-4.000
-5.000
-6.000
-7.000
17.000
22.000
27.000
32.000
37.000
Figure 8 Flowline-Structure Connection
42.000
Loadstep
After Sleeper
Series3
Figure 6 Typical stress response at sleeper
Fatigue design of buckle mitigation sleepers is especially
problematic, since conventional span intervention measures are
not available, as the pipeline must remain free to move
sideways. Multiple support arrangements, augmented by
oversize collars at touchdown regions to further reduce span
lengths, have been considered to reduce slug fatigue damage at
buckle initiation locations. Figure 7 shows a proposed design
for a pipeline crossing which also acts as a buckle initiator.
This features multiple supports to limit slug induced fatigue
damage to acceptable levels. It is evident that such a
construction would be more expensive than a simple crossing
design, but could be implemented if more economical solutions
are unavailable.
0.8
Vertical Deflection (m)
Before Sleeper
0.6
0.4
0.2
0
-0.2
0
20
40
60
80
100
120
-0.4
Distance (m)
Figure 9 Typical Span at Flowline Structure Connection
This prop-type support can generate significant span
lengths, which must be assessed for gravity load effects of slug
flow. Structural modelling of the global response of the span to
variable gravity loads is therefore required. Where excessive
fatigue stress range is indicated, stiffeners can be applied to the
“stem pipe” adjoining the structure, as shown in Figure 8.
These stiffeners require careful design in order to gradually
increase the bending stiffness of the span, so that stresses are
diffused. Detailed FE analysis such as that shown in Figure 10
has been used to refine stiffener designs and demonstrate
adequate fatigue resistance.
Figure 7 Fatigue Tolerant Buckle Initiation/Crossing Design
Another example of an “engineered” span is at connections
between flowlines and pipeline end or in-line structures. In
some cases, these can be designed so that the pipe centerline
remains close to the mudline, so that no span is introduced.
However, installation vessel constraints can preclude this
approach, requiring a vertical upset at the structure-pipe
connection, as shown in Figure 8 and Figure 9 below.
Figure 10 FE Analysis of Pipe-Structure Connection
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DESIGN RECOMMENDATIONS
Fatigue effects due to slug flow have been identified as a
critical design driver in recent projects, requiring extensive
engineering effort to develop robust, constructible solutions. It
is shown that careful analysis of fatigue stresses is required at
the following locations:
• Terrain-induced flowline spans
• “Engineered” spans (Crossings, buckle mitigation
sleepers, flowline/structure connections)
• Interconnection spools.
All span analysis should consider gravity load effects, and
spool designs (which incorporate pipeline bends) should also
consider momentum effects. This may require dynamic
analysis.
The use of unprotected carbon steel flowline elements in
locations subject to significant slug-induced fatigue, such as
free-spanning spools, should be approached with care.
Corrosion mechanisms associated with reservoir souring are
likely to limit fatigue resistance in these situations. Corrosion
resistant alloy materials should be considered in such locations.
The potential for chronic slug flow conditions, either due
to hydraulic flow regime, terrain or other causes, should be
carefully assessed at the outset of a multiphase flowline system
design. This assessment should characterize, as fully as
possible, the following parameters required for mechanical
design as described in this paper:
• Slug frequency
• Slug and bubble length
• Slug and bubble density
If possible, these key input parameters should be defined
as variables, which change over the life of the field and the
location in the flowline system. Failing this, average values of
parameters should be defined, for application at all locations
throughout the design life. Clearly, there is considerable
uncertainty in these parameters. However, undue conservatism
in definition of slug parameters should be avoided, as this can
have a substantial adverse impact on flowline design and
ultimately construction cost.
valuable. A recent example of research effort in this field is
described in [4].
Analysis of flowline spans is relatively straightforward,
even when dynamic effects are to be included. However,
rigorous dynamic analysis of spools, including momentum
effects at bends, is challenging with conventional software
tools. Improved structural analysis methods, allowing
convenient definition of slug flow inputs and automatic
generation of dynamic load time histories, would be useful.
ACKNOWLEDGMENTS
The authors wish to acknowledge the contributions to this
work from all members of the INTECSEA pipeline engineering
team, from the UK and elsewhere, who have participated in
elements of the work described in this paper. Input from
colleagues in client organisations and project partners,
including 2H Engineering, are also gratefully acknowledged.
REFERENCES
[1] Butterworth, D. & Hewitt, G. F. “Two-Phase Flow and Heat
Transfer.” Oxford: Oxford University Press (1977)
[2] Rabih Kansao, Euro Casanova, Armando Blanco, Frank
Kenyery, Mayela Rivero. “Fatigue Life Prediction Due To Slug
Flow In Extra Long Submarine Gas Pipelines.” OMAE200858005 (Lisbon)
[3] Recommended Practice, Det Norske Veritas, DNV-RPF105. “Free Spanning Pipelines”, February 2006
[4] Melvin Brown (Smith Institute), Paul Dellar (Imperial
College London) “Gas entrainment at a propagating slug front”
http://www.smithinst.ac.uk/Projects/ESGI59/ESGI59NorskHydro/Report (2007)
[5] Det Norske Veritas, DNV OS-F101 “Submarine Pipeline
Systems”, October 2007
[6] BS 7608:1993 “Code of practice for fatigue design and
assessment of steel structures” British Standards Institution
Effective design of multiphase flowlines with slug flow
requires close collaboration between individuals and teams
responsible for flow assurance and mechanical design,
particularly in the early stages of the design
FURTHER WORK
Accurate definition of slug flow regime at the design stage
remains a challenging task. Development of improved
prediction methods, including validation by comparison with
slug flow patterns observed in operating flowlines, would be
6
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