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
Contents Page Abstract Nomenclature Introduction Model tests Evaluation of results Conclus ion Acknowledgement
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
-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
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
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
(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
IIunder 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 niwave direction(PH)
1800Current 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. Theanalysis method is
convertio-nal one.
Version
Q,, Z1,F,
I (convert
ional)
10. 72. 04
m2050
KN II(witho
ut bottom)
9.8 1.88
ni 1800 KN 20pitch
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