Ocean wave data analysis and ship dynamics

OCEAN WAVE DATA ANALYSIS AND SHIP

DYNAMICS

by A.E. Mynett and J.A. Keuning

Report nr. 876-P

June 1990

Symposium to Prof. R..E.D. Bishöppon.thè

Dynamics of Marine Vehicles and Structures

in Waves.

IJTAM- London

24-27 June 1990.

Deift University of Technology Ship Hydromechanics Laboratory Mekelweg 2

2628 CD Deift The Netherlands Phone 015 - 786882

OCEAN WAVE DATA ANALYSIS AND SHIP DYNAMICS

**

A.E. MYNETr and J.A. KEUNING

Wave loading and the dynamics of ships in ocean waves are important factors for structural design and optimal routing. Although computer programs are available to determine the wave fòrces and ship motion response, information on the input wave conditions is essential for the assessment-of the computational results With- the -advent of directional wave buoys, mathematical wave prediction models and oceanographic satellites this type of information is

becoming available. The multi-national LEWEX measurement campaign was set up to assess various prediction and measurement procedures. Results of the Dutch oceanic research vessel HN1MS Tydeman are presented together with laboratory experiments and numerical computations. Implications for on-board analysis procedures are outli-ned.

1. INTRODUCTION

Wave loading- and the motion response of ships- in ocean waves are- important factors for structura-1 design and optimal routing. Numeriôal compütations enable a detailed analysis of wave forces on various types of ship geometries over a ui-de range of input

conditions-. Also, computer programs- are available to optimise ship routing procedures given expected weather

conditions-.

in both cases, however, knowledge of

the actual and expected wave conditions is an essential factor in the assessment of the computational results. Quite often only limited information is

available -on the environmental

conditions that détermine the design or the optimal route. For example, if

atlas data based on ship's observations aré used, the emphasis is often on wave heights rather than wave periods and hardly any -- if at all -- information is available on wave directionality, in

particular in case of combined sea and swell conditions.

* _{Research and Development Division,}

DELFT HYDRAULCS, P.O. Box 177, 2600 NH Deift, The Netherlands

** Department of Maritime Technology.,

DELFT- UNIVERSITY OF TECHNOLOGY, P.O. Box 356, 2600 AS DeIft, The Netherlands

- On the other hand, it is- well known that the -incoming wave direction is an important parameter in ship motion response. If detailed information on the local wave conditions along the planned sai]ing route were available, the expected motion response

could be

computed' on-board to minimise extreme wave loads and motions or to optimise the proposed route.

With the advent of directional wave

buoys, mathematical wave prediction models and oceanographic -satellites this

type of information is becoming

available. The LEWEX (Labrador Extreme Wàve Experiment) campaign, involving muti-national participation, was- -set up

to assess various prediction and measurement procedures. Computational results of wind and wave conditions and ship response are compared with wind and

- wave measurements from airborné synthetic' -aperture radar, ocean surface measurements- from -directional wave buoys and actual motion measurements of marine vessels. The complete results- of the

measurement campaign are to be published this year.

-In this paper,, measurement results are presented of the Dutch oceanic

research vessel HN1MS Tydeman,

participating in the campaign. Also,

results of extensive laboratory

experiments carried out to measure the motion response of the model ship in

various directional sea states, are given. The measurement results are discussed and compared with numerical computations based on strip theory methods. Implications for ocean wave data collection and on-board analysis procedures are outlined.

2. SHIP DYNAMICS

The ship motion response due to wave excitation is investigated in three different ways. First, full scale motion measurements are considered as 'obtained during the LEWEX measurement campaign. Next, results of laboratory experiments 'are discussed. Finally,

computational resùlts are presented of the hydrodynamic loads and motion response.

