Experimental study on the behaviour of a two tension leg buoy mooring system in wind, wave and current

CHINA SHIP SCIENTIFIC RESEARCH CENTER

Experimental Study on the Behaviour ofa Two Tension Leg

Buoy Mooring System in Wind, Wave and Current

Qi Xinynan

Xie Nan

Cheng Tianyang Ql Tao

CSSRC Report

November 1994

English Version 94007

P. 0. BOX 116, WUXI, JIANGSU

CHINA

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Mekelweg 2, 2628 CD Oeft

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015 786873 - Fax:_{015 781S33}

Contents

Page

Abstract

I

Nomenclature

1

Introduction

2

The Two TLB Mooring System

2

Mode! Tests

3

3.1

Modelling of Wind, Wave and Current

3.2

Elastic Modelling

3.3 Measurement and Data

Processing

Mode! Test of the Complete System

under OperatIon Condition

3

4.1 Test States

4.2 Motions of the Moored Tanker

4.3 Motions of TLB

Conclusions

s

Acknowledgements

6

EXPERIMENTAL STUDY ON THE BEHAVIOUR OF

A TWO-TENSION-LEG-BUOYMOORING SYSTEM

IN WiND, WAVE AND CURRENT

Qi Xinyuau

Xie Nati

Cheii Tanyang

China S/zip Scientific Research Center

Qi Tao

China Ocean Enjineerz,z Service LTD, Shanqhai Office

The two-Tension-Leg-Ouoy (TLB)-mooring system is a new type of early production system and has been designed to open up the oil fields in

South China Sea. The system consists of two tension leg buoys and a

mooring tanker. The mooring tanker is connected with the TLBs by using the wires and this mooring is a slack one, while the TLB is the taut mooring. The system is operating In the water depth of 40m. In order to confirm the behaviour of this system in various combinations of wind, wave and current, model tests of this system in wind, wave and current was conducted in the wave tank of China Ship Scientific

Re-search Center (69x46x4 o). During the tests, the Local current and. wind generating facilities wore used. Hodel tests of this system were

performed with 9 different combinations of wind, wave and current.

In the present paper, experimental results of the moored tanker and

tension leg buoy, as well as those of tension variation in the leg

of TLB were given. The behaviour ol the system was analyzed at the

end of this repert.

Nomenclature

Acc. acceleration at the midship of tanker 6 breadth of tanker

tension in No.1,",No.6 leg of TLBs

g acceleration of gravity H depth of tanker

H113 significant wave height

transverse radius of gyration longitudinal radius of gyration

LOA overall length of tanker

T draught of tanker TA after draught

fore draught

T mean period of wave X surge of tanker

1-displacement of TLB in x-direction Y sway of tanker

displacement of TLB in y-direction

z9 height of center of gravity of tanker

4, roll of tanker

e pitch of tanker yaw of tanker wave direction

I. Introduction

Following the development of the offshore oil _{fields, various kinds of offshore} structures were envisaged. _{In order to open up the middle and small}

scale oil field and marginal field, attention has been _{paid on the early production system. To develop} a oil field of 40 meters water depth in South China Sea, _{Goungzhou Marine and Offshore} Engineering Oesign Company has designed a _{two-TLB mooring system. It is a new type} of early production system1 and consists of jack-up platform, two TLBs and a shuttle

carrier, while the shuttle carrier and _{two TIBs compose a floating mooring system.}

The system has the merits of low _{cost, safe operation and good mobility. In order}

to

verify the expected behaviour of this _{system, model test was conducted on the model} with scalo of 1:30 in the _{wave tank of China Ship Scientific Research Conter (CSSRC).} During the model test, _{the environmemt conditions have been truly simulated.}

That is the model tests were performed _{under the combination of wind, wave and}

current. Use has been made of the blower _{to simulate the wind, while the local}

current generating facility was used to simulate the _{current. The irregular wave was made by using the} pneumatic wave maker. _{The experimental results indicate}

that under the 9 different

combinations of wind, wave and current, the system has satisfactory behaviour.

II. The Two-TLB Mooring System

The early production system designed _{by GUMECO for exploration of}

oil, in North Gulf of South China Sea can be split into the following parts:

jack-up platform; tanker: and

tension leg buoy (TLB).

The two-TL8 mooring _{system which is the theme of this paper consists}

of two TLBs and a tanker (See Fig.1). The coordinate system was defined as shown in Fig.1.

