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AN INVESTIGATION ON THE EFFECT OF CURRENT DIRECTIONALITY
ON RISER VORTEX-INDUCED VIBRATION
S. Manayankath, Lloyd’s Register, UK
S. Huang, University of Strathclyde, UK
ABSTRACT
The paper presents the results from a preliminary study on the influence of current direction ondeepwater riser VIV. The theoretical investigation was carried out using SHEAR7 and the VIV analysiscodes within Orcaflex. It was observed that compared to a unidirectional current over the full riser length,
in a multidirectional inflow the fatigue damage reduced significantly. The reduction in fatigue damagewas noted over the complete riser length though the current direction was varied only in the lower half of
the riser. Based upon the results, it appears that we can conclude that using a unidirectional current inVIV analysis would lead to results which are likely to be highly conservative and this needs to beconsidered and studied further in future VIV modeling and prediction.
1. INTRODUCTION
Fatigue damage due to vortex-induced vibration
(VIV) is an important issue in deepwater riser
design. The mechanism of VIV is complex and not
fully understood. Apart from risers, other offshore
installations like tethers, pipelines, members of
jacketed structures and even deepwater pile
installations are affected by problems arising from
alternating vortex shedding. VIV suppression
devices such as helical strakes are used to suppressVIV. Though these devices do improve the VIV
performance to some extent, it comes at additional
penalties such as drag increase and costs in
fabrication and handling.
Notwithstanding its complexity, progress has been
made both numerically and experimentally in
understanding the fundamentals of marine riser
VIV. Numerous papers have been published on
this topic in the last decade as the offshore
engineering industry relentlessly pushes into everdeeper waters. A number of papers such as
Sarpkaya (2004), Vandiver (1993), Pantazopoulos
(1994), and Gabbai & Benaroya (2005) are
excellent sources of information that trace the
developments in this field over the years and
provide a comprehensive review.
Along with numerous efforts to investigate various
aspects of marine riser VIV, a great deal of work
has been carried out to synthesise these results to
yield theoretical VIV prediction models and codes.SHEAR7 (Vandiver and Li, 2005) is an example
of these models and codes and is probably the
most commonly used design tool in the industry
for riser VIV analysis at the moment.
In the recent years, as the riser monitoring devices
become better understood and more and more
reliable, efforts have been made to calibrate the
riser field-monitoring data with the SHEAR7
predictions (Tognarelli et al, 2009). Many
technical issues and fundamental questions
however arise in this calibration effort. For
example, SHEAR7 is based upon some empiricalinput data which are derived in low Reynolds
number model tests. These data are not necessarily
applicable to full-scale risers (Huang and Kitney,
2009).
Another issue relates to the key assumption used
in SHEAR7, i.e. the current profile is
unidirectional across the water column. In the real
ocean environment, particularly in deepwater,
however, a riser will be subjected to multi
directional currents along its length. In calibratingSHEAR7 by the use of riser field-monitoring data,
this issue of discrepancy between the modeling
assumption and the reality has yet to be addressed
and its effects quantified.
Most of the model tests on VIV are carried out
with rigid or flexible cylinders placed in a
unidirectional flow due to practical limitations.
There is very little published data on the effect of
current directionality on VIV. It is generally
considered that a unidirectional inflow would leadto conservative results, but with further research
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on VIV in multidirectional currents it is hoped that
the design conservatism could be reduced in the
future.
The paper presents the preliminary results from a
theoretical investigation into the effect of currentdirection on riser VIV. The currently popular
theoretical models were used for the investigation.
It was believed that such a study would provide
some insight, however limited it may be, on the
riser VIV response in a multi directional current
environment. It was also felt that such a theoretical
study would draw attention and R&D efforts to
this so-far largely ignored issue.
2. INVESTIGATION METHODOLOGY
A top tensioned riser was analysed using Orcaflex
with its associated VIV analysis tools and
SHEAR7 programs in different flow conditions as
described below with a view to make a qualitative
assessment of the influence of current direction on
riser VIV. The inflow conditions were as given
below.
• Uniform flow over the full length of the
riser.
• Flow direction in the riser lower half at
different angles of incidence with respect
to the uniform flow on the upper half.
SHEAR7 is a program widely used in the industry
for VIV analysis which uses a modal analysis
technique and iteratively calculates the lift and
damping coefficients to attain a balance of power
input from lift force and power output throughdamping (Vandiver and Li, 2005).
