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DESIGN CONSIDERATIONS FOR SELECTION OF FLEXIBLE RISER CONFIGURATION
N. Ismail, R. Nielsen, and M. Kanarellis
Wellstream Corporation Panama City, Florida
ABSTRACT
A brief review of recent literature on riser
design is presented and a concise description of
the design process is given. Effects of
hydrodynamical design parameters on marine
flexible riser design are then reviewed. Riser
dynamic analyses are described which
substantiate design criteria for the selection of
riser configuration in deep and shallow water.
The results of dynamic analyses using
computerized numerical models are presented in
this paper in graphical form for the selected riser
design cases. Output includes envelopes of riser
coordinates, axial force and time histories of
forces and wave surface profiles. The results
obtained highlight the significance of motion
spatial gradients for the combined flow of waves,
currents and vessel heave motion on the selection
optimum of riser configuration to address design
requirements.
NOMENCLATURE
A = flexible pipe cross-sectional area
H = water pressure head (h-s)
L = wave length
T = wave period
a = wave amplitude
g = acceleration of gravity
h = water depth
K = wave numberL
2π
Nomenclature (continued)
s = elevation above sea bottom
p = fluid pressure
Greek Symbols:
ρ = fluid density
σ = wave frequency L
2π
Subscript:
i = pipe internal properties
o = pipe external properties
INTRODUCTION
Though flexible pipe as a marine product
was introduced to the offshore market in the early
seventies, it was not until 1978 that flexible risers
were specified and installed in the Enchova field
offshore Brazil (Machado, 1980) as part of a
floating production system.
Since 1980, the use of flexible pipe has
spread worldwide and is used in almost every
offshore oil development today as witnessed in
papers by Mahoney (1986) for North Sea
application, Tillinghast (1990) for Gulf of
Mexico, Gulf of Suez applications, Tillinghast
(1987) and Beynet (1982) for the Far East.
This type of dynamic application is
typically used for floating production systems for
high pressure production risers, export risers,
chemical/water/injection lines and gas lift lines.
Currently, the main manufacturers of flexible
pipes are Coflexip, Wellstream and Furukawa.
References and Illustrations at end of paper
PD-Vol. 42, Offshore and Arctic Operations - 1992 ASME 1992
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At the present time, much interest in riser
systems is shown by the operators as evidenced
by papers by Beynet (1982) and Ashcombe
(1990).
RISER CONFIGURATION SELECTION
Industry practice calls for several types of
riser configurations typically used in conjunction
with Floating Production/Loading Systems. The
standard five configurations generally used are:
Free-Hanging Catenary, Lazy-S, Lazy Wave,
Steep-S, and Steep Wave. Figure 1-a illustrates
these typical types of riser configurations. Figure
1-b illustrates a schematic of a new riser
configuration proposed by Wellstream for the
Alcorn Linapacan Field Development Project.
The motivation for and validity of this new riser
configuration is presented in this paper.
The dynamic response of a particular riser
system is directly related to the environmental
loadings due to the combined wave-current field
flow and the dynamic boundary conditions of the
riser top end at the water surface, coupled with
the interaction arising from the structural non-
linear behavior of the riser itself.
To illustrate, design parameters impacting
the suitability of a particular configuration for a
particular water depth, two riser design cases
were studied in deep and shallow water. The
computer analyses were carried out at
Wellstream’s engineering offices using computer
program FLEXRISER-4 developed by Zentech,
UK.
The results of the dynamic analyses for
these several design cases are the essence of this
paper. The output illustrates the critical aspects of
wave and current hydrodynamics as well as
vessel motion response affecting the selection of
the riser configuration.
FLEXIBLE RISER ANALYSIS AND DESIGN
Flexible pipes and risers are critical
components for offshore field developments
because they provide the means of transferring
fluids, or power, between subsea units and a
topside floating platform, or buoys. These risers
accommodate floating platform motion and
hydrodynamic loading by being flexible. In storm
conditions, they undergo large dynamic
deflections and must remain in tension
throughout their response. They are consequently
manufactured to possess high structural axial
stiffness and relatively low structural bending
stiffness. Their global dynamic behavior can be
considered as more mechanical, or force
dependent, than structural. In contrast, behavior
near the end connectors of a system is governed
by local structural stiffness properties.
