At-sea experiment of a floating offshore structure characteristics of directional wave spectra at a test field

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6. Atsea Experiment

of a Floating Offshore

Structure

Characteristics of

Directional Wave Spectra at a

Test Field

Hirofumi YOSUIMOTO*

(From J.S.N.A. Japan, VoL 168; Dec. 1990)

Summary

The atsea experiment of the moored floating structure 'POSEIDON" has been carried out at Japan Sea for 4 yearsl). In this experiment, three ultrasonic type wave probes wereinstalled as a line array at the sea bottom of the test field and themeasured three time series of wave surface elevations were used to analyze directional wave spectra by Maximum Likelihood Method. This paper describes the characteristics of directional wave spectra measured at the test field.

Nomenclature

Cd : Drag coefficient

f : Wave frequency

f : Peak frequency of S(f)

Nondimensional peak frequency

(=27rfUi9.s/g) Fetch

Coefficient to equate the area of

Smj(f) with that of Smi(f) Nondimensional fetch

(=g F/U19.5)

g : Acceleration due to gravity

G0 Normalizing function

G (f, O): Directional function

h : Water depth

H113 : Significant wave height H* : Nondimensional wave height

I

TY *2

g ril/3/u

L Wavelength (=gTHi,32,2 7r)

N Spreading parameter proposed by the

Norwegian Petroleum Directorate

P : Root mean error between the input

and MLM estimation S (f, O) : Directional wave spectrum

s(f): Frequency spectrum Smt(f) : Modified ISSC spectrum

Smj(f) Modified JONSWAP spectrum

S Spreading parameter of the Mitsu.

yasutype function

Sguax : Spreading parameter at peak

frequen-* Ship Research Institute

I

cy of S(f)

S* Nondimensional peak value of

S(f) (=s(f) f/H1132)

TH1,3 Significant wave period

T* Nondimensional period

(gTH1,3,u

*)

u10: Mean wind velocity at the position of

10m height above the mean sea

surface

U195 : Mean wind velocity at the position of

19.5m height above the mean sea surface

Friction velocity of air

a :

Ratio of the water' depth to the

wavelength (=h/L)

Y : Peak enhancement factor

Mean wave direction

1. Introduction

Interests in directional wave spectra has

been increasing in recent years as a result of both scientific studies and engineering require. ments. As for engineering, information on wave directionality is a necessary requirement for the design procedures of offshore structures and ships. However, there is uncertainty as to the properties of directional wave spectra. because the measured data in the field is quite a

few because of the constraint of cost and

operational difficulty of oceanic measurement. In the POSEIDON project, the measurement of directional wave spectra has been

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consi-2

dered to be important to study the effect of

directional sea

states on the

motion, the

structural strength, and mooring line tensions of POSEIDON. The main purpose of this paper is to discuss the characteristics of directional wave spectra measured at the test field. In general, the directional wave spectrum can be

expressed as

a product of

the frequency

spectrum and the directional function. The particular attention should be given to the shape of frequency spectra and directional

functions.

The outline of the paper is as follows. The author first describes the measurementsystem

and the data

analysis. The author

then

discusses the shape of the frequency spectrum. The spectral shape of the high frequency range

is not proportional to f

but f

This is

confirmed by applying Toba's

3/2 powers

law2 to the measured data and the author presents a new formula of frequency spectrum

expressed by _4 in high frequency range.

Finally, the characteristics of directional

function is discussed.

2.

Full Scale Measurement

and Data

Analysis

2.1 Full Scale Measurement

The test field is 3km offshore of Yura Port, north western coast of Honshu district. Fig. i shows the topography of sea bottom around the test field. The water depth where the wave probes have been installed is 43m. It is a part of the eastern area of Japan sea, where the severe wind and wave strike periodically from WNW direction in winter.

A variety of methods are available for the

Fig. i Topography of sea bottom aroundthe test field

Hirofumi YosHflIoTo

measurement of directional wave spectra. In this experiment, the array technique is used.

The array consisted of three

ultrasonic type

wave probes, which were installed as a line array at the sea bottom of the test field shown in Fig. 2.

