Numerical Solutions and Model Test Design for Anti-Typhoon ... · before the hurricane arrives....

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

1 Copyright © 2019 ASME

Proceedings of the ASME 2019 38th InternationalConference on Ocean, Offshore and Arctic Engineering

OMAE2019June 9-14, 2019, Glasgow, Scotland, UK

OMAE2019-95196

Attendee Read-Only Copy

Learn more at www.2hoffshore.com

(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



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 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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EFFECTIVE TENSION ALONG RISER UNDER CONNECTED MODE Base Tension 550kips, Buoyancy Can Position 400m Below Water


Figure 3. Effective tension along riser under connected mode

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BENDING MOMENT ALONG RISER UNDER CONNECTED MODE Base Tension 550kips, Buoyancy Can Position 400m Below Water


Figure 4. Bending moment along riser under connected mode



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DISPLACEMENT ALONG RISER UNDER FREE STANDING MODEBase Tension 550kips, Buoyancy Can Position 400m Below Water

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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


50Yr Current 100Yr Current Figure 6. Effective tension along riser under freestanding mode

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BENDING MOMENT ALONG RISER UNDER FREE STANDING MODEBase Tension 550kips, Buoyancy Can Position 400m Below Water


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.



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



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)



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



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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