_F,F2,

are designated as the tension variations in six _{legs of the two TLBs} respective-ly. Use was made of a 5000 E7PT tanker, the particulars of the vessel _{are given in Table}

1. The _{body plan is shown in}

Fig.2.

The TLB

is

_{made up of three parts as follows:}

buoy;

base: and

three groups of mcoring tethers of tension type (each group contains two wires). The arrangement and main _{particulars of TLB are illustrated in Fig.3.}

-2-III. Model Tests

1. Modelling of wind, wave and current

The test reports on the behaviour of offshore structures under the influence of

combinations of wind, wave and current in the basin aro scare at the present time. In the published investigations, two modelling methods have been introduced

To simulate wind and current field

This can be done by modelling the wind velocities, current velocities and their

distributions in the basin.

To simulato forces (or moments) of wind and current

Wind and current forces (or moments) acting on the offshore structures can be

modelled by controlling RFt1 of fans and propellers mounted cn the offshore structures with the aid of computer and the numerical values of forces (or moments) that had been obtained by calculations or wind tunnel tests prior to tank tests.

CSSRC has succeeded in the simulation of wind and current force in model tests

of single point mooring system. Tri the present experiment, wind and current were

modeled physically by means or respective local wind and current generating facilities

of CSSRC, which could create local wind and current field at the site in the basin where the model of ILS mooring system was located. The current maker is illustrated in Fig.4. As shown in Fig.5, from the free surface to the water depth of 0.5 m, the

vertical distribution of the current velocity can be considered as uniform approximate-ly. The exit of the current maker was 0.5 m below free surface. So there was no ap-parent effects ori the wave. The measured results of current at the site where the model was located indicated that the current velocity had no significant change within the rango of 3.5 m in y-direction. Therefore, at the space (4.5x3.5x0.5 m) where the model

was located in the basin, uniform current field could be obtained.

'lavos wore generated by the pneumatic wave maker in the wave tank of CSSRC.

2. Modelling of elasticity

Correct modelling of elasticity of tether is important for model test of any moor-ing system. In this test, it was performed by using six steel wires with correct scal-ing of vertical stiffness of prototype.

3. Measurement and dta processing

As seen in Table 2, the 18 parameters which must be measured in the tests can be

split up into four categories. All measurements were recorded on magnetic tapes and

an oscillograph to facilitate the data processing.

The data given in the present paper wore converted into those of prototype. For

all figures appeared in this paper, the units of force, displacement and angle aré kilo-Newton (kN), meter Cm) and degree Y) respectively.

IV. Niodel Test of the Complete System

under Operation Condition

1. Test states

The early production system will be installed in South China Sea. Based ori the

particular environmental condition in that region. 9 sea states in the water depth of 40 o have been selected for the model tests (see Table 3). Since the shuttle carrier

-3-works on the operation sea conditions, so. in this paper, the behaviour of the system

oniy under these sea conditions are discussed.

2. Motions of the moored tanker

The forces on the moored tanker are very complicated. They include the following

components:

wind force;

current force;

wire force acting on the tanker induced by the motions of ILS;

wave frequency exciting force;

second order wave force due to different frequency components in irregular waves;

a r.d

second order wave force due to sum of frequency components In irregular waves.

Considering the small influence of this force on the motions of the tanker end TL3,

it is always negligible.

Because of low wind velocity and small projected area above waterline of ILS,

ef-fects of wind forces on the motions of 118 are very small. But attention must be paid

on the effects of wind and current forces acting on the tanker. A yaw of tanker was

observed under the action of wind and current forces during the tests. This is because

the superstructure area and underwater configuration of

the

tanker were asymmetric

about midship section. The two-TLB mooring system can also be regarded as a combined

one which consists cf a slack mooring system (between tanker and TLB) and a taut

moor-ing system (ILS itself).

From the spectrum and time histories of motion of the moored tanker under operation

conciiticns (see Fig.6),

o1lowing comments can be made:

(i) Roll, pitch and heave

From Fig.6,

it. is evident

that

low frequency motions are negligible ccmponentS

in

these moticns, and, the dcminant one was wave

f reuency motion. When T=6 s.

the

emplitudes of these motions incresse with the increase of wave height.

Surge. sway and yaw

Being different

rorn roll, pitch arid heave motions, low frequency motions

complete-ly dominate the surge. sway and yaw motions, while wave frequency motion is quite small.

When T01=6

s,

the higher the wave height.

the larger will be

the amplitudes of

the

low frequency motions.