'1 No.2 Wave Probe Q

No.1 J e Probo

No.3 Wave Probe

Fig. 2 Coordinate system and

position of wave probes

2.2 Data Analysis

In the array technique, the degree of

directional resolution depends on the number

of the wave probes, their spacing

and the

method of analysis. The estimation of the directional wave spectra follows the Maximum Likelihood Method (MLM)

The directional resolution

of MLM is

investigated by the numerical simulation. Fig. 3 shows the comparison of the input and the

directional functions estimated by MLM at Oo

= 0, 30, and 60 degrees

respectively. The

input directional function is the

MitsuyasuType function4 as follows.

G(f, 0)Gocos2S 1(0 - 8)/21

(1)

where G0 is a function needed to ensure that Eq. (2) is satisfied.

L

2r

G(f, 0)dO=1

(2) x(:) y(:) No.1 -4.93 25.02 No.2 5,80 92.12 No.3 0.00 0.00

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r

At-Sea Experiment of a Floating Offshore Structure Characteristics of Directional Wave Spectra at a Test Field 3

It is

found that MLM is

available for

analyzing the directional sea states. However there is a slight difference between the input and the MLM estimation near the mean wave directions.

Fig. 4 shows the effect of wave frequency on

the MLM estimation'

for S= 10, 25, and 75

respectively. In this figure, "P" means the root

c3 o -4 u_u g 2--.4 a g o -4 J_u

-a k -4 g o SO 40 30 20 10 o 0° -Inpuc 4, No 3 No.2 o 3 o 3-o 0.0 .f -0. 084Hz

8 -30°

Wave No. No.3 I; No.2 _900 0.1 0.2 Fr.qu.ncy 0.3 0411z

Fig. 4 Effect of tvave frequency on estimation of accuracy

mean square error between the input and the MLM estimation. It is found that the MLM estimation yields a good representation to the input function in the low frequency range (less than 0.11 Hz), although it gives not so good estimation in the high frequency range. There-fore, the data whose significant wave period is more than 9.0 seconds (about 0.11 Hz) have only been used.

3. Analyzed Data

The analysis has been carried outusingthe 4 continuous data measured when the low

atmospheric pressures approached to the

experimental site in winter. The sampling time interval is 1.0 second. The data have been divided into 132 runs of 30 minutes each. Fig

5 shows the changes of H113 and TH1,3in every

30 minutes for each data.

Table i shows statistical valuesof wind and waves at its highest H113 for each run. F* has

been calculated by Wilson's empirical

equations. The wind velocity and direction has been measured by an anemometer on the mast of POSEIDON installed at the position of 19.5 m height above the mean sea surface.

The maximum THI,3 of the analyzed data is

10f-. -'... . ... ... ... ...

O t

20 0 10 hour 10 20 0 hour

Fig. 5 Changes of H113 and T11113 in every 30 minutes

Table i Characteristic values for the analyzed data

D71217 090202 081214 091119 DOtS 17 Dcc. 87 2 l'cb. '86 14 Dcc. 86 19 NO.,. 89 Dct LcnqtQ,øuci 22.4 24.7 30.1 47.1 lion. 35 23 30 44 7.29 5.87 5.86 6.64 Th,,,(z.ci 11.10 11.39 10.65 10.70 U1.,úi./c) 18.52 15.65 21.10 20.40

Wind Dizccr ion 181W 1151W iW *1W

F' 7.5510' l.3510' 2.9510' 4.5510' I 5 0I, 071217 080202 ¡II,,! -i i 10 z , 1cc 20 0 hour 081214 20 0 1Ohor 091119 -'-I 4 ¿

_{6_-}

00

- .1

_Slave o: -

-2 /

No.3 No 2 -Input No.141... av. ....'..o 8 60° No 2 Fig. 3 -do° O ¿0 30° 60° 900 Dir.ctton Numerical simulation

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about 12 second. It is well known that the properties of waves are influenced by the water depth. The range of water depths is conveniently divided into the shallow water, intermediate depth and deep water ranges, depending on the parameter a

(=h/L). The

threshold value of a is usually about 0.5

between the deep and

intermediate depth ranges and about 0.05 between the intermedi-ate depth and shallow wintermedi-ater ranges6. The analyzed data belong to the intermediate depth range. From a geographic viewpoint, they belong to fetchlimited waves. Bearing these features of the analyzed data in mind, the author has to investigate thecharacteristics of the directional wave spectra.

4.

Characteritics of Frequency Spectra

4.1 Result

Fig. 6 shows the development of frequency spectra at every 2 hours for the data (D80202, D81214) shown in Fig. 5 when waves are growing. The frequency spectra are estimated

by using an implementation of the Auto

Regression Model. It is found thatthe spectral shape of high frequency range is proportional

to i_4, Frequency spectra shift to the low

frequency range according to f with the

growth of waves keeping a very similar form of spectrum.