2.1. Full Scale Motion Measurements The Labrador Extreme, Wave Experiment took place in march 1987 in the Labrädor Sea off the coast of Newfoundland. The measurement campaign was initiated and

coordinated by the NATO research groups RSG1 and RSG2.. Participants in

the experiment caine from Canada, France, Germany, The Netherlands, Norway, Spain

FIGURE '1

Body plan and ship charactéristics of the oceanic research vessel HN1MS Tydeman

and the USA. The objectives of the measurement campaign were threefold: - to check the results of mathematical

wave prediction models by measuring actual conditions on-site

- to compare the performance of a number of different ocean wave measuring

devices 'and analysis techniques with particular emphasis on directional wave properties

- to carry out full-scale ship motion measurements

The experiments were performed: from two ships while two airplanes, equipped with special radar equipment, participated 'in the wave measurements. One 'of the ships

involved in the LEWEX campaign was the Royal Netherlands Navy oceanographic

research 'vessel HN1MS' Tydeman. The main interest of the DELFT UNIVERSITY Ship Hydronautics' Group was in the full-scale ship motion measurements and thé '.comparison of'different types of wave

.'buoys., Especially interesting«was the

fact' that directional 'wave data were available during LEWEX from several 'directional wave buoys. On all earlier occasions but one, 'wave directional information' consisted only 'of visual estimates. Unfortunately, on' the one occasion that directional wave date were

'available the weather conditions were not particú'laly suitable for the tests.

The main characteristics añd the body plan of the HN1MS Tydeman are given in

Length over all

Length on the waterline '(CWL)

Maximum breadth Draught (CWL) Weight of displacement. (CWL) Maximum speed Service speed 90:.15 s 84,. 50 Zn 14.40 m 4.75 m 2977 tons 15 knots 12 knota

Figure 1. The ship was equipped with a

number of sensor in order to measure the

wave notion' response. Roll and pitch angles were measured using a vertical

gyroscope which also stabilised a

platform which

contained

three

servo-accelerometers which measured surge, sway and heave 'acceleratïons. These sensors were located in the gravimeter room around the centre of gravity of the ship. In this way the measured accelerations are virtually undistorted, i.e. contain hardly any contributions due to centripetal and rotational accelerations.

A total of 30 ship motion runs of 30

minutes each was performed from march, 1:4 until march 25. Unfòrtunately, the

weather conditions were rather light during most of the tests, with often very confused seas which were not ideal for scakeeping experiments. The tests were carried out, by following .one of the two preset patterns. The difficulty due to the confused seas was. that', when

starting a pattern, 'the mean wave direction was determined visually,. At some occasions this estimate proved to be wrong afterwards, when the results of the WAVEC and WAVESCAN diréctional wave buoys became available. This does not imply that the test runs are useless,

but stresses the importance of reliable information on wave direotionality. A complete set results of seakeeping experiments, obtaïned after spectral analysis of the measured time series,

has been reported by Corns (1'). Typical values for one particular day are

summarised in Table 1.

2.2. Laboratory Experiments

Further research on ship motion response in directional seas was carried out in the laboratory using a 1:30 scale model of the HN1MS Tydeman. A special measurement system, designed by the DELFT UNIVERSITY Ship Hydromechanics

Laboratory, enables independent

measurement of all modes of motion except yaw, which was kept f ixed to

maintain a constant heading with respect to the incoming waves. Details are given by Mynett et al. (2).

The .àhip model and' measurement system were installed 'in the 'DELFT HYDRAULICS Directional Wave Basin, as indiôated in Figure 2. The segmented wave generator consists of 80 elements with a total

length of' 26.5 m. Computer controlled wave generatIon. . enables direct specification' of the required wave

characteristics', ranging rom a single component. uni-directional monochromatic waveS to multi-directional seas including mixed sea-states of . combined sea . and

swell, conditions.

Computer-control led wave generati'on

also enäblös the generation of oblique waves over a wide range of angles. By positioning the ship model at. different

headings, the entire angular

distribution and motion response could be covered. A definition sketch of the angle of wave incidence is given in Figure 3.

Sample . results of measured and

computed heave transfer functions for oblique wave incidence at zero forward speed are presented in Figure .4. The

TABLE 1

Results of seakeeping experiments on 15 March 1987

Relative Speed Roll Roll Pitch Pitch Heave Heave Bow Bow Wave Wave

wavedi'r. vis. /WAV. ampi. 1f3 Tp. ampi. 1/3 Tp. ampi. 1/3' Tp. '.ampi. 1/3 Tp. ampi. 11.3 Tp.