The Orcaflex VIV toolbox had two wake oscillator
models, i.e. the Milan model and Iwan & Blevins
model. The Milan model is a wake oscillator
model proposed by a group in Italy and described
in the paper by Falco, Fossati and Resta (1999). In
this model, the effects of VIV are simulated using
a series of equivalent oscillators that are connected
to the structural model nodes. The equivalent
oscillator is a non-linear one degree of freedom
system which transmits to the structure forces
equivalent to vortex shedding mechanisms. The
Iwan & Blevins wake oscillator model uses a Van
der Pol type equation with a flow variable todescribe the effects of vortex shedding. Model
parameters are determined by curve-fitting
experimental results for stationary and forced
cylinders in the Reynolds number range between
103 and 105 (Blevins, 2001).
A top tensioned drilling riser subjected to a
uniform flow of 1.0 m/s over its full length was
selected as the base case. This base case was
analysed using SHEAR7, a frequency domain
model. The same riser was then analyzed using theOrcaflex VIV tool box with its wake oscillator
models which are time domain methods.
With the differences in modelling techniques and
assumptions, as well as the amount of empiricism
in place, close matching of the results from
different models was not expected. It was hoped
that, in the absence of riser field-monitoring data,
a comparison of the base case in SHEAR7 and
Orcaflex would give a qualitative indication on the
conservativeness of the results, assuming
SHEAR7 results to be the reference case.
The cases with multidirectional current inflows
were analyzed using Orcaflex only, as SHEAR7
can only deal with unidirectional current profiles.
The current speed was again 1.0 m/s, as in the base
case. But the angle of attack in the lower half was
varied with respect to the flow direction in the
upper half. The angles analysed were 30, 45, 60,
75 and 90 degrees.
A schematic diagram of the riser model used for
the study is shown in Figure 1. The riser details
are given in Table 1.
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Figure 1 Riser Model
NameHeight
[m]Mass/m[kg/m]
OD[m]
ID[m]
Submerg
ed wt/m[kN/m]
Ca CD
A Pup in air 19.00 769.4 0.603 0.529 -7.467 1.596 1.516
BPup inwater
4.33 769.4 0.603 0.529 -4.732 1.596 1.516
C Slick 18.29 769.4 0.603 0.529 -4.605 1.596 1.516
D Riser(Buoyant)
310.99 1042.8 1.107 0.529 -0.561 1.176 1.301
E Pup 7.92 769.4 0.603 0.529 -4.532 1.596 1.516
Density of sea water 1.025 tons/m3
Density of contents 1.138 tons/m3
Table 1 Main Particulars of the Riser
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3. ANALYSIS PROCEDURE
SHEAR7 is based on frequency domain solution
and the program outputs rms values of
displacement, acceleration, stress and fatigue life,
all in the cross-flow direction. Orcaflex is a timedomain based program and the results in both the
cross-flow and in-line directions are given. At
every node, the minimum, maximum, mean and
standard deviations of different parameters such as
displacement and acceleration are calculated by
the program along with time histories.
In comparing the results from SHEAR7 and the
wake-oscillator models, only the cross-flow
displacements and accelerations were used.
Standard deviation data from Orcaflex wascompared with SHEAR7 rms data as the mean
value was close to zero in the cases compared.
In comparing the fatigue damage rate, a simplified
procedure was used to analyse the data. In the
following equations M is the mass per unit length,
T is the tension in the riser, E is the Young’s
modulus, I is the moment of inertia and r is the
radius of the extreme fibre.
The dynamic equation of the riser in tension for
free vibration is given by
02
2
2
2
4
4
=∂
∂+
∂
∂−
∂
∂
t
y M
z
yT
z
y EI
Considering the riser as a string in tension, i.e.
ignoring the bending stiffness, the dynamic
equation can be simplified to
y M
T y ′′=
where double dots represent differentiation with
respect to time and double dashes represent
differentiation with respect to distance z along the
riser.
Assuming sinusoidal modal shape and applying
simple bending theory, the bending stress may be
approximated as
T
MEr ys =
The stress range S is then given by
T
MEr yS std 22=
where subscript std denotes standard deviation.