It is well known that the frequency spectrum

of growing windwaves has an

equilibrium range in the high frequency range. Pillips7

ICC 102 S (t) 10 100 10 10 _0.1 _0.5 _H: _0.1 0.5 Hz

Fig. 6 Changes of frequency spectra in every 2 hours

proposed the existence of an equilibrium rar ge

expressed by ¡9 gf° (-s powers

law) and it has been generalized to be involved in the presentation of PiersonMoskowitz or JONS.

WAP spectrum. It disagrees with the author's

result. This suggests the existence

of an

equilibrium range expressed by

f

(-4

powers law).

4.2 Validity of 4 Powers Law

The existence of an equilibrium

range expressed by f has been confirmed by '3/2 powers law proposed by Toba2 as follows.

where A = 0.062. The 3/2 powers

law means that there is a strong combined

relation-ship given by Eq.

(3) between the wind velocity represented by u * and the waves

represented by H113 and THI,3 when the

windwaves are growing. If it is satisfied, the

frequency spectrum in the high frequency

range can be expressed by f " (See Appendix A).

Fig. 7 shows relations between H* and T* obtained from the measured data, which have been grouped into three classes of the para-meter a to investigate the shallow effect on 3/2 powers law. The data of those H1,3 are greater than 1.0 m in winter seasons through 4

years have been selected in Fig. 7 (a). In this case, the data measured for about 34 minutes regularly in every 6 hours have been used. The continuous data measured when a typhoon approached to the experimental site have been used in Fig. 7(b). u' is estimated by Bulk law shown by Eq. (4).

*- I

u vC Ui9.5

where C40.0016. The solid lines show Eq. (3). The measured data agree quite well with

3/2 powers law regardless of the shallow

effect or the particular case such as a typhDon, This result confirms that an equilibrium range of frequency spectrum is expressed by i

(5)

to

*

K

10

101

At-Sea Experiment of a Floating Offshore Structure Characteristics of DirectionalWave Spectra at a Test Field

S io2 to3 T (a) )(,,3.GT.t.Om J I I

/

O Sballo. a<O.O5 A lateriediate O. 05(a(O.5 O Deep O.5<a

/

102 10 . (b) Typhoon

Fig. 7 Verification of 3/2 powers law using measured

data

4.3 Present fo1mula

The above discussion leads to the following formula representing that the spectrum consis-tent with the 3/2 powers law is proportional to

f in the high frequency range.

Smi(f)

=0.173(H1132f') (f/fe)

_i _i

_-6

i

Xexpy-2/3f/f,

I

The non-dimensional peak value of

spec-trum, Smi* (fn) (Smi (f)f/H1132), has been

assumed to be equal to thatof ISSC spectrum. Furthermore, the following modified JONS.

WAF spectrum can also be represented

including the peak enhancement factor Y. Smj (f) Smj (f) F1 y

exp H (f9-02/(2 e ₍₆₎

w here

° =0.07

= 0.09

f>f

is assumed to be the same value as the original JONSWAP spectrum given by Hassel-mann et al.8. Table 2 shows the values of F1

with respect to each 7.

Table 2 Values of F1 with respect to 7

4.4 Peak Enhancement factor

In the engineering design work, the evalua-tion of the peak enhancement factor is very important. Fig. 8 shows the relation between S' of measured frequency spectra and - The

solid lines show the

nondimensional peak

values of modified JONSWAP spectrum given by Eq. (6) for 7 = 1.0,

2.0, 3.3 and 6.0

respectively. S * corresponding to 7 = 2.0 becomes almost a constant in the range of a greater than 0.24, but it increase?s at the range of small a This tendency of measured data may be due to the transformation of spectrum associated with shoaling, but

the author is

unable to resolve this phenomenon.

A0 O

7"2.O A07u11 0080202 0081214 0091119

02OO

00 °0

I

/

0.20 _a _{.A/(gT,I,./2x )} 0.30

Fig. 8 Distribution of nondimensional peak values of

frequency spectra

5.

Characteristics of Directional

Func-tions

5.1 Directional Function

Fig. 9 shows a typical example of directional

wave spectrum estimated by MLM. The data

have been measured at 10:30. 3rd February.