(dog)' ' (kn.) (dog)' (s) (dog) (s) Cm) (s)' Cm) _{(a') (n)}

()

180/ 169 4.0 2.99 10.5 3.36 9.0 1.13 9.0 2.58 9.0 4.09 10.5

210/200 4.4 2.49 9.7 3.58 8.4 1.13 9.0 2.74 8.4 3.76 9.7

240/230 5.3 4.21 9.7 3.20 9.0 1.31 9.0 2.57 9.0 3.55 9.7

CONTROL ROOM S.S re DIRECTIONAL WAVE GENERATOR (80 SEGMENTS) WATE RDEPTH h-80 ¿Orn

verttcpl

reflecting- sidewall

bench

vertical

reflecting sidewall

FIGURE 2

The Directional Wave Basin at -DELFT

HYDRAULICS used in the experiments

FIGURE 3

Angle of wave incidence definition

results are both qualitatively and quantitatively in good agreement.

In-order to determine the motion response

in case of non-zero forward speed,

different computational methods were

used,, as d-iscùssed in the ñext section.

Lt ta 1.0 '0.5 O wave generator

--H

0n30

ß00

90 ß.450

a -

angle of wave incidence

O +300 _300 O 0 +300 O 300 - o 0.7 0.9 1.0 1.2 Tt7 -FIGURE 4

Measured and computed heave transfer functions for oblique wave incidence

Considerable effort was spent on trying to verify the principle of superposition by measuring the, motion response in waves from different directions either separately or in.

combinatïon. In- analogy with the frequency interactions of two wave components due to the free surface non-lïnearity, investigations were

carried out to detect possible

directional interactions. Unfortunately, the accuracy of wave generation, measurement, and: analysis procedures did not allow to draw f Im- conclusions. However, the ::- experimental results certainly did -not indicate that linear sperposition _{is -not a- válid principle}

for engineering applications of

directional wave modelling.

2.3. Numerical--Computations

All transfer functions for the motion response presented above refer to the situation of zero forward speed. In

order to account. for the forward- motion of the- vessel, the strip theory

formulation of Gerritsffla and Beukeiman

'I-ti _E 0 gravel-ship usoaau remen t area

X

(.3) was used. Since the main emphasis

in _{the present investigations, is on the}

heave and pitch motion, the only hydrodynamic _{problem is associated with} forced oscillation in the vertical

plane., The SEAWAY program, developped 'byJourne '(4), was used to solve this problem and' determine the resulting motion response. Following the Ordinary Strip theory _{Method, SEAWAY determineS} the sectional values of the addéd sass

and damping coefficients from

two-dimensional computations. in' this

way, no hydrodynamic interactions are accounted for,, effectively assuming infinitely long cylinders of constant crbss-sectjoña'i value.

On. the other hand, for zero forward

speed, _{three-dimensional radiation and}

diffraction programs capable. of dealing

with arbitrary geometries, are

available., even on Personal computers. One such program package is WA}i'IT, developped by Newman and Sclravounos (5). This progran was used'. by Keuning and:

Adegeest '(6') in combination _{with a}

standard strip theory formulation, thüs combining 3D-effects and forward speed.

The computational procedure l's as

follows.

The input panel distribution for

WAMIT' is generated in such a way that the required number (20, say) of cross-sectional strips can directly be obtained from the panel distribútjon. This implies that the areas and normal vectors of each panel have to be calculated and stored. After solving the fUIl three-dimensional _hydrodynamic problem, the sectional values are readily obtained by integrating the computed pressures along the particular sections, using the. directional cosines and areas of the panels. In this way, results of three-dimensional d'iff]'actjon theory; are combined with classical strip theory. Conputationaj. results of thé quasi three-dimensional approach are presented hereafter togetter with

results based on conventional

two-dimensional' computations. The.

hydrodynamic reaction forces,. wave exciting forces and motion transfer functions are discuséed separately..,

The sectioñal values for' the heave

added mass and damping coeffidients _are obtained from the. radiation problem in the vertical plane:. Both. in the 2D and

in the 3D .computations a total of 20 cross. sections were distinguished. 'The

results are presented in, Figure '5 for'

three different frequencies,

corresponding to 'a wave length to ship length ratio of 3.O,, 1.5 and. 0.3 resp.

4-t' o I I I SEAWAY WOMIT + HYCOMO Fn _0.34

/

/

/

/

I i I I. I I I

Fn = 0.34

L5 STATION NR. = 1.5 I I Il 6 8 10 12 STATION NR

--ADDED MASS FIGURE 5.

