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TECHNICAL PAPER
Numerical Solutions and Model
Test Design for Anti-Typhoon
Drilling Riser
J. Wang, L. Li, F. Lim, H. Zhang, L. Xu, L. Sheng, R. Jin - TIWTE
OMAE June 2019
NUMERICAL SOLUTIONS AND MODEL TEST DESIGN FOR ANTI-TYPHOON
DRILLING RISER
Jinlong Wang 2H Offshore Engineering
Beijing, China
Lihui Li 2H Offshore Engineering
Beijing, China
Frank Lim 2H Offshore Engineering
Beijing, China
Hui Zhang 2H Offshore Engineering
Beijing, China
Liangbin Xu CNOOC Research
Institute Beijing, China
Leixiang Sheng CNOOC Research
Institute Beijing, China
Ruijia Jin Tianjin Research Institute for Water Transport Engineering
Tianjin, China
ABSTRACT Anti-typhoon drilling riser, a solution to overcome high
time cost of offshore drilling riser emergency retrieval under the
situation of imminent arrival of a typhoon, is to modify the
existing drilling riser to make it disconnectable closer to the
surface and leave the long riser string below (and subsea
blowout preventer (BOP)) in a safe and freestanding mode to
survive the typhoon. The freestanding riser is held up by a
buoyancy can system. And during the normal drilling operation,
the buoyancy can maintain neutral thus it has limited effect on
the riser overall global performance. However, locally the
buoyancy can system will have some effect on the riser system
nearby. To study these effects, numerical analytical
methodology and results of anti-typhoon drilling rise under
connected mode and freestanding mode are proposed, and a
model test of 1:21 scale factor is designed. Three configuration
modes: freestanding mode, connected mode and disconnecting
mode are simulated in the tank test. A series of load cases under
various current and wave, buoyancy upthrust are conducted in
the experimental test to evaluate the hydrodynamic and strength
performance of the riser near buoyancy can. The numerical
solution and model test design can be a significant basis of
water tank test for anti-typhoon drilling riser and a valuable
reference for deepwater drilling engineering.
INTRODUCTION The frequently hurricanes and typhoons occurred in ocean
that battered the offshore facilities have prompted many
operators and drilling contractors to rethink the way their
drilling risers are designed for disconnection and retrieval
before the hurricane arrives. When drilling in very deepwater
depths, retrieval of the drilling riser is time consuming and
tedious; and in emergency situations like the imminent arrival of
a hurricane or typhoon, the drilling rigs risk not being able to
recover the drilling risers in time before the evacuation of
personnel or sailing for shelters, with potentially devastating
consequences.
Figure 1. Schematic layout of anti-typhoon drilling riser system
A freestanding drilling riser (FSDR) designed for anti-
typhoon, illustrated in Figure 1, is proposed as an available
solution to overcome this concern [1-3]. Through installing the
buoyancy can and a near surface disconnection package
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Proceedings of the ASME 2019 38th InternationalConference on Ocean, Offshore and Arctic Engineering
OMAE2019June 9-14, 2019, Glasgow, Scotland, UK
OMAE2019-95196
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(NSDP) in the middle of conventional riser, it can be
disconnected in the position of NSDP. The NSDP and buoyancy
can modules can be pre-installed or assembled onto the riser
and should be located below the wave zone and the high surface
currents This technology needs to pull a short riser section from
NSDP to the drilling vessel and the remaining string of riser is
supported by a cluster of buoyancy cans.
Some experts have conducted a lot of theoretical researches
on the free-standing drilling riser, but there is still a big gap to
use in the deep and ultradeep water. Lim [4] proposed a
freestanding surface BOP drilling system together with the
structural analysis and operational assessment to demonstrate its
feasibility. Liu [5] conducted structural design and analysis of
buoyancy can for a freestanding riser. Guo [6] designed the
integral structure and chamber structure of the buoyancy can
and discussed the effect of vortex induced vibration (VIV)
response and fatigue damage. Liu [7] reviewed the anti-typhoon
riser technology and Yang [8] analyzed the stress performance
of an NSDP using finite element method. Guo [9] described a
freestanding mid- depth BOP drilling riser system and
conducted the trial with monitoring the novel components (the
tieback string and buoyancy unit) to study their motion behavior
for correlation against analytical predictions. Guo [10]
introduced a field trial of a new drilling system using both free-
standing drilling riser and mid-depth BOP in the South China
Sea. Zhou [11] conducted a model test to research on vibration
response of storm-safe drilling riser after disconnecting, but it
ignores the influence of buoyancy can. Yong [12] studied the
hydrodynamic response of the FSDR mechanism under typhoon
generated swell using finite element method. Yong [13] also
presented an optimal disconnection location based on extreme
weather condition analysis on FSDR at various disconnection
location and mechanical behavior under extreme weather
conditions are studied.
