The wavescan second generation directional wave buoy

lb. V.

cheepsbouwKunGe

2

jULi

1988

(ii,r prest.d at the

sanoigy Intaatj

Technsche HogschooI

Ciar

3riita%, U.K., Nar 19U)

De1fL

ARCHF

THE WAVESCAN SECONDGENERATION DIRECTIONAL WAVE BUOY

Stephen F. Barstow

Oceanographic Center (OSS). Trondheirn, Norway

Gird Ueland

Continental Shelf and Petroleum Research. Institute Ltd (IKU),

Trondheim, Norway

BjGrn A. Fossum

SEATEX A/S. Trondheirn, Norway

ABSAC

This paper describes the new ml.tipurpose data buoy VESCAN specially designed for directional wave uasurementa. The overall objective _has been to produce a second generation high capability metocean data buoy, with full in situ processing, real time telemetry and onshore result

presentation. One of the main goals was to design a buoy hull with the wave following capability needed to accurately measure wave slope whilst at the same time retaining the stability to operate and collect meteorological data. under the extren conditions the buoy is likely to

meet It is this aspect of the design that we concentrate on in this paper. However, it was seen 6f equal inçortance that the buoy should be conçact and modular in design for ease of operation and maintenance, and should take full advantage of recent microprocessor technology for

real time data processing, system monitoring and event controlled _data collection. The buoy system should also be based on the ARGOS satellite system for world wide surveillance _{and data transmission and have a} user-friendly data presentation. Finally, the cost of the system should be reasonable conared to other systems presently available.

The present paper puts the present design into perspective by briefly reviewing the advantages and disadvantages of the various buoy hulls whidh have been ençloyed for collection of metocean data. We then proceed by discussing the stability and dynamic response of the final design followed by _a discussion of results from a field test intercomparison during which a prototype buoy was deployed for several weeks offshore mid-Norway. The VESC?N system functions and the

directional wave analysis are sunmarised in the Appendices.

1.

INTRODUCI'ION

VESCAN is a second generation metocean data buoy specially designed for directional wave spectrt measurements. It is an integral part of the SEASCAN real time data collection and presentation system, which is

015/2/2/Els

2

the result of a 3 year R & D prOject at _{The Continental Shelf Institute} (IJ) in Trondheith. _{The overall objective of this project has been to} expand the capabilities of medit size data collection buoys _{to include} full, in-situ processing of directional _{wave and meteorological}

para-meters and to tranit the _{results to the user in real time by}

the Argos satellite system. _{However, equally iortant was that the}

buoy should have a ixdular

and ccact design for

_{ease of operation and}

maintenance, be capable of event controlled data collection and finally to have a. reasonable, cost without sacrificing reliability.

I1 deployed its first buoy with wave direction measurement _capability in 1980. Since then oceanographic arid _{meteorological measurements have} been carried out with several _{buoys of this type, here referred to}

as (XAS-buoys, totalling about 10 buoy-years. _{Measurements have been} carried out. at. a nunher of _{locations fr the tropics to the}

high arctic. Operational experience had shown that to obtain acceptable

data recovery by iroving instrntation

_{reliability, an intensive}

maintenance and inspection program _{was needed. This lead naturally' to} high operational costs, _{especially when coupled with the logistic}

problems of operating at offshore locations which were far from the coast and widely sparated geographically. _{The. need to develop a tre} reasonably priced cost-effective system therefore rapidly became evident.

The increasing requirement from the offshore industry for the acqui-sition of wave _{directionality data alified this need for new data}

collection equipnt. Even though the analytic methods for obtaining directional wave data from wave slope following buoys was well developed, it was evident' that the lack of adequate and reliable

in.str.ntation

was a limiting factor. With this background IKEJ in 1981/82 forilated a project with the aim to design a second generation data buoy system to meet these requirements.

The objectives for the develoent project were to design, build and test a buoy based data collection system for directional wave data acquisition and presentation within a period of 2 years, based on the following criteria:

015/2/3/E].s

Re&cti.on of the overall cost of threctj.ona]. wave _data acquisition and presentation.

Increase in the _{flexibility and ease of operation}

together with extension of the unattended period at sea.

Irovemant of data recover'by _performance

nitoring.

- Enhancement of the _potential

use of the buoy system by

providing a set _{of metocean data together with directional} wave data in real time.

Inclusion of facilities for event controlled data collection. Provision of a system _{capable Of opeçating on a worldwide} basis.

In order to meet these objectives the project was divided _{into logical} sub-units corresponding to the main develoçznent efforts:

Buoy Null Design.

-. _{In-situ Processing and Control Software.}

- Electronics for the Data Acquisition _Nodule. - Real-tima Monitoring and Data _{Presentation.}

- Enhanced Storage and Data

_Cojnication.

In this paper we concentrate on the buoy hull and a first assessment of the overall measuring performance of _{the buoy system. we first discuss} the various buoy hull designs which were considered _{at the. project} outset and si.arise their _{strengths nd weaknesses based on a brief} literature survey, we then proceed

by

discussing the .thods used in

designing the final buoy hull. Based on cOnuter _{siilat-ions and a}

comprehensive series of wave tank _{tests we s1.mmarise the response. and} stability characteristics of the buoy system.