Computed. heave added nass and' damping

coefficients along, the ship's. length

The results indicate that only for the longer wave length., the différence between 'the standard and' the extended

SEAWAY DAMPING

- - WAMIT + IIYCOMO

approach can C)early be Óbserved, as was to be expected. The other results are

in quite close agreement.

In addition to the hydrodynamjc reaction forces, the wave exciting forces can be determined, following the standard strip theory formulation. The computed _{vertical wave force and moment} are presented in Figure 6, again distinguishing between a 2D and 3D

approach. '0

f

1.0 o

f

2.0 I.0 o SEAWAY WAIIOT + IIYCOMO 0.5 FIGURE 6.

Computed vertical wave fórce and moment

It is readily observed that the

differences between the two methods is

only marginal over the entire frequency

range. This need not be surprising if

it is kept in mind that these forces not

only include the effects of the

hydrodynamic contributions but also the undisturbed Froude-Xrilov components. Apparently these components are dominant

factors in the formulation.

C '0 1.0 0.5 1.0 0.5 SEAWAY WAM1T + HYCOMO Fn 0.34 SEAWAY - - - WÄHlT + HYCOMO

Fn = 0.34

[11800

- - -- r'0

1500 u lZ0 ñ:7

-FIGURE 7.

Computed heave and pitch transfer functions for different angles of wave incidence HEAVE p 90 " PITCH 1.0 15 o 0.5 1.0 15 0.5 1.0 1.5

Having obtained the hydrodynamic reaction and exciting forces, the

equations of ¡notion can be solved

provided all other ship's

cahracteristics are known. The computed transfer functions for both heave and pitch motion are presented in Figure 7

for different wave headings.

Again, the curves indicate only minor

differences between the two

computational methods for any of the

investigated directions. However, it is also observed that the effect of wave heading is much more pronounced than the effect of the computational method. This implies that the motion response of the vessel can be determined within a

reasonable accuracy, provided that the directional information of the incoming waves

j:

known., This subject is dealt with in the next chapter.

3. OCEAN WAVE DATA ANALYSIS

The characteristic wave parameters necessary for the design and operation of marine structures, can be obtained from ocean wave data analysis. After introducing a common mathematical description of the sea surface, both direct measurement techniques and remote sensing techniques are discussed. Particular attention is paid to the

sensitivity and accuracy of determining the various parameters.

3.1. Sea Surface Description

A well established way to describe irregular wind generated surface waves

is based on the assumption of linear superposition. The sea-state is assumed to be composed of a great number of individual wave components, each having

a particüÏar wave height, périod,

direction of propagation and' phase angle. A detailed 'description of stochastic analysis techniques and probabilistic prediction methods for

random seas can be found' in Ochi (7). A

common representation of the

two-dimensional energy density spectrum

is given 'by

Sq(w,O) - Sw) D(w,9) (3.1).

where S denotes

the

one-dimensional

energy

density

spectrum

and

D is' the

directional

distribution

function,

satisfyIng the condition

JD(ò,'9)dO - i

(3.2:)

Although a number of distribution functions can be found in the literature (7), the most common expression is the so-called cos-2s model introduced' by Longuet-Higgins (8),' given by

D(w6) -

D0()cos2''(")(

_lei

w

(3.3)

where

the

spreading

parameter

s

determines the width of the

distribution

around the; mean approach' direction while

the

normalization factor -follows from

condition (3.2).

Thedirectional

wave parameters are disòussed hereafter in

some detail.,

3.2. 'Direct. Measurement Techniques

One class of wave analysis procedüres

is based on cross-épectral analysis of

three mutually orthogonal point

measurements. In case of a pitch and roil buoy, time serias of surface elevation and wave slopes are used. in

laboratory

situations,

the

analysis

procedure

is often based on the surface

elevation and two perpendicular

orbital

velocity components. In both cases,

however, the analysis procedures are

quite

similar,.

Since

results

of

laboratory experiments wil be presented

the

formulation

based

on

velocity

components will be given here. From cross-spectral analysis, the following

results are obtained'

spiri

sxx

cuy

(3.4)

s _Qxy

where S denotes the' auto-spectrum while Cand Q represent the' co-incident the

quadrature

component

of

the

cross-spectrum. Ail 'spectra are functions of wave frequency. In order to derive the directional parameters from the cross-spectral analysis

results,

the

directional distribution

function

is

expanded

into,

a

Fourier

a0, N

- - +

_n1

w(a

cosnO + b

sinn0)

(3.5)'

where the number of expansion terms, N, depends on the number of available measurement signals and wn are weighting functions to account, for the series truncation. For point measurements involving three orthogonal components,

sch as

for a pitch and roll 'buoy, the number of expansion terms is, limited to

N2.. The particular expressions for the Fourier coefficients in terms of the

auto-' and cross-spectral components are given by Longuet-Higgins (8).