Though freestanding hybrid riser (FSHR) is used in
production, it’s also supported by the top buoyancy can below
the wave zone and is similar to the FSDR. Sun [14] emphasized
on main components of freestanding hybrid riser, boundary
conditions, loading conditions, and provided the scaled design
for top and bottom structure of FSHR. Tan [15-16] proposed a
numerical model and model test device for the upper assembly
and lower assembly of a free-standing hybrid riser based on
similarity criterion and correlative elastic-plastic mechanics
theories. Kim [17] has researched several factors that have
influence on the behavior of FSHR and presented an
optimization method through a parametric study.
Model test is an efficient method to research the strength
performance of drilling riser. There exists a lot of full-scale
measurements, an example of using in situ monitoring data to
provide realistic VIV fatigued damage to drilling riser and
wellhead [18]. The full scaled test is difficult to conduct due to
complicated environmental load and structure, which are high
cost and unrealistic. Scaled model tests are realistic methods for
this purpose. Jaap [19] undertook an exploratory research
model test campaign was undertaken to study the vortex
induced motions for a range of current speeds in a large towing
tank. Yin [20] carried out a comprehensive model test program
on drilling riser in towing tank to validate and verify software
predictions of drilling riser behavior under various
environmental conditions by use of model test data.
Numerical analytical methodology and results of anti-
typhoon drilling rise under connected mode and freestanding
mode are proposed, and a scaled test model for anti-typhoon
drilling risers is designed. A series of load cases under various
current and wave, buoyancy upthrust are considered in the
experimental test to evaluate the hydrodynamic and strength
performance of the riser near buoyancy can. The present
experimental study provides a simplified but well-defined anti-
typhoon drilling riser model. Due to the ongoing test, the test
data which will be obtained in future can be compared with the
riser’s numerical analysis result.
NUMERICAL SOLUTIONS Two configuration modes are considered in the numerical
analysis: connected mode under normal operation state and
freestanding mode under anti-typhoon survival state.
FSDR is the critical case for anti-typhoon riser due to harsh
ocean environment. A bottom or submerged current loading is
the only external hydrodynamic loading that the FSDR may
experience, but it can be the driving factor to the design of
FSDR in the freestanding mode. Strong bottom currents
generate high drag force along bottom region of the riser
causing excessive flex-joint rotation, high bending moment near
wellhead and high stress in the conductor. These parameters
such as, the location of buoyancy can below water surface,
amount of buoyancy upthrust, hydrodynamic coefficient and
limiting current speed are significant to design and analyze the
anti-typhoon drilling riser to maintain safety. The mechanic
performance under various current and wave are studied.
Analytical Methodology
A finite element (FE) analysis model is developed using the
non-linear time domain analysis software FLEXCOM-3D [21]
to model the anti-typhoon drilling riser under connected and
disconnected (freestanding) mode. A typical anti-typhoon
drilling riser in South China Sea is taken as an example: 1500m
water depth, buoyancy can position 400m below water, base
tension 550kips at lower marine riser package (LMRP). The
conventional riser, buoyancy can, lower flexible joint, LMRP
and BOP are modeled as pipe elements with appropriate
properties to represent their mass, stiffness and hydrodynamic
properties and the NSDP is modelled as a concentrated mass.
Numerical Results
(1) Connected mode
The drilling riser under connected mode is in normal
operation state. Two mild ocean environment cases: 1-year
return period current and 1-year return period wave & current
are considered in numerical analysis. The horizontal
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displacement, effective tension and bending moment along riser
system are shown in the Figure 2~4.