Using the stress range and applying a simplified S-
N curve approach the annual fatigue damage and
fatigue life was estimated with a view to obtain a
qualitative comparison of the different empirical
model results.
The number of cycles to failure N is given by
mS
A N =
where A = 1.04x1012 and m = 3 were used in the
calculations. The number of stress cycles n in a
year is determined from the frequency of vibration
and the damage rate is calculated as n/N [1/year].
4. UNIFORM CURRENT
The top tensioned riser was subjected to a uniform
current of 1.0 m/s and the analysis was carried out
using both SHEAR7 and Orcaflex Milan and Iwan
& Blevins wake oscillator models. As SHEAR7
only models the transverse response, the
comparisons are only for the transverse (Y)
components and the in-line (X) effects were notconsidered for the study. As can be seen from
Figure 2 the transverse displacements estimated
using both Orcaflex Wake Oscillator models and
SHEAR7 were closely comparable. Both programs
indicated a single mode response though the mode
numbers were different. For the two wake-
oscillator models the results were closely
comparable except for the slightly higher peak in
the Iwan & Blevins model.
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Figure 2 Transverse displacements in uniform current
Figure 3 Transverse accelerations in uniform current
Figure 4 Maximum fatigue damage in uniform current
The accelerations using the two programs are
compared in Figure 3. It may be seen that the
accelerations estimated by SHEAR7 was almost
twice that determined using both models ofOrcaflex. This implies that, the bending stress,
which is directly proportional to acceleration
would have a similar distribution, which in turn
would affect the fatigue damage calculations.
Once again the results from Milan and Blevins
models were closely matching except for the peak
magnitude estimated by Blevins model beingslightly higher than that by Milan model.
Max Fatigue Damage (Uniform Current)
0 50 100 150 200 250
Shear7
Blevins
Milan
Damage [1/year]
0 . 0
0 . 2
0 . 4
0 . 6
0 . 8
1 . 0
1 . 2
1 . 4
1 . 6
1 . 8
2 . 0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0
R i s e r l e n g t h [ m ]
Y - A c c [ m / s q . s
]
S h e a r 7 - Y - a c c - 0 d e g
M il a n - Y - a c c - 0 d e g
B l e v in s - Y - A c c - 0 d e g
0 .0 0
0 .2 0
0 .4 0
0 .6 0
0 .8 0
1 .0 0
1 .2 0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0
R i s e r l e n g t h [ m ]
Y [ m ]
S h e a r 7 - Y - d i s p - 0 d e g
M il a n - Y - d is p - 0 d e g
B l e v in s - Y - d is p - 0 d e g
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The maximum fatigue damage anywhere on the
riser estimated from SHEAR7 and Orcaflex data
are presented in Figure 4. As fatigue damage
depends on both frequency and stress range,
SHEAR7 predicted the peak damage to be almost
four times that estimated by the Blevins model andeight times as estimated by the Milan model.
5. MULTIDIRECTIONAL CURRENTS
For the multidirectional current case the analysis
was carried out using Orcaflex wake oscillator
models, with the angle of attack on the riser lower
half varying from 0 to 90 degrees with respect to
the current direction on the upper half. The anglesconsidered were 0 (i.e. entire riser subjected to
unidirectional current), 30, 45, 60, 75 and 90
degrees. The flow speed was 1.0 m/s throughout.
The results presented here are for the direction
perpendicular to the current direction on the riser
upper half.
The results from the Milan wake oscillator model
are plotted in Figures 5 to 7. It may be seen from
the displacements given in Figure 5 that the
displacements over the full riser length
progressively decreased as the angle of attack inthe riser lower half increased from 0 to 90. This
decrease was seen over the full length of the riser.
The transverse accelerations are plotted in Figure
6. The trends shown in displacements were
repeated here. The calculated fatigue damage over
the riser length is plotted in Figure 7. As expected
the maximum damage in the Y direction is for the
case of fully unidirectional flow. As the angle of
attack in the lower half increased, the damage in
the Y direction decreased all along the riser.
The riser in multidirectional current analysed in
the foregoing was further analysed using Iwan &
Blevins wake oscillator model implemented in
Orcaflex. The transverse displacements and
accelerations from the Blevins model are plotted
in Figures 8 and 9 respectively. It can be seen thatthe trends seen from Milan model results repeated
here as well. Consistent trends were shown for all
of the angles analysed.