For practical applications', the

directional wave parameters of. the

cos-2s model can be expressed in terms of the expansion coefficients, which yields

b1

- tan

(, - )

_a1

s

(3.6)

J a12 + b1

_(3.7')

a0

- fa12 +

b1T

for the princpal approach direction and'

the spreading. parameter resp. Yet,

another way of, representing the

directional spreading is by introducing the standard-dev1aton of the cos-2s model, in analogy with the Gausian distribution. . The relation with the

Fourier expansion coefficients is

0 0.1 0.2 '0.3 0.4 0.5

Frequency in' 'Hz

FIGURE a.

Sample result of directional buoy output

Having 'established' the appropriate

parameters .. describing the

.two-dimensi'onal-energy "density spectrum,

-the most important wave 'characteristics can be . derived from the first few moments of, the -' spectrum. .The most important wave' ''parameters used in

engineering analyses are discussed separately hereafter with particular emphasis on aspects of accuracy and sensitivity.

3.2.1. Wave Height änd Period

An estimate of the significant wave

height is directly obtained by

integrating the two-dimensional energy

0.1 0.2 0.3 0.4 0.5 Frequency in Hz e e e 00 360 80

s

. 270 60 k '8 00 e 8 . 180 40 ..4a

J

u

e

k 00 :8 1 -4 8, e 90 20 e e k U) 0'

û -

J 2(1

-

J

aj2

+ b s (:3.8')

which represents the angular spreading (often presented in degrees) around the mean 'direction. Again, all parameters are functions of the wave frequency. Some typical results obtained from directional' buoy measurements are shown. in Figure

8.

density spectrum with respect to both

frequency and dIrection. Also,

estimates of

the meañ

zero-crossing period are readily derived from the zero-th, first and second moment by

standard analysis procedures:.

Indications of the accuracy and

variation of these parameters

can be

obtained by comparing meaéurement results of a number of (different types

of.) buoys. Typical variations in wave height and period are on the order of 10 - 15 percent.

3.2.2. Mean Direction

In order to determine the directional wave characteristics, it is necessary to make a-priori assumptions On. the shape

of the directional distribution

function4 If., for example, the

previously introduced cos-2s model is assumed, only one mean wave direction

will be estimated for each wave

frequency. Hówever, if the actual.

sea-state is composed of mültiple wave fields (e.g. in case of combined sea and swell conditions coming from two distinctly different directions) the analysis results will not distinguish between the different directions and only provide average values for each

frequency component.

Extensions of the analysis techniques to multi-modal directional wave spectra are discussed by Van Heteren (9), again based on tri-orthogonal measurements. By introducing a double cos-2s model it

becomes possible to estimate. the significant wave height, dominant wave period and direction of the' sea and swell components separately.. Still., for

different types of sea-states different assumptions have to be made. In this respect, the reliability of the analysis results are directly related to the correctness of the assumptions and the knowledge of the underlying physical processes. Depending on the' type of sàa-state, the accuracy may range f ròm

10 - 25 degrees.

3.2.3. Directional Spreading in addition to the mean wave direction, the directional spreading arouñd the mean is of interest, In

particular' in case of wave loading and motion response of marine structures. However, it can be observed that the

spreading parameter sig-o given by (4.7)

iS

sensiIve to

variations in Fourier coefficients, in particular in case of

long-crested seas. In fact, extensive laboratory experiments carried out In

the DELFT HYDRAULICS directional wave basin indicated that it is quite difficult to detect long-crested waves from tri-orthogonal measurements, even under controlled laboratory conditions. A comparison of measured and generated

standard' deviations of directional

distributions is summarised in Figure' .9..