The _{VESCA1J prototype was deployed}

on Haltenbanken for its first full scale sea trial in Autuim 1984. _{One of the first wave directional buoys} extensively used by IKU, QCAS.!492 (manufactured

_by

_{Bergen Ocean Data),}

4

was also moored nearby. The results of this field

trial interconcarison are presented here and are used to _{qualify the}

fulfillmant of the design objectives. Finally, although _{lack of space does}

not allow us to describe in detail the advanced electronics fQrming the backbone of the system, we do give a brief overview of _{the basic VESCN}

functions in Appendix 2.

2.

DIRECiL

_{IVE BtY5; A BR.tEF H STOR}

_AL

_PSPECTIVE

A n'.er of generic buoy hull, shapes have been suggested with the view to collecting metocean data. These _{range fr the}

lOm monster buoys

designed

_{for long term unattended operation, to small 2 utre}

research buoys designed for short _{term deployints from research ships.}

The first real effort _{towards an wxferst.anding of the}

total buoy system

dynamics was

inde uier the

National

Data

_{Buoy Developent Project} which started in the United States in _{1967, later coordinated by the}

AA Data Buoy Office (0). airing _{the early 1970s the NDSO was fore-.} most in wave tank testing of various types of data buoys and the

develont of

mathematical models from knowledge carried over from

ship theory. This rk has _been well. doci.nnted in a series of publications. Kaplan et al (1972) _{and later Hoffman et al (1973) report} on the concarison of mathematical udel results _{with model wave tank.} tests of a ntunber of generic buoy shapes including discus, boat, catamaran and spar types, the latter report being an excellent state-of-the-art review.

Marechal (1973) tested three generic shapes (Fig. 2.1) which have later been applied for the maasuremant of wave direction. One was _{the discus} solution used for the DEl and )B monster buoys, a number of research buoys such as the original

los

pitch-and-roll buoy (Longuet-Higgins _et a.l,1963) and the Colombia University 1.5 m discus

_buoy

(Stewart,1977).

It is perhaps riot too, _{unfair to state that we can sum up the problems}

of a discus type buoy by the fact that the Colombia _{University buoy was} equipped with radio antenna on both faces of the disc, one piercing downwards into the water I Other directional _{wave systems, apart from} E and the DB-series, which have been developed are of much smaller size (typically 2.5 m diamater).

015/2/S/E1S

L)

Disk

Disk w/IglOo Disk wIC'wwgflt

Figure 2.1. The tIree generic buoy hull shape tested by Marechal

(1973) in

breaking waes

ai later adopted to asure directional wave spectra.

The response characteristics of the three hull types used for

directional nasurements may be st rised as follOws:

Discus:

A free

floating disttth buoy has near perfect wave

following ability up to a frequency determined by the buoy's diauter (the buoy will tend to begin to "go through" waves as the wavelength approaches the buoy diameter). However, the stability is basically poor, but this may be içcoved by enloying specially

designed noring systems which in turn can adversely affect dyna.ic response. Such systems may only then be eloyed to collect a full range of mateorological paraters.

Disc with counterweight: Typical

2.5 diater buoys which

have been used in Norway are of this type. aid sho good wave following ability up to about 0.3Hz frçm which frequency the buoys show enhanced pitch and roIl. The pitch/roll eigenfrequency for OPAS-492 is at 0.36Hz. This seems to be satisfactorily corrected for by

assing that the buoy behaves like a foçced linear reSonator in response to the surface wave slope (Barstow and Krogstad, 1984), thus giving directional wave information up to approX. 0.5 Hz where again buoy diameter is the limiting factor. Stability is good allowing mateorological data also to be collected.

Disc with igloo: Pynamic response is presumably good (no _published in.formation apart from so-'called check ratios which, _{as we show}

in

Appendix 1, give an indication of amplitude response - Ezraty et al., 1981, 1983; Pitt et al., 1985). Meteorological data collection may be difficult due to stability problems.

3.

BWY

_{HULL DESI; S3ILITY A}

_{NIC RESPJSE}

The disc/counterweight generic buoy _{hull type used for the ODAS-buoys} was also chosen for the second generation buoys as it was considered to be the st promising Solution

_{for a clete}

_{tocean buoy system,}

i. e one that would allow both _{teoro1ogia1 and directional wave data}

to be collected. _{Based on this choice the aim was to design a new buoy} huLl with sufficient righting nt in the extre breaking waves

likely to occur in its applicatian areas. In this respect experiences

with the CAS type design, with its _{proven stability, could be used} In addition, it was seen as an added advantage if the natural pitch frequency could at the sa _{tims be increased over O..4 Hz in order to}

reve the resonance fr the major ocean wave frequency range.

Although a good deal has been written about the resonse àf the larger discus buoys, very little has so far been published on the response characteristics and other design considerations for the smaller buoy systems. It is clear that for all directional systems there has been made soma choice giving a compromise solution between stability and dynamic response.

. approaches re made towards the design objectives apart from assessing experiences from the ODAS buoys. First, computer simulations ware made using the

AA Data

Buoy Office's siimilation dél of the total system

buoyAoring

configuration for various design

alternatives., Although useful experience was qained from this exercise, it was not easy to relate results directly to physical values. Secordly, a comprehensive series of model wave tank tests (Barstow, 1984) were run to test the theory and quantify differences between the various alternatives. The following variables were considered in the design optimisation:

- Buoy hull shape

- Length of understructure - Counterweight

- Mooring type

- Mooring attachnt point

The design proceeded by intercoaring response and stability of three possible buoy hull designs, all of the _{disc/counterweight type. The} dels were fabricated to a 1:20 scale. A del of the ODAS 492 buoy was used as a control. The following tests _{were. carried out ui a towing} tank at the Norwegian Hydrodynamic Laboratories

_{() in Trondheim:}

- _{Determination of natural. frequencies in pitch and heave in still} water

Dynamic response in reilar waves covering a. wide raxige of frequencies and at two wave alithdes corresponding _{to '1/2 and} 2/3 of the breaking height for the given frequency.