It was felt that it would be interesting to have a
comparison of the two wake oscillator models and
so the acceleration results from the two models are
compared as shown in Figure 10. In general it can
be said that the displacements and accelerations
computed by the two models at different angles of
attack are reasonably close except when the angleof attack was 90 deg.
The maximum fatigue damage anywhere on the
riser length at different angles of attack predicted
by the two models is given in Figure 11. As
discussed earlier the damage progressively
reduced with increase in the angle of attack. It may
be noted that a current direction of 45 degrees in
the lower half reduced the annual fatigue damage
by more than half, relative to the fully
unidirectional inflow case. In the cases considered
here the riser fatigue damage reduced over the
complete length though the attack angle was
varied in the lower half only.
The results indicate that the current direction does
influence the overall riser response and so it needs
to be taken into account during the design stage.
The results also seem to suggest that assuming a
unidirectional current in the riser design may
result in highly conservative designs.
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0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300 350
Riser length [m]
Y [ m ]
Y-disp-0deg
Y-disp-30deg
Y-disp-45deg
Y-disp-60deg
Y-disp-75deg
Y-disp-90deg
Figure 5 Transverse displacements at different angles of attack
00.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250 300 350
Riser length [m]
Y - A c c [ m / s q . s
]
Y-acc-0deg
Y-acc-30deg
Y-acc-45deg
Y-acc-60deg
Y-acc-75deg
Y-acc-90deg
Figure 6 Transverse accelerations at different angles of attack
-5
0
5
10
15
20
25
30
50 100 150 200 250 300 350
Riser length [m]
D a m a g e [ 1 / y e a r ]
Y-damage-0deg
Y-damage-30deg
Y-damage-45deg
Y-damage-60deg
Y-damage-75deg
Y-damage-90deg
Figure 7 Fatigue damage at different angles of attack
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0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300 350
Riser length [m]
Y
[ m ]
Y-disp-0degY-disp-45deg
Y-disp-60deg
Y-disp-75deg
Y-disp-90deg
Figure 8 Transverse displacements at different angles of attack (I & B model)
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300 350
Riser length [m]
Y - A c c [ m / s q . s
] Y-acc-0deg
Y-acc-45deg
Y-acc-60deg
Y-acc-75deg
Y-acc-90deg
Figure 9 Transverse accelerations at different angle of attack (I & B Model)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 50 100 150 200 250 300 350
Riser length [m]
Y - A c c [ m / s q . s
]
Milan-Y-acc-45degMilan-Y-acc-90deg
Blevins-Y-acc-45degBlevins-Y-acc-90deg
Figure 10 Transverse accelerations (Milan vs. I&B)
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Max Fatigue Dam age( Diff angles of attack on r iser low er
half)
0 20 40 60 80 100
0 deg
30 deg
45 deg
60 deg
75 deg
90 deg
Damage [1/year]
Blevins
Milan
Figure 11 Maximum fatigue damage at different angles of attack
6. CONCLUSIONS
Based upon the preliminary results presented in
this paper, the following conclusions can be drawn.
• For the unidirectional current, all models
predicted a single mode response for the
uniform flow condition. However there
were differences in the mode number and
magnitude of the accelerations. The fatigue
damage calculated from SHEAR7 outputs
by assuming a sinusoidal modal response
of the riser was four to eight times larger
than that calculated from Orcaflex wake
oscillator models’ results.
• The investigation revealed that compared
to the case of unidirectional current inflow
over the full riser length, in amultidirectional inflow the fatigue damage
was significantly reduced. It was found
from the study that a current direction of
45 degrees in the lower half reduced the
annual fatigue damage by a factor of two to
five relative to the fully unidirectional
inflow case. The reduction in fatigue
damage was observed over the complete
riser length even though the angle of attack
was varied only in the riser lower half.
• The study appears to indicate that using a
unidirectional current in VIV analysis
would lead to results which are likely to be
highly conservative. Future VIV model
developments and calibrations need to
consider this issue.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the support fromDr. Dave Thomas and Mr. David Heffernan of
Orcina and Dr. Richard James of BP Exploration.
DISCLAIMER
The views expressed in this paper are those of the
authors alone.
REFERENCES
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Gabbai,R.D., Benaroya, H., 2005. An overview ofmodeling and experiments of vortex-induced
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