A detailed disáüss±on is given 'by Sand and Mynett (10). 40 o w E 30 w z .20 50 10

2

/

THEORY MEASUREMENTS & 30 40 50 60 00 GENERATED FIGURE 9'

Generated and measured standard

deviations in laboratory experiments'

Yet another interesting result oñ the behaviour of the directional spreading parameter was obtained: from directional

wave measurements in laboratory

generated, long- crested, irregular

seas. . Although in this. 'case the

,standard deviation 'should "theoretically

'be zero, the measured results were found to vary' between typically lO degrees around the spectral peak to 'about 20

-30'. degrees at the lower and hIgher frequencies. In order to eliminate

possible distortions due to

instrumentation and' measurement noise,

numerical experiments were carried out Time signals for the surface elevation

and the orbital velocity components in long-crested irregular Seas were generated numerically based on linear

wave theory. These simulated

signals for the cross-spectral analysis procedure. The results are presented in

Figure 10,. It is interesting to observe

that even in this situation the

calculations suggest a directional spreading ranging from about 3 degrees around the spectral peak to sorne 10

degrees at the lower and higher

frequency components. 0.02 STANDARD . DEVIATION 60

-

HEAN DIRECTION J

30-1.00

2.00'

FREQUENCY (Hz) FIGURE lO.

Directional analysis results of a

numerically simulated long-crested sea-state with oblique wave incidence

Based on the results of experimental and numerical simulations it

can be

concluded that the spreading parameter

is quite sensitive away from the

spectral peak. This need not be

surprising if it is kept in mind that the analysis procedure is based on

cross-spectral techniques of

tri-orthogonal measurements. In these.

situations the signal-to-noise ratios of the measurement signals and the accuracy and stability of the instrumentatiOn are

of consderabie importance. In

particular in the ocean environment the conditions are often quite unfavourable. for sensitive measurement equipment. Unfortunately this implies that hardly any if at all information can be obtainedon the actual values of the spreading parameter other than some typical 20 30 degrees around the spectral peak that often enòountered.

3.3. Remote Sensing Techniques

In addition to direct measurement techniques of the ocean surface, remote sensing techniques enable wave data cóllection at, arbitrary locations throughout the ocean environment. Results of..earljer..sateflites like the SEASATshowed that .useful: information can 'be obtained on windfields near the 'ocean surface by means of a microwave

.scatterométer., In 1990. the. European Space _Agency (ESA) will launch the ERS-1, 'thefïrst EuropeanRemote Sensing satellite. On. board.thespacecraft are

a number of instruments for

oceanographic measurements that are briefly discussed hereafter in relation to wind wave generation. More specific details can be found in the. brochure by ESA (11).

.3.3.1. Wind Scatterometer

One of the important parameters in air-sea interaction is the wind shear stress at the ocean surface. The wind scatterometer of the ERS-1 uses three sideways looking radar ' beams at

different angles, which provide

measurements of radar 'backscatter from

the sea surface. From these

measurements, the surface wind vector can be calculated in terms of speed and direction. In order to verify the computational . algorithm and to. 'improve

the understanding of 'the

'physical-processes involved in' the scattering of radar waves 'by the' 'wind-driven gravity waves, the mutual. ' interaction was studied in the Wind Wave Flume of DELFT 'HYDRAULICS. The results are preseñtéd in the progress report on the VIERS-'l

project by Van Halsema et al. (12). it

is anticipated» that 'the wind speed can be measured in the range between 4 24

m/s wïth an accuracy of 2 In/s or 10% of the measured valúe. The wind direction can be. determined over a range from O 360 degrees with an, accuracy of

_+1-

20

3.3.2. Radar Altimeter

The Radar Altimeter is a nadir pointing pulse radar designed to measure echoes from ocean surfaces. It will be

used to measure the sea-surface

elevation, wind speed and wave height.

The significant wave height is

determined from the slope of the leading edge of the return wave form. The wind speed over sea surfaces is derived from the power level of the return signal.

On board processing of the return echoes from sea sürfaces will typically yield one-second mean values. Quasi real-time estimates of wind and wave parameters are therefore possible. Altitudes can be. measured in the range between 745

-825 km with an accuracy of 10 cm.

SIgnificant wave .heights can be obtained ranging from 1 - 20 n with an accuracy of 0.5 m or lO% of the measured value.