DESIGN CRITERIA
Efficient design of flexible riser systems is
made possible by using computer-based solution
techniques.
The design criteria of flexible riser systems
is usually based on allowable pipe curvatures and
tensions prescribed by the pipe manufacturer,
clearances between the riser and other structures,
and boundaries during its dynamic response. The
allowable curvatures and tensions are based on
full-scale test procedures and stress analysis
carried out by the manufacturer. These limits
ensure the pipe is not over-stressed when
responding to dynamic loads and vessel motions.
The system is generally designed so the pipe is
tensioned throughout its dynamic response cycle.
Minimum clearances are also specified to avoid
clashing problems between riser and seabed, or
riser and vessel, and between the riser or other
adjacent risers, cables, or mooring systems.
DESIGN PARAMETERS AND PROCEDURES
The main problem in designing flexible
riser systems is the large number of design
parameters. The environmental conditions, vessel
or calm buoy motions and riser properties are
usually well-defined. The main design parameters
are the choice of riser configuration, the length of
riser, the system geometry and the sizing of
buoyancy modules, subsurface buoy or arch. The
choice of riser configuration is usually based on
economic criteria, position of the wells, wave and
current forces, motion response and excursions of
the vessel or surface buoy as well. The design
procedure can be described as consisting of three
stages.
First Stage
The first stage in designing a flexible riser
system is determining an acceptable system
layout. The first stage is based on static analysis.
It is normal to carry out a parametric study
assessing the effect of changing the design
parameters (i.e., system geometry and length) on
the static curvature and tension. Based on the
results of this parametric study, the design selects
a suitable range of system geometries and lengths
satisfying the design criteria. The parametric
study will also assess the static effects of vessel
offset (displacement of the top end) and the
current loading in different directions.
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Second Stage
The second stage in the design procedure is
performing a dynamic analysis of the system to
assess the global dynamic response. A system
layout and length is chosen from stage one and a
series of dynamic load cases are considered.
These load cases combine different wave and
current conditions, vessel or surface buoy
positions, and riser contents in order to prove an
overall assessment of the riser suitability in
operational and survival conditions. The corre-
sponding analyses are then carried out and
dynamic curvatures, tensions and clearances are
checked against the design limits.
The majority of riser dynamic analyses
packages, including FLEXRISER-4, make use of
the “concept of effective tension” (Sparks, 1983).
Sparks addressed the drilling riser case where the
riser is essentially restrained. A catenary riser on
the other hand turns 900 to meet the sea bed. It is
subject to friction and can be subject to
compression due to these conditions. This
concept accounts for the effects of external and
internal hydrostatic pressure acting on the
internal and external surfaces of the pipe wall. It
is the effective tension which controls the
stability of the riser from the point of view of
deflection. The relationship between effective
tension, Teff, and the “true wall” tension, Twall,
that acts on the pipe wall and contributes to stress
in the pipe wall is:
Twall = Teff + (pi ± ρjg Hi) Ai – (ρ0g HO)AO………….(1)
where:
Twall =Wall tension to be used for stress
calculation in flexible pipe wall.
Teff = Effective tension as predicted by the riser
analysis computer program.
The effective tension is independent from
internal and external pressure. Given the effective
tension, as predicted by the riser global analysis
program, the true wall tension may be simply
calculated from the equation (1). Since internal
pressure affects the Twall, it is important to
carefully note the internal pressure conditions in
the pipe under the maximum load cases as well as
the limiting operational conditions when pressure
in the riser may be released or maintained.
Third Stage
The third stage in the design procedure is
performing detailed static and dynamic analyses
of local areas to design particular components.
This state is presented in a separate publication
(Brown, 1989).
Key papers by operators in this regard are
Out (1989), Boef (1990) of SIPM and de Oliveira
(1985) of Conoco. This third stage of design also
includes a question of life expectancy which has
recently been addressed by Claydon, et al (1991).