The tanker is moored under the influence of combinations of wind, wave end

current. So long period motion is the dominant horizontal motion, and

a steady yaw

and roll occured due to the action of second order forces. This can be seen frcm the

figures of time histories of yaw and roll.

The significant double amplitude values of motions with six degrees of

freedom

and the maximum value of acceleration at the midship of the tanker in each sea state

are listed in Table 4.

3

Motions of TLS

As a

taut macring system in wind, wave and current,

the dominant motions of TLS

are horizontal motions (surge. sway and yaw).

When wind, wave and current are in

the same direction (j=9O),

the dominant

motion of TLB is sway (Y );

B

When wind and current are in the sanie direction (=9O'), while wave is heading

(p=IBO'), the surge becomes the significant motion of TLB;

4-(3) When wind, wave and current are all in the direction of i=lO5°, the motions

of TLBs

are both surge and sway. However, yaw motions of TLB are of a small amplitude in any sea states during the course of testing.

The maximum excursions

of TLB

in each sea state are listed in Table 5. A typical motion spectrum and time history of TLB are shown in Fig.7.

4. Variation of tension in the legs of TLBs

As shown in Fig.1, the arrangement of 6 legs of the two TLBs are given, and F,

F2, F6 designated tensions in each leg of TLBs respectively. Fig.8 is a sample of

tension spectrum in the legs under one sea state.

The results based on analysis of the tension variations in legs of TLB in various test cases are:

(1) The wave frequency component of the tension variation significantly dominates the tension variation in each leg and the low frequency (slowly-varying) component of tension variation is much smaller compared to the wave frequency component. Exist-ence of the low frequency component is mainly due to the following reasons:

influence of the low frequency slowly-varying second order wave (orces; influence of the long period motions of tanker and TUL

When i=90', F1 and F4 are much larger than F, F, F and F5.

When i=18O, F and F are much smaller than F, F3, F5 and F6.

When i=105, F

and F are still larger than F2, F3 F5 and F5, but without apparent regularity.

The double amplitude significant values of tension in 6 legs of two TLBs in all sea states are shown in Table 6.

V. Conclusions

From the above series of experiments of the two TLB mooring system conducted in wind, wave and current, following conclusions could be drawn:

Under the 9 combinations of wind, wave and current, the motions of six degrees

of freedom of the tanker and the acceleration in the midship of the tanker are not

tiarge, when }-1 3=3.5 rn, the roll of the tanker is larger in beam and bow waves, and

a light green water occurs. The motions of tanker in the horizontal plane (surge, sway and yaw) are dominated by the long period motions.

Motions of TLB occur mostly on the horizontal plane. When wind, wave and cur-rent are in the same direction and ii=90, the sway is the largest. While in the heeding

sea, the surge is dominant. In the present 9 sea conditions, the maximum displacemeOt of the buoy is 2.5 meters, and the yaws of TLB ere very small.

The large part of tension variation in each leg of TL9 is wave frequency compo-nent, there is low frequency component in the tension variation. There are differences among the tensions of each leg, their quantities depend on the incident wave direction. In the 9.different sea conditions, the maximum double amplitude significant value of the tension in the legs of TLBs is 1200 kM, it is very small if compared with the de-sign value of 11,500 kM.

In the above sea ccnditions, the snap does not occur.

It can be concluded that the behaviour of the system under the 9 different combina-tions of wind, wave and current is satisfactory.

-5-Acknowledgemeri ts

The authors would like to thank Cuangzhou lionne and Offshore Engineering Design Company for permission to publish the results of the experiments. Thanks are due to

their colleagues in the Seakeeping and Ocean Engineering Division of China Ship Scien-tific Peseerch Center for their assistance in this research program.

R e fe retice s

(IJ. Takezawa S, Hiraynma T and Hua Duyu: 'Numerical Time Domain Simulation of

Behaviour of a Moored Semi-Submersible Platform under Compound External Forces.

Journal of the Society of Naval Architects of Japan,Dec.1988.

(21. Ql Xin-yuan, Liti JI-ru, He Cheng-yuen and Sun Bo-qi: 'Behaviour of TLP In South

China Sea", 5th Offshore East South Asia, Singapore, Feb1984.

(3J. Yan Shen-hua, Xie Nan: " A Current Generating Facility In a Large Seakeeping Basin",

Proceedings of the 3rd China National _Conference on Experirnenlal Fluid Mechanics,

Tianjin, Oct 1990.