There is little difference about displacement, effective
tension and bending moment between the two load cases. The
maximum displacement occurs in the middle of riser, and the
tensioner ring supports the whole riser and is subjected to the
maximum tension. A peak bending moment occurs at a few
meters below the seabed due to the soil constrains to the
conductor.
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Displacement (m)
DISPLACEMENT ALONG RISER UNDER CONNECTED MODE Base Tension 550kips, Buoyancy Can Position 400m Below Water
1-Year Current 1-Year Wave and Current
Figure 2. Displacement along riser under connected mode
(2) Freestanding mode
The drilling riser under freestanding mode is in the anti-
typhoon survival state and subjected to the harsh ocean
environment. The mechanic performance of FSDR under
typical current in South China Sea, from 1-year return period up
to 100-year return period are considered in this study.
The horizontal displacement, effective tension and bending
moment along riser system under various current are shown in
the Figure 5~7. It can be concluded that with increasing current
magnitude, the horizontal displacement along riser increases
while the current has negligible effect on the riser effective
tension. The bending moment along the riser are generally small
except for the region near the wellhead and conductor under the
seabed. A peak bending moment occurs at a few meters below
the seabed due to the soil constrains to the conductor.
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Tension (kN)
EFFECTIVE TENSION ALONG RISER UNDER CONNECTED MODE Base Tension 550kips, Buoyancy Can Position 400m Below Water
1-Year Current 1-Year Wave and Current
Figure 3. Effective tension along riser under connected mode
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Bending Moment (kN.m)
BENDING MOMENT ALONG RISER UNDER CONNECTED MODE Base Tension 550kips, Buoyancy Can Position 400m Below Water
1-Year Current 1-Year Wave and Current
Figure 4. Bending moment along riser under connected mode
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-100
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Displacemnet(m)
DISPLACEMENT ALONG RISER UNDER FREE STANDING MODEBase Tension 550kips, Buoyancy Can Position 400m Below Water
No current 1Yr Current 10Yr Current
50Yr Current 100Yr Current Figure 5. Displacement along riser under freestanding mode
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EFFECTIVE TENSION ALONG RISER UNDER FREE STANDING MODEBase Tension 550kips, Buoyancy Can Position 400m Below Water
No current 1Yr Current 10Yr Current
50Yr Current 100Yr Current Figure 6. Effective tension along riser under freestanding mode
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leva
tio
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m)
Bending moment (kN.m)
BENDING MOMENT ALONG RISER UNDER FREE STANDING MODEBase Tension 550kips, Buoyancy Can Position 400m Below Water
No current 1Yr Current 10Yr Current
50Yr Current 100Yr Current Figure 7. Bending moment along riser under freestanding mode
EXPERIMENTAL METHODOLOGY
Scaled Model Design
Due to the real water depth 1500m, it’s unrealistic to do the
full-size test, and a scaled model test for the significant zone
near buoyancy can is proposed. And the similarity theory and
gravity similarity criterion are used in the test.
The water depth of anti-typhoon drilling riser is 1500m and
the outer diameter of buoyancy joint is 21 inches. To achieve
the appropriate diameter in the water tank test, the scaling factor
is set to 1:21, which is suitable with respect to the dimension of
the water tank and the model size. The diameter of scaled model
riser is 1 inch. And the riser model length is lowered to 71.4m.
To obtain the strength performance of the zone of riser near
buoyancy can, the zone near buoyancy can is truncated due to
the restriction of water tank depth. The length of scaled model
near buoyancy can zone (shown in Figure 8) is set to 8.5m
finally, including the 7.5m lower section and 1.0m upper section
with the intermediate connector which simulates NSDP.
Test Set-up
The overview schematic of the model test setup is shown in
Figure 9. A steel fixed beam is used to fix the upper riser end
and the loading mechanism located on fixed beam which can
impose various tension to the riser by thread and spring. The
steel fixed steel beam is installed onto the wall of water tank.
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On the bottom side of riser end, it’s fixed to the plate and
remain stationary. There is a quick connector between the upper
riser and the lower riser section. There are 3 buoyancy cans
installed in the lower riser section. Two 3-axial force sensors
are installed on the top end and bottom end, respectively. The
lead plate is used to decrease or balance the buoyancy force
provided by buoyancy cans. There are two observation windows
in the wall of water tank, and the motion of the rise in the water
could be observed.