All of the stated design aspects are important but
the solution to each problem starts with the
selection of an optimum configuration which is
the subject of the remainder of the paper.
HYDRODYNAMIC LOADING ON RISERS
The determination of surface wave
hydrodynamic loads on the marine systems are
based on two major techniques. For large
structures, the scattering of incident waves is
considered and a diffraction theory is employed.
The wave loads on the small members of flexible
pipes are determined by applying the Morrison
equation. This equation separates the
hydrodynamic loads into inertia and drag forces.
Considering the pseudo static design approach
(deterministic), it has been customary to use
irrotational wave models to predict fluid particle
kinematics for a certain design wave. Drag and
inertia force coefficients are then used to relate
the particle kinematics to the hydrodynamic
forces expressed by the Morrison equation.
Further, it is important to determine appropriate
drag and mass coefficient as well as to adopt the
appropriate wave theory representing the design
wave characteristics.
To include currents in wave force
calculations, the oil industry has traditionally
used the technique of linear super-position.
When doing numerical analysis of the
hydrodynamic loading and selection of flexible
risers configuration, one has to recall the fact the
global riser response in a particular water depth is
affected by the spatial and temporal distributions
of the integral properties of waves (mass,
momentum, pressure and energy); therefore,
these distributions of water-wave properties
significantly dictate the suitability of a particular
riser configuration.
To illustrate some of these water wave
properties, consider a progressive wave moving
in the positive horizontal axis (figure 2). For
simplicity, considering the small amplitude wave
theory, the wave kinetic energy concentration
averaged over one wave period at any elevation
above the bottom is given by:
2kscosh 2kh)(cosh 24
2k2g2a KE(s)
σ
ρ= …………...(2)
In order to represent the above relation in
dimensionless form, the ratio of the kinetic
energy at any elevation ‘s’ to the mean free
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surface is given by
2khcosh
2kscosh
KE(h)
KE(s)= …………………………..(3)
Equation (3) is plotted in figure 3 for the
cases of h/L= 0.05 (conventional shallow water
limit) and h/L = 0.5 (conventional deep water
limit). The figure shows in the case of shallow
water waves, the energy concentration is nearly
uniform with depth; in the case of deep water
waves, the energy is concentrated near the
surface.
It should be noted that effects due to wave-
current interaction should always be considered
in the design and analysis because it might cause
a significant change in the magnitude and
distribution of fluid forces. This change of fluid
forces could be dramatic in coastal waters, as
seen by Ismail (1984), not only due to the strong
current shear, but also due to the change in
characteristics of shoaling waves. The parameters
usually sensitive to wave-current interaction
effects include hydrodynamic force coefficients,
current velocity profiles, and relative direction of
waves and currents.
PRESENTATION AND INTERPRETATION
OF RESULTS
In order to illustrate the potential and the
limitations, of adopting specific riser
configurations in a particular water depth, the
results of riser dynamics analyses for two design
cases in deep water and one design case in
shallow water are presented. The computer
analyses were conducted at Wellstream’s
engineering office using the FLEXRISER-4
personal computer program developed by
Zentech, UK.
FREE-HANGING CATENARY RISERS
A Free-Hanging catenary configuration was
considered for riser systems in two design cases.
The first case is in a 600 m water depth and the
second in a 350 m water depth. Table I illustrates
the main design parameters used as input for the
computer analyses. For the first case, figure 4
shows a snapshot of the riser configuration and
the distribution of axial force along the length of
the riser depicting a small amount of compression
near the seabed. In contrast to this case (for the
350 m water depth), figure 5 shows an
appreciable amount of compressive force in the
riser near the seabed. To maintain the integrity of
a flexible riser system, it is imperative that the
riser pipe remains in tension throughout its
operational life. Because of the development of
this excessive amount of compressive force in the
pipe, it can be safely concluded a Free-Hanging
catenary configuration is unsuitable for this
design case in 350 m water depth. A major reason
of the development of axial compression forces is
that the heave motion associated with the calm
buoy is appreciable. To mitigate the effects of
this loss of tension in the riser pipe, which could
adversely affect the service life of the pipe, one
of the following measures may be adopted:
• Disconnection of the riser under strong
wave conditions;
• Increase the weight of the riser pipe section
near the seabed by wrapping with heavy
material;
• Modify the buoy design to reduce the heave
motion;
• Attach a subsurface buoy, near the seabed.