1988. The wind had been coming from WNW

direction at the mean wind velocity 20 rn/s at that time. The estimated directional wave spectrum shows that the component waves propagate with almost the same direction as the wind direction.

Fig. 10 shows the directional function at the peak frequency of directional wave spectrum shown in Fig.

9. The solid line means the

measured directional function and the broken line means the Mitsuyasu-Type function for S =14. The Mitsuyasu-Type function gives a good description of the measured directional

V

t

Tobo

/

7 1.0 2.0 30 33 4.0 5.0 6.0 r, v.00 0.8) 0.09 0.66 0.61 0S4 0.49 0,4 0.3 0.2 0.1 0.0

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function. The majority of measured directional wave spectra have unimodal directional dis-tributions shown by Fig. lo.

u C C. I. t.. 0.2 m2sec/rad. 0:30 3rd February, 1988 li,,. - 6.87 11.39 sec .2Hz -. t -s u N Dtrcc te

Fig. 9 Bird's eye view and contour curvesof directional

wave spectrum

Fig. 10 Example of directional function

5.2 Directional Spread

The characteristics of the directional func-tions the author is particularly interested in are the directional spread.

The MitsuyasuType

function has the feature that the spreading parameter takes a peak value of Sniax at the peak frequency. Although the original proposal of Mitsuyasu et relates the spreading parameter to the wind speed, Goda et al.9 has rewritten the

original formula into the following forms for the purpose of engineering applications.

Smax= 11.5(2 ir

fUjo/g)

2.5

SSmax(f/fp)

ff

SSmax(f/ í)

2.5 f>f

These equations have been applied to the deep water waves.

Fig. 11 shows the relation between the Smax obtained by the method of least squareand f The solid line shows Eq. (7). The measured

Smax distributes around

10-25, and is 10 in

the range of f

*

less than 1.0.

It remains

uncertainty that the solid line shows a

discrepancy with

the measured data. The

broken line, SmaxlO, has been proposed by

Goda'° for windwaves in deep water. The

measured data may appear to support the value

recommended by Goda for the

windwaves.

But the author must note that analyzed data belong to the intermediate depth waves and there is a possibility that the value of Srnax decreases more at a deeper depth area than the test field.

30

0.5

r -2z ¡,U,. .. h

Fig. 11 Relation between Sm.. and nondimensional frequencyf*

In order to investigate the characteristics of S with respect to the frequency, the means and standard deviations of (S/Smax) where the growth of waves almost becomes peak has been shown in Fig. 12 as a function of (f/fe). The solid line in the figure shows Eq.(8) and the broken lines shows the following formula by Izumimiya et al.'1) from the measured data

D11217 Gro,tI A Decay D10202 D Gtoit D812L4OGrotth 091t19 OCrowtb Decay

00

o

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r

AtSea Experiment of a Floating Offshore Structure Characteristics of Directional Wave Spectra at a Test Field 7 1. D1L211 òD80202 D81214 D9U19 0. _1.0 _f/fe 1 5

Fig. 12 Changes of the spreading parameter S with

respect to the frequency

around the coastal area.

SSmax/ [1+ 10

3log(f/fp)I 2] (g) The measurea data shows the tendency of frequency dependence as suggested by Mit-suyasu, but the change rate of S obtained from the measured data is not so rapid as Eq.(8). The measured S rather agrees well with the

result of Izumimiya.This suggests the analyzed

data has been influenced by refraction due to shallow effect.

Finally, the author introduces the concept of

directionality in the Norwegian Petroleum

Directorate (NPD) guidelines'2. The proposal of NPD is very simple. For the sea states with

1-11/3 less than 10m,NPD suggests the following

directional function.

G(0)=GocosM(O - O)

(10)

where

N=H113 (in meter) H113<lOm (U)

For the sea states withH113 greater than 10

m, NPD recommends to disregard

directional-ity, because it may give rise to a reduction of loads and responses. The author shows the measured N as a function of FI1,3 in Fig. 13 in order to compare with it. The solid line shows Eq.(11). The measured N shows the similar tendency to the proposal of NPD.

071217 o 080202 0 081214 o 091119

o,

0 0 0 000 °cP 0 0% 8 5.0 Hua (m)

Fig. 13 Relation between the spreading parameterN

and FI11,

6. Conclusion

The conclusions from the present investiga-tians can be summarized as follows:

The shape of frequency wave spectrum in

the high frequency range is not

prop-ortional to f

but f4. This has been

confirmed by applying 3/2 powers law proposed by Toba to the measured data. A new formula of frequency spectrum

ex-pressed by f

in the high frequency range has been proposed.