- Dynamic response in irregular waves. - Stability in breaking waves.

- Towing test in still water.

Al]. tests were repeated for different combinations of understructure length/counterweight and mooring _{attachment 'point. TWO types Of} mooring were also tested the standard mooring (Fig. 3.1) and a vertically pretensioned mooring solution. Measured

paraterS

included wave, height and period and the buoy's heave, pitch arid. surge and mooring loads.

Response alitude operators, which are _{rton-dimensional paratCrs} specifying the ratio of model to real values of pitch and surge were computed for all tests and interconcared.

Oi.5,'2/7/E1S

015,'2/8/EIS

Figure 3.1 The cVESCPIN buoy and its standard oririg system.

Based on the response and stability characteristics of the three

dels and the god agreennt of the

_{ODAS-492 n*del}

response

with

full scale asLirents, a prototype de1 (closely resembling the full scale prototype depicted in Fig.

3.2) was

manufactured. It was subseiently Subjected to a similar series of tests to those described, above, this time in a larger towing tank at NHL with emphasis on dyiamic response iii regular waves and stability in breaking waves Both pliuging and spilling breakers up to ii. m (full Scale) were used. This may appear soWhat low, but an analysis of wave records carried out by Kjeldsen and Myrhaug (1978) had shown.that only waves in the range up

to 13 m have extreu _{steepUess racteristjcs. These tests showed that} the prototype udel fulfilled both the stability and dynamic response requ.irents set at

the

outset. _{The next stage was}

the manufact,.re of

the first prototype

buoy

based on

this

_{de1, to test}

it out in

_the real environznt, compare with the ixdel test results and, if

necessary, adjust. sô of the design paratérs. The field testS are described in. the following section.

015,'2/9/Els METEOROLOGICAL

-9-SENSORS ARGOS ANTENNA MOORING ATTACHMENT POINT COUNTERWEIGHT

Figure 3.2 The final WAV CAN design.

Buoy _650kg

015,'2/l0,t].s

10

-4. _{FID TESTI1'}

The main irpose of the _{field tests were: to assess the}

operationa].

characteristics of

the

system; to get a first impression on. system durability and to uncover possible weak _{spots in the system; and}

to obtain a first assesnt _{on PVESCAN' s ability to measure,}

and also compute in real time, the _{direction4.wave spectn.}

We have tried to meet the latter objective by assessing _{the physical realism of}

the

directions.], data based. on _{meteorological and geographical}

conditions, by internal check _{parameters and by intercomparing}

the VESCAN data with data fr _{the cAS-492 buoy}

ored neirby. The two

_{buoys crise}

different electronics, _{compasses and wind sensors. Although}

VESCAN should have somewhat better wave following ability than _{AS-492 it was} not expected that _{this would have a large}

effect on the _directional

wave spectn _{(see Appendix 1).}

4.1, _{Prototype tests}

The first _{VESQN prototype was completed during}

Autunri 1984. _{After a}

series of function tests in the fjord off Trondheim, _{the buoy was}

deployed

on 18th October approximately _{150 km off the Norwegian}

coast on Haltenbanken (Fig. 4.1). The water depth at

the

_{deployment area is}

about 270. metres ,and _{as may be seen from the map, fetch}

lim.itations

will be strong for a _{sector of width arotnd 200°, whilst outside}

this sector the ocean stretches _{south westwards out into the Atlantic}

and northwards up into the Barents Sea. The location is, therefore, situated in one of

the ust exposed areas of the

_{Norwegian Shelf.}

Data

recorded by the ODAZ-buoys during 1980 to 1985 in this area has

produced significant wave heights up to 14 m and single waves up to 24

m. The. area is also exposed to _{energetic long period swell from the} Atlantic. Currents in the area are generally small with maximi.wi near surface current events (60,'s) associated with _{inertial oscillations.} No current measurements were available at the time of the field trial.

ThIVESCAN was posit-ioned on Ha].tenbanken for two _{nth long deployments}

during the

1984-85 winter. _{Operational experiences have been good with} over 95% wave data return. Both buoys were equipped with the real time wave processors described in Appendix 2 allowing us to follow the testing in real time. _{Preliminary results based on the real tjme ARGOS} data has been presented in Jeiand and Barstow (1985).

a)

b)

I

Figure 4.1 Th* location of

th

field test site to

_{the west of}

Ealtenbanken on the Norwegian

Shelf.

I -- I I- 4 4

18 18 20 21 22 23 24 .23 31

OCTOBER

Figure 4.2 Coarison of

aYprincipal wave direction,

iDtR,

and b) wind direction for real. ti ARCOS

data C)

and post!-processed data recorded on the bi.oy' $

_Sea

Data loggers

12

-Fig. 4.2 shows that the data collected in _{real time by the ARGOS}

system

is in good agreent with the directional _{wave data poSt-analysed}

at

IKLT fr the Sea Data tapes.