EASY-TOUCH CATENARY RISER
Each of the above remedies to eliminate
compression forces could be viewed as a possible
solution depending on the technical and
economical constraints of the project in general
and the riser system in particular.
The latter concept of adding a subsurface
buoy (= 60 kN) to the free hanging catenary riser
in the 350 m water depth design case to eliminate
compression forces was implemented. The results
of dynamic analyses demonstrating the success of
the concept are shown in figure 6. It is important
to highlight this, if the position of the surface
buoy is to increase above the seabed, the riser
will be approaching the Steep-S riser
configuration, compression forces would still
develop over the lower catenary portion.
Wellstream had applied this modified concept for
the catenary type configuration in the riser design
for Alcorn’s West Linapacan Development
Project in the South China Sea.
The Easy-Touch Catenary was coined as the
name for this modified catenary configuration.
The validity of the newly proposed riser
configuration was tested under various operating
scenarios for the calm buoy and under normal
and maximum environmental conditions.
STEEP WAVE RISERS
Dynamic riser analyses were conducted for
a design of a single oil export system in a 40 m
water depth site. An extensive analysis was
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carried out prior to arriving at the proposed steep
riser configuration (either 6-inch or 8-inch
flexible pipe diameter) resulting in many unac-
ceptable configurations, primarily due to the
riser’s inability to withstand the severe envi-
ronmental conditions imposed by extreme wave
heights of 15 m for the I year-storm and 20 m for
the 100 year-storm. The analyses showed neither
Lazy-S nor Free-Hanging systems were feasible.
Moreover, the need to eliminate the Steep S
configuration, as an alternative, became apparent
when the sensitivity of the riser’s dynamic re-
sponse to the intensity of additional buoyancy
distribution was determined. Thus, the Steep
Wave was left as the only feasible configuration
for the riser of this oil export system. The
distribution of buoyancy along the Steep Wave
riser was determined (figure 7) to prevent any
buckling instability and to reduce any excessive
angle at the riser base. A snapshot of the riser
configuration under the dynamic effects of waves
and currents is shown in figure 7 for both cases of
far and near field. The corresponding axial
tension force along the Steep Wave riser is shown
on figure 7.
CONCLUSIONS
It was evident the most apparent riser
configuration does not necessarily provide the
appropriate solution for the design cases of
flexible marine risers in deep and shallow water
considered in this paper. The deep-water cases
emphasize a solution based on a simple riser
configuration to facilitate modularity and ease of
installation and removal either the standard
Catenary in the 600 m water depth case or, the
Easy-Touch Catenary in the 350 m water depth
case. In the shallow-water case, the design is a
more complex riser configuration due to the
severe environment loads requiring particular
design configuration loads. Both cases represent
new frontiers for use of flexible pipes as marine
nsers.
The sensitivity of the riser dynamic
response, in particular configuration to
environmental data and vessel/floater motion
data, warrants a careful review of design basis
prior to the dynamic analysis and design of
marine risers. The selected configuration then
determines many design parameters, among
others, design life and bending stiffener and
restrictor requirements.
REFERENCES
American Petroleum Institute, 1987, “RPI7B -
Recommended Practice for Flexible Pipe,”
Houston, TX.
Ashcombe, G.T., and Kenison, R. C., BP
Engineering, 1990, “The Problems Associated
with NDT of High Pressure Flexible Pipes,”
Society of Underwater Technology Conference,
Aberdeen.
Beynet, P. A., and Frase, J. R., 1982,
“Flexible Riser for a Floating Storage and
Offloading System,” Proceedings of Offshore
Technology Conference, Paper OTC 4321,
Houston, TX.
Boef, W. I. C., and Out, J. M. M., 1990,
“Analysis of a Flexible Riser Top Connection
with Bend Restrictor,” Proceedings Offshore
Technology Conference, Paper OTC 6436,
Houston, TX.