The majority

of measured directional function has unimodal directional

dis-tributions because of the fetchlimited condition of the test field. The measured directional functions can be approximated by the MitsuyasuType function.

The measured spreading parameters show the tendency of frequency dependence and take the peak values at the peak frequency suggested by Mitsuyasu et al., although the tendency of measured peak values differs

from the proposal of Mitsuyasu.

The change rate of measured spreading para-meters with respect to the frequency is not

so rapid as the

relation given by

Mit-suyasu, but it rather agrees well with the result of Izumimiya.

The proposals of Goda and of the NPD are very helpful to estimate the spreading parameter for windwaves.

The characteristics of spectral parameters

such as a peak enhancement factor

and a

spreading parameter, which may be influenced by shallow effects, have not yet been clarified

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in this paper. Furthermore, bimodality in the directional distribution will be a subject for research in the years to come.

Acknowledgement

The POSEIDON project has been planned by

the former director

of Ocean Engineering

Division of Ship Research Institute, Mr. Sadao Ando who passed away before the ending of the

POSEIDON project. The author expresses

sincere thanks to him to for giving the author the opportunity to study this interesting theme.

The POSEIDON projecthas been carried out

in corporation with Nippon Kaiji Kyoukai

(NK) , IshikawajimaHarima Heavy

Indus-tries, Ltd., Taisei Corporation, NKK

Corpora-tion and KUMAGAI GUMI Co., Ltd. The author acknowledges all their superior supports. The author also appreciates all staff member of Ocean Engineering Division of Ship Research Institute for their efforts on data acquisition and analyses.

REFERENCES

Ohmatsu, S. et al., "AtSea Experiment of

Floating Platform POSEIDON. 8th

OMAE, 1989

Teramoto, T., Y. Toba et al.,

"KAIYOUGA-KU KOUZA i KAIYOU BUTSURIGAKU

I", University of Tokyo Press, 1974 (in

Japanese)

Capon, J., "Highresolution frequency

wave number spectrum analysis. Pro.

IEEE, Vol. 157, 1969, pp.1408-1418

Mitsuyasu, H. et al., "Observation of the

directional spectrum of Ocean Waves

using a cloverleaf buoy, Jour. Physical Oceanogr.. Vol. 5, 1978, pp.750-760 Wilson, B.W., "Numerical prediction of ocean waves in the North Atlantic for December 1959, Deut. Hydrogr. Zeit., Vol. 18, 1965

Sarpkaya, T., "Mechanics of wave forces on offshore structures", VON NOSTRAND

REINHOLD COMPANY, 1981

Phillips. O. M., "The equilibrium range in the spectrum of windgenerated waves,

Jour. Fluid Mccli., 4, 1958

Hasselmann, K, et al., "Measurement of

windwave growth

and swell decay during the Joint North Sea Wave Project

(JONS WAP) ", Deutsche Hydr. Zeit, Reihe

A(8°), No. 12, 1973

Goda, Y et al., "Computation of refracticn and diffraction of sea waves with Mit-suyasu's directional spectrum",Tech. Note Port and Harbor Res. Inst., No. 230, 1975, 45p. (in Japanese)

Goda, Y., "Random Sea and Design of

Maritime Structures", University of

Tokyo Press, 1985

Izumimiya, T. et al., "Field observation of wave field in the diffraction region", Proc. 33th Japanese Conf. Costal Engg., 1928, pp.129-133 (in Japanese)

Norweigian Petroleum Directorate,

"Guidelines for determination of load and load effect", January 1987

APPENDIX A

The author gives the

outline of 4 powers

law proposed by Toba2.

Eq. (3) can be written asthe following form.

H1132= A2gu *TH1,33

(A-1)

H113 and TH1/3 can be expressed as

H1132=16

f0s

(f) df

(A-2)

T113 1/fr

(A-3)

Assuming the similarity of the frequency spectra as 'oo S'df'

_J

S(f)df/S(f)fconstant

(A-4)

where

s'=s(í) /s(f2)

(AS)

f'f/f

(A-6)

Combining these equations the following rela-tion is derived.