Figure 8. Scaled model of riser zone near buoyancy can
Figure 9. Schematic of test model
Figure 10. Non-homogeneous hybrid riser scaled model
Table 1 Riser scaled theoretical and model value comparison
Item Scaled Theoretical
Value
Scaled Model Parameter Model
Value Perspex Annular Tubes UHMWPE Tube Aluminum Tube Lead Bar
OD (mm) 25.4 65 (WT 3) 25 12 10 25
ID (mm) - 45 (WT 3) 12 10 0 0
Drag OD (mm) 65 - - - - 65
EI (Nm2) 59.29 - 10.89 36.89 8.35 56.13
Weight in water (kg/m) 0.271 - - - - 0.284
Riser Model Material and Sizing
(1) Riser model
Based on the similarity theory and gravity similarity
criterion, at least three parameters must meet the scaled
requirements: geometry, bending stiffness and weight in water.
There is no one appropriate material which can meet the
requirements to manufacture the riser scaled model under model
scaling factor 1:21. Based on previous research work [18, 22], a
non-homogeneous hybrid pipe with 4 different layers (given in
Figure 10 and Table 1) can satisfy the requirement. From inside
to outside, the first layer is the lead bar which provides main
weight. The second one is the aluminum tube, which provides
main bending stiffness. The third one is UHMWPE tube which
provides some weight. These three layers are core pipe to give
correct bending stiffness. The outermost layer is Perspex
annular tubes which provides main buoyancy to obtain outer
geometry and wet weight. These four layers are bonding and
friction forces between core pipe and outer buoyancy are
avoided. The gap between Perspex annular tubes cluster ensures
that bending stiffness of core pipe is not influenced by a cluster
of Perspex annular tubes.
(2) Buoyancy can model
Buoyancy can model provides large buoyancy force and the
geometry is determined by scaled ratio. Solid buoyant material
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is the appropriate material and selected to fabricate the
buoyancy can. The shape is cylinder and scaled theoretical
value and model value are listed in Table 2. There are 3
buoyancy cans which are installed to the riser model.
Experimental measurement
The lower riser section near buoyancy can zone which
remains in the sea to survive in the typhoon is the main
concerning zone and its strain and strength performance should
be monitored in the experimental test. Optical fiber
measurement system is used to measure the strain of lower riser
section. As shown in the Figure 11, there are 2 groups of 16
optical fiber grating sensors installed in the direction of 0 and 6
o’clock of core pipe cross section, and there are 2 groups of 10
optical fiber grating sensors installed in the direction of 3 and 9
o’clock of core pipe cross section. The position of sensors
(listed in Table 3) is determined by the modal analysis results.
Figure 11. Experimental measurement system
The forces at the top end and bottom end are significant
parameters to riser strength performance. Two 3-axial force
sensors are utilized to measure the forces of the riser top end
and bottom end. During the test, the overall displacement of the
system is observed by a high-speed camera.
All the data are saved in the computer in real time and will
be processed after experimental test.
TEST PROGRAM AND LOAD CASE MATRIX Three configuration modes are simulated: connected,
disconnecting, and freestanding mode. The detailed test
procedure and load matric under various current, wave
buoyancy upthrust are introduced.
Table 2 Comparison of buoyancy can model
Item Scaled Theoretical
Value Model Value
Material Steel plate can Solid buoyant material
OD(m) 0.203 0.200
ID(m) 0.065 0.065
Single length(m) 0.871 0.871
Clearance(m) 0.218 0.218
Density(kg/m3) - 460
Mass in water (kg) -13.21 -13.80
No. of cans 3 3
Table 3 Position of optical fiber grating sensors
Item Position of Sensor (Length from Bottom End, m)
A-A B-B
P1 1.105 -
P2 1.743 1.743
P3 2.465 2.465
P4 3.315 -
P5 4.335 4.335
P6 5.228 -
P7 6.02 6.02
P8 6.8 6.8
Connected Mode
Under connected mode, the net buoyancy force induced by
buoyancy cans is balanced to zero by determined weighted lead
plate. As shown in Figure 9, the procedure of anti-typhoon
drilling riser scaled experimental under connected mode are as
follow: (1) Load predetermined bottom tension using tension
loading mechanism; (2) Measure top & bottom tension and
strain; (3) Repeat the model test under various current and
wave.