Brooks, D. A., Kenison, R. C., and BP
International Limited, 1989, “Research &
Development in Riser Systems,” Subsea, London.
Brown, P. A., Soltanahmadi, A. and
Chandwani, R., 1989, “Problems Encountered in
Detailed Design of Flexible Riser Systems,” Int.
Seminar on Flexible Risers, University College,
London.
Claydon, P., Cook, G., Brown, P. A.,
Chandwani, R., Zentech International Ltd.
London, 1991, “A Theoretical Approach to
Prediction of Service Life of Unbonded Flexible
Pipes under Dynamic Loading Conditions,” J.
Marine Structures, preprint.
Hoffman, D., Ismail, N., Nielsen, R., and
Chandwani, R., 1991, “Design of Flexible Marine
Riser in Deep and Shallow Water,” Proceedings
Offshore Technology Conference, Paper OTC
6724, Houston, TX.
Ismail, N. M.,1984, “Wave-Current Models
for Design of Marine Structures,” Journal of the
Waterway, Port, Coastal and Ocean Division,
ASCE, Vol 110, No.4.
Kastelein, H. J., Out, J. M. M., and Birch,
A.D., 1987, “Shell’s Research Efforts in the Field
of High-Pressure Flexible Pipe,” Deepwater
Offshore Technology Conference, Monaco.
Kodaissi, E., Lemerchand, E., and Narzul,
P.,1990, “State of the Art on Dynamic Programs
for Flexible Riser Systems,” ASME Ninth
International Conference on Offshore Mechanics
and Artic Engineering, Houston, TX.
Lopes, A. P., de silva Neto, S. F., Estefgn, S.
F. and Da Silveria, M. P. R.,1990, “Dynamic
Behavior of a Flexible Line Design Installation,”
Proceedings Offshore Technology Conference,
Paper No. OTC 6437, Houston, TX.
6
Machado, Z. L., and Dumay, J. M.,1980,
“Dynamic Production Riser on Enchova Field
Offshore Brazil,” Offshore Brazil
Conference, Latin America Oil Show, Rio de
Janeiro.
Mahoney, T. R., and Bouvard, M. J.,
1986,“Flexible Production Riser System for
Floating
Product Application in the North Sea,”
Proceedings Offshore Technology Conference,
Paper
OTC 8163, Houston, TX.
Oliveira, J. G., de Goto, Y., and Okamoto, T.,
1985, “Theoretical and Methodological
Approaches to Flexible Pipe Design and
Application,” Proceedings Offshore Technology
Conference, Paper OTC 5021, Houston, TX.
Out, J. M. M., 1989, “On the Prediction of the
Endurance Strength of Flexible Pipe,”
Proceedings Offshore Technology Conference,
Paper OTC 6165, Houston, TX.
Sparks, C. P.,1983, “The Influence of
Tension, Pressure and Weight on Pipe and Riser
Deformation and Stresses,” 2nd International
Offshore Mech and Artic Engineering
Symposium,Houston, TX.
Tillinghast, W. S.,1990, “The Deepwater
Pipeline System on the Jolliet Project,”
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Design data Deep water Shallow water steep wave
configuration Case A
Free-Hanging
Catenary
Case B
Easy-Touch
Catenary
Flexible Riser Pipe Data:
Internal diameter, M
Outside diameter, M
Axial Stiffness, N
Bending Stiffness, Nm2
Wt in Air, kg/m
0.1016
0.1631
5.45E7
2.00E3
44.0
0.13
0.2
1.28E8
4.E3
62.0
0.15
0.23
2.00E7
5.24E3
63.0
Environmental data:
Water depth, m
Wave height, m
Wave periods, s
Current speed, m/sec
Top, Bottom
625.0
11.8
11.2
1.1,0.6
350.0
17.0
12.0
1.9,0.7
40.0
15.0
13.0
2.3,1.07
Vessel/Floater Excitation:
Surge, m
Heave, m
1.8
2.0
2.2
5.5, -9.5
2.5
2.2
8
9
10
11
12
13
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