4.2 Data Analysis

Both the. VESCAN and the ODAS-492 buOys (Fig.4..3) _{asured synchronous}

time series Of heave, pitch, roil _{and ccmpa5a together}

_with

wind speed, direction, air pressure, _{sea and air temperature.}

Time series were 34 mine, in length and were sampled _{at 1. Hz giving}

a Nyquist frequency at 0.5 Hz. Measurement sequences _started

every third hour, ending on the hour. The _{AS-492 buoy was equipped with a Dataweil Hippy}

12QA wave

sensor whilst

the

_{'WIVESCAN system had a Hippy}

40. Installed, The.

crmpariscn between the t système is based on _{the period fr 18th} October - 9th November 1984. Wave _{conditions varied,}

_including

a nter

of storms, maxi

significant wave height reaching 6.5 metres. A swell event and a number of situations with crossing wave trains _producing .iltindal wave spectra also occurred. _{Wave heights were. rarely under} 2 metres.

4

4.2.]. Dynamic response

The first stage of the analysis was to derive the slope, _transfer

functions. Both buoys were subjected to

this analysis, i

_{.e. we did not}

'as a priori that the nominal ODAS 492 transfer

functions

_(Barstow and Krogstad, 1984) were correct. The

method

by which the transfer functions are derived is described _{in Appendix 1. The buoys' response}

to wave slope is assud to be that of a forced linea,r oscillator with associated pitch/roil _{eigenfrequency and damping ratio. Before we} correct for the buoy's dynamic _{response, the phase of the heave/slope} cross-spectra is as shown in _{Fig. 4.4a. In Figures 4.4b and c we show} the phases _{after correction for transfer functions accocding to the} best fit eigenfrequency and damping ratio. _{The corrected phases have} nearly ideal behaviour for both systems. The eigenfrequency derived for VESCAN was fomd to be _{0.43 Hz which meets the design objective and} agrees closely with the measured ndel 'eigenfrequency.

a)

I

90-

0.-f--000

025

050

Frequency (Hz)

13

-WAVESCAN

_{0DM 492}

Figure 4.3 1VQN and

S

492 bioys

at Halteribanken during the

inte tcciari

son test.

- WAVESCAN

OOAS 492

go.

0. 0

000

025

050

000

025

050

Figure 4.4 Cross spectral phases fo VESCAN (- )

and ODAS

492

(

.);

a) before. correction by transfer functions, b) 2 In

< HMO < 4 m (best fit transfer functions) and C) kfi10 > 4 in

(best fit transfer fwctions). The t curves shown for each system correspond to elevation/east

and

eievatioiViorth phases.

-ff

015/2/1 4/E1S

- 14

By using this approach for deriving the slope transfer functions it

s1u1d. be pointed out that only the phase is checked. _{The check ratio,} RD (see Apçendix 1) is dependent Only on the dulus of the transfer

functions for a sytric boy systen. In

_{Figure 4.5, the. nan RD is}

plotted vs. non-dinsjonal frequency, f/f1 _{for all the 173 records in}

the first depioynnt period.

RD is very cloSe to i. near the. spectral peak frequency as we would expect. _{We also notice that}

RD increases slightly over unity for the _{higher frquencies. This may partially}

be due to _{.zrent advection. The deviation below f is caused}

by spectral estimation leakage (Ezraty et _{al., 1981) and Second order effects in} the wave field (Krogstad, bc. cit.).

Figure 4.5 Deep water dispersion (check) ratio RD vs.

_{non-dinsional}

frequency, f/fe where.f is the peak wave frequency

-l/I'

The second major design criterion was that the system should be stable in extren waves. Experience so far has riot revealed any stability problems. At. the ti of. writing, the VE CAN buoy has successfully survived storms with single waves up to 20 itres and significant wave heights over 10 ntres without incident.

015,'245/EIs

15

-4.2.2 Qiobal directional wave paramsters

Qe of the standard

checks

carried cut on directional data is to

corre.-late the wind direction and a

high

_{frequency wave direction estimate} (usually at about 0.4 Hz). This has been done for the october deploy-nt data sets (Fig. 4.6). _{The correlation is good for both systems,} VESCPN's data

_showing

less _variability. This is probably at least partially due to ure accurate wind djrection _{zasuremsnts for VESCAN}

which coutes a vector average _{over. 10 minutes whilSt for}

_OAS-492

only 6 instantaEieous measurements are averaged. Wind speed and direction are plotted together in Fig. 4.7, and the measurements are generally very close.

In figure 4.8 we show significant wave height (0), peak _{period (TP,),}

main wave direction _{(1IR) and wiidirectivity index (Ut) for both}

systems. All parameters are

defined

in Appendix 1. we draw the following conclusions to this carison:

Generally no significant difference

has

been fow between, the

estimates. The salinq variability of *1O is about 0.3 in.

TP : for HNO, but son deviations, _{particularly when the wave} spectri. has iltidal characteristics. This is to be expected when t spectral peaks have similar energy levels. Note that TP has relatively larger statistical variability. IR Generally little bias apart from when directions are around

north. This effect is. also seen in Fig. 4.6 and is caused by a minor instrtnt scaling error for ODAS-492.

This parameter is a good indicator of xw.iltiidality in the wave field (see Appendix 1). The period from 23rd to 25th shows a particularly marked deviation under. 1 and corresponds to a period of offshore winds (Fig. 4.7) with at the same time swell arriving from at various times the Atlantic and the Barents Sea. The estimates of UI show no large. deviations between the t systems.

Based on the above the VCAN system appears to measure and record wind and wave data. satisfactorily.