Sinôe most wave models use wind field data as primary Input parameter, global measUrements of wind speed and direction may significantly improve forecasts of sea conditions. In particular, data assimilation techniques based on actual observations .can be used to correct numerical simulations of wind wave generation. An algorithm for the assimilation of wave- and wind data in the third generation numerical wave prediction model WAN is discussed by De Valk and Calkoen (13).' The objective.is to make effective use of data from the ERS-1 to correct errors in the wind fields driving the numerical wave simulations.

3.13.3. Synthetic Aperture: Radar One of the components of the Active Microwave instrument CANI.) on board the

ERS-1 is a synthetic aperture radar. When operated in wave mode the SAR provides small (5 kin x 5 kin) images at intervals of. 200 km along the track. It

provides a global, sampling of wave

spectra suitable for the daily

measurements of the wave lengths and directions of the main ocean swell wave systems. Sample size and sample rate have been chosen to give useful global data on ocean waves, but kept smaÏl

enough to facilitate rapid data

processing. The images show differences

in radar backscatter from the sea.

surface which are related to the dominant wave lengths and directions of the wave systems. Automatic processing is carried out to derive information on the wave spectra. The parameter range for the wave direction is between

o -

180 degrees with an accuracy of.

+1-

20 degr. Wave lengths ôan be determined in the range between 100 -1000 n, having an accuracy, of +/- 25 percent.

In summary, remote sensing techniques are capable of providing valuable information on characteristic wind and wave parameters. However, the accuracy

is at most about 10% for wind speed and wave height, and between +/- 20 degrees for wind and wave direction. Again, no reliable information - if. at all - is

available on the directional

distribution of the 'wave fields.

4. CONCLUSIONS

In 'order to determiner the dynamics of

¡

- ships in ocean waves, the principle of superposition is often aasumed, in

particular for. the 'motion- response lin highly irregular seas.. In analogy with control system theory,. a "wave input

-ship transfer -- .motion response" modél is quite commonly used.

Considerable effort has been spent over a great number of years. to establish the properties of ship transfer functions in great detail.

In general, both measurements' and computations indicate quite good agreement. In the present paper results

are presented of full scale

measurements, laboratory experiments and

numerical computations. Special

attention is paid 'to the effect of incoming wave direction on the ship motion response. It was observed that

the difference between either

computational or experimental methods is less dominant than 'the effect of the wave direction itself. Hence, it is important to properly account for the directional input in order to be 'able to compute the correct motion response.

Unfortunately, detailed knowledge of oceanwave conditions is not readily available. . Although .directional wave

buoys have been developped . to measure the sea-state . parameters on-site, they are only limited in number

and not

operational except at a few locations. Moreover, in order to derive directional wave information from these buoy measurements, assumptions have to be made regarding the properties of the

wave field' itself. It has been shown here that both the mean direction and the directional distribution around the

mean are sensitive to these assumptions. in fact, _{indications are that}

directional buoys may be useful to

determine mean wave directions, _{but do} not _{provide relevant information on the} directional spreading.

Numerical wave prediction models are operational on a real-time basis for the North Sea and the North Atlantic. In

principle, these models provide

information on the distribution of the

wave energy over frequency and

directional bands. In order to derive the relevant wave parameters, however, similar assumptions have to be made as

in the case of directional buoy

measurements. Also, if buoy

measurements are used to validate numerical, models, only limited means are availablé to verify the directional information. The same applies to remote sensing techniques, whiòh are becoming more and more available. In this case

the interpretation ôf radar signals in relation to the required ocean: wave

parameters is still subject to

cnsiderable research.

in summary, for a particular ship,

the motion transfer functions can be determined within a reasonable accuracy. This information could readily be stored

in an on-board omputer. Also,

information on the ocean wave conditions can be obtained from either wave buoys,

real-time wave prediction models or even

satellites. However, a careful

interpretation of the predicted ship motion response is required, in view of the limitations in determining the input wave parameters.

ACKNOWLEDGEMENTS

Measurement results on the motion response of the Royal Netherlands Navy oceanic research vessel HN1MS Tydeman, participating in the LEWEX campaign,

were provided by Mr.. J. Ooms. The

laboratory wave experiments including validation, of the measurement procedures were performed by Mr. J. Bosma. Special thanks are due to Mr. J.M.J. Journee and Mr. L.J.M. Adegeest for carrying out the SEAWAY and WANIT computations. The assisteñce of Mr. P.W. de Heer in

preparing the figures and the layout of the final paper is greatly appreciated.

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