S(f) =Bgu*fI)_I

(A-7)

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At-Sea Experiment of Floating Platform

POSEIDON"

At-sea experiment using the prototype floating platform "POSEIDON" has finished in summer of 1990. The main objective of this field measurement is to validate the design methodology of floating offshore structures based on the model experiments and on the theoretical approaches.

i

Floating Platform "POSEIDON"

The POSEIDON means Platform for Ocean Space Exploitation.

The structure of POSEIDON are shown in Fig.1. It consists of 12 legs with footings which support the upper structure.

The upper structure is mainly composed of the box-type girders around four sides. The instrumentation house is arranged on the upper deck for power supply and data aquisition.

Machinery Room

Principal Dimensions of POSEIDON

OIKENSIONS

ITEMS

Lenith overall Breadth overall height of cain structure

Draft

34.0 . 24.0 a

13.5 i

5.5 I

Distace between columns 10.0 i

Column diameter 2.0 i

(partially 2.5i)

Coloco height 8.5 u

Postini dia.ter 4.0 a Footing heIght 2.5 i

Data Acquisjton Room

1

Fig.1 Structure of POSEIDON

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2 Location of test area

The location of test area is about 3 km offshore Yura port, Tsuruoka City, Yamagata Prefecture, southwestern part of Japan Sea as shown in Fig.2.

The sea depth is about 41 m. We have constantly severe sea conditions due to the strong seasonal wind in winter.

The POSEIDON was constructed at Naruto, Tokushima Prefecture. After ari inclining and free decaying test were carried out at there, it was towed by a tag boat from Naruto port to Yura port, about 800 miles over 9 days, and installed in the test areaof Yura port in July, 1986.

The POSEIDIN was siackly moored by six chain lines as shown in Fig.3. The forward direction of the POSEIDON was WNW, which corresponds to the dominant direction of seasonal wind and waves in winter.

POSEIDON \ 3k, offshore JAPAN SEA \ \ 4, \ . \ . TURA PORT YAMAA1A Pref. o

Fig.2 Location test area

2

i

l5toni

,/

/

f

/

I,

/

l4tonI

i4tons\0Sz

/J //

-.

(

\/

/

//

14to&

/

\\

/

\1oton pOStIi0

---

_{_;}

N Fig3 rrengefleflt of chain lines 1

(11)

3 Measuring items and devices

The POSEIDON has many measuring items. They can be categorized as follows: Environmental conditions(wind,waves and current etc.)

Motions

Structure strength Mooring line tensions Corrosion and paints

Table 1. Measuring Items and Devices

3

ITEMS No. Devices and Remarks

Wind

Waves

Current

Temparature air

Temparature water surface Temparature bottom Humidity

Solar radiation

Temparature deck plate Temparature house wall Relative water level Impact pressure Wind pressure Acceleration Roll Pitch Yaw

Slow drift motion Mooring line tension Strain 2 3

i

1

i

i 3 2 5

i

1

i

6 8 12

Ultrasonic, 3 axes, 19.5m above W.L. Ultrasonic, 1 axes, lO.Om above W.L.

Ultrasonic, sea floor of 180 m offshore of POSEIDON line array

Impeller type, under 10 m below W.L.

Resistance temparature type, on the top of house Semiconductor type, at the footing

Semiconductor type, on the sea floor

Thin film capacitive type, on the top of house Thermopile type, on the top of the house

Resistance temparature device Resistance temparature device

Ultrasonic, at the centre column of offshore side Strain type, lm,3m and 6m above W.L.

on the center colunm of offshore side

Semiconductor sensor, difference of wind pressures on fore & aft side of upperstructure and

starboad & port side of upperstructure

servo type, surge, sway and heave (center, fore and aft) Vertical gyroscope

Vertical gyroscope Directional gyroscope

Ultrasonic, 2 transmitters on the footings and 3 receivers on the sea floor

Load cell type, strain gauge type Strain gauge installed indirectly

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The three ultrasonic wave probes were installed as a line array on the bottom of 180 m offshore of the POSEIDON. They were used to estimate the directional wave spectra. The measuring system of slow drift motion consists of two ultrasonic transmitters attached on the footings and three receivers on the sea floor.

4 Data Aquisition System

Fourty eight items are automatically recorded by a personal computer at every six hours. One record time was 34 minites and 8 seconds. The sampling interval was 0.5 sec. thus 4096 data are recorded for each items.