The detailed load cases matrix under connected mode are
given in Table 4. The tension at bottom end can be obtained by
adjusting tension loading mechanism. Three kinds of bottom
tensions are considered, and total 39 load cases will be
conducted in the water tank test under connected mode. The
current and wave are scaled from the actual environment
conditions by scale factor.
Disconnecting Mode
Under disconnecting mode, the buoyancy upthrust induced
by buoyancy cans should work to obtain various bottom tension
and it can be adjusting by different weight lead plate (shown in
Figure 12): (1) Remove all lead plates by diver; (2) Unlock the
quick connector; (3) Measure the top & bottom tension, and
strain; (4) Reconnect the upper and lower riser by diver; (5)
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Repeat the model test under various current, wave and
buoyancy force.
The load case matrix under disconnecting mode is the same
as that under connected mode which is listed in Table 4 and
there are also 39 load cases.
Figure 12. Test model schematic under disconnecting mode
Figure 13. Test model schematic under freestanding mode
Freestanding mode
Under freestanding mode, the upper riser section is
removed and there is only lower riser section freestanding in the
water and is upthrusted by buoyancy cans. By using different
weighted lead plate, three kinds of the bottom tension can be
induced. As shown in Figure 13, the procedure of anti-typhoon
drilling riser scaled experimental under freestanding mode are
as follow: (1) Remove the upper riser completely; (2) Impose
different buoyancy force; (3) Measure the top & bottom tension
and strain under various current and wave.
There are 30 load cases (given in Table 5) considered in the
freestanding test. In the reality, the buoyancy cans are designed
to be below the wave and large surface current zone. There is
no influence on the freestanding riser induced by wave load. To
achieve the strength performance in the wave zone, some mild
wave is induced additionally in freestanding riser test.
Table 4 Test load matrix under connected mode
Case
No.
Tension at Bottom
End (N)
Current
(m/s)
Maximum Wave
Height (m)
Wave
Period (s)
1~3
403.8
/355.8
/307.7
0 - -
4~6 0.04 - -
7~9 0.08 - -
10~12 0.124 - -
13~15 0.16 - -
16~18 0.2 - -
19~21 - 0.327 1.9
22~24 - 0.409 2.12
25~27 - 0.514 2.64
28~30 0.04 0.514 2.64
31~33 0.08 0.514 2.64
34~36 0.124 0.409 2.12
37~39 0.124 0.514 2.64
Table 5 Test load matrix under disconnecting mode
Case
No.
Net Buoyancy
Force (N)
Current
(m/s)
Maximum Wave
Height (m)
Wave
Period (s)
1~3
388.5
/311.0
/233.5
0 - -
4~6 0.137 - -
7~9 0.151 - -
10~12 0.2 - -
13~15 0.25 - -
16~18 - 0.327 1.9
19~21 - 0.409 2.12
22~24 0.06 0.409 2.12
25~27 0.137 0.409 2.12
28~30 0.151 0.409 2.12
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CONCLUSIONS Numerical analytical methodology and results of anti-
typhoon drilling rise under connected mode and freestanding
mode are proposed. And a model test of 1:21 scale factor for
anti-typhoon drilling risers to evaluate the hydrodynamic and
strength performance of the riser zone near buoyancy can under
various current and wave is designed. Three configuration
modes: freestanding mode, connected mode and disconnecting
mode with a series of load cases under various current and
wave, buoyancy upthrust are simulated in the water tank test.
The present numerical and experimental study provides a
simplified but well-defined anti-typhoon drilling riser model.
The experimental data which will be in the ongoing test
obtained can be compared with the riser’s numerical analysis
result in future.
ACKNOWLEDGMENTS The authors thank 2H Offshore cooperation to publish the
paper. This research is supported by National Science and
Technology Major Project of China (Grant No.: 2016ZX05028-
001).
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