C C

0

b)

0

0

I. I I -( I I i 1 I OCTOBER

16

-a) WAVESCAN

b) ODAS 492

I-- I I I I I I I I 1 I I I $ I I

I,v,

I I I- I I I I I I i I I

i8

₂ 24

.27

30 I 2 8 NOVEMBER

Figure 4.7 Cariscn of

a) wind speed and b) wind direction for

VESQN (' ) and ODAS 492

C

+ 4 .4 .4 4 % 4 te.,..

4/i.

4 .4 4.4I 4

- --

-4 4

4.'-+ + * : 4 ' 4 4 + 4 MG -4 .

.4

-.

0 180 270 360 0 90 180 270 360 0tR

Figure 4.6 High frequency wave direction (at ca 0.4 !!)., T!ThF _against

bi.ioy

nasured wind direction

_{for a) VESCAN and b)}

OCS. 492.

C

C

a AL

a

N

a

c) I I I-- -I I I I I -, I I

I-

I- I -I 1 ' 17 -I I I I

I--I.

I I I I -I---I--I--I I I I. I I l I -I I I -I 1 I I I I 0) i

0

0-0

a

0

0

N a o

d

21 1 24 27 OCTOBER I - I I -I I I --1 I -I- -I I I I I

'p

I I

I-I

I I --I-- I

I-I I --

1- I

I I I I 30 2 8 8 NOVEMBER

Figure 4.8 quiparison of a) Significant wave height, HMO, b) peak wave period, TP, c) principal wave direction, IIDIR, and d)

unidirectivity index, UI for WAVESCAN (.'.) and OCAS 492

(

).

18

-4.2.3 Special Events

By studying the full directional wave spectr _{we can get a fairly good} idea of the systems ability to correctly respond to given

teorologica]. conditions. Three special _{events from the first} deploymsnt are described below.

EV A (Fig. 4.9a) Unidirectional _{wave field,}

_high

sea state. Within 3 days of the WAVESCAN prototype's _{first deployment}

it was subjected to an extre _{test as an energetic low ixcved}

north eastwards from the North Sea towards the _{Q.ilf of Bothnia}

_on

_{the 21st.}

The winds

on the

depression's rear were _{strong and northerly.}

Significant wave

height

rose _{quickly to over 6.5 metres with}

single waves up to 11 metres. _{The directional wave spectri. for}

0600 G'TI on _{the 21st i3}

shown for both buoys in Fig. 4.lOa. _{The mean wave direction}

may be

seen to vary little with frequency.

EVENI' B (Fig. 4.9b) Low sea state, bida1 wave field, swell/windsea. Total significant wave height was _{approximately 3 metreS early on the} 25th October.

The

one dinsional wave spectrt _{(Fig. 4.lOb) could}

easily be mistaken for a _{single peaked unidirectional}

spectrun but in fact the plot of mean wave direction (Thi(f)) _{shows up a bithodal wave}

field with an easterly wind _{wave part and a south westerly}

Swell,

separated in direction by about 130°. _{Both systems show these features} although there is a small bias in directions. _{The wave spread shows a} characteristic dip at the spectral peak with a frequency band with high spread in the region _{between the swell and wind wave part due to}

the

bindality of the directional, distribution. _{That these features are} physical may be seen from _{the accompanying weather chart for the 25th} (Fig. 4.9b) which shows a system of twin _{depressions the nre easterly} of the t giving the local easterly winds whilst the second had remained fairly stationary in that position for about 2 days producing the coincident swell on the 25th.

EVr C (Fig.

4.9c) Veering winds, biidal wave _{field, ixderate sea}

state..

A small cyclone _{ved. northwards along the Norwegian coast on the 30th} October resulting in a rapid change in local wind direction from south east through south to west. _{The cyclone intensified as it nved north}

over Haltenbanken producing the strongest winds of the period.

19

-In addition, a strong swe].i. signal arrived _{a3..st simultaneously}

to the cyclone passage through the area. This _{swell was generated a}

couple of days earlier by a strong depression to the south west _{of Iceland.}

The contour plots of

the

_{deve1oent of}

_the

_{directional wave spectrt}

(Fig.

4.11) during this event nicely _{illustrate this bidal}

event and the

ability of the heave, pitch _{and roil buoy to both follow the}

response of wave direction _{to the veering winds,}

_and

secondly to resolve the

swell arid

wind

_{wave parts of the spectr.}

a) 2lstOctoberl984

_{b) 25th October 1984}

Figure 4.9 _{Surface weather charts from 12 QT for the events discussed} in the text (COurtesy of the journal Weather).

O15,'2/19/Els

a-a

3-0. TO 0.*O 0.5.3

.r_"_3_

:.

0.00 Q.?0 .2O O.O O.0 0.50

5ECY

20

-a) b) a WAVESCAN

- 00AS492

a a a C a 0.00 0.20 0.30

Figure 4.10 Directional. wave spectra for a) 21st October 1985 and )

15th October 1985 and for WAVESCP,N ( ) .and OAS 492

:). _S(f) - elevation wave _{spectr.=; 1*J(f)} mean wave direction; SPR(f) - circular Standard deviation (see Appendix 1).

-3.40 3.30

2 8

r

F

2 2 9 8 2 8 2 9 2 S S 2

'4

<

(-I

9 S I' - -- -t 8 SO 120 'SO 240 300 5 0 av.v,J

-

21

-ODAS 492 WA VESCAN _WAVESCAN

'20 ' zic 3 S S '00 240 500 Q.Ot

rt

0.10 '.00 '0.00

Figure 4.11 COntOur plots of the directional distribution D(f, 9) during

a m.tidal event with veering wind. Wix direction is

sh.'n by the vertica.1. 1ixe. The one- di.nsionsa1 ve spectr is shown, on the rit land side and is. clearly double peaked to start. with, the t peaks -rging at 0900 as the wind speed increases and tI wind sea veers

roi. to near the swell direction.

