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Experimental study on behaviour of an open bottom floating platform in wave, wind and current

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CHINA SHIP SCIENTIFIC RESEARCH

CENTER

Experimental Study on Behaviour of an Open Bottom Floating Platform in Wave, Wind And Current

Qi Xinyuan

June 1995 CSSRC Report

English version 95002

Presented at the 94'ISOPE

P. 0. BOX 116, WUXI, JIANGSU

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Contents Page Abstract Nomenclature Introduction Model tests Evaluation of results Conclus ion Acknowledgement

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ABSTRACT

The behaviour of a mdöred open bottoni floating platform has been investigated by a series of model tests at the wave basin of CSSRC (69 x 46 x 4m).The model tests were divided into two

versions. i.e.,versi.on I for a conventional semisubmersible

and version II for an open bottom floating platform.

Comparison was made under the same mooring and environmental

conditions including waves, steady wind and current. The

resu-its of model tests indicate that the open bottom floating platform is more stable and with less mooring loads than the conventional semisubmersible.

KEY WORDS: Open Bottom Floating Platform.

Convent mal Semisubmersible.

NOMENCLATURE

a acceleration

F forces acting on mooring wire F maxium value of F

max

Il, significant wave height

S wave spectral density

T,, mean period of wave

V wind velocity V current velocity wave direction current direction u wind direction r- wiiI w wave frequency

Q,. significant value of pitch motion

z,, significant value of heave molion

I NTRODUCT1ON

With the exploitation of offshore oil fields various offshore

structures have been niade.T!ìe floating platform studied in this paper consists of an upper deck,four columns with open

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-1-bottoms, and trusses connecting any two adjacent columns.Every column is a hollow cylinder without bottom,but has a water

tight pressure bulkhead installed inside at a certain

dis-tance form the open end, which is different from a conve-ntional semi-submersible.Compressed air in the bottom air

house of each column acts as a cushion, and brings bouyancy to

the floating platform.While the platform is floating in water. the water level inside the columns is lower than the

free surface outside because there exsists an internal air pressure higher than atmospheric pressure.A general confi-guration of the mooring system is given in Fig 1.

Fig i Two versions

Evaluation to the behaviour of this new floating system under survival conditions was made by model tests in waves combined

with steady wind and current.

In order to identify the alvantage of this system,the model tests were extended to conventional semi-submersible under the

same mooring and environmental conditions.

MODEL TESTS

The model tests were conducted at a wave basin of China Ship Scientific Research Center(CSSRC),which consists of two

ver-sions. i.e.version I for a conventional semi-submersible and

version II for an open bottom floating platform.

The 1: 16 scale models were used for both Versions.

The only difference between the mooring systems used by the

tests of version I and version II is whether the each column

of the platform has an opened bottom.And the one with opened

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The main particulars of prototype for the two versions are the

sanie as listed in Table 1.

Table i Main particulars

Displacement 1740 T

Column diameter 10. 0 ni

Length Width 48. 5 39. 0 m

Operating water depth 30 m The platlorm has four mooring wires.

The slack mooring system has a rectangular deck supported by

four large columns of 10m in diameter, interconnected by

trus-ses.

In order to simulate the operating water depth a berth

supporter was fitted at the center of the wave basin. The model is moored to the supporter.

In the present experiment,wind and current were physically

simulated by means of loca! wind and current generating facillities respectively, which can generate local wind and current fields at the site where the model of mooring stru-cture is located.The current maker is illustrated in Fig 2.

From the free surface to the water depth of 0. 5m, the vertical distribution of the current velocity can be considered as uniform approximatelv.The location of the current maker is

0. 5m below free surface, so there is not apparent effect on

waves.The measured current data from where the model was

located shows that there is not significant change in current velocity at a distance of 3. 5m in Y-direction. Hence within the range of water body (4. 5 3. 5 0. 5)where the model was

located,an uniform current field can be reasonably

approxi-ma ted, as shown n Fig 3.

Pneumatic wave makers are located along two orthogonal sides

of the basin,which can generate regular and irregular long-crested waves in both x and y directions.

The model tests for version I and II were conducted in

long-crested irregular head waves.The simulated spectrum is

illu-strated by Fig 4.The calculated extreme values of H, and are 4.9 meter and 8.9 second respectively based on the

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ction of storm condition with the return period of 50 years at

China Bahai sea.

V pump support 0. 5 O (in) z 1

I

V(m/s)

Fig 3 Profile of current

Fig 2 The current maker velocity in Zdirection The pi tcli and heave of the

s 06354(M**2/SEC) (peak value)

mooring system in each test were measured by a gyro and an accelerometer installed at the center of the deck. The mooring forces were mea-sured by strain gauges.

All measurements were reco-rded on magnetic tapes and also by an oscillograph to

facilitate the data proces Fig 4 Wave spectrum

s i n g.

The measured parameter as shown in Table 2. Table 2 apparatus for measurement

o

L O o' (RAD/S)

The data given in this paper have been converted into Si units.The units of force,displacement and angle

are Kilo

Newton

items quantities apparatus for measurement wind wind velocity anemometer

current current velocity current meter

wave height and period ultrasonic wave probe

motion pitch gyro

Acc. Acc. at center of deck accelerometer force Tension of moored wire tension meter

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(KN),meter(m).and degree(') respectively.

EVALUATION OF RESULTS

To evaluate the behaviour of the moored open bottom floating

platform..Model

tests

for

the comparison were performed with

versions I and

II

under the same enviromental conditions.

The detailed informations are given bellow:

The main particulars of two versions are the same

The depth of water and mooring wire are the same

The envionmental conditions are same. i.e.

wave period(T(,1)

8.9 sec

wave height (H,,)

4. 9 ni

wave direction(PH)

1800

Current direction(p)

90°

Current velocity(V.)

l.2knot

wind direction (

) 180°

wind velocity(VHe,,d)

19m sec

Measured record of model test in survival condition

as show in

Fig 5.

-2 u

heave

Fig 5 Measured record of

motions in survival

condition

The analytical results of model tests are listed in Table 3.

The above table indicates that:

the behaviour of the new

moor-ing systeni(Version II)

was better than a conventional moored

semisubmersible(Version I).

5

Table 3 comparison

of

experimental results

Data obtained

in

the

tests

were processed by statistical

analysis in

the data

proces-sing

center

at

CSSRC. The

analysis method is

convertio-nal one.

Version

Q,, Z1,

F,

I (convert

ional)

10. 72. 04

m

2050

KN II

(witho

ut bottom)

9.8 1.88

ni 1800 KN 20

pitch

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CONCLUS ION

Comparing the results of the model tests from the version I

and version II. it can be concluded that an open bottom float-ing platform is more stable and with less moorfloat-ing loads than a conventional semisubmersible under the same mooring and

envi-ronmental conditions.

The air cushion among the opened bottom can absorb some energy during the dynamic process so that a reduction can be achieved in the motions and in the mooring loads.Regarding the station keeping capability of a mooring system, an open bottom floating platform is therefore better than a conventional

semisubmer-sible.

ACKNOWLE DG EMENT

Mr. Chen Tian yang,Mr.Feng Yue and Mr.Fu YuFei assisted this

Cytaty

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