Table 1 Hain particulars of the tanker

-designation Symbol unit quantity

displacement volume t 5416.0

overall length _LOA m 109.25

breadth B m 17.40

depth H m 8.50

height of center of gravity Z9 n 5.59

draught T n 4.65

fore draught T m 4.38

after draught T n 4.92

A

longitudinal radius of gyration m _O.2SLOA

transverse radius of gyration m 0.358

item quantities apparatus for measurement

wind current wave wind velocity current velocity height/period anemometer current metor

ultrasonic wave probe motion of buoy

motion of tanker

surge, sway resistance-wire type displacement meter surge, sway, heave. servocontrolled motion pitch, roll, yaw tracer

force tensions in 6 legs of buoys

tension meter

Acc. Acc. et midship of

tanke r

accelerometer

Table 3 9 sea states

Table 4 Moasured tanker motions (significant double amplitude)

Table 5 M,ximum values of displacement of RB

Table 6 Measured tensions in the legs of TLBs

(significant double amplitude value)

-7

state I No. wind velocity (mis) current velocity (m/) direction of wind & current

() significant wave height (m) wave period (s) wave J direction () 1 15 0.75 90 2.0 6.0 co 2 15 0.75 90 2.5 6.0 90 3 15 0.75 90 3.5 7.8 90 4 15 0.75 90 2.0 6.0 180 5 15 0.75 90 2.5 6.0 180 6 15 0.75 90 3.5 7.8 180 7 15 0.75 105 2.0 6.0 105 8 15 0.75 105 2.5 6.0 105 9 15 0.75 105 3.5 7.8 105 state No. surge (e) sway Cm) heave (a) roll () pitch

()

yaw () max. Acc. at midship (g) 1 0.61 0.84 1.13 2.19 1.11 1.06 0.17 2 0.72 1.22 1.37 2.43 1.31 1.39 0.21 3 0.76 2.38 2.34 13.7 1.32 1.42 0.25 4 1.06 0.60 0.63 1.75 1.92 0.98 0.08 5 1.86 1.18 o.go 2.02 2.87 1.69 0.10 6 2.40 1.43 1.24 3.05 4.90 2.24 0.11 7 0.96 1.06 1.16 3.84 0.73 1.20 0.22 8 1.31 1.53 6)4 1.00 1.58 0.27 9 1.49 3.73 17.0 2.34 1.96 0.30 state No. X (rs) B Y (ra) B wave () 1 1.70 90 2 2.20 90 3 1.75 90 4 1.50 180 5 1.90 ---- 180 6 2.50 180 7 1.50 1.90 105 8 1.40 1.70 105 9 1.10 1.70 105 state No. F C kM) F 2 ( kM) F 3 C kil) F 4 C kM) F

5'

C kM) F 6 (kM) wave ( 1 414 309 250 430 331 175 90 2 765 515 380 900 625 363 90 3 668 445 325 576 392 293 90 4 298 474 454 477 580 550 180 5 423 800 765 700 860 780 180 6 580 1080 1080 770 1124 1070 180 7 50g 411 222 361 300 105 8 864 570 276 500 460 105 9 619 405 340 350 335 105

BO

o

y

Fig.1 The two TLB mooring system

/

/

/

/1

/ /7

\\ \

_y

) / I

i

/ I

1 I

4 J 1 '

'I

. 1-. f / >

J

ì1

.

L\

t

/

it

Y! / /

i'

/j

Fig.2 The body plane of the tanker

buoy

leg

base

-Fig.3 The tension leq buoy

/

tanker O -0.1 -0.2 -0.3 -0.4 -0.5 3000

F-Fig.4 The current maker

z 0 0.1 -1 y (m/s) o o

Fig.5 Profile of current velo-city in z-direction

(ßJ 0. 5039(M"2/S) 0 0.5 1. 0.453762(M"2/S) Surge 0.256335(M**2/S) sec 280 560 280 560 (radis) (radis) sec 0.5 Sway 0.315334(M"2/S) (radis) sec

280 434

560 666

Fig.6 _{Sample of motion of the moored tanker}

i 85885(KN"21S)

Fi

0.5 1 8805 1(KN**2/S) 0 0.5 1 60823(KN"2/S) F3 0 0.5

9

1. Heave 0.713184(M**2/S) 0 0.5 1 (rad/s) O (radis) sec 459294(KN'2/S) 0 0.5 1.

i 78072(KN2/S)

F5 0 0.5 0.5 1. _(rad/s)

Fig.8 _{Sample of tension spectrum} Fig.7 _{Sample ott sway motion of ILS in logs of ILS}

O