WIND SPEED _9.Orn/s

0000 GMT

WIND SPEED 9.8m/s 0300 GMT

WINO SPEED 13.6m/s 0600 GMT

WIND SPEED 16.6 mIs

0900 GMT 3.10 1.30 '0.30 I00.301.30 WOY. Sctr.. (s-.) 500 0 SO 240 0

22

-5.

causias

A new metocean data buoy, _{ThIVESCN, has been developed.}

This system extends the capabilities of medit size data collection buoys to include full in-situ data processing of _{wave directional data which}

is transmitted to the user in real time by _{the ARGOS satellite system.} This gives the user both security ir the _{correct functioning of the}

buoy and the _{possibility to access a].l wave,}

and _{meteorological} parameters in real time. _{Event cOntrolled data 'collection}

is also an available option. The new design _{is dular and compact which}

eases operation and maintenance. The onboard _{microprocessor may be prograd} to meet special user needs. _{Finally, a cost-effective}

system has been

pro&ced without sacrificing reliability. _{Experiences so far have} shOwn that:

- _{Buoy handling and ease of operation is good.}

Onboard data processing of wave and meteoro].ogicai data is

adequate.

- Data recovery has been very good (>95%) in the two _{xrnth test} periOd.

- Wave directional data is of sufficient quality, is _{corisi stent} with internal check parameters arid with data _{from another buoy}

system.

- Wind and other meteorological data is of good quality. - The buoy's stability is good.

Thus, we may conclude that _{VESCAN has reached its design goals and it} has been further emphasized that _{the basic buoy hull shape consisting} of a disc with counterweight can be employed to collect accurate directional wave data whilst at the sa time allowing the collection of standard meteorological data.

AS

The authors acknowledge with thanks _{the support of Norsk Hydro A/S,} Statoil and the Royal Norwegian Council of Scientific arid Industrial Research for sponsoring in part the development _{0 VESCM4.}

- 23

we also wish to thank Norsk. Hydra A/S, Saga Petro1ei., _{A/S, BP Petrole} Develcsi&pt, Phillips Petrolei. Co. Norway, _{Elf Aquitaine Norge A/S,} Conoco Norway Inc., Esso Expi. and _{Prod. Norway Inc. and the Royal} Norwegian Cotmcil for Scientific and Industrial Research (NnJF) who supported the CLAP project under which the ODAS-492 data _{was collected.}

BARSTi, S.?., 1984: cperimantal

study

_{of. the ztions of idel data}

b.ioye, BS Project/IJ Rep. _{no. 01.0078.04/3/84, 176pp.}

(Confidential).

3MSTi,

5.1'. and

GSD,

LE.,. 1984: General Analysis of

directional ocean wave data fr _{heave/pitch/roll buoys,} Modelling, Identification and Control, 5, No. 1,. 47-70.

,

BAT, R. and CAVANI!, A., 1981: Evaluati de la sure. de la

direction des

_{vagues a partir des dees d'une}

_bcu6.

instrntê.. Oceanologica Acta, 1981, 4, pp. 139-149. AT1, R.,

_{OtLINiLT, N.}

_{andAYELA, G.., 1983: Awave directional}

spectrue buoy

using AS. Proceedings of the Oceans 83 Conference, San FranciSco, 1, _{pp. 258-261.}

HOF)N, D.,

(!TTWq,

E.S. and NIElAN,

C.S., 1973: _Mathematical SiAlation and del tests in the desiç of data buoys, Trans. Soc. Nay. Arch. Mar. big. _{81: 243-73.} KAPLAN, P., RAFT, A.I. and T.P., 1972: Experimantal and

Analytical Studies of Buoy Hull

_NOtions

in Waves, _Report

- 72-89, Oceanics Inc., Plainview, N.Y., U.S.A.

Prepared for National Data Buoy Center, Contract .S 8-26879,

L61pp.

t., R.3;

1980: The Statistical evaluation of directional spectri.

estimates derived from Pitchofl buoy data, J. Phys.

Ocean.

₂

373-81.

LaUJT-IGGINS, LS.,

CAR1WRIGT, D.E. and

SNI, M.D.

1963: (servations of the directional spectr of sea waves using the

ition of a floating buoy.

_{Ocean Wave Spectra,} pp. 111-36., Prentice-Mall, Mew York.

Z'1ARECAL, N., 1973: Ocean Data _{Collection Plat-form Survival.} Iaterocean '73, pp. 1058-68, D(isseldorfer _{Messegelschaft}

mba.

PIT1', E.G., PASCALL, R.W.

_{and van HErJ, J., 1985: A Cciarison of}

the Measuremants made by t Pitch-Ro1l buoys during the

NUEC Project. Proceedings Ocean Data Conference

-Evaluation,

Caarison

and _{Calibration of Ocean} Instrtmnts,

_{Stir, London,}

Jwle .1985.

STR, R.H., 1977:

A discus-hulled wave asuring buoy,

_{Ocean Eng.}

4, 101-7.