In order to record a long duration data, another recording system is also used. The system is that the lond duration measurement can be started by a command sent from the beach by means of a telemeter system. In this case, the sampling interval was 1.0 sec. The recorded data were store on boad into the hard disc of 40 MB.

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The list of the papers about the at-sea experluient

Hara Shohichi et.al; A Field Tests of Proto-Type Floating Offshore Structure POSEIDON. Proc

of OCEANS '89 (1989) at Washington

Ohmatsu Shigeo et.al; At-Sea Experiment of Floating Platform POSEIDON, Proc. of OMAE (1989) at Hague

Kitaniura fumitoshi et.al; At-Sea Experiment of Floating Offshore Structure POSEIDON, Proc. of Techno-Ocean '90 (1990) at Kobe

Ohkawa Yutaka et.al; A Field Measurement of Floating Platform POSEIDON, Proc. of IMSDC

(1991) at Kobe

Ohkawa Yutaka et al. ; Results of the Field Experiment of the Floating Offshore Platform POSEIDON, TJJNR 1991 Meeting (1991) at Tokyo

Ohniatsu Shigeo; At-Sea Experiment of Floating Offshore Structure - Environmental Conditions

responses of Structure - Proc. of Techno-Ocean '92 (1992) at Yokohama

Ohniatsu Shigeo et.al; Wind, Waves and Currents at the Test Field of Floating Platform

POSEIDON, Proc. of OMAE (1990) at Hague

Yoshimoto Hirofuini et.al; Characteristics of Directional Wave Spectra Measured at Japan Sea,

Proc. of OMAE (1992) at Calgary

Kato Shunji et.al; At-Sea Experiment of a Floating Offshore Structure Part 1. Wind Character-istics of the Test Field, Jour. of SNAJ Vol. 167 (1990) ( in Japanese and English )

Ando Sadao et. al; At-Sea Experiment of a Floating Offshore Structure Part 2. On the Distribu-tion of Temperature by Solar RadiaDistribu-tion for Experimental Structure, Jour. of SNAJ Vol. 167

(1990) (in Japanese )

Yoshiinoto Hirofumi et.al; At-Sea Experiment of a Floating Offshore Structure Part 3. Charac-teristics of Directional Wave Spectra at the Test Field, Jour. of SNAJ Vol.168 (1990) (in Japanese and English )

Yoshirnoto Hirofuni et.al; On the Statistical Properties of Waves Measured at the Japan Sea,

OCEANS' 93 (to be submitted

QIotions of structure>

Ohinatsu Shigeo et.al; Motion Responses in Directional Waves of Prototype Floating Platform

'POSEIDON, Proc. of OMAE (1992) at Galgary

Ohmatsu Shigeo et.al; At-Sea Experiment of a Floating Offshore Structure Part 4.Motion

Response in Directional Spectra Waves, Jour. of SNAJ Vol. 169 (1991) ( in Japanese )

Saitoh Masakatsu et.al; At-Sea Experiment of a Floating Offshore Structure Part 8. Analysis of free Decaying Test, Jour. of SNAJ Vol. 171 (1992) ( in Japanese )

Kato Shunji et.al; At-Sea Experiment of a Floating Offshore Structure Part 9. Time Domain Simulation and Statistical Predictions of Slo'* Drift Motion, Jour. of SNAJ Vol. 172 (1992)

(14)

Q,looring system>

Ohinatsu Shigeo et.aI; On the Tensions of Mooring Lines of Floating Platform POSEIDON. Proc. of OMAE (1991) at Stavanger

Oniata Sadao et.al; At-Sea Experiment of a Floating Offshore Structure Part 7. Characteristics of mooring line Tension, Jour. of SNAJ Vol.171 (1992) ( in Japanese )

Yago Kiyokazu et.al; At-Sea Experiment of a Floating Offshore Structure Part 5. Measurement of Stress Range Distribution for Fatigue Analysis, Jour. of SNAJ Vol. 170 (1991) (in Japanese

Hoshino Kunihiro et.al; At-Sea Experiment of a Floating Offshore Structure Part 6. Deforma tion and stress of Structure due to the Ununiformnity of the Temperature Distribution, Jour. of SNAJ Vol. 171 (1992) (in Japanese )

OMAE : Offshore mechanics and Arctic Engineering

IMSDC : International Marine System Design Conference

UJNR : United States/Japan Cooperative Programmi in Natural Resources