T.JELIAND, G.

and BARSq, S.?.,

1985:

A wave direction buoy alls

presentation of wave direction data in near real _tirn. Presented at the AOS Users

_{Conference, xiel, May 1985.}

A:PpENDIx

1.

DIRECrIL

VE

_{MLYSIS AND BY TRANSFER FUNCrI4S}

The directional wave

_pectr

_{may be expressed as the product}

_{E C f,}

_e)

-5(f) D( f, 8) where S is the one dinsiona.L

_{fiequency spectri.}

_{and D is}

the directional distribution,

2zD(f,$) - 1 +

2Z_1(a(f)cos nG

₊

b(f)sin

_nG).

The Fourier coefficients

a1, b1, a2 and b2

are related to the auto- and

cross-spectra, S - C - iQ, of the ti series of heave and slopes,

- (C

-

C)/(C

C)

b2

- 2C/( C

₊

C).

Here x an y refer to slopes and h tO heave (Long, _1980).

A real buoy does not record

(flfl1rL) perfectly.

_{However, if}

hori-zontal buoy cvement is neglected

and

linear theory is asSund, it

turns out that

the corresponding

measured signals

_{(bbsb) relate}

_to

(flrisn)

by linear filters. For the Fourier

_{transforms we thus obtain}

015,'2/24/Els

a1 -

'(cth(c

C))1"2

b1 -

+

c))

-

24

-where T,

and T

are the transfer functions.

T is primarily

diffe-rent from

1

due tO integration of the acce].eroiter signal,

_{whereas T}

and T have hydrodynamic origin.

-Tfl

-

(Al.2)

Oi.5/2/25/E1s

25

-It is easily verified that the _{urier éóefficients based on the}

buoy signals satisfy

a a cos(arg. T - arg T)a1

b - cos(arg T arg b

a2 - a2, b

b2b2

= T1,. _{Thus, for a circularly syetric}

System the scaled Fourier coefficients are completely _{thdependent of. the amplitude}

transfer fwctions and (a21b2) are also independent of phase transfer fmcticns. The definitions given in (Al.l) are this. robust with respect to

transfer fcUort

errors. In

general, C, C

_{and Q,}

_have expectation zero (Barstow and Krogstad, 1984) Therefore,

as T is

known, arg(T) followS from t.e equation

Sbb(f) T(f) T(f) Sfl _(A1f4)

where . is thea complex conjugate.

If we asse that the. buoy acts as a forced linear OScillator, then

f2

T(f)

- +

+ f)

(A]. .5)

where the eigenfrequency (f0) and damping ratio _{(A) may be estinted}

from the. _{asuted arg( Ta).}

The above approach checks Only the phase of the transfer ftthction. An

idea of

the amplitude response my be derived _{y considering the ratio} between the linear wave theory _{wavenurnber, k(f) - (2itf)2/g (in deep} water), and the iasured

_{wave mber:}

It is easily sh that b

ITI

Rb-T RD x 015/2/26/E1S C

+C

xx 1/2 26 -(kl.6) (Al.7)

Hver, unlike the cross-spectra]. phases, the CheOJI ratio may depart

fr 2. due to current advection and the presence of nCc-1-inear

contributions in. the wa"e field (see Barstow and Krogstaci, 1984 for an

asSesnt of

these effects). In practice. departures fr the

expected values of the cross-spectral

phases and

dispersion ratio could be minimized using the linear oscillator del to give the best fit to theory, or by directly using the nasured transfer functions.

The following directional parazters have been used in this paper

Mean wave direction, e0(f) - THE'(f) arctan(b1/a1)

Circular standard deviation or spread,

0(f)

a (2(1 -, (a

+b"2

/2

Main wave direction, YWIR - arctan(b/a)

UxtidirSctivity index UI a (a2 + b2)1"2

Here a - (IS(f) cos(e0(f))df]/EIS(f)dfJ and similarly for b

with

"Sin"

replacing "cos".

U gives a good indication of the directional homogeneity of the wave fIeld. If all .an wave directions are the. sam, then UI a 1. When UI is close to I, IR is close to

at T.

For a sytric buoy, e

is insensitive to transfer functions, whereas c depends only on the

27

-APPENDIX 2

SYST

DESIPTI

The SESCAN data col.lectiori _{and data dissemination System consists of} three main elements; the _{VESCIN data buoy, the data telemetry link,} and the monitoring and presentation sstena. The datalink is implemented using Service Argos,

_which

_{in addition provides positioning fac1ities} for the buoy. Figure A2.1 indicates the operational principle and a.lternatie dataflows using the Argos system.

VESCN( provides a complete _{software controlled data acquisition and}

processing imit including a set of utility

routines for quality checks

and

acquisition

control. Changes

of

sampling rates, sensor set-up,

prcessing algoritm, data trarsmiSsion software etc. _{are all carried}

0 either by

changing

control parameters or adding subroutines. The on-board microprocessor is progranmd in Micro Concurrent Pascal, providing the system with capabilities not previously available in this type of data collection equipnt. Fig. A2.2 shows the data flow between the main parts o the system and a more detailed block diagram of the data, acq sition

d1e

iS given in Fig. .3. It consists of the following items:

Single board S microcomputer with 192 memory

- 12 bit CbS analog to digital converter with 32 input channels

- Digital input channels - "Watch dog" logic

- Power save logic

- Power supply with subsystem on/off switching

- AS P

- Sea Datatape recorder -. Field and lab test terminal

Ml the

electronics are housed in special rugged, splashproof boxes placed inside

the

splashproof

thstrflt cylinder in

the buoy centre. Low power components are used whenever possible and all parts of the system are turned off when riot in use. The niicrocomputer turns into a power save mode when idle. SincC the system is desigTted using the

standard bus structure (Eu), new function modules can easily be added.

Sensors

Figure A2.1 Real !j Z'bnitoring of Metocean 'Paraters by the SESQN

system. Field test terminal Acquisition and

proceing unit

User specifications

Figure A2.2 Buoy Data System.

28 -Satellite transmitter (ARGOS) Onboard recorder

DAM

UANI

Figure A2.3

Data Acquisition ModuIe.

AOOML I2Ia.

I

ri nfl

_cAH:

WATCH gus

3.iesr $tpSI

t. Oat.i

wIutIsn

U

Dii.

_t.iirel

OL

Ns50 PrecssuIn VeCiW A,crsglii rn. Oslo NUIoi. Hiss HicW Niad: Hi_i DiolrsI I Hici DwrsM Osuidis Have Pousr scirus SD Q..i. sdrs Hiss DIIW'IIssid lru

Figur. A2.4

Data flow in-Situ processing.

SAflUIU ISV _ZIIA, ISSV

fl$fliM

. ISV UAT1 'A"

I"

'Vs

-0

'UT IaUHI. uo vilo Mta VN SD

'--44

II. TI WAVI I I

RIC

_"(U.

('U

_I4'

F

cA

I COIMU lb

-)

U

'OW" 'OW" 'AVI yrimialislin F.r.sltsr i.ra.ilsr L _

--

__.J

U

_'I'll

_DI.,'" .5.1.IIII. I.,.

13

U

015/2/30,tls

30

-A2.i. Basic system configuration

The prototype buoy is equipped with the following _sensors:

Heave, pitch, roll: _{Datawell Hippy-40}

Compass: _{Digicourse 1O1E (fluxgate)}

Wind: _{Srod1eS and Gateh use Model}

152,211.

(Sensor height 3.5 m)

Barozter: _{ANES8O Solid State Sensor}

Air and Sea Temperature: _{Aanderaa 2775 and 1.229}

The Hippy 120 sensor

Will

also be used in future versions. These sensors form a basic set, _{but additional sensors may be fitted to the} sensor mast or the buoy hull.

The data is collected at regular intervals, _{at present at synoptic ti} every 3 hours at 1. _{Hz and over 34 minutes. These and other parameters} are user progrnn.ihle within limits set by the operational period _and the data storage capacity. Typical Opératiotial period is 3 rcnths. The satellite trariitter sends 256 bits every minute, updating _the transmitter buffer every observation period. _{The standard real time} data set consists of the following parameters:

Meteorological data

- Wind Speed (10. mn. vector average))

WindDirection

B5rtric Pressure

- Air Temperature

Sea Surface Temperature

Oceanographic data

- Mxiim wave height and its corresponding period

- Heave spectrum, S ( f)

Diretiotal Fourier coefficients a1(f), b1(f), a2(f) and b2(f) - Spectral 1I1mnts

- Significant wave height, O

31

-- Mean wave direction, e(f)

- Main spectral wave direction, 1IR - tidirectivitj index, UI

- sigh frequency wave direction - Wave direction at spectral peak - Swell energy and direction

Operational status

- Absolute ti Of last

asurent

- Battery Voltages

Active status of each sensor - Data .ia]4ty Control Flags - System Status Flags

A2.2 User selectable paraters

The stafldard system software covers a wide range of applications by specifying new sets of constants in the paranter lists. For the data acquisition one may specify the sensor configuration with individual sampling frequency,

ntber of

samples,

senor

type etc. Selected results and raw data. are recorded as files on tape.. The file format may be specified for each application, and event controlled recording is

possible; Likewise, results may be specified for transmission via satellite, the transrnission. format may eaSily be altered and conditional transmissions are possible.

Analog sensors may in general be added directly to the system, provi4ed that they are electrically coatible. Inclusion of new digital sensors

wil].

generally require s prograimning.

Inclusion

of custom designed processing is, in st cases, a straightforward task, provided enough

mary is available.

A2.2.I In-situ processing

The comptitational parts of the system cOnsist of general and special processing. the general processing c be specified for any sensor, whereas the special. processing is Sensor specific. The general processing includes varioUs interpolation and averaging procedures as well as data control features such as checks for spikes, short and long term trends and signal drop out. The standard system special

015/2/32,'ElS

32

-processing includes full wave _{directional analysis based} on

mearnts from

the Hippy sensor. _{Typical processing time is}

13

minuteS for a 1O24-sa'le record length _{and five data controls.}

A2.2.2 Recording

Nearly any file format can be acconb,dted, _{and the processor can} handle several file types if _{necessary, e.g. one file could}

contain

only raw data and

another results _{from. the in-situ processing.} Recording features include:

- Alternate file types

- EVSflt controlled recording

- Easy definition of new file formats

-

Remaining

length of tape status _{(available for transmission} to shore)

- Default tape format for standard syst

A2.2.3 Data Transmission

- Multiple ARGOS pages in

_one

message Alternate transmission format

- Event controlled transmission format

- Easy definition of new transmission format

Default transmission format fOr the standard system

A2.2.4, Field operator cr1Im1nicatiOn

This utility serves as an aid to verify correct operation of the data buoy and includes

Menu driven coiunication - Self explanatory operation - Field test utilities

Site report

- System setup display.

- Raw data and results inspection

- AOS